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-
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 highand 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 mechanismsleading 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 theiranalysis 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 breedinglines 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 beeninferred from the general observation that lower coefficient of variability (CV) value isassociated with hybrids but this linear relationship accounts only for a small part of thetotal variation in environmental sensitivity. They concluded that F) shows superiority notbecause 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).
576 Darbeshwar Roy and M.C. Kharkwal
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 environmentsHigh Specifically adapted to favourable
environmentsLow Specifically adapted to unfavourable
environmentsAbsolute 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.
Breeding for Wider Adaptability 577
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
578 Darbeshwar Roy and M.C. Kharkwal