Mating System and Response to Selection

MATING SYSTEMS and RESPONSE TO SELECTION :

MATING SYSTEMS and RESPONSE TO SELECTION AK Chhabra, Pratiksha Mishra, Meenakshi Jain and Bharti Aneja

INTRODUCTION AND HISTORY:

INTRODUCTION AND HISTORY May be defined as the method by which individuals are paired for crossing OR various schemes which are used for crossing or mating of individuals. Five systems of mating were given by Sewall Wright in 1921

SEWALL WRIGHT:

SEWALL WRIGHT He has given the mating system. He gave five type of mating system

VARIATION OF MATING SYSTEMS IN PLANTS:

VARIATION OF MATING SYSTEMS IN PLANTS Plants vary in their mating system from completely selfing to completely outcrossing . Anther - stigma distance is a useful measure of mating system . Anther stigma distance determine if the mating system differed between the two species Ref – www.digitalnaturalhistory.com

TYPES OF MATING SYSTEMS:

TYPES OF MATING SYSTEMS THERE ARE FIVE DIFFERENT TYPES OF MATING SYSTEMS 1. RANDOM MATING 2. GENETIC ASSORTATIVE MATING 3. GENETIC DISASSORTATIVE MATING 4. PHENOTYPIC ASSORTATIVE MATING 5. PHENOTYPIC DISASSORTATIVE MATING

RANDOM MATING SYSTEM:

RANDOM MATING SYSTEM Each female gamete has equal chances to unite with every male gamete. Form of outbreeding FACTORS THAT AFFECT RANDOM MATING: Difference in flowering time Self-incomatability Position of plants in the field Wind Direction In plant breeding some form of selection is practiced such mating always practised SUCH A IS system called as random mating with selection

RANDOM MATING SYSTEM:

RANDOM MATING SYSTEM Each female gamete has equal chances to unite with every male gamete. Form of outbreeding In plant breeding some form of selection is practiced such mating system called as random mating with selection With selection – 1. Increases frequency of alleles for which selection is practiced 2. Reduces frequency of other alleles 3. Increases variance 4. Changes mean of character

CONTINUE…….:

CONTINUE……. Random mating with selection increases the frequency of alleles for which selection is practiced and reduces the frequency of other alleles . Random mating with selection alters the gene frequency and population mean, but has little effect on homozygosity, population variance and genetic correlation between Relatives. Random mating without selection does not change gene frequency, variability, population mean and genetic correlation between relatives in a population…………….. Hardy Weinberg Law Hardy Weinberg

USES OF RANDOM MATING IN PLANT BREEDING:

USES OF RANDOM MATING IN PLANT BREEDING Progeny testing Production and maintenance of synthetic and composite varieties Production of polycross progenies Evolutionary advantages – maintain high level of diversity

GENETIC ASSORTATIVE MATING SYSTEM :

GENETIC ASSORTATIVE MATING SYSTEM

GENETIC ASSORTATIVE MATING SYSTEM :

GENETIC ASSORTATIVE MATING SYSTEM Mating occurs between individuals that are more closely related by ancestry than in random mating. More commonly known as INBREEDING. Inbreeding has following effects in the population: Increases homozygosity and decreases heterozygosity Characters/alleles are fixed. Genetic variability of the population is increased but within line is reduced (under selection). The Prepotency of individuals increases under inbreeding. (Prepotency is the property of an individual to produce…………………….

Prepotency :

Prepotency Prepotency is the property of an individual to produce progeny which are similar to each other and to parent. Progeny Progeny Has more prepotency

Prepotency :

Prepotency Prepotency is affected by: Homozygosity Dominance Epistasis Linkage Homozygosity is the most important factor and is under the control of the breeder. As homozygosity increases the prepotency of the individuals also increases. An individual completely homozygous for all the dominant allele will be most Prepotent. Genetic Assortative mating is useful in making partial and complete inbreeds

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GENETIC DISASSORTATIVE MATING SYSTEM

GENETIC DISASSORTATIVE MATING SYSTEM:

GENETIC DISASSORTATIVE MATING SYSTEM Such individuals are mated which are less closely related by ancestry than random mating . Commonly called as outbreeding. Totally unrelated individuals are mated. These individuals belong to different populations. For example – Intervarietal & interspecific crosses. Popn1 Popn2 Popn1 Popn2 Popn1 Popn2 Popn1 Popn2 Popn1 Popn2 Popn1 Popn2 Popn1 Popn2 Popn1 Popn2

GENETIC DISASSORTATIVE MATING SYSTEM:

GENETIC DISASSORTATIVE MATING SYSTEM Effect similar to those of migration. A. Variability – Increased due to combining of two or more genes from two or more different sources. B . heterozygosity – Increased due to combining of genes from different lines.

Continued…:

Continued… C . Homozygosity – reduced rapidly because outbreeding favours heterozygotes. D. Population mean – increased due to combining more dominant genes from different lines. E. Genetic correlation – decrease due to decrease in homozygosity. F. decrease in prepotency- because heterozygosity increased

PHENOTYPIC ASSORTATIVE MATING SYSTEM:

PHENOTYPIC ASSORTATIVE MATING SYSTEM

PHENOTYPIC ASSORTATIVE MATING SYSTEM:

PHENOTYPIC ASSORTATIVE MATING SYSTEM Mating between individuals which are phenotypically more similar than would be expected under random mating. Refers to mating of extreme types, i.e., cross between AA & AA and aa & aa . Only two extreme phenotypes i.e., lowest and highest remain in the population Variability : Increases since it divides the population into two extreme phenotypes. Homozygosity : Leads to complete homozygosity in single generation

…….PHENOTYPIC ASSORTATIVE MATING SYSTEM CONTD…:

…….PHENOTYPIC ASSORTATIVE MATING SYSTEM CONTD… Genetic correlation : Perfect genetic correlation between number of progenies is achieved in one generation. In some breeding schemes like recurrent selection… Useful in isolation of extreme phenotypes. IMP Note : The changes due to this mating system disappear randomly when random mating is restored

PHENOTYPIC DISASSORTATIVE MATING SYSTEM:

PHENOTYPIC DISASSORTATIVE MATING SYSTEM

PHENOTYPIC DISASSORTATIVE MATING SYSTEM:

PHENOTYPIC DISASSORTATIVE MATING SYSTEM Mating between phenotypic dissimilar individuals belonging to same populations. I.e., mating between individuals having genotypes AA & aa and Aa & aa Variability : Constant, since it reduces inbreeding. Heterozygosity : Remains constant or slight Increase Genetic correlation : Decreases due to decrease in prepotency. Prepotency : decreased due to decrease in homozygosity

USES OF PHENOTYPIC DISASSORTATIVE MATING SYSTEM:

USES OF PHENOTYPIC DISASSORTATIVE MATING SYSTEM In making population stable i.e., maintaining variability. Progeny more desirable than parents. Useful when desirable type is an intermediate one and the available parents have the extreme phenotypes. Most notable – Maintaining variability in relatively smaller populations.

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Response to Selection In a Random Mating Population

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M aize © A.K. Chhabra The pollen grains are very small, barely visible to the naked eye, light in weight, and easily carried by wind. The wind-borne nature of the pollen and protandry lead to cross-pollination, but there may also be about 5 per cent self-pollination. RANDOM MATING POPULATION Out-crossing in maize

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Hardy-Weinberg law

HERITABILITY & GENETIC ADVANCE:

HERITABILITY & GENETIC ADVANCE Heritability :-the ratio of genetic variance to the total variance i.e. phenotypic variance is known as Heritability . Heritability (H)=Vg/ Vp or =Vg/(Vg+Ve) Where Vg ,Vp and Ve are the genetic ,phenotypic and environmental components of variance respectively.

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1.From analysis of variance table of a trial consisting of a large number of genotypes. 2.Estimation of Vg and Ve from the variance of F 2 ,P 1 ,P 2 and F 1 generation of a cross. 3. Parent offspring regression upon doubling provides an estimate of Heritability. Heritability can be estimated by three different methods

HERITABILITY:

Heritability estimated from above three methods is known as broad sense heritability. Broad sense heritability estimates are valid when homozygous lines are studied. Heritability (in broad sense )= [ VF 2 -VF 1 )/VF 2 ]x100 Narrow sense heritability :-it is the ratio of additive variance to the total phenotypic variance. Heritability (in narrow sense)=[1/2 D/VF 2 )x100] HERITABILITY

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selection is most important activity in all plant breeding programmes. The various types of selection schemes are mass selection, progeny selection and cyclic selection are used depending upon the mode of pollination of crop species. The efficiency of selection largely depend on the extent of genetic variability present in a population. Selection is generally more effective for characters with high heritability than those of having low heritability. Basis to selection

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estimates of heritability serve as a useful guide to the breeder. if heritability of a character is very high e.g . 0.8 or more ,selection for such a character should fairly be easy. but for a character with low heritability i.e. less than 0.4 ,selection may be considerably difficult. Estimates of heritability serve as a useful guide to the breeder

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the difference between the genotypic value of the selected plants and of the original population is known as genetic advance under selection. It depends upon : 1.The genetic variability among different plant or families in the base population. 2.The heritability of the character under selection. 3.The intensity of selection i.e. the proportion of plant or families selected. Genetic advance under selection

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Genetic advance under selection may be calculated as: Gs=(k) ( p ) (H) Where Gs is the genetic advance under selection, K is the selection differential, p is the phenotypic standard deviation of the selected population and H is the heritability of the character under selection . Genetic advance

Expected genetic advance in segregating populations…….. :

Expected genetic advance in segregating populations…….. the expected genetic gain as calculated by above formula is applicable to a mixture of pure lines or clones, but it is not applicable to segregating generations. this is because pure lines or clones give rise to progeny which are identical in genotype or parent family. Therefore, the genotypic value of progeny remains the same as that of parent plant or family. but in segregating generation, the selected plants are likely to be heterozygous for few or several genes.

……..Expected genetic advance in segregating populations :

Therefore when dealing with segregating populations, heritability in narrow sense (i.e . H=additive variance / phenotypic variance ) is more appropriate for estimating Gs. the use of broad sense heritability estimates would give higher estimates of Gs than would be practically realized. ……..Expected genetic advance in segregating populations

k vs. i:

k vs. i Selection intensity in percent=q/n x100 1 2 5 10 20 30 Value of k 2.64 2.42 2.06 1.76 1.40 1.16

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Selection in a random mating population is able to: Change the gene and genotypic frequencies Produce new genotypes due to the changed gene frequencies Cause a shift in the mean in the direction of selection Change the variance of population to some extant. The magnitudes of these effects are influenced by: the number of genes controlling the character The degree of dominance The nature of gene action Heritability of the character under consideration aaBBCC Aabbcc AAbbcc AAbbCc gene frequencies genotypic frequencies x 2 Change in Selection pressure aaBBCC aaBbCc

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The Effects of these Factors:……….. A. Characters governed by one or few genes GENETIC GAIN NO. OF GENERATIONS slow rapid slow Characters governed by rare allele The gain under selection would be slow if the allele being selected for it is rare. The gain would become more rapid as the frequency of selected allele increases. It will again become slow as the selected allele becomes predominant in the population. After some e.g., 4-6 generations under selection, the gain would be small. The genetic variance would also be much smaller than the original population. Rare allele Frequency increased Predominant in Population

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…………………The Effects of these Factors: B. Characters governed by polygenes GENETIC GAIN NO. OF GENERATIONS slow slow Characters governed by polygenes The gain under selection would be slow and would continue for many generations. The mean would change in the direction of selection. The genetic variance would not be affected much.

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…………………The Effects of these Factors: C. Dominance and non-additive gene action GENETIC GAIN NO. OF GENERATIONS Slow progress These tend to slowdown the progress under selection .

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…………………The Effects of these Factors: D. Heritability GENETIC GAIN NO. OF GENERATIONS Slow progress Heritability is of great importance in determining progress under selection <100 % H will cause slow progress Low H……either low progress or no progress No progress Fast progress High heritability Low heritability

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Response to selection: 5 broad groups 1. Rapid gain followed by slow progress 2 . Continued slow response for a long period 3. Slow response for a short period 4. Little or no response 5. Rapid gain-plateau-rapid gain

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Response to selection: 5 broad groups 1. Rapid gain followed by slow response Colour intensity NO. OF GENERATIONS UNDER SELECTION genes with larger effect /genes Plant Ht, disease resistance, colour etc. Change in the mean in the direction of selection. Appearance of new genotypes Reduction in variability as indicated by variance. GENETIC INTERPRETATIONS: Few major genes Several minor genes

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Response to selection: Slow progress for a long period 2. Slow progress for a long period Per cent oil content NO. OF GENERATIONS UNDER SELECTION Selection for high oil content Burr White OP Var. of maize 10 20 30 40 50 60 70 80 Selection for low oil content Selection for high and low oil Selection for high and low protein 2 4 6 8 10 12 14 16 18 20 Mean = 4.7 % Range= 3.7-6.0 % Oil content increased for 76 generations…..reached 19% second generation had some plants with more than 6% By tenth generation all plants had more than 6 %

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Response to selection: Slow progress for a long period Per cent oil content NO. OF GENERATIONS UNDER SELECTION Selection for high oil content 10 20 30 40 50 60 70 80 Selection for low oil content 2 4 6 8 10 12 14 16 18 20 appearance of new genotypes in the population. The variability in 50 th generation was comparable to the first generation. The evidences came from the facts: The variances for different generations were comparable Suspension of selection resulted in decrease in oil content , i.e. tendency to revert back towards original parent Burr White. Selection for the low oil content was effective.

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Response to selection: Slow progress for a long period Per cent oil content NO. OF GENERATIONS UNDER SELECTION Selection for high oil content 10 20 30 40 50 60 70 80 Selection for low oil content 2 4 6 8 10 12 14 16 18 20 These findings clearly indicate that sufficient variability existed even after 50 and then 76 generation of selection. The effects of selection are summarized as: Slow changes in mean in the direction of selection Appearance of new gene combinations after few generations of selection Maintenance of variability even after long period of selection

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Response to selection: Slow progress for a long period Per cent oil content NO. OF GENERATIONS UNDER SELECTION Selection for high oil content 10 20 30 40 50 60 70 80 Selection for low oil content 2 4 6 8 10 12 14 16 18 20 GENETIC INTERPRETATIONS Since quantitative characters are generally governed by several genes each with a small additive effect, in such cases q will be smaller, consequently the progress would be slow under selection. The progress will be further slowed because H<100 %. As a result genes are seldom fixed.

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Response to selection: Slow progress for a long period GENETIC INTERPRETATIONS Generally a Correlated Response to Selection is observed for quantitative traits, i.e. changes in other characters is also observed. For example: Maturity Plant Height Tiller No. Cob Size Grain characters Yield Were also changed when sel. For high and low oil and protein content was made.

SLOW RESPONSE FOR A SHORT PERIOD:

SLOW RESPONSE FOR A SHORT PERIOD Per cent oil content NO. OF GENERATIONS UNDER SELECTION Selection for high oil content 10 20 30 40 50 60 70 80 Selection for low oil content 2 4 6 8 10 12 14 16 18 20 Some character like low oil content in Burr White maize show slow change for some generations, then reach at a plateau and do not show any response in later generations. Oil content from 4.68% decrease to 1 % in 25 generations but then became constant. Reason: oil is present in embryo, embryo determines viability of seed, below certain threshold level, plant does not survive. EMBRYO SIZE IS REDUCED WITH REDUCED OIL CONTENT Genetic interpretations

LACK OF RESPONSE TO SELECTION:

LACK OF RESPONSE TO SELECTION Example: Yield in Maize It has more variance, additive type but less heritability. Therefore, phenotype doesn’t represent genotype so selection for yield is not effective. The problem can be overcome by following Recurrent Selection Schemes . Simple selection will not be effective in this case.

RAPID GAIN-PLATEAU-RAPID GAIN RESPONSE:

RAPID GAIN-PLATEAU-RAPID GAIN RESPONSE Bristle No. in Drosophilla Mather and Harirson Increase in Bristle No. was associated with reduced fertility , so increase in bristle no. continued for some generations. Selection was suspended for some generations. Reselection in new population again responded favorably and bristle no. increased. This popn had less bristle no. as compared to the state when selection was suspended.

RAPID GAIN-PLATEAU-RAPID GAIN RESPONSE:

RAPID GAIN-PLATEAU-RAPID GAIN RESPONSE Bristle No. in Drosophilla It reached at plateau. Selection was suspended again. Reselection again increased the bristle no. which again reached at a plateau.

RQPID GAIN-PLATEU-RAPID GAIN RESPONSE:

RQPID GAIN-PLATEU-RAPID GAIN RESPONSE 68 60 52 44 36 28 0 14 28 42 56 70 84 98 112 126 Abdominal Bristle No. Generations under selection Response No Selection Plateau Response Plateau

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