Selection in cross pollinated crops.pdf

8/10/2019 Selection in cross pollinated crops.pdf

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Selection in cross pollinated cropsIn breeding SP crops, homozygous nature of plant is exploited. A pop of SP plants may be

composed of either a single, homozygous genotype, or a mixture of homozygous genotypes.

Plant selection is effective only within populations that are mixtures of genotypes. Massselection for a particular phenotype in a mixed population of a SP crop will reduce genetic

variability and increase the frequency of genes affecting the characters being selected. Pure line

selection involves isolation of a single, best pure line Breeder may resort to hybridization among

selected pure lines to promote recombination and select desirable recombinants

In breeding CP species, heterozygous nature of individual plant exploited. In pop of a CP sps

each plant has both homo-, and heterozygous loci, but it is heterozygous loci that give CP plants

their characteristic genetic structure. As a consequence of natural CP, the genes are reshuffled

each generation and regrouped into new genetic combinations. With an almost limitless number

of gene combinations possible within the gene pool, almost never two plants be found with

identical genotypes.Under natural environmental influences, CP populations are relatively fluid,genes favouring adaptation and increased seed production tend to increase at the expense of

genes unfavourable for adaptation or fitness to reproduce. In a breeding population, the shift

toward more adapted genotypes may be accelerated by selection, and by environmental stresses

to which the breeding population is subjected

In CP crops, the focus of breeder is on populations instead of individual plant. A population is a

group of sexually interbreeding individuals. The capacity to interbreed implies that every gene

within the group is accessible to all members through the sexual process. All the individuals in a

population share the common gene pool. A gene pool is the total number and variety of genes

and alleles in a sexually reproducing population that are available for transmission to next

generation. Due to extensive heterozygosity in CP crops, there is an abundance of phenotypic

variation; hence cultivars of CP crops less uniform than SP crops. Genetic variability for

qualitative characters may be drastically reduced by rigid selections, but genetic variation in

quantitatively inherited characters continues to be present, due to inability of breeder to select

accurately for individual gene effects and to the influence of GxE interactions

Important difference between SP and CP crops, how breeding materials is evaluated. SP crop,

homozygous genotype is true breeding, evaluated by progeny tests. In CP crops, individual

plants are heterozygous, & largely pollinated by pollen from other plants, genotype is not

faithfully reproduced in progeny. A better test would be, if plant is pollinated with a

heterozygous/homozygous collection of pollen of known origin. Performance can then be

compared among progenies of plants pollinated with same source of pollen. A test comprising

progeny performance of plants or strains pollinated with a known tester line is known as a test

cross and evaluates the combining ability of mother plants or strains with the common tester line.

Average or overall performance of a plant or a genetic strain in a series of crosses with different

tester lines is measure of its general combining ability, whereas performance of a plant or genetic

strain in a specific combination in comparison to the performance of other cross combinations is


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a measure of SCA. Combining ability tests used to identify desirable combinations of inbred

lines to develop hybrid cultivars or synthetic cultivars

HARDY WEINBERG LAW: Given by Hardy (1908) and Weinberg (1909). The law states that in a

large random mating population, gene and genotype frequencies remain constant generation after

generation in the absence of selection, mutation, migration or random drift. Consider a large population

in which random mating occurs, with no mutation or gene ow between this population and

others, no selective advantage for any genotype, and normal meiosis. Consider one locus, A,

with two alleles, A and a. The frequency of allele A in the gene pool is p, while the frequency of

allele a is q. Also, p + q = 1

Assume a population of N diploids (have two alleles at each locus) in which two alleles (A, a)occur at one locus. Assuming dominance at the locus, three genotypes AA, Aa, and aa are

possible in an F2 segregating population.

Assume the genotypic frequencies are D (for AA), H ( Aa), and Q (aa). In diploid populationthere will be 2N alleles in it. The genotype AA has two A alleles.

Hence, the total number of A alleles in the population is calculated as 2D + H.The proportion or frequency of A alleles (p) in the population is : (2D+H)/2N=(D+1/2H)/N=p

The same can be done for allele a, and designated q. Further, p + q = 1 and hence p = 1 q. If N

= 80, D = 4, and H=24, p=(4 +12)/80 =16/80 =0.2 and hence q=10.2 =0.8.Random mating involving locus (A/a) will yield the genotypes: AA, Aa, and aa, with the

corresponding frequencies of p2, 2pq, and q

2, respectively and p

2+ 2pq + q

2= 1.

This mathematical relationship is called the HardyWeinberg equilibrium. Equilibrium betweengenes and genotypes is achieved in large populations. The frequency of genotypes in a

population depends on the frequency of genes in the preceding generation, not on the frequency

of the genotypes.

Considering the previous example, the genotypic frequencies for the next generation following

random mating can be calculated as follows:AA=p2 =0.22 =0.04 Aa =2pq =2(0.2 0.8) =0.32 aa =q2 =0.82 =0.64 Total =1.00

HardyWeinberg equilibrium is hence summarized as: p2AA+2pqAa+q

2aa=1(or 100%)

In a population of 80 plants, about three plants will be AA, 26 will be Aa, and 51 will be aa. The

frequencies of the genes in the next generation : p= (3 +13)/80 =0.2, and q=1 p=0.8 The allelefrequencies have remained unchanged, while the genotypic frequencies have changed from 4, 24,

and 52, to 3, 26, and 51, for AA, Aa, and aa, respectively.

However, in subsequent generations, both the genotype and gene frequencies will remain

unchanged, provided: Random mating occurs in a very large diploid population.

Allele A and allele a are equally fit

There is no differential migration of one allele into or out of the population The mutation rate of allele A is equal to that of allele a.

The variability does not change from one generation to another in a random mating population.

The maximum frequency of the heterozygote (H) cannot exceed 0.5.The HardyWeinberg law states that equilibrium is established at any locus after one generation

of random mating.


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In order for HardyWeinberg equilibrium to be true, several conditions must be met. However,

some situations provide approximate conditions to satisfy the requirements.

The issue of population sizeThe HardyWeinberg equilibrium requires a large random mating population. However, in

practice, the law has been found to be approximately true for most of the genes in most cross-

pollinated species, except when non-random mating occur.The issue of multiple lociIt is possible for alleles at two loci to be in random mating frequencies and yet not in equilibrium

with respect to each other. Further, equilibrium between two loci is not attained after onegeneration of random mating but is attained slowly over many generations. Also, the presence of

genetic linkage will further slow down the rate of attainment of equilibrium. If there is no

linkage (c = 0.5), the differential between actual frequency and the equilibrium frequency is

reduced by 50% in each generation, it would take about seven generations to reach approximateequilibrium. At c = 0.01 and c = 0.001, it would take about 69 and 693 generations, respectively,

to reach equilibrium.

Factors affecting change in gene frequency Migration: Migration is important in small populations. It entails the entry of individuals

into an existing population from outside is via pollen transfer (gamete migration). The

impact this immigration will have on the recipient population will depend on theimmigration rate and the difference in gene frequency between the immigrants and

natives.

Mutation: Natural mutations are generally rare and would have little impact on genefrequencies. Recurrent mutation may affect the gene frequency of the population.

Selection: Selection is the most important process for altering population genefrequencies by plant breeders . Selection decides which individuals will be allowed to

contribute to the next generation. Its effect is to change the mean value of the progeny

population from that of the parental population.

Different types of selection

There are three basic forms of selection Stabilizing Disruptive Directional

Directional selection being the one of most concern to plant breeders.

These forms of selection operate to varying degrees under both natural and artificial selection. Innatural selection, the goal is to increase the fitness of the species, whereas in plant breeding

artificial selection is usually imposed to direct the population toward a specific goal (not

necessarily the fittest).

Stabilizing selection: Regarding characters that directly affect the fitness of a plant (e.g.,

viability, fertility), selection will always be directionally toward the optimal phenotypefor a given habitat. However, for other characters, once optimal phenotype has been

attained, selection will act to perpetuate it as long as the habitat remains stable. Selection

will be for the population mean and against extreme expressions of the phenotype. It isalso called balancing or optimum selection. Eg for flowering , stabilizing selection will

favour neither early flowering nor late flowering. Stabilizing selection promotes additive

variation.


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Disruptive selection: Natural habitats generally consist of a number of ecologicalniches distinguishable in time (seasonal or long-term cycles), space (microniches), or

function. These diverse ecological conditions favour diverse phenotypic optima in form

and function. Disruptive selection is a mode of selection in which extreme variants havehigher adaptive value than those around the average mean value. It promotes diversity

(polymorphism). Directional selection: Plant breeders impose directional selection to change existing

populations or varieties (or other genotypes) in a predetermined way. Artificial selection

is imposed on the targeted character(s) to achieve maximal or optimal expression. Hence,

directional selection leads to the establishment of dominance and/or genic interaction

(epistasis).

Selection in Mendelian populations

Of the various forces causing disturbance in gene frequencies, selection and non random mating

are of interest to the breeders.Consider a single pair of genes A/a, a random mating population

will consist of AA, Aa, aa genotypes. If AA or aa plants are selected, frequency of selected allelewill be 1 and of other allele will be 0. ( assuming AA or aa can be identified without error). In

the next generation only selected allele will be present (fixed). Here disadvantage in reproduction

i.e, coefficient of selection,s will be 1. s is proportionate reduction in contribution of a particulargenotype compared with a standard genotype, usually the most favoured one. Generally, s has

values less than 1 due to problems in exact identification (dominance, >100% heritability). This

is particularly true for quantitative characters. Thus, selection is expected to change genefrequencies, rather than fix or eliminate one or other allele

When s is less than 1, rate of change in gene frequency (q ) would depend upon the intensity of

selection (s) and the gene frequency, q. If s is fixed, that is, for a given value of s, q will

depend on the gene frequency, q, and to some extent by the degree of dominance. When a allele

is rare in a population, q for this allele will be small.

As the value of q increases due to selection,

q also increases, reaching a maximum at aboutq=0.3 in case of dominant allele A, at q=0.5 when there is no dominance, and at q=0.7 in case of

recessive allele a. After this delta q again declines as the favoured allele becomes more frequent

in the population. Thus, selection in a RMP is highly effective in increasing or decreasing thefrequency of alleles, but is unable to fix or eliminate them. However, when combined with

inbreeding, selection is highly efficient in fixation or elimination of alleles.

When selection favours heterozygote, Aa, both alleles retained in population. q and p reachequilibrium when q=Sa/(Sa+SA), where Sa and SA are selection intensities against aa and AA,

respectively. Equilibrium value of q is quite different from 0.5 although at this frequency,

frequency of heterozygote, favoured class, is maximum. This is because fitness of population asa whole is greatest at equilibrium point of q, although frequency of heterozygote may not be

maximum. Fitness of a genotype may be defined as its reproduction rate in relation to those of

other genotypes. Gene frequency of q=0.5 does not necessarily produce best average phenotypefor population, although it produces maximum proportion of best phenotypic class.

For quantitative characters, selection intensity is measured as difference between the mean of the

population and that of selected individuals. Selection of extreme phenotypes will increase

frequency of desirable alleles in the population. q for each gene becomes progressively smalleras the number of genes governing the character increases. With an increase in the frequency of

desirable alleles, the frequency of desirable genotypes will also increase. New genotypes


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producing more extreme phenotypes will also appear and mean of the population will change.

q for each gene would be greatest when q=0.5 since additive gene action is assumed. With

higher/lower values of q, q for each gene will become smaller. Thus selection will be unable tofix/eliminate alleles in case of quantitative traits. Further, variance may decrease to some extent,but selection will show continuous gain for several generations

Progress under selection for quantitative characters is retarded by Non additive gene action Low heritability

Permissible selection intensity

Heritability for quantitative traits

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Selection in cross pollinated crops.pdf