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Can anyone help me in understanding how backcrossing helps hybrid to achieve pureline?
I have been looking into the references i possess but couldn't seem to find anything
Back crossing is the process by which a hybrid organism is crossed back (backcrossed) into a pure organism. Let's see an example
Organism 1 is heterozygous for a single gene (e.g. a hybrid), which has alleles that we can call $A$ and $a$. We can write that organism's genotype.
Organism 1 Genotype: $Aa$ (100% chance)
Right now, there is a 0% chance that organism 1's genotype is $AA$.
Goal: increase probability that organism 1's genotype is $AA$
One way to achieve this goal is to cross Organism 1 with another organism, let's call it Organsm 2, that is has a homozygous (e.g pure) genotype of $AA$.
So if Organism 1 is crossed with Organism 2 and produces a new Organism 3 what is its genotype? Well, we're not sure.
Organism 3 Genotype: $AA$ (50% chance)
Organism 3 Genotype: $Aa$ (50% chance)
Now we do the same thing again, to get Organism 4
Organism 4 Genotype: $AA$ (75% chance)
Organism 4 Genotype: $Aa$ (25% chance).
This effect keeps compounding, until there is a very high chance that the final organism has a pure (e.g. $AA$) genotype.
What is a test cross in biology?
Medical Definition of testcross : a genetic cross between a homozygous recessive individual and a corresponding suspected heterozygote to determine the genotype of the latter.
Similarly, what is Dihybrid cross in biology? A dihybrid cross describes a mating experiment between two organisms that are identically hybrid for two traits. A hybrid organism is one that is heterozygous, which means that is carries two different alleles at a particular genetic position, or locus.
Beside above, what is test cross with example?
In a testcross, the individual with the unknown genotype is crossed with a homozygous recessive individual (Figure below). Consider the following example: Suppose you have a purple and white flower and purple color (P) is dominant to white (p). A testcross will determine the organism's genotype.
What is the importance of test cross in genetics?
A test cross is pretty important in genetics as it helps you determine an unknown genotype. In a test cross, a homozygous recessive(both alleles are identical) individual is crossed with an individual with unknown genotype, exhibiting a dominant phenotype.
Varying Rates of Speciation
Two patterns are currently observed in the rates of speciation: gradual speciation and punctuated equilibrium.
Explain how the interaction of an organism’s population size in association with environmental changes can lead to different rates of speciation
- In the gradual speciation model, species diverge slowly over time in small steps while in the punctuated equilibrium model, a new species diverges rapidly from the parent species.
- The two key influencing factors on the change in speciation rate are the environmental conditions and the population size.
- Gradual speciation is most likely to occur in large populations that live in a stable environment, while the punctuation equilibrium model is more likely to occur in a small population with rapid environmental change.
- punctuated equilibrium: a theory of evolution holding that evolutionary change tends to be characterized by long periods of stability, with infrequent episodes of very fast development
- gradualism: in evolutionary biology, belief that evolution proceeds at a steady pace, without the sudden development of new species or biological features from one generation to the next
Varying Rates of Speciation
Scientists around the world study speciation, documenting observations both of living organisms and those found in the fossil record. As their ideas take shape and as research reveals new details about how life evolves, they develop models to help explain rates of speciation. In terms of how quickly speciation occurs, two patterns are currently observed: the gradual speciation model and the punctuated equilibrium model.
In the gradual speciation model, species diverge gradually over time in small steps. In the punctuated equilibrium model, a new species changes quickly from the parent species and then remains largely unchanged for long periods of time afterward. This early change model is called punctuated equilibrium, because it begins with a punctuated or periodic change and then remains in balance afterward. While punctuated equilibrium suggests a faster tempo, it does not necessarily exclude gradualism.
Graduated Speciation vs Punctuated Equilibrium: In (a) gradual speciation, species diverge at a slow, steady pace as traits change incrementally. In (b) punctuated equilibrium, species diverge quickly and then remain unchanged for long periods of time.
The primary influencing factor on changes in speciation rate is environmental conditions. Under some conditions, selection occurs quickly or radically. Consider a species of snails that had been living with the same basic form for many thousands of years. Layers of their fossils would appear similar for a long time. When a change in the environment takes place, such as a drop in the water level, a small number of organisms are separated from the rest in a brief period of time, essentially forming one large and one tiny population. The tiny population faces new environmental conditions. Because its gene pool quickly became so small, any variation that surfaces and that aids in surviving the new conditions becomes the predominant form.
Backcross Method: Meaning and Features | Crop Improvement | Botany
In this article we will discuss about:- 1. Meaning and Features of Backcross Method 2. Genetic Basis of Backcrossing Method 3. Breeding Procedure 4. Achievements 5. Merits and Demerits.
Meaning and Features of Backcross Method:
Backcross refers to crossing of F1 with either of its parents. When the F1 is crossed with homozygous recessive parent, it is known as test cross.
A system of breeding in which repeated backcrosses are made to transfer a specific character to a well-adapted variety for which the variety is deficient is referred to as backcross breeding. The backcross method of breeding is commonly used in self and cross pollinated species. In vegetatively propagated crops like sugarcane and potato this method is rarely used and that too with some modifications.
Main features of this method are briefly presented below:
The backcross method is generally used to improve specific character of a well-adapted variety for which it is deficient such as resistance to a specific disease. This method is more commonly used for transfer of monogenic or oligogenic characters than polygenic characters.
In other words, it is more successful when the character has high heritability. Oligogenic characters have high heritability than polygenic traits. Backcross method is applicable in all three groups of crop plants, viz. self-pollinated, cross pollinated and asexually propagated.
Backcross method involves two types of parents, viz. recipient parent and donor parent. The parent which receives a desirable character is known as recipient parent. The recipient parent is repeatedly used in the backcross method hence it is also called as recurrent parent.
The recipient parent is generally a well-adapted high yielding variety of an area which is deficient in one or few characters. The parent which donates the desirable character is known as donor parent. Since donor parent is used only once in the crossing, it is also known as non-recurrent parent. The donor parent is generally poor in agronomic characters. Thus backcross method is used when one of the parents is un-adapted type.
3. Genetic Constitution:
Backcross method retains the genotype of original variety except for the character which is improved by backcrossing. In other words, the new variety resembles the parent variety in all the characters except for the character under transfer.
4. Number of Backcross:
Generally 5 to 6 backcross are sufficient to retain the genotype of original variety with new character.
The basic requirements to start a backcross programme are:
(iii) High heritability of the character under transfer.
Genetic Basis of Backcrossing Method:
Backcross increases the frequency of desirable individuals in a population. For example, from a cross involving single locus (AA x aa), we will get only 1/4 desirable individuals (AA) in F2 through selfing (1AA: 2Aa: 1 aa). In case of backcrossing (AA x Aa), we get 1/2 desirable individuals in the BC F1 (1 AA: 1 Aa).
The same is expected for each gene pair. The population gradually becomes identical to the recurrent parent. The population is not divided into 2 n homozygous genotypes as happens in case of selfing.
However, in backcrossing homozygosity is attained at the same rate as with selfing which is given below:
Proportion of homozygous individuals = [(2 m – 1)/2 m ] n
m = number of backcrossing or selfing and
Moreover, the chances of breaking linkage between desirable and undesirable genes are more with backcrossing than with selfing. Suppose, gene A is desirable and it is linked with undesirable gene b. The desirable gene A has to be transferred from a donor to a well-adapted variety.
The cross between adapted and donor parents will produce AaBb hybrid. The genes A and ‘a’ have the tendency to, inherit together to make it difficult to obtain AB combination. Since gene B is reintroduced with each backcross, there will be several chances for the crossover to take place.
Thus the probability of elimination of b gene is as given below:
Probability of eliminating of b gene = 1 – (1 – p) m+1
p = recombination fraction and
Breeding Procedure of Backcross Method:
Some characters are governed by dominant gene and others by recessive gene. The breeding procedure of backcross method depends on whether the character under transfer is controlled by dominant or recessive gene.
The breeding procedure for both the situations is briefly presented below:
1. Transfer of Dominant Gene:
Suppose wilt resistance in cotton is controlled by a dominant gene (RR). The donor parent is a strain (B) from the germplasm. The resistance has to be transferred to an adapted variety (A) which is susceptible to wilt. The adapted variety (A) will be used as recurrent parent and strain (B) as donor parent.
The F1 will be wilt resistant but heterozygous (Rr). Backcrossing of F1 (Rr) with susceptible variety (rr) will produce resistant and susceptible plants in equal number in BC1 F1 (1Rr: 1rr). The resistant cotton plant (Rr) can be identified by growing the material in wilt sick plot. The resistant plants (Rr) are then backcrossed to the adapted variety.
Generally, 6-8 backcrosses are sufficient to obtain plants identical to adapted variety except for the added genes for wilt resistance. The wilt resistant plants are heterozygous (Rr). They are selfed for one generation to obtain homozygous (RR) resistant plants. All the resistant true breeding plants are bulked and new variety is released. The variety developed in this way is identical to the adapted variety (A) expect for wilt resistance (Table 20.1).
2. Transfer of Recessive Gene:
Suppose wilt resistance in cotton is governed by a recessive gene (rr). In such case, the progeny of each backcross will segregate into two genotypes (RR and Rr) which cannot be identified. Therefore, it is necessary to self the population after each backcross to obtain resistant homozygous recessive plants (rr).
The resistant plants are identified by growing the F2 material in wilt sick plot. The resistant plants are backcrossed with adapted variety. Here each backcross is followed by one selfing, whereas with dominant gene continuous backcrosses are made.
3. Transfer of Quantitative Traits:
Backcross method is generally used for transfer of monogenic or oilgogenic characters. It can also be used for transfer of polygenic traits. However, transfer of polygenic characters is somewhat difficult due to low heritability of such characters and more influence of environment in the expression of polygenic characters. For successful transfer of polygenic character, the non-recurrent parent with extreme phenotype for the polygenic character under transfer should be chosen.
For example, if we want to improve protein percentage from 20 to 25%, we should select non-recurrent parent with 30% protein. This will make identification of the character easy. Moreover, after each backcross one or two generations of selfing are required, to get the desirable segregants. Furthermore, large populations have to be raised to achieve the desired combination. In other words, the observations should be based on large samples.
Sometimes, several characters have to be transferred into an adapted cultivar through backcrossing.
This can be achieved in two ways:
(i) Transfer of genes in separate backcross programme and then combining them into single genotype, and
(ii) Simultaneous transfer of genes into single genotype in one backcross programme.
For simultaneous transfer of multiple characters, backcross seeds have to be produced in more quantity to be sure to get a genotype with all desirable genes.
Achievements of Backcross Method:
Backcross method has been widely used for the development of disease resistant varieties in both self and cross pollinated species. It has also been used for interspecific gene transfer and development of multiline varieties in self-pollinated species. Several varieties resistant to various diseases have been developed, by this method in wheat, cotton and several other crops.
In cotton varieties V797, Digvijay, Vijalpa and Kalyan which belong to Gossypium herbaceum have been developed by backcross method. A brief comparison of pedigree, bulk and backcross breeding methods is presented in Table 20.2.
Merits and Demerits of Backcross Method:
1. Backcross method retains all desirable charters of a popular adapted variety and replaces undesirable allele at a particular locus.
2. This is a useful method for transfer of oilgogenic character like disease resistance. It is also useful in the incorporation of genes for quality such as protein content.
3. This method is extensively used in the development of varieties with multiple disease resistance. Multiline varieties carrying resistant genes for different races of pathogen are also developed by backcross method. This is used for development of isogenic lines and multiline variety, a mixture of several isogenic lines.
4. The male sterility and fertility restorer genes are transferred to various agronomic bases by this method.
5. This is the only breeding method which is used for interspecific gene transfer.
6. The variety developed by this method does not require multi-locational testing, because it is identical to parent variety except for the character under transfer.
1. This method is used to rectify the defect of an adapted variety. The new variety differs from the old one only in respect of defect which has been rectified.
2. It involves lot of crossing work. The backcrosses have to be made for 6-8 generations. In pedigree and bulk methods hybridization is done only once.
3. Sometimes, undesirable character is tightly linked with desirable one, which is also transferred to the new variety.
The importance of mating between species in nature is becoming more apparent as molecular studies reveal extensive evidence of hybridization events. Thus, the study of hybrids and reproductive isolation is now central to our understanding of the origin and maintenance of species [4, 6, 34]. In the present study, the use of experimental hybrids between the anther smut fungi, M. lychnidis-dioicae and M. silenes-dioicae, helps to illuminate the effect of mating type during reproductive isolation in recently-diverged sister species. The results show significant support for pre-mating barriers depended on the combinations of specific mating type chromosomes and less obvious signs for post-mating isolation driven by the origins of the mating types. In contrast to previous hybrid studies in this system, the use of controlled backcross experiments further reveals effects which are likely to play important roles in both maintaining species separation and the nature of backcrossed hybrids lineages that may emerge in the presence of backcrossing potential.
Assortative mating in Microbotryum
Pre-mating barriers between sister species, which may contribute directly to their evolutionary viability and isolation, is an issue complicated by the multiple influences upon contact and mating between individuals. In the anther smut fungi that infect the Caryophyllaceae, where the pathogens are specialized to their particular hosts [18, 19, 22, 30], the strength of pre-mating barriers in sympatry is poorly understood. Sympatric populations of M. lychnidis-dioicae and M. silenes-dioicae are common but the frequency of hybrid genotypes seems to be low [23, 24]. Previous studies have generally not detected causes for pre-mating isolation upon contact between species , apart from the potential contribution of a developmentally influenced high selfing rate in combination with sibling competition . The current study shows that assortative mating, in the form of recognizing species-specific variation at the mating type locus, might serve as pre-mating barrier that is active between sister species of Microbotryum. Although the signal is significant, patterns for assortative mating among the two species are weak. This might be due to the fact that the most closely related Microbotryum species have been used - out of the necessity to have viable F1-hybrid meiotic products, and we may expect stronger MAT- predicted assortative mating in more distant other species pairs that come into contact in nature. Moreover, the large variation in conjugation rates among replicates emphasizes that mating in Microbotryum might be also influenced by other factors that have not been isolated yet.
For example, in the study by Le Gac et al. , assortative mating was evaluated by testing for the correlation between mating rates (intra- and interspecific) and the genetic distances between several Microbotryum species. Wide variation in mating rates was seen across the species pairs, but it was not correlated with genetic distances. That result also suggests that species differences besides the compatible mating types may be affecting conjugation rates (i.e. environmental responses, phenology, etc.). In M. lychnidis-dioicae, previous studies have shown that temperature, available nutrients, and the presence of the plant exudate alpha-tocopherol can affect the propensity of haploid cells to mate [35–37], as other organisms may respond similarly to extrinsic signals such as pH or light [38, 39]. In the current study, gametes were produced from F1-hybrids, where the identity of the non-recombining mating type chromosome was controlled for and the autosomal component of the genome is generally expected to be a mixture of the two parental species, thus potentially homogenizing the influence of contrasting cellular responses to non-pheromone-based environmental cues. Therefore, with this approach the influence of mating types upon behavior could probably be better resolved than in prior studies to reveal preferences in the mating compatibility signals.
With evidence that M. lychnidis-dioicae and M. silenes-dioicae originally diverged through allopatric isolation , we see now that the pathogens show adaptation to specific hosts that could further contribute to their isolation . Neutral divergence or selection for assortative mating upon secondary contact (i.e. reinforcement, but see ), remain plausible explanations for the evolution of the patterns observed here.
Sources of post-mating isolation
Fitness reductions due to mal-adaptation to parental environments and genomic-level incompatibilities that are typical of hybrids have been experimentally demonstrated using crosses between Microbotryum species. Interspecific Microbotryum hybrids are less successful at infecting host plants than the progeny of intraspecific crosses . Also, hybrids often show incomplete sporulation on host plants [31, 32]. Moreover, the study of Le Gac et al.  revealed the existence of host-dependent factors that influence hybrids fitness, where identical F1-hybrid genotypes between M. lychnidis-dioicae and M. silenes-dioicae differed in infection ability on their two hosts, S. latifolia and S. dioica. Our results may support the conclusion of host-dependent effects upon hybrid fitness, where backcrossing that was homospecific for the M. lychnidis-dioicae mating type was significantly favored on S. latifolia but not on S. colorata, consistent with the expectation that host adaptation to S. latifolia is an extrinsic post-mating factor . This coincides with the meta-analysis study of Giraud and Gourbier  that also emphasizes that the occurrence of post-mating barriers in Microbotryum is more likely caused by extrinsic factors than genetic incompatibilities.
In our design, the second natural parental host environment could not be used, (i.e. Silene dioica), but it would be very informative to test forces of extrinsic isolation in models that can include both parental hosts as environments. The use of a novel host environment did, however, allow our study to assess mating compatibility based upon the species-specific mating type chromosomes. In the unbiased novel host environment backcrossed pathogens with genomes with a higher percentage from a single Microbotryum species (i.e. homospecific backcross) should perform better than offspring with a more mosaic genome (heterospecific backcrosses), but this study did not provide evidence for such an effect. The lack of evidence for negative epistatic interactions in these backcrosses may be reasonable considering the very small genetic distance between M. silenes-dioicae and M. lychnidis-dioicae[21, 44] even though these two fungal species show reduced hybrid fitness in the form of sterility .
In addition, results obtained on the novel host environment suggest that greater genetic contribution from the M. lychnidis-dioicae species provided an infection advantage. The direction of backcrossing toward M. lychnidis-dioicae was higher in both of the mating-type treatments, where the most successful infection were mating type heterospecific, and significantly higher in one case than the 0.5 neutral expectation. A greater infection potential of M. lychnidis-dioicae than other Microbotryum species has been previously observed [21, 44]. Thus, regarding post-mating isolation in Microbotryum, it is important that species-specific characteristics be considered in addition to the classification of intrinsic and extrinsic factors, and such an effect may also have contributed to higher infection rates by M. lychnidis-dioicae-backcrossed pathogens on the S. latifolia host.
Potential for hybridization and backcrossing in Microbotryum
While a large number of studies utilize molecular tools for the analysis of present and past hybridization, the current study takes a different approach to illuminate the potential impact of inter-specific mating through controlled backcrossing experiments. F1-hybrids and backcrosses between the two closely related Microbotryum species M. lychnidis-dioicae and M. silenes-dioicae are highly viable on a natural host and a novel host, which supports the idea that hybridization and introgression have the potential to impact natural Microbotryum populations. There are several examples in plants and animals where hybridization seems to facilitate new evolutionary lineages [4, 6, 45], and in fungi hybrid speciation events also have been described [1, 46, 47]. The current study provides insights into the potentials for hybrid speciation in Microbotryum and for introgression via backcrossing of alleles from one species to another, which have both been suggested by molecular analysis of natural Microbotryum populations [24, 44].
Reproductive isolation from the parental species is essential to the emergence of a new hybrid species. This can be achieved by changes in ecology or genetics (i.e. ploidy) that favor the production of offspring between hybrid genotypes [4, 48, 49]. The preference by F1 Microbotryum hybrids for conjugating with compatible mating type alleles from the same parental species may instead favor backcrossing over hybrid selfing, because F1-hybrid selfing is necessarily heterospecific at the mating type while backcrossing can be favored as homospecific. However, it should be noted that this would only be the potential influence of the mating type upon the process of backcrossing, which may not be strong enough to counter-programmed effects upon development that favor selfing in this organism.
Dominant AAB hybrids with CaMa cytotype
To explain the absence of a B genome plastid and mitochondrion contribution to the cytotype of AB and AAB types, Carreel (1994) suggested that a fertile primary AB hybrid from a cross AAfemale × BBmale with CaMb cytotype may have been pollinated by an AA donor of cytotype CaMa. This would have ensured that its AA or AB progeny were all CaMa. The pollination of an AB type by AA is known to produce viable diploid progeny, but their frequency is thought to be dependent on the genotype of the primary AB diploid's B genome progenitor ( Shepherd, 1999). The secondary AB hybrid of cytotype CaMa could then produce ABA (AAB) offspring of CaMa cytotype when pollinated by an AA type.
The rarity of edible AB types raises the question as to whether the (AB) × AA route (parentheses indicate the source of female meiotic restitution) could have, in reality, made a contribution to the occurrence of the AAB types which predominate among African and Pacific plantains. The seeming absence of edible AB types outside of India makes the route rather implausible. The alternative, starting from a less-fertile edible AA and via the (AA) × BB cross, appears to be more realistic, since the AAB hybrid would have the CaMb cytotype, and its pollination by a male-fertile AA parent would generate an AAB with the CaMa cytotype. Such a scenario is more than feasible in the situation (as obtains in the lowlands and islands of south-east Asia) in which a small number of wild BB types is surrounded by many AA types. The ‘wild’ BB had probably been introduced in the remote past to this region by human intervention, and since become naturalized ( De Langhe and de Maret, 1999).
AAB hybrids with CbMa cytotype are unusual
To date, only one accession (‘Pisang Radjah’) appears to possess the CbMa cytotype. A possible origin for this type may have passed through a primary BA diploid formed by the cross (wild)BB × (edible)AA (CbMa), with the edibility and female restitution of the triploid BAA (CbMa) inherited from the AA pollen parent involved in the (BA) × AA cross.
Multiple origins of ABB hybrids
Boonruangrod et al. (2008) observed two cytotypes among ABB accessions: CaMb in ‘Pelipita’, ‘Saba’, ‘Monthan’, ‘Ney Mannan’ and ‘Bluggoe’ and CbMb in ‘Pisang Awak’, ‘Peyan’ and ‘Klue Teparod’ (Table 1). The Indian accessions ‘Monthan’, ‘Ney Mannan’ and ‘Bluggoe’ would have been generated from the cross (AB) × BB. However, for the Philippine cultivars ‘Pelipita’ and ‘Saba’, the (AB) × BB route is unlikely, since no edible AB types have been recorded in this region. Because edible AA types are endemic, the probable origin is [(AA) × BB] → (AAB) × BB → ABB.
This leaves the problem of the ABB (CbMb) types. The presence of Cb dictates that a BB type was the maternal parent. If the paternal parent of the primary hybrid was an AA type, then this BBA hybrid would have a CbMa cytotype, which has not to date been observed among ABB types. A theoretical route can be imagined, passing through a BA diploid derived from a cross (BB × AA), and its backcross to BB to produce BAB (CbMb) progeny. While this route is imaginable for the Indian ABB accession ‘Peyan’ and perhaps also for ‘Klue Teparod’, it does not provide an acceptable explanation of the origin for ‘Pisang Awak’, since no edible AB types are known in Thailand while the exceptional somaclonal diversity of ‘Pisang Awak’ indicates its possible origin as a triploid hybrid in this region.
In an alternative scheme, we would assume that edible BB types having female restitution do exist. Then the pedigree of the BBA (CbMb) types could have been via a [(BB) × AA] hybrid (BBA of cytotype CbMa), followed by its pollination with BB. The underlying assumption remains controversial, even though balbisiana-like plants bearing more-or-less seedless fruits have been described. Thus, Swangpol et al. (2007) have provided cpDNA sequence-based analysis to show that some ABB accessions must have M. balbisiana as a maternal ancestor. Furthermore, a DNA analysis, based on six discriminating nuclear gene fragments, of four balbisiana-like edible banana specimens from north Thailand (one of which was diploid and the others triploid) showed that no diagnostic A genome sequences were present (G. Volkaert, Kasetsart University, Thailand, pers. comm.).
Hybrid organisms are those born as a result of the combination of the traits of two organisms of distinct varieties, breeds or species through sexual reproduction. Not just plants, but animals also form hybrids in nature. For instance, when a male lion mates with a female tiger, the resulting offspring is a hybrid &ndash a liger.
Liger, a lion/tiger hybrid bred in captivity (Photo Credit: Ali West /Wikimedia Commons)
Similarly, take the example of hooded and carrion crows. These are different groups of crows that usually mate within their own group, but sometimes, they mate with each other and hybridize. The offspring of such a union usually possess physical traits of both hooded and carrion crows.
It&rsquos important to note that not all hybrid organisms, or simply hybrids (or crossbreeds), are intermediates between their parents some hybrids only show hybrid vigor, which means that they can grow taller or shorter, or demonstrate other traits at a different degree of intensity than their parents,
Methods Employed in Plant Breeding (With Diagram)
The below mentioned article provides an overview on the three methods employed in plant breeding. The methods employed in plant breeding are: (I) Selection (II) Hybridisation and (III) Mutation Breeding.
Method # I. Selection:
The oldest method of plant improvement is by the selection of the best plants and by growing only the seed from them. This is useful only so long as the population of plants is a mixture of pure lines. Selection within a pure line is useless according to Mendelian Genetics.
The Pure Line Concept: Pure Line Selection:
Louis de Vilmorin (France, 1856) developed the method of progeny test. Individual plants are isolated and their progeny tested to find out if all the off-springs show uniform characters (e.g., sugar content of sugar beets), i.e., if characters are segregating. By strict progeny tests W. Johannsen (Denmark, 1857-1927) established the Pure Line Theory. He took a commercial variety of beans, selected for the weight of beans and found that a single variety could be broken up into 19 pure lines.
Each one of the 19 pure lines had a constant average weight of beans (different for the different lines) and this average within a line could not be altered by further selection. This is because the pure line is homozygous for the character or characters studied, no further segregation takes place and, therefore, any further selection within it is futile.
The Lysenko school denied the existence of pure lines since, according to them, heredity is not anything fixed but varies with a variable and unpredictable environment. Even in the Mendelian sense it is not possible to get a perfectly homozygous plant for all characters although one may get a plant homozygous for all predominant characters.
The case becomes even more complicated if polygenes or modifying genes be present. The term pure line should, therefore, be used in a relative sense.
Pure line selection is important whenever a new variety of uncertain origin is obtained, or when investigations are begun on a new crop. It loses its importance, as soon as all the pure lines are isolated. But, it has already been pointed out that the word pure line is relative. Whenever investigations are taken in hand for a new character, fresh pure line selection is necessary. Thus, one may even now carry on pure line selection for protein content or vitamin content of rice.
In the methods of selection two courses are usually followed:
Not individual plants but whole groups of plants are selected out. This is the simpler method.
2. Individual Plant selection or Pedigree selection:
Individual plants are selected out, isolated, and its seed grown separately (Progeny test). The same process may be repeated for a few generations. The process is naturally more rigorous but yields better result. A very well-known method is the ear-to-row or panicle-row method. Ears or panicles of cereal crops are selected out and each ear or panicle is grown in a separate row for future selection.
Clones are plants propagated vegetatively from single original stocks and it has already been pointed out that the genotypic constitution of plants propagated in this way is not likely to change. They are as stable as pure lines and no segregation or varia­tion is expected among them. So, selection within a clone is not likely to be fruitful. But, in nature, bud mutations have been found to occur occasionally and the selection of such bud mutations has played an important role in the breeding of clonal crops like potato, sugarcane, pineapple, apple, citrus, etc.
In clonal crops, even better results are expected if clonal propagation be combined with actual hybridisation—sometimes in places far away from the actual fields. That is why there are special sugarcane and potato breeding stations on the hills where very important work is done on the hybridisation of such clonal crops.
It can now be realised that all plant breeding stations must maintain different sections for different purposes. First, there must be the variety plots where hundreds and thousands of varieties (introductions and selections of varieties usually grown) are grown year after year as a living herbarium from which seed of any variety may be obtained for further selection or hybridisation. Secondly, there are the pedigree culture or progeny test plots where the pure lines are found out. Thirdly, there must be the variety test plots.
In whatever way may a variety be obtained (by introduction or selection or hybridisa­tion), its performances must be tested before it can be recommended to the farmers. For this, the varieties under test are grown side by side with standard varieties and their qualities compared. There may be different plots for testing different qualities, viz., yield trial plots, disease nursery plots, etc.
Selection played a very important part in the early days of plant breeding. It was largely employed in the selection of Indian wheat varieties by Mr. and Mrs. Howard. Valuable strains have similarly been obtained in India by pure line selection in rice, millets, cotton, etc.
Method # II. Hybridisation:
Hybridisation is a very important method in plant breeding. It has played a big role in the development of improved sugarcane strains at Coimbatore where sugarcane has been experimentally crossed with sorghum, maize and even with arundinacia bamboo. Hybridisation has also been usefully employed in getting good strains of wheat, rice, cotton, etc. There was phenomenal improvement of wheat varieties of England by hybridisation.
There may be a local variety which is good in all respects but inferior in one particular character, e.g., the grain may have an unwelcome red colour. Let it have the constitution AABBCCRR (A, B & C are good qualities and R for red grain). The plant breeder will then find out another pure line which may not be a good variety but it will have white grains. Its constitution may be aabbccrr (a, b & c are bad recessives and r white recessive).
If a cross be made, the F1 hybrid plant will show the dominant characters and will have the constitution AaBbCcRr. In the F2 and sub­sequent generations the characters will segregate and recombine in all different ways. Some will be inferior types with white grains, some superior with red grains but there will be a very few superior types with white grains.
The plant breeder will now go on selecting for a few generations only the desirable combinations, i.e., good qualities with white grains (phenotype A-B-C-rr). The progeny will be all white as it is homozygous for rr but there will be segregation for the A, B and C characters as most of the selected plants will be heterozygous for these. If he goes on selecting for a few more generations he will ultimately find out an AABBCCrr plant which will be a pure line, being homo­zygous, and this will be the desired combination.
The plant breeder may carry on this selection of the F1 progeny by two well established methods:
The plant breeder grows every one of the F2 selected plants separately. In the F3 he again selects the suitable plants (A-B-C-rr) and grows every one of the selected plants separately keeping clear pedigree records of each plant. The advantage of this method lies in the quickness with which he will get the true breeding AABBCCrr plant. Among the F2 plants that he selects (A-B-C-rr), there may be some AABBCCrr plants which he will be able to identify in 3 or 4 generations if he is lucky enough from the simple fact that these pedigreed lines will not show any segregation.
While the pedigree method is advantageous in being very quick it is disadvanta­geous in demanding much more attention and labour. In plant breeding stations, usual­ly a large number of crosses are handled simultaneously and it is impossible to carry on with all those crosses by the pedigree method. The second method (Bulk method) is, therefore, used to save labour.
This is a mass selection method. The F2 selected plants are not grown separately but are bulked together to form a single F3 population. In the F3 again, the suitable (A-B-C-rr) plants are collected and bulked together. This goes on for a few years after which the A-B-C-rr plants are tested separately to find out the true breeding AABBCCrr plant. This method involves more time but minimises labour as the plants do not need individual attention. Very often the plant breeder has no other option but to follow this method.
The table below shows two programmes, one according to pedigree method and the other according to bulk method, which are meant for rice- but may also be followed for other small grains like wheat or barley.
When a plant breeder wants to hybridise between two varieties he must first gather all information about the flowers, viz., the time of flowering, the exact time when the anther and the stigma become mature for pollination, which flowers give healthy seeds, how long do the pollens remain viable, etc. He must take all precautions so that hybridisation takes place only in the way he desires, precluding all chances of self- pollination and must ensure that no foreign pollen can contaminate the result.
He should follow the following stages:
1. Selection and preparation of parents: Isolation:
The plant breeder first selects the plant that he will use as the mother parent and keeps the male parent ready so that the anther will be ripe just at the desired time. If there are too many flowers on the branch of the mother parent he clips off a number of them.
This is specially true in the cereals (wheat or rice) where there are a big number of flowers on the spike or panicle. In rice, about ten or twelve flowers of the same age are kept. It is necessary to isolate the female parent and, sometimes, even the male parent, by growing on isolated plots or by bagging or caging. Necessity of isolation increases with the percentage of natural cross-pollination.
The anthers must be plucked off the female flowers just before the anthers are ripe (anthesis) without causing injury to the flowers and, specially, the car­pels. Care should also be taken not to break the anthers. This is easily done with a pair of fine pointed forceps in the case of larger flowers like those of tomato. Rectified spirit should be used freely in sterilising the instruments during crossing.
In the case of small flowers the process is rather painstaking. In ordinary cereals where the bracts are not brittle (e.g., wheat or oats) the process is simpler but it is rather different in rice. Fig. 882 shows the plant breeder’s kit specially necessary for emasculation.
After the flowers are emasculated they are to be kept isolated which may be done either by keeping the whole plant in a muslin cage or by enclosing the flowers in muslin or oil paper or plastic bags so that foreign pollens may not come in contact with the stigma. Fig. 883 shows different types of bagging. Usually these bags are kept till seed-setting is complete.
When the stigma of the emasculated flower is mature the bag is temporarily removed and the stigma pollinated by dusting with complete broken anthers or pollens from the male parent. Special study should be made as to the viability of the pollens. Flowers are bagged again after pollination.
Care must always be taken to keep the crossed flowers properly labelled or tagged. The label should be as brief as possible but complete. It should bear the names of the parents (female parent first) and, at least, a number referring to the field record book as shown in Fig. 882. All or her necessary particulars should be entered in a handy field record book with a number and the number referred to on the tag.
The hybridisation technique must be adapted to the particular crop on which work is being done.
Techniques usually followed for three important crops—rice, wheat and cotton are give below:
In rice, the lemma and palea are rather hard and the flowers remain open for only about half an hour, sometimes after the sun warms up. Emasculation must be done before the time of an thesis which can be easily ascertained by placing the closed flowers against the sun (Fig. 884) and looking through the semitransparent lemma-palea.
Just before anthesis the anthers rise from the base to the top of the closed spikelet.
Emas­culation may be done in four ways:
(1) By slightly forcing open the lemma and palea just before the opening of the flower. This is the method for wheat and other cereals and this is the method usually adapted for rice in India.
(2) By clipping off the tip of the open flower with a pair of scissors and taking out the anthers through the opening by a pair of fine pointed forceps or a needle. Care must taken not to break the anthers and to take out all the six anthers of rice.
(3) The panicle is covered in the early morn­ing, before blooming, by a dark or brown paper bag (Ramiah). The heat inside forces the flowers open and emasculation is carried on as usual.
(4) The last method of hot water emasculation, developed by N. E. Jodon, is very interesting and is now widely used in the U.S.A. It has also been found very fruitful in India (Gangulee 1959).
Warm water (about 43°C.) is taken in a thermoflask, a mature tiller of rice is tilted and a whole panicle kept immersed within the warm water for about 10 minutes (Fig. 884). Within a few minutes of taking the panicle out of the water, just the mature flowers open out. Not only that, the temperature renders all the anthers sterile while the carpels are not injured, thus causing automatic emasculation. Pollination is to be completed within the next half an hour after which the flowers automatically close.
For pollination whole broken anthers, from flowers in which anthesis is about to take place, are used. When emasculation is carried on by forcing the flowers open, the latter are kept closed by small rubber rings so that bagging is unnecessary. Bagging is necessary for the -second and the third methods of emasculation in rice and necessary only for the half an hour while the flowers are open after hot water treatment. Only a few flowers in a panicle should be pollinated and all others scissored off.
After the female spike is selected, 5 to 10 healthy flowers are chosen and the rest scissored off. Emasculation is easily done with a pair of narrow forceps by gently opening the lemma and palea as the latter are soft and not brittle as in rice. The emasculated spikes are kept bagged with paper or plastic bags supported on stakes. Pollination is done after two days with whole broken anthers.
Emasculation is done on the afternoon of the day previous to the normal opening of the flower. An incision is carefully made round the base of the corrolla and the latter is removed taking care not to injure the pistil. The anthers are removed by plucking or scraping them off very carefully. Anthers must not be broken in the process. Emas­culated flowers are now bagged.
A still simpler process is to take a small bit of drinking straw, to seal one of its ends and to slip the open end on the emasculated pistil so that it fits tightly on the ovary. With straw it is not necessary to scrape off the lower anthers which get rubbed off when the straw is fitted. The straw is now fastened to the stem with soft and fine copper wire. Pollination is done next morning by plucking whole flowers from previously bagged (to ensure purity of pollen) male plants and dusting the pollens on the stigma.
Special Methods Involving Hybridisation:
1. Breeding for Disease Resistance:
Disease often causes serious havoc among plants. Effective control of such diseases by germicides, etc., is often too expensive and inconvenient. On the other hand, strains of crop plants (which may even be wild, distantly related varieties or species) are some­times found to be naturally immune (resistant) while other strains are normally sus­ceptible for specific diseases.
This resistance and susceptibility to diseases (e.g., rust in wheat or Helminthosporium = Ophiobolus in rice) are usually genetic factors. Sometimes this resistance is found in wild varieties which are otherwise useless. When this is so, it is possible to get the resistance factor from the resistant variety combined with the good qualities of some suitable cultivated variety.
The cultivation of the new resistant variety will then be an efficient as well as cheap measure in controlling the disease. In breeding disease resistant strains, every hybrid generation is to be subjected to artificially created disease producing conditions. Special inoculation tents are sometimes found useful for this purpose. All plants are artificially inoculated with fungal spores, etc., and the proper environment (specially, humidity) created.
The resistant segregates are then easily spotted in every generation. Selection is carried on for a number of years till the homo­zygous strain is obtained. Breeding for disease resistance is not always simple as it is difficult to get strains resistant under all conditions and the inheritance of disease resistance is often multi-factorial. Moreover, most important diseases have a large num­ber of strains of the pathogens and it is difficult to get any variety of the crop resistant to all the strains. Some success has been attained in breeding disease resistant strains of various Indian crop plants.
One of the serious limitations of the present method of raising disease resistant varieties is that it does not take into account the potentiality of the disease producing organisms to undergo mutation. That is why it has often been found that a disease resistant variety once obtained does no remain so for a long time. Later, it becomes susceptible due to mutation of the organism causing the disease, which can then infect the so-called resistant plant.
2. Backcross and Testcross Methods:
It has already been shown how the crossover percentage of two linked genes may be determined by the testcross, i.e., by backcrossing the hybrid F1 plant back to the recessive parent. This testcross method is very usefully utilised by the plant breeder and the geneticist in determining the genotypic constitution of any plant.
As backcrossing takes place with the recessive parent, the latter does not show itself in the progeny and the backcross segregation ratio represents the gametic ratio of the plant in question and, hence, its genotypic constitution.
Thus, Fig. 885 shows the result of a testcross with a tall x dwarf heterozygous pea plant. The backcross ratio can only be explained by assuming that there are T and d gametes on the plant in equal proportions, i.e., it is a Td plant. If the backcross plant is a pure line (with only one type of gametes), the backcross generation must show plants of one type only.
Another use of the backcross method in plant breeding is introduction of a character from either (i.e., recessive or dominant) of the parents more quickly. Thus in breeding for disease resistance, the hybrid disease resistant plants of successive generations are repeatedly backcrossed with the disease resistant parent. In this way homozygosity for disease resistance may be attained a few years earlier. Backcrossing is rather easy when a monogenic character is to be transferred while it may also be adapted for multi-factorial characters.
3. The use of Hybrid Seed:
F1 plants are always more vigorous because of heterosis. Yield of a crop can be greatly increased if F1 seeds can be directly used as seed by the farmers. But, production of such seed is usually so costly that it can only be used for experimental purposes and is hopelessly uneconomic if used as the farmers seed.
However, an exception has been found in the use of hybrid com (maize) as seed. Methods have been developed in the U.S.A. for obtaining hybrid corn seed in large scales at low cost. Production of maize has been greatly increased in recent years by this process.
Production of hybrid corn has now taken hold in India. The author’s own experiments show that it is possible to cultivate hybrid rice on a small plot and obtain a much higher yield.
Maize is a naturally cross-fertilised crop. So, the first step towards producing hybrid corn is isolating the pure lines by inbreeding. The inbred pure lines are rather weak.
Two such pure lines are planted in the field alternately, one as the male stock or polli­nator and the other as the female stock or the seed producer. Such fields must be isolated from all other varieties of maize to get good uncontaminated hybrid corn The-usual method is to plant one pollinator row for every two seed rows. Male flowers of maize are borne on apical panicles or ‘tassels’ while female cobs are borne on the axils.
So, emasculation is rather easy by simply lopping off these panicles and it is now possible to carry on large-scale emasculation or ‘detasseling’ mechanically. All kernels developing on the detasseled variety are hybrids, pollination being possible only from the other variety. But, as the inbred pure line plants are weak, the production of single- cross grains on them is rather low. So, double-crossing is resorted to by making a hybrid of the second order out of two F1 single-cross hybrids in order to get a large quantity of double-cross grains.
The cobs on the double hybrids (Fig. 886) are even larger than on the single hybrids. For double crossing, two rows of pollinator single-cross plants are planted for six to eight rows of single-cross seed rows. As the single-cross plants are much more Vigorous than the pure line plants, the production of double-cross grains is much more bountiful. Four pure lines are involved in producing a double-cross hybrid and the method of such production is diagrammatically represented in Fig. 887. The fanner gets a heavy yield when he uses double-cross grains as his seed.
A new development in the production of hybrid corn is the utilisation of the ‘male- sterile’ character. Plants having this gene or character have sterile pollens so that they are naturally emasculated. If a strain having this character is used as the seed parent then it is no longer necessary to emasculate or detassel it. If it be possible to get such suitable ‘male-sterile’ genes in other crops it may be possible to get cheap hybrid F1 seeds as the labour involved in emasculating will be reduced to the minimum.
Method # III. Mutation Breeding:
A new line of plant breeding has opened up in recent years—that of mutation breeding. Important crop varieties are known to be mutants. Mint- zing thinks that more than half the species of flowering plants are polyploids. A plant breeder has to keep his eyes open to select out any naturally occurring mutant (point mutation or intergenic mutation) that may look promising. Selection within clones and pure lines is possible only when such mutants occur.
Artificial induction of mutations has been extensively employed in recent years. The most important agents for such mutation induction are (i) X-rays and other types of ionising radiation explained in (giving rise to the science of Radiation Biology) and (ii) Chemical mutagens like mustard gas or colchicine.
All growing organs, seeds, pollens, eggs, etc., may be subjected by such irradiation or chemical treatment. There is no fundamental difference between natural and artificial mutations. It is true that most mutations are of no practical importance or even harmful and also that there is no way of predicting what type of mutation one is going to get.
But, this should not give rise to any scepticism and a plant breeder should be satisfied if he gets a new beneficial character in a million. Some very useful radiation-induced mutations have already come to the use of agriculturists and horticulturists. Sweden has greatly advanced in mutation breeding since 1929 with workers like Gustafsson and Miintzing at their Svaloff station. They have obtained improved ‘ereictoid’ stiff-strawed barley varieties by X-ray treat­ment and similar treatment has yielded better varieties of white mustard, Phaseoltis, ground-nut, oats, peas, etc.
Similarly, there, are reports of improved barley from Germany rust resistant wheat from Austria improved barley, peanut and short-strawed rice (by Beachell) from the U.S.A. and improved wheat varieties in India. Radiation induction has been even more fruitful in horticulture. New flower varieties have been raised and then propagated vegetatively. In India attempts have also been made to get better varieties of rice, sesame and jute in this way. A short-strawed, high-tillered rice mutant has been reported by Ramanujam and Parthasarathy. Other results are still under investigation.
Induction of mutation with chemical mutagens has become even more popular as the method is accessible to all types of workers. Colchicine has been extensively used in the production of tetraploids and amphidiploids involving hundreds of species. Although most of these polyploids are of no practical value, they have been found to be very useful material which can possibly be improved by further breeding.
Thus, some useful Triticale (wheat X rye, i.e., Triticum x Sec ale amphidiploid) varieties have been obtained by hybridising different types of Triticales. Muntzing has obtained tetraploid winter rye. Kihara and his associates in Japan have reported triploid sugar beet with higher sugar content, triploid water-melon, tetraploid radish, etc. Tetraploid grapes, also, have been produced.
Cytogenetics has placed more synthetic breeding material in the hand of the plant breeders in the form of plants with altered genomes or substituted chromosomes and the study of trisomies, monosomies, nullisomics, etc., has enabled them to work on the definite positions of the desirable genes in the chromosomes.
Finally, the latest attempt of inducing mutations in a novel way is well worth mentioning. It has been mentioned that the most important chemical constituents of genes are nucleic acids and there is evidence that free nucleic acid may control heredity.
There has been a claim from France that deoxyribose-nucleic acid (known so cytogeneticists as DNA) extracted from the eggs of the Khaki Campbell variety of ducks when injected into the Peking variety, has induced some Khaki Campbell characters to the Peking ducks and this change has been found to be hereditary.
As DNA has been proved to be the gene substance, it is quite likely that in near future it may be possible to use just the DNA extract from one of the desirable parents in the hybridisation of higher plants as has already been done in the case of Pneumococcus strains of bacteria. If this method of changing heredity be established and improved, possibly it will be the most important tool in the hands of the plant and animal breeders, (c.f. Genetic Engi­neering).
The Importance of Plant Breeding in Modern Agriculture:
With the present population explosion the cry in the densely populated under­developed countries, specially of Asia, is for food and more food. Many, such countries like India does not produce enough cereals to feed their own populations. The need is to develop more amount of food on the same area of land.
For this, besides improved methods of agriculture, better and high yielding varieties and strains must be developed. Fortunately, great success has been achieved in this direction in recent years. A green revolution has taken place in India and some other count­ries so that these countries would soon produce more food that what they presently need.
In this connection one must men­tion Norman E. Borlaug (Fig. 888), the plant pathologist plant breeder devoting his life at the International Maize and Wheat Improvement Centre at Sonora in Mexico. His splendid work in developing new high yielding, rust resistant, non-lodging dwarf wheat varieties and strains, which are now being cultivated in many countries, is the basis of the present ‘green revo­lution’.
His Sonora 64, with Sonalika, Kalyansona and other varieties developed in India, is working a miracle. Most deservirtgly, Borlaug was awarded a Nobel Prize for Peace in 1970. Nothing contri­butes more to peace than self-sufficiency in food.
Similarly, in the field of rice also high-yielding varieties have been developed by plant breeders. IR-8 developed in the International Rice Research Institute located in the Philippines and the strains Padma and Jaya developed in India are contributing towards ‘green revolution’ here.
Backcrossing in Hybrid - Biology
Speciation is the underlying process that leads to formation of new species, and therefore is the basis of biodiversity. Genes involved in each stage of speciation, such as those involved in interspecies sterility, remain elusive. Male hybrid sterility and postzygotic isolation between Drosophila pseudoobscura and D. persimilis was examined in this study through backcrossing of female hybrids into each parental line (introgression), selecting for a sterile sperm phenotype, needle-eye sperm. Sperm phenotypes did not separate through backcrossing instead, males presented with multiple sperm phenotypes. A relationship between the phenotypes observed and the potential genes involved was examined through whole genome sequencing and SNP analysis of the DNA of 20 introgressed male hybrid samples. One finding was SNPs for hybrid sperm sterility were species specific. Also, sperm sterility and heteromorphism appear to be controlled by many loci. Further analysis of SNPs isolated in this study has the strong potential to identify candidates for loci involved in formation of needle-eye sperm, and postzygotic male hybrid sterility in other species.
Summary for Lay Audience
Speciation is the process of two populations of organisms of the same species evolving over time until they are unable to reproduce with each other. Some species have not completely separated, and are still able to create viable, but oftentimes sterile, hybrid offspring. A common example of hybrid sterility comes from horses and donkeys, who separated approximately 7.7-15 million years ago (Huang et al. 2015). When a male donkey and a female horse reproduce, they sire a mule. All male mules are sterile and most female mules are sterile. In rare cases female mules are fertile when mated to a horse or donkey (Savory 1970).
Similar to horses and donkeys, the crossing of two species of fruit flies, Drosophila pseudoobscura and D. persimilis, produce all sterile male hybrids. However, in the case of these fruit flies, all female hybrids are fertile. These two species of fruit flies also diverged more recently, 0.55 million years ago. These sterile hybrid male fruit flies can still produce sperm, but these sperm are not able to fertilize female eggs to make more hybrids. Fruit flies are used because they are less expensive to maintain, have shorter life cycles, and can be in a tightly controlled environment. My research focused on genetic differences cause the male fruit flies to be sterile. Hybrids receive genetic material (DNA) from both parent species. The DNA of both fly species studied here is split into two pairs of five separate chromosomes, X/Y, 2, 3, 4, and dot. The pairs of each chromosome can interact with each other through proteins. Instead of ten separate assembly lines for proteins, pairs of chromosomes are connected to each other by networks integral to protein production and cell function. In hybrids, the chromosomes are unlikely to all function properly because each species has differentiated chromosomes that might not be able to form proper pairs. The failure of some of these networks could be the basis of sterility. My study supported the species-specific differences in the pieces of the network contributing to hybrid sterility. This work can be continued to identify specific points in the DNA that lead to hybrid sterility and applied to other species.
Achieving Consistency When Breeding Marijuana Plants
If you have ever purchased a hybrid strain from a dispensary, you should appreciate the effort that has gone into ensuring your weed has consistently desirable traits. In all likelihood, the weed you buy has gone through generations of breeding to ensure it doesn’t carry unwanted characteristics.
The seeds created from cross-pollination will have different attributes to their parent strains. Every seed is unique with various characteristics and different combinations of traits derived from its parent. Seeds with different expressions of traits are known as phenotypes. If you purchase cannabis seeds , the best kind is ‘homozygous’ that means they have the same set of genes.
When you have homozygous seeds, you know that your plants will consistently produce seeds with the same genetic makeup every time. This consistency is highly desirable because it means breeders and consumers know what to expect 100% of the time. Heterozygous seeds produce a wide variety of phenotypes which makes them a lot less predictable. When you cross a strain, you need to select the phenotype you prefer.
Other files and links
In: Plant Breeding , Vol. 97, No. 4, 12.1986, p. 315-323.
Research output : Contribution to journal › Article › peer-review
T1 - Simple Genetic Control of Hybrid Plant Development in Interspecific Crosses between Phaseolus vulgaris L. and P. acutifolius A. Gray
N2 - Crosses were performed between nine Phaseolus vulgaris lines (as females) and seven P. acutifolius lines (as‐ male to examine parental compatibility for the production of vigorous hybrid And backcross plants, in vitro embryo rescue techniques were required to secure hybrid and backcross proseny following interspecific crossing. Seedling development appeared to be dependent on which allele the P. vulgaris parent carried at an interspecific incompatibility locus. Seven of the nine P. vulgaris lines tested carried an allele at this locus which interacted with a nuclear factor in the P. acutifolius genome resulting in stunted, sub‐lethal hybrids. The lines, ICA pijao' and ‘Sacramento Light Red Kidney’ did not carry this allele and produced vigorous hybrid progeny in combination with all P. acutifolius parents. Intensive backcrossing produced progeny which also segregated for sub‐lethal and viable plant development. The observed segregation patterns suggest that a bridge crossing scheme would facilitate the introgression of P. acutifolius germplasm into incompatible P. vulgaris lines. Similarities, with an intraspecific incompatibility system are discussed.
AB - Crosses were performed between nine Phaseolus vulgaris lines (as females) and seven P. acutifolius lines (as‐ male to examine parental compatibility for the production of vigorous hybrid And backcross plants, in vitro embryo rescue techniques were required to secure hybrid and backcross proseny following interspecific crossing. Seedling development appeared to be dependent on which allele the P. vulgaris parent carried at an interspecific incompatibility locus. Seven of the nine P. vulgaris lines tested carried an allele at this locus which interacted with a nuclear factor in the P. acutifolius genome resulting in stunted, sub‐lethal hybrids. The lines, ICA pijao' and ‘Sacramento Light Red Kidney’ did not carry this allele and produced vigorous hybrid progeny in combination with all P. acutifolius parents. Intensive backcrossing produced progeny which also segregated for sub‐lethal and viable plant development. The observed segregation patterns suggest that a bridge crossing scheme would facilitate the introgression of P. acutifolius germplasm into incompatible P. vulgaris lines. Similarities, with an intraspecific incompatibility system are discussed.