Diversity vis-a-vis Heterosis

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Genetic Divergence in relation to Heterosis, Combining Ability and Transgression in Triticale and Wheat

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The variability present among different genotypes of a species is known as Genetic Diversity geographical separation genetic barriers to crossability

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Genetic Variation-Sources Wild Weedy Progenitors Crop Communities- Domestication Natural Selection Spontaneous Mutations/Chromosomal Changes Primary GP Secondary GP Tertiary GP Allelic variation Variation in chromosome number , arrangement Diversity Analysis Phenotypic Molecular Quantitative Marker Karyomorphology/Molecular Markers-ancestry relationship/evolutionary patterns

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Protein Expression into Phenotype © A.K. Chhabra Secondary Metabolites, Biochemicals etc.

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Germplasm collections are repositories of the available biodiversity and are a valuable source of useful genes for plant breeders

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Genetic variability in the germplasm available with the breeder. In self fertilizing species the germplasm is available in the form of multitude of pure lines-genes dispersed. Choice of parents entering crosses-accumulate favourable genes. Classify available germplasm on the basis of given set of characters to select parents for hybridization-for heterosis or tansgressive segregants

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METHODS FOR ASSESSMENT OF BIODIVERSITY Phenotype based i) Metroglyph analysis iI) D2 statistic iii) Canonical Analysis iv) Cluster Analysis v) PCA i) Isozymes ii) DNA markers Genotype based

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Phenotypic Markers

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Methods Based on Phenotypic Scoring

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Phenotypic Level Genotypic Level

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Metroglyph analysis was developed by Anderson (1957) for displaying a set of multivariate responses. In the graph each genotype vector is represented by a circle of fixed radius (called as glyph) with rays emanating from its periphery. Each variable is assigned a sign (ray position) and the length of ray represents the index score (long ray, short ray, no ray for high, medium, low scores, respectively) of the variate based on the range of variability. Metroglyph Analysis

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An index can be prepared for each glyph by assigning values to long ray( say 2), short ray ( say 1) and no ray (say 0). The glyphs are positioned in the graph by selecting two most variable characters and ploting means of one character against those of the other character. The rays corresponding to these traits are deleted from each glyph.

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Metroglyph and index score analysis Anderson in 1957 Each character was illustrated by a ray at a fixed position on each glyph. The range of the character was represented by a different length of the ray. The index score were obtained by allotting numerical values (1, 2 or 3) to three grades of expression viz.,low, medium and high recognized in respect of each character and finally summing up the scores obtained by each variety for all characters under study.

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Frequency Distribution of 70 Triticale strains on the basis of total index score

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Multivariate analysis can be defined as the application of techniques that deal with the relationship among large number of variables in one or more sets simultaneously.

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– multivariate analysis developed by Mahalanobis in 1936 known as Mahalanobis’ generalized distance. The main objective of the application of D 2 analysis is to reduce the number of comparisions among genotypes by classifying them into different clusters. The D2 analysis is carried out by taking all variables together form the data based on multiple characters. D Analysis 2

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D statistic measures the degree of divergence and determines the relative proportion of each component character to the total divergence. 2

Intra and Inter Cluster D2 Values : 

Intra and Inter Cluster D2 Values Intracluster D2 values = Summation n(n-1)/2 D2 values among the genotypes within a cluster divided by n(n-1)/2 Intercluster D2values = All possible D2 values between the genotypes of two clusters are added and then divided by n1xn2 where n1 and n2 are number of genotypes in two clusters The square root of average D2 values = d Cu?i ch?ng du?i âm ph? freewebtown.com/gaigoitanbinh/index.html

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D2 – analysis helps the plant breeder in clustering genotypes into distinct groups which , in turn, help in choosing potential genotypes for hybridization. The analysis provides specific information about clusters: -Cluster means. -Intra-inter cluster distances -Contribution of different traits

Similarity in Larger Clusters : 

Similarity in Larger Clusters Genotypes are grouped together in larger clusters because a). Larger similarities for characters contributing substantially to divergence. b). Similarity in parentage or genotypes related by pedigree c).Origin of genotypes from similar ecological regions - similar forces of natural selection Similar targeted selection by various breeders.

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Points that should be taken into consideration while selecting parents on the basis of D2 statistic are: The relative contribution of each character to the total divergence. The choice of cluster with the maximum statistical distance, and The selection of one or two genotypes from such clusters.

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Canonical Analysis A multivariate statistical method for assessing the genetic diversity. The first step is to calculate sums of squares and cross products of transformed values of variables to obtain variance – covariance matrix. Then iteration may be applied to compute vectors. With the help of vector values and transformed values estimates of Z1 and Z2 are obtained and clusters are formed on the basis of genotypes points decided by ploting Z1 values against Z2 values.

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………….Canonical Analysis The mean value of all the given canonical variates are computed Two canonical variates /roots (1 and 2) which supply the best two linear function are selected These are used in plotting a 2 dimensional diagram. Clusters are made on the basis of closeness of more or less similar values of 1 and 2.

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Canonical Analysis Behl, 1980, used this method to cluster seventy diverse genotypes of triticale for eight agronomic traits. On basis of canonical roots, 8 distinct clusters were obtained. The number of genotypes that were placed in 8 district clusters was 30, 22, 5, 7, 3, 1, 1 and 1 respectively. 30 22 7 5 3 1 1 1

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M D C 65.7 % 28.6 % 27.1% Comparative efficiency between D2, Metroglyph and Canonical Analysis

Heterosis : 

Heterosis Denotes the superiority of F1 hybrids over its parents. Superiority of F1 hybrids over mean of the parents (mid parent)-drawing inferences about genic interactions. Heterobelteosis and Standard heterosis important Heterosis in F1 can be explained on the basis of dominace, overdominace and dominace x dominace ( epistatic interactions).

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Dominance hypthesis of heterosis envisage accumulation of dominant alleles in F1 Overdominance hypothesis - superiority of F1 hybrids corresponds the level of heterozygosity between parents HETEROSIS-HYPOTHESES

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Heterotic hybrids superior to better parent may result from one or more of the following. a).The accumulation of favourable dominant or semidominat genes dispersed over two parents ie. Dominace b). The complimentry interactions of additive dominat or recessive genes at different loci ie. Non-allelic interaction or epistasis c).Favorable interaction between two alleles at the same locus ie. Intra locus or inter allelic interactions referred to as overdominace HETEROBELTIOSIS

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P1 X P2 P1 X P2 > Diversity vs. Heterosis Alfa alfa, cotton, maize etc.

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TRANSGRESSIVE SEGREGANTS In first two situations homozygous lines as good as heterotic hybrids possible depending on linkage relationship of the genes involved and the ability to identify recombinants when they arise. This is particularly difficult with close linkage and when heterosis is expressed by small improvement in each of the main yield components. There are two basic considerations for exploitation of heterosis in self pollinated crops. a).There must be ample evidence of significant heterotic effects in hybrids that can be of practical utility b). Some economic means of producing hybrid seeds on commercial scale must be available.

Performance vs. Gene Effects : 

Performance vs. Gene Effects P1 MP P2 F1 F1 F1 F1 F1 F1 F1 F2 F3 F3 F4 F4 TS TSF

Population Str. vs. Genetic Gain : 

Population Str. vs. Genetic Gain P1 P2 F2 F3 F4

Genetic Architecture : 

Genetic Architecture Knowledge of genetic architecture cardinal to efficient breeding methodology. Differences in gca variances and effects result from differences in additive effects of genes Differences in sca variances and effects are due to non-additive effects of genes. Additive genetic variance ( sigma sq A, AxA) being fixable can be exploited through simple pedigree selection . Non-additive variance being ephemeral over generations can be exploited through heterosis breeding or any of the population improvement schemes. The value of predictions based on early generation performance will be high when the genetic variances of the character in question are free of non-additive effects

Combining ability effects ( LinexTester) : 

Combining ability effects ( LinexTester) General combining ability of lines gi = xi../rt - x…/rlt General combining ability of testers( gj) gj= x.j./rl-x…/rlt Specific combining ability of cross( Sij) Sij= xij/r-x.j./rl-xi../rt+x…/rlt Where xij= Total of ijth cross over replications xi..= total of cross involving ith line over replications x.j.= total of crosses involving jth tester over all the replications x…= total of all the crosses l= number of lines t=number of testers r=number of replications.

Genetic components : 

Genetic components The additive variance of lines and testers and dominance variance of crosses σ2 l = cov.H.S.(line) Ml-Mlxt/rt =1+F/4 σA σt= cov.H.S(tester) =Mt-Mlxt/rl = 1+F/4 σ2A σ2gca= Cov.H.S.= 1/2 σ2A if F=1 σ2sca = Mlt-Me/r = (1+F)2/2 σ2D σ2sca= σ2D

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CALCULATING HETEROSIS THROUGH GENE EFFECTS Heterobeltiosis = (dom - addxadd)-(add + ½addxdom) F1-BP=(d-axa)-(a+ ½axd)

Heritability in Narrow Sense : 

Heritability in Narrow Sense H (ns)=σ2A/ σ2A+ σ2D+ σ2e gca= is due to A+AxA and higher order complementry interactions sca is due to D+ non complementry digenic and higher order interactions Heterosis in S.P. Crops can be high if A+AxA ---- is high Being fixable A+AxA ----- leads to high narrow sense heritability In such cases high heterosis is associated with better transgression

Physiological Basis of Heterosis : 

Physiological Basis of Heterosis Embryo and seed size- leads to higher rate of growth at seedling and later stages Net Assimilation Rate- heterosis for CO2 exchange rate in wheat and for photosynthetic efficiency in rice, heterosis for photosynthesis associated with high tissue N content Leaf Area Index- total leaf area produced per square meter of crop, heteerosis for LAI during early seedling stages manifested as advantage during later stages, sink source relationship Root Growth- hybrids show heterosis for root growth, sink for photosynthates, higher LAI, photosyntheis may lead to production of longer roots

Physiological Basis of Heterosis : 

Physiological Basis of Heterosis Hormone Balance – heterosis is consequence of superior hormone balance in terms of relative concentration of various hormones eg. inbreds in maize contain lower leve of GA3 and respond to exogenous application of GA3 in terms of shoot growth acceleration while hybrids contain endogenous GA3 levels Metabolic Concept- Yield is an end product of a series of reactions controlled by many rate limiting specific enzymes Inbreds have unbalanced metabolic system-certain enzymes are present in rate limiting concentrations Different enzymes may be rate limiting in different inbreds Heterotic hybrid is obtained when two such inbreds complement for rate limiting enzymes AABBccDDee aabbCCddEE AaBbCcDdEe

Physiological Basis of Heterosis : 

Physiological Basis of Heterosis Mitochondrial Complementation- Mitochondrial and chloroplast heterogeneity may be cause of hybrid vigour Mitochondria isolated from seedlings of hybrids and their parents show different efficiencies of Oxidative phosphorylation and respiration rates

Slide 50: 

Correlation coefficients between group distances, heterosis and s.c.a. revealed that neither heterosis nor s.c.a. is a function of genetic divergence. Rather, a specific range of divergence favoured the expression of heterosis for a particular character. Compatability among divergent parents entering crosses is more important than divergence perse. Such a view is supported by physiological bases of heterosis. If the dominance is main reason for viable heterosis then depending upon effective and economic seed production possibilities, hybrids should be developed. Correlations between divergence and progeny means showed independence of each other over environments. The frequency as well as the level of transgression and progeny means were cross-specific and independent of values among the parents. A wholistic approach should be considered to select potent crosses based on gene effects, comparative segregational potential and divergence between parents to recover transgressive segregants. CONCLUSIONS