Plant Breeding || Breeding for Wider Adaptability

Plant Breeding - Mendelian to Molecular ApproachesH. K. Jain and M. C. Kharkwal (eds.)Copyright © 2004 Narosa Publishing House , New Delhi , India

Breeding for Wider Adaptability

Darbeshwar Roy1 and M.e. Kharkwal-

Abstract

24

The success of a variety depends not only on its high yielding potential but alsoon the stability of its yield over years across environments. If a variety gives high

and stable performance over years at a location within an agro-climatic zone, it iscalled a stable variety . When the same variety is grown in different agro-climaticregions and if it shows high yield with little or no change in performance incomparison to other varieties, it is called a widely adapted variety . Theadaptability, thus refers to the reduced variation in performance across locations,whereas stability refers to the reduced variation in performance across years .Before embarking on a breeding programme aiming at developing a widelyadaptable variety , it is essential to know the genetics of the trait, the mechanisms

leading to the development of this trait and finally the methods of measurementof the trait. This paper describes some of the concepts and methods underpinningthe development of high yielding, stable and widely adapted varieties .Mechanisms and measures of stability and adaptability of varieties and their

analysis under various agro-climatic conditions have been discussed.

IntroductionThe different agro-climatic regions differ with respect to climatic (temperature gradient,photoperiod and rainfall distribution) and edaphic (soil fertility and type) factors andmanagement practices . Phenotype is the product of genotype and environment. In thepresence of genotype x environment interaction the phenotype will be the product ofgenotype, environment and genotype x environment interaction. Same genotype canproduce different phenotypes in different environments and different genotypes canproduce same phenotype in a particular environment. A stable genotype is one whichinteracts less with the environment or shows a minimum of genotype x environmentinteraction . Further, the environment consists of controllable or uncontrollable,predictable or unpredictable factors . So while talking about stability we are consideringminimum or less genotype x uncontrollable or unpredictable environmental interactionand further less genotype x unfavorable environment interaction. The predictableenvironmental conditions are topography, soil type and climatic condition such as daylength.

'o.n. Pant University of Agriculture and Technology, Pantnagar 263 145, India2Indian Agricultural Research Institute, New Delhi 110 012, India

574 Darbeshwar Roy and M.C. Kharkwal

When the genotype x environment (g x e) interaction is significant, we must furtherinvestigate other specific types of interactions such as genotype x location, genotype xspecific treatment such as fertility level, irrigation schedule, sowing date etc , genotype xseason (year) and genotype x location x year. In case of stability, a variety showsminimum or low variety x season (or year) interaction . Here the environment isunpredictable. In case of higher adaptability, genotype x location x year interaction is lowor minimum and here the environments consist of predictable as well as unpredictablefactors.

The Need for Wider AdaptabilityThe need for breeding a variety well adapted over wide areas arises because of the factthat the breeder is already developing a variety for specific adaptation . They areknowingly or unknowingly developing a variety which is highly responsive, as they areselecting material in the best possible environment. Their variety will give stableperformance in environment similar to their experimental sites. Variety(ies) with wideradaptability will increase the production quickly and it will be much easier to control thequality of seed. Further, any effect of disease/insect or any other specific factor on thecrop can be monitored very precisely and correction measures applied effectively .

Mechanism of StabilityThere are two ways in which stabilization of yield over locations and over years can occur(Allard and Bradshaw, 1964).

1. Individual Buffering: In case of pure line variety (homozygous, homogeneous),single cross hybrid (heterozygous , homogeneous) or clones the individualsthemselves may be well buffered . Each individual of the population is well adaptedto a range of environments.

2. Population Buffering: In case of mixtures in self-pollinated species (made up ofhomozygous and heterogeneous individuals) and in double or three-way hybrids,improved open-pollinated populations, synthetics and composites in case ofcross-pollinated species, where the population is made up of a number ofgenotypes, each genotype is adapted to a somewhat different range ofenvironments resulting in the stabilization of yield.

Thus what we see is that the variety's yield is stabilized in the face of variousenvironmental influences as a result of buffering, which signifies protection againstvariability. In other words, this character (stability of yield) is buffered or canalized, sothat its development is unaffected by environmental stresses or by underlying geneticvariability. Further, it can be said from the above that buffering is not only the property ofan individual (homozygous or heterozygous) but also that of a population (homozygous,homogeneous or heterozygous, heterogeneous) .

In case of out-breeding species, buffering is a characteristic of heterozygosity .Heterozygous individuals have better buffering ability than homozygotes. Lerner (1954)used the term genetic homeostasis to describe this property. It refers to the tendency of a

Breeding for Wider Adaptability 575

physiological system to react to external disturbances in such a way that the system is notdisplaced from its normal values. In other words, it means resistance to change which isjust opposite of plasticity . Further it depends on the particular array of gene frequenciesbuilt up by a population over a long period of time. Darlington and Mather (1949) calledthis as co-adapted gene complex or super gene which is of fundamental importance in theadaptation of a population to their environments.

Jinks and Mather (1951) observed that in case of inbreeding species, pure breeding

lines differ in their buffering capacity and F) does not show increase in stability . That F)

shows higher stability in comparison to homozygous pure breeding parents has been

inferred from the general observation that lower coefficient of variability (CV) value is

associated with hybrids but this linear relationship accounts only for a small part of the

total variation in environmental sensitivity. They concluded that F) shows superiority not

because of heterozygosity per se but because of gene contents.

The population buffering arises from interaction among different constituent genotypes .Stability in performance of such a population is often associated with genetic diversity(Allard, 1961) . Mixtures are more diverse than pure breeding varieties . Three-way anddouble-cross hybrids are more diverse than single-cross hybrids and have shown higherstability based on estimates of CV, although certain single-cross hybrids have also shownhigher stability (Jones 1958; Sprague and Federer. 1951). Thus heterogeneity or diversityprovides a higher degree of population buffering. Results from varietal mixtures ormultilines, have shown that they yield higher than pure line cultivars in the face of bioticstresses (Barrett, 1978). As the population is made up of a number of genotypes,favourable g x e interaction may be high for some, low for some others and evenunfavourable for certain other genotypes. Thus overall the population appears to be givinga low g x e interaction . Reduced g x e interaction can also be due to inter-genotypecompetition occurring among different coexisting genotypes, but so far little is knownabout the mechanism underlying population buffering. Whatever may be the mechanismunderlying individual or population buffering, it must be measurable in terms of g x einteraction. It is difficult to measure g x e interaction in case of population.

Stability of yield can arise because of plasticity of the individual traits as well. Yield isa function of a number of traits. Almost similar yield can be obtained via slight increase inthe value of a component trait of yield and the corresponding decrease in the values ofother yield component traits or vice versa . Stability in yield thus arises because ofcomponent compensation.

Measures of Stability/Adaptability

Various measures of stability proposed are as follows:I. The contribution of ith genotype to the g X e interaction sum of squares is a

measure of stability of ith genotype (Wricke , 1962 and Plaisted and Peterson,1959).


2. Finlay and Wilkinson (1963) proposed regression coefficient (b) of ith variety yieldon the yield of all the varieties for each site and season as a quantitative measure ofadaptability of the ith genotype. They described the nature of adaptabilityconsidering the regression values and the varietal means as follows:

Regress ion coefficient (b)

b = 1.0 (Average stability)

b < 1.0 (Below average stability )

b > 1.0 (Above average stability)

b = 0

Yield level Nature of adaptability

High Well adapted to all environmentsLow Poorly adapted to all environments

High Specifically adapted to favourableenvironments

Low Specifically adapted to unfavourableenvironments

Absolute phenotypic stability

Here the varietal means serve only to discriminate between regression coefficientsof equal value or to specify performance within a set of environments.

3. Hanson 's (1970) measure of stability combines the contribution of the ithgenotype to g x e interaction sum of squares with its response to environmentalchange.

4. Eberhart and Russell (1966) and Tai (1971) suggested the use of two parameters,regression coefficient (b) in conjugation with deviation from regression (S2d) as ameasure of stability and a variety is called stable if b = I and S2d = O. The twoparameters can be estimated following Perkins and Jinks' (1968) analysis as well.

5. Breese (1969) advocated the use of deviation from regression (S2d) as a measureof stability, which according to him, provides the measurement of unpredictableirregularities in the response to environment. Witcombe and Whittington (1971)observed that more generally S2d, the non-linear environmental sensitivity willbe accounted for by the variance of the genotype's response to all the differentenvironmental variables present, so S2d is not always analogous to an unpredictableirregularity in response to the environment and it is predictable when theenvironmental changes, which are not accounted for in the analyses are known.Thus deviations from regression are not due to developmental noise (Waddington,1957), or related to the concept of developmental homeostasis (Lerner, 1954).

The other measures of stability are Shukla's (1972) stability variance, coefficient ofvariability (CV) and the relative performance of an entry (Yau and Hamblin, 1994).

The different measures of stability lead to three concepts of stability as follows:I . A variety is stable if its variance among environmental is small, i.e. the genotype is

stable in absolute sense (b= 0). This variety is interacting less with theenvironment or shows minimum g x e interaction.

2. If b = 1.0, its response to environment corresponds to the mean response of all thevarieties in the experiment. The variety can be said to have average stability.

3. If its deviation mean squares, S2d is smaller or zero , the variety is called a stablevariety.


Thus the definitions of stability are many and varied (Hill, 1975) . The relationshipsbetween these have been discussed by Witcombe and Whittington (1971), Easton andClements (1973), Freeman (1973), Marquez-Sanchez (1973) and Lin et al., (1986).

In principle we should look for a variety for which b =0, but in practice we should gofor a variety for which b =1.0 and s'2d =0, as we need a variety which is responsive andan ideal variety is one which responds favourably when the environment is favourable (i.e.b > 1.0). Such a variety is difficult to find. In other words ideal variety having generaladaptability is one with maximum yield potential in most favourable environments and amaximum phenotypic stability (Finlay and Wilkinson, 1963).

Genetics of StabilityAs the genotype x environment interactions depend on genotype as well as environment,they are partly heritable. Further the variation in response of genotype to environment isas widespread as variation in yield, so the trait determining the stability of performance isconsidered a quantitative trait. The genetical architecture of this trait can be worked outusing analytical procedures of biometrical genetics (Mather and Jinks, 1982) and furtherthe relative sensitivities of any genotype can be predicted. As the selection programme iscarried out in the high fertility environment, selection for high mean performanceinvariably selects genotypes with high b values and thus there appears positive correlationbetween mean performance and environmental sensitivity (bD . That yield and yieldstability, are, at least, in part, under independent genetic control was shown by Perkins andJinks (1968a, 1968b, 1971 , 1973) and Jinks and Connolly (1973). So selection ofgenotype(s) for b = 1.0 (average stability) should not be a problem. Further since the lowremainder mean sum of squares, s'2d is not correlated with either bi or mean performance(yield) , selection for this character will not be complicated. Thus the aim should be todevelop genotype(s) with b =1.0 and showing uniformity in response to environmentaleffects , i.e. low remainder mean sum of squares (s'2d = 0).

Measurement of Stability ParametersFor calculating the stability parameters of different varieties, they are grown in replicatedtrials in multilocational environments. For detecting g x e interaction and estimating bi'sand S2d/ s either joint regression analysis (Finlay and Wilkinson, 1963 and Perkins andJinks, 1968), or Eberhart and Russell's (1966) stability analysis is carried out. With snumber of varieties and t number of environments, the A:tj.OVAs in Perkins and Jinks andEberhart and Russell's analyses take the form as given in Table 1.

The joint regression analysis of Finlay and Wilkinson (1963) and Perkins and Jinks(1968) combines the individual regression analysis being carried out for each of thevariety for estimating b, and S2d

i . The individual regression analysis takes the form asgiven in Table 2.

Having calculated the bi' sand S2d/ s of the different varieties, testing of theirsignificance is done and those varieties for which b = 1.0 and s'2d = °are classified asstable variety. After that means of the varieties are compared and a variety with high mean


Table 1. ANOVAs in Perkins and Jinks and Eberhart and Russell's analyses

Sources of variationPerkins and Jinks's analysisGenotypesEnvironmentsGenotype x EnvironmentHeterogeneityRemainderErrorEberhart and Russell's analysisGenotypesEnvironment + g x eEnvironment (linear)G x E (linear)Pooled deviationGenotype 1

GenotypePooled error

r is the number of replications

df

(s-I)(t-I)(s-I ) ((-I)(s-I)(s-I) (t-2)st (r-l )

(s-I)s (t-I)1(s-I)s ((-2)(t-2)

(t-2)(s-I) t (r-I)

Table 2. Regression analysis for individual variety

Sources of variation df

Regression 1Deviation from regression (t-2)

mean and b = 1.0 and S2d = 0 is finally selected and recommended for commercialcultivation or used as parent in the crossing programme aimed at breeding a high yieldingvariety with general adaptability .

Multivariate AnalysisAs we have seen above, the stability of performance of a variety can be the result ofplasticity in different traits and compensation between different component traits. In otherwords, stability is as a result of balance among responses in different traits and sostability/ adaptability of genotype can also be studied using multivariate techniques(Hardwick and Wood, 1972). The different multivariate analyses that are being used arecanonical analysis and factor analysis (Grafius and Kiesling, 1960) and principalcomponent analysis (Freeman and Dowker, 1973). Further, in case of either jointregression analysis by Finlay and Wilkinson (1963), and Perkins and Jinks (1968) orEberhart and Russell (1966) analysis, the linear model was assumed to be able to explainthe genotype-environment interaction. In other words, the phenotype of an individual wasassumed to be the result of additive effects of genotype and environment. However, Fisherand Mackenzie (1923) observed that a product formula (multiplicative model) provided abetter fit to the yields of varieties in different environments. Considering this Mendel(1971) used principal component analysis to study genotype-environment interaction. Inthe biplot method (Gabriel, 1971; Gauch Jr ., 1988) principal component analysis isapplied to genotype-environment interaction to generate multiplicative model. The biplotis a plot which simultaneously displays both the genotypes and the environments, whereas


the GGE biplot (Yan and Hunt, 2002) is a plot which displays the genotype main effects(G) plus the genotype-environment interaction (GE). It is constructed by plotting the firsttwo principal components, PCI and PC2, derived from the principal component analysisof the environment-centered data. This analysis identifies ideal cultivar (s), the ones with alarge PC 1 score (representing high yielding ability) and a small PC2 score whichrepresents stability . This analysis also identifies the test environment - an environmentwith one large PC 1 score (discriminating genotypes more in terms of genotypic maineffect) and a low PC2 score (more representative of the overall environment). The onlyassumption underlying this graphical analysis is that the genotype and thegenotype-environment interaction which are the two sources of variation that are relevantto the cultivar evaluation are sufficiently accounted for by the two principal components,PCl and PC2.

Non-Parametric Measures of StabilityThe various non-parametric measures of phenotypic stability such as ranks and the othertwo statistics, namely mean absolute rank difference and variance of the ranks for testingand comparing the stabilities of different genotypes have been suggested by Htihn (1979),Nassar and Htihn (1987) and Htihn (l990a&b).

Utility of Stability AnalysisThe breeders who generally pick up genotypes with high means and stability on the basisof their performance and ranks in different environments, will have problems if thenumber of varieties and number of environments in the multilocational trial is large andthis is where the stability analysis will be of help. These analyses (un ivariate ormultivariate) will categorise the varieties into the following groups.

1. Varieties with high yield and general stability/adaptability2. Highly responsive genotypes in favourable environment3. Genotypes showing better response in poor environment

Besides the analysis will indicate the test environment which can discriminate betweengenotypes, which is essential for making selection effective.

Proven Adaptable VarietyIf a crop variety is in commercial cultivation over wide areas for more than 10-15 years,that variety can be said to have wider adaptability and should be used as one of the parentsin the hybridization programme, aiming at developing a high yielding variety havingwider adaptability. There are a number of varieties in each crop , which are high yieldingand have been in cultivation for a long time and thus can be said to possess wideradaptability, for example, Kalyansona and Sonalika in wheat, IR-24 and Jaya in rice. If welook at the characteristics of the different varieties within a crop species, we find that thewider adaptability is associated with different combinations of traits values . Further, if wecompare these high yielding semi -dwarf varieties with the land races which were lowyielding but having wider adaptability, we find that the adaptability is associated with bothlow and high yield, which again confirms that these two traits, yield and adaptability areunder different independent genetic control system.


Breeding for Wider AdaptabilitySelf-Pollinated Crops: In case of self-pollinated crops, a variety of proven adaptability isselected and used as a parent in the crossing programme and the pedigree selection ispracticed for extracting recombinant pure breeding line(s) superior in yield andadaptability . The segregating generation population is raised in a number of contrastingenvironments. This method of breeding uses the principle of disruptive selection. Here theselection is for genotype(s) showing extreme phenotypes at the same time. InternationalMaize and Wheat Improvement Center (CIMMYT), Mexico called this method ofbreeding a widely adapted variety as "Shuttle breeding" . Here the individual/familyhaving plasticity in different traits resulting into stability of yield over differentenvironments is selected. Family/individual is selected on the basis of its meanperformance in such environments and advanced. The development of an individual isthus channeled into one optimal phenotype or another by a developmental switchmechanism (genetic, environmental) . The general adaptability can thus also be attributedto variation in phenotyptic plasticity . In the later stage of breeding programme stabilityparameters can also be worked out to see the agreement between the conclusions drawnon the basis of observed mean of a variety and its variance over different environmentsand the conclusions drawn from the estimates of stability parameters.

As we have seen above the genetic diversity within a variety also leads to populationbuffering. To exploit this, lines which are similar in height, maturity, grain colour, etc. inF6 - Fg generation in pedigree or SSD, can be bulked to constitute a variety which should

be tested in contrast ing environments, in order to examine its adaptability . Similarly,varietal mixtures can be constituted to provide diversity to the variety , and thus differentvarietal mixtures can be made and tested in different environments and mixtures(s) havinghigh yield and adaptability can be identified.

Cross-Pollinated Crops : In cross-pollinated crops while developing single cross

hybrids, inbreds should be thoroughly evaluated and only those inbreds having higher

adaptability should be used to produce hybrids. Again as in case of self-pollinated crops

while developing inbreds through pedigree method, family/individual should be tested in

contrasting environments and selection should be based on the means of inbreds in

different environments.In case of developing improved populations through either inter- or intra-populations,

improvement methods, full-sibs or half-sibs or SI or S2 families developed should betested in markedly different environments. Also, full- sibs or half-sibs or SI or S2 familieshaving higher means over environments should be allowed to intennate to reconstitute thepopulation for starting another cycle of selection. Here it must be noted that the selectionon the basis of means in multilocational trial will not work. The environments in whichevaluation/testing of lines or families is done must represent contrasting environments .For details the reader is referred to Roy (2000) .


Yield at a location or in a season may be affected by a specific factor or factors such assusceptibility of the crop variety to diseases and pest, water logging condition, drought,cooler weather, nutrient deficiency, alkalinity, salinity, acidity, etc . Then these defectsshould be corrected and thereafter the yield and the adaptability of a variety will improve.Under such unfavourable conditions the b value of a widely adapted variety will be low.Further, the S2d value of a variety susceptible to disease/insect will be very large . So if agene for resistance to disease/insect is transferred to that variety, its adaptability willimprove . If a particular abiotic stress is a characteristic of a location in a particular region,it would be better to breed a variety specifically adapted to such conditions. For example,in case of deep water (flooded) condition, one should develop a deep water rice varietyand also one can develop a maize variety specifically suited to flooding condition.

Molecular Approach to Breeding for Wider Adaptability

As there is no mechanism which precisely determines the stability of performance of anindividual or population over locations or seasons and with the availability of molecularmarkers such as Randomnly Amplified Polymorphic DNAs (RAPDs), AmplifiedFragment Length Polymorphisms (AFLPs), mini- and micro -satellites (Williams et aI.,1990 and Rafalski and Tingey, 1993) it would be worthwhile to detect, locate and estimatethe effect of individual Quantitative Trait Loci (QTLs) determining the stability/adaptability and thus the genetic architecture of this can be precisely worked out. What isrequired at present is to carry out QTLs analysis in proven or tested, widely adaptedvarieties in different crops and try to seek the answer to the following questions :

1. How many QTLs are explaining most of the variation in this trait?2. Whether QTLs determining the adaptability are orthologous or not.3. Whether or not they show syntenous relationship.

Once the desired QTLs are found they can be transferred to an otherwise desirablegenotype. High yielding selections with desired QTLs in the segregating generations canbe isolated using molecular markers .

Declining Importance of this TraitAs agricultural research stations are now established in different agro-climatic zones andin regions of specific problems such as flooding, drought/dry areas, high altitude, alkaline,sodic or acidic soil etc. , plant breeders are developing varieties for such specificconditions. Further, they are engaged in developing variety with specific adaptation tohigh fertility, low fertility, irrigated or rain fed condition and there they are moreconcerned about stability of performance over years/seasons in such conditions rather thanabout the general adaptability of the variety. They are also engaged in developing varietyspecifically adapted to multiple cropping systems such as inter-cropping, relay cropping,strip cropping, mixed cropping, ratoon cropping, etc . To fit well in a particular croprotation system, they are developing early maturing varieties.

With the restriction on free trade vanishing, the breeders are now engaged indeveloping varieties with specific quality, such as variety with specific level of protein, oil

582 Darbeshwar Roy and M. C. Kharkwal

(saturated and unsaturated fatty acid per cent) , vitamins, storage/transportation quality,seed size , anti-nutritional factors, infection/infestation of grains with pathogens/insects.There is also a need of variety suitable for mechanization (combine harvesting). Besidesthese, plant breeders are engaged in developing disease/insect resistant variety, so thatthere is less or no use of chemicals, which are detrimental to environment as well ashuman health. Also, here the objective is to control the epidemic through the use ofvarietal diversity in space and time. Further, they are trying to develop variety which ismore suitable to organic farming and is thus less dependent on use of chemical fertilizer.as the people are now more conscious about their health.

So in summary it can be said that the breeders are now engaged in developing qualityproducts which can fetch higher price in the international market. It can also be seen fromthe above that there is a long list of objectives before plant breeders and they are engagedmore in developing varieties which can give stable performance with respect to thesequality characteristics, thus, the objective of breeding widely adapted varieties has beenlost in this era of globalization . Further, as discussed above, for selection ofadaptable/stable genotype(s) the segregating generation population must be grown in thecontrasting environments and because of resource crunch the breeder cannot generateenough segregating material and further cannot conduct multilocation trial in the earlystage of breeding programme. Therefore, they cannot isolate line(s) with generaladaptability and thus they are not making it as one of the objectives. But the internationalinstitutes like CIMMYT, International Rice Research Institute (IRRI) have got enoughresources and can conduct multilocation/international trials and because of that they are ina position to develop material with wider adaptability.

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Plant Breeding || Breeding for Wider Adaptability