Genetic Component and Vr-Wr Graphical Analysis in Blackgram [Vigna mungo (L.) Hepper]

Blackgram [Vigna mungo (L.) Hepper] is an important crop of South East Asia cultivated in all three seasons viz., kharif, rabi and zaid either as a monocrop or as an intercrop. Besides being for human consumption, its foliage is served as a nutritive fodder to milch animals. Blackgram can not only fix upto 80 per cent of its nitrogen requirement but can also contribute to the yield of subsequent crop through biological nitrogen fixation (Lauren et al., 1998). The production of blackgram globally is around 8.5 million tonnes. In spite of being the largest producer and consumer of blackgram (also known as urdbean), India has to import nearly 85 per cent of its domestic blackgram requirement from countries like Myanmar, Singapore and Thailand (Anonymous 2018). Such a wide gap between its production and demand can be attributed to the fact that majority of Indian farmers are small and marginal. They prefer growing staple cereals for subsistence, while the farmers having large landholdings prefer growing cash and commercial crops. In whatever meager land pulses are cultivated, are often provided with low inputs and generally suffer from various problems like water unavailability, salinity and poor drainage leading to water logging. All these constraints make it precedent to breed varieties that can thrive well under such conditions. This can be achieved by developing new recombinants suitable for the target environment by following an effective breeding strategy well suited to the genetic architecture of various desirable traits of the crop.
 
Diallel provides a systematic approach and a comprehensive genetic evaluation of the breeding material (Johnson 1963). The Hayman’s approach (1954a and 1954b) of diallel analysis provides information not only about the gene action but also regarding various statistics revealing nature of the genes involved. It further tests the validity of assumptions like absence of linkage, maternal effect and multiple alleles and presence of diploid segregation, homozygous parents and random mating. It also provides information regarding presence or absence of epistasis which otherwise involves complex analysis. Therefore, the purpose of present investigation was to provide enough chance for the breeding material to produce desirable recombinants and to get the complete information regarding genetic architecture of the complex traits, so that appropriate breeding strategy can be formulated for advancement and effective selection of the material for yield enhancement along with varietal development for constrained marginal lands.

The research was carried out at the Experimental Farm, Department of Genetics and Plant Breeding, CSK HPKV, Palampur, Himachal Pradesh, located in the foothills of Himalayas at 32°8’ N latitude and 76°3’ E longitude, 1290.8 m above mean sea level. The area is characterized by humid sub-temperate climate with high rainfall (2500 mm) and acidic soil (pH 5.0 to 5.6). The breeding material comprised of nine fixed diverse lines of blackgram viz., DKU-98, DU-1, Him Mash-1, HPBU-111, HPBU-124, IC-281994, IC-413304, KU-553 and Palampur-93. These genotypes were crossed in a half diallel system in kharif 2016  in a glasshouse and the 36 F₁’s so obtained along with nine parents and one check (Palampur-93) were laid out in fields in three replication CRBD in kharif 2017. One row of each genotype was sown keeping 30 cm × 10 cm spacing and the crop was raised following recommended package of practices. The data was collected on randomly chosen five competitive plants on thirteen traits viz., days to flower initiation (DFI), days to 50 per cent flowering (DFF), plant height (PH), branches per plant (BP), days to 75 per cent maturity (DM), pods per plant (PP),  pod length (PL), biological yield per plant (BY), seeds per pod (SP), seed yield per plant (SY), harvest index (HI), 100-seed weight (SW) and crude protein content (CP). Crude protein content for each entry was obtained through Inframetic Analysis System. The data were analysed as per the procedure of Jinks and Hayman (1953) and Hayman (1954a and 1954b) for diallel analysis using software WINDOSTAT VERSION 9.3.

Various genetic components so obtained were as follows:
 
E is the non-heritable or environmental variation associated with individual means; D is the component of variance attributed to the additive effect of genes; H₁ is component of variance attributed to dominance effects; F is covariance of additive and non-additive effects in all the arrays; H₂ is component of variance attributed to non-additive effects corrected for gene distribution; h² is dominance effects (as the algebraic sum over all loci in heterozygous phases in all cases). The significance of the parameters was tested. If the value of a parameter by its standard error exceeds 1.96, then it is significant and vice versa.
       
The analysis also provided following proportional values for detailed insights into the genetic architecture of these traits.
1.   Mean degree of dominance. Value less than 1 indicates partial dominance while, the greater than one indicates overdominance.
2.  H₂/4H₁ = Proportion of genes with positive and negative effects in parents. The ration equals 0.25 indicates symmetrical distribution of positive and negative genes.
3. =  Distribution of dominant and recessive genes in the parents. Less than one value indicates preponderance of recessive alleles while, greater than one indicates excess of recessive genes.
4.  h²/H₂ = Number of gene groups exhibiting dominance.
5.  Heritability (narrow sense) = ½D/(½D+¼H₁+E).

In the present study Hayman’s graphical and numerical approach (1954a and 1954b) were followed to get information regarding the genetic architecture of the blackgram traits under study. To check the validity of the assumptions laid by Hayman for diallel analysis, t² test was conducted (Table 1). The significance of the t² for PH, PL, HI and CP indicated the failure of the assumptions for these traits. Further testing of the assumptions was done by plotting the Wr-Vr graph. The regression line (b) should deviate significantly from zero but not from unity in case of the absence of the epistasis. Deviation of the regression line indicated the absence of epistasis only for one trait i.e., DM. None of the traits had unit slope of regression line confirming the partial fulfillment of the assumptions. The Wr-Vr graph further indicated partial dominance (negative point of intercept of regression line) for DFF, DM, BY and CP and overdominance for rest of the traits (positive point of intercept of regression line). The parents lying close to the origin had maximum dominant genes for the trait while, the farthest one had maximum number of recessive gene (Fig 1 to 5). KU-553 had preponderance of dominant alleles for DFI, DFF and SP; DKU-98 and DU-1 for CP. IC-281995 had preponderance of recessive alleles for DM, PH, BP, PP, PL and SY; DKU-98 for DFI, DFF, PH, HI and SW; DU-1 for DFI, DFF, PP, PL and SY; HPBU-124 for SP and HI; Him Mash-1 and HPBU-111 for SY.
 

 

 

 

 

 

Various statistics obtained through Hayman’s numerical analysis (Table 2) revealed the role of both additive and non additive gene action in the inheritance of PH, BP, DM, PL, BY, SP and SW however, rest of the traits were exhibiting non-additive gene action. Significant h² indicated presence of unidirectional dominance in genes governing DM, PH, BP, PP, BY, SY and CP while others had bi-directional dominance. Environmental component was predominant in PH, BP, DM and PL. Genetic component F has a positive or negative sign depending upon whether most of the genes governing the trait were dominant or recessive.  The positive value of F and less than 1 value of kD/kR for all the traits except SP and CP revealed that relative frequency of dominant genes in the parents was high in these traits. Dominance deviations were higher than the additive gene effects for all the traits except SP. High narrow sense heritability was observed in SY, SP, SW and CP; low in PL and medium in rest of the traits. All the traits had asymmetrical distribution of positive and negative genes. One major gene group was controlling every trait except BP and DM where, two genes groups were involved. Parents having maximum number of dominant and maximum number of recessive genes did not show high variation in the mean values of the traits except for DM and BY.
 

 
Non-additive gene action had major role in the inheritance of all traits which can be best exploited through hybrid seed production. However this is a rare possibility in blackgram therefore, selection of desirable transgressive mutants should be done in later generations. In traits like PH, BP, DM, PL, BY, SP and SW, population improvement can be employed to exploit both additive and non-additive gene action.
 
The role of both additive and non-additive gene action in the inheritance of yield and its components traits have also been reported by many other workers in blackgram (Singh and Singh, 2005; Karthikeyan et al., 2007; Bhagirath, et al., 2009; Chakraborty et al., 2010; Panigrahi et al., 2015; Vadivel, et al., 2019), greengram (Singh et al., 2007; Patil et al., 2011; Thangavel and Thirugnanakumar, 2011; Singh et al., 2016) and cowpea (Egbadzor et al., 2013; Dias et al., 2016). The substantial presence of non-allelic interactions has also been reported by Gupta (2005), Khan et al., (2004), Chand (2000), Adeyanju et al., (2012), Ramakant and Srivastava (2012) and Bindra et al., (2017) in backgram; Thangavel and Thirugnanakumar (2011) and Singh et al., (2016) in mungbean and Tchiagam et al., (2011) in cowpea. However, contrary to our findings absence of epistasis has also been reported by Rehman et al., (2009) in mungbean for seed yield.

In a self-pollinated crop like blackgram where exploiting heterosis commercially is still a remote possibility, the utilization of generated variability for varietal development needs efficient and effective breeding and selection methodology to prevent loss desirable recombinants in the segregants population. Such a methodology can be devised only through precise knowledge regarding the genetic architecture of the traits so that the potential parents and crosses can be carried forward to obtain maximum transgressive segregants. Most of the traits under study had both additive and non additive gene action. Therefore the population improvement can yield maximum high yielding transgressive segregants.