Detection of Epistasis for Biometrical Traits in Urdbean [Vigna mungo (L.) Hepper] under Mid-hill Conditions of Northwestern Himalayas

Urdbean [Vigna mungo (L). Hepper] also known as blackgram is an important short duration, diploid (2n=2x=22) leguminous pulse crop, belongs to Fabaceae family. Vigna mungo var. silvestris, grows wild in India and believed to be progenitor of blackgram (Lukoki et al., 1980).  Pulses contain a remarkable amount of proteins, minerals, vitamins and carbohydrates. Among the various pulses, blackgram is an important one which contains approximately 25-28% protein, 4.4-5.5% ash, 0.5-1.5% oil, 3.5-4.5% fibre, 62-65% carbohydrate on dry basis (Sohel et al., 2016). It also contributes a major portion of lysine in the vegetarian diet and fairly good source of vitamins like thiamine, niacin, riboflavin and much needed iron and phosphorus. It is one of the most important legume crop utilized in the food, fodder, soil conservation, integrated farming systems, reclaiming of degraded pastures and symbiotic nitrogen fixation. Despite their significant importance, there is a big gap between potential and actual yield which is more notably due to lack of suitable ideotypes for variable cropping systems, low harvest index (HI), abiotic/biotic stresses and their cultivation in marginal and harsh environment.
       
Seed yield is a complex trait and it is the result of expression and association of several plant growth components. Qualitative characters controlled by one or few major genes are more readily manipulated in a breeding program as compared to quantitative traits controlled by many genes. Nevertheless, the breeder is concerned mainly with quantitative characteristics which could be of use in both formulating and performing the breeding program. Thus, the inheritance of characteristics chosen has a major influence on the strategy employed for whether self or cross pollinated cultivar development. For improvement in yield of any crops such as blackgram, it depends to a large extent on the nature of gene action involved in the control of complex yield contributing character(s) and accordingly the breeding programme.
       
Triple test cross (TTC) design developed by Kearsey and Jinks (1968) is an extension of North Carolina Design III of Comstock and Robinson (1952) that is applicable to any population irrespective of its mating system and its gene and genotype frequencies (Kearsey and Jinks 1968). This analysis provides a test of epistasis and in its absence gives independent and equally precise estimates of additive and dominance genetically components. Earlier, few scientists had reported significant role of epistasis governing inheritance of target traits viz., Kumar et al., (2011) in lentil, Barona et al., (2012) in soybean, Moreto et al., (2012) in common bean, Lal et al., (2014) in peanut, Keerthi et al., (2015) in Dolichos bean, Parihar et al., (2016) in grass pea, Yadav et al., (2017) in mungbean and Bindra et al., (2017) in urdbean. The importance of epistasis has been reported in several species for many economically important traits like yield, using the TTC or Modified TTC. Until now there have only been a few publications concerned with epistasis in Vigna Species. Keeping this in view, the present investigation was conducted with the objective to understand the nature and magnitude of gene action for yield and yield related traits.

Experimental site
 
The experiment was conducted at experimental farm, department of crop improvement, CSK HPKV, Palampur  Himachal Pradesh, during Kharif seasons from 2015-2017. The location of experimental station was 36^o6'N latitude and 76^o3'E longitude with an elevation of 1,290 m above mean sea level with annual rainfall (2,500 mm).
 
Material used
 
The experimental materials comprised of 27 indigenous collections from IIPR Kanpur used as Lines (P₁ - P₂₇) of urdbean (Vigna mungo L.) and three Testers (P₂₈ - P₃₀), of which first is local selection of CSHKPKV, Palampur, second is a released variety by CSHKPKV, Palampur and third is cross between first and second (HPBU-111 × Him Mash-1) presented in Table 1.
 

 
Crossing plan
 
Two testers viz., HPBU-111(macrosperma) and Him Mash-1(microsperma) designation as L₁, L₂ and third one which is cross between (HPBU-111 × Him Mash-1) designated as L₃ respectively. Twenty-seven lines (P₁ - P₂₇) were crossed with the three testers (L₁, L₂ and L₃) during Kharif 2016 and also during zaid (off-season) under glass house and eighty one triple test cross (TTC) hybrids were developed of which 54 are single and 27 are three-way crosses. 
 
Sowing plan
 
Each cross/parent was raised in single rows, 1.5 m long with row to row and plant to plant spacing of 30 cm and 10 cm, respectively. Recommended package of practices by CSK HPKV Palampur were followed for raising the crop.
 
Recording of observations
 
The data was recorded for twelve traits i.e., days to 50% flowering, days to 75% maturity, plant height (cm), number of branches per plant, number of pods per plant, pod length (cm), number of seeds per pod, 100 seed weight (g), biological yield per plant (g), seed yield per plant (g), harvest index (%) and crude protein content (%) as per micro-Kjeldhal method, AOAC, 1970.
 
Statistical analysis
 
The 81 triple test cross progeny families (L_1i, L_2i and L_3i) were subjected to statistical analysis with the help of window stat software developed by Indian Agricultural Statistical Research Institute, New Delhi, India. The data were analyzed for (i) the analysis of variance as per triple test cross design (Kearsey and Jinks, 1968), (ii) analysis of variance to test epistasis and its components (Jinks and Perkins, 1970), (iii) analysis of variance for testing of adequacy of testers (Jinks et al., 1969; Jinks and Virk, 1977; Virk and Jinks, 1977) (iv) estimation of additive and dominance components of variation (Jinks and Perkins, 1970).

The two testers (HPBU-111 and Him Mash-1) used under study showed considerable amount of genetic variation and these tester were selected from true breeding lines of urdbean by selection generation after generation. Thus fulfilling the basic requirement of triple test cross analysis. The analysis of variance revealed significant differences among all the traits viz., days to 50% flowering, days to 75% maturity, plant height (cm), number of branches per plant, pod length (cm), number of pods per plant, number of seeds per pod, seed yield per plant (g), biological yield per plant (g), 100 seed weight, harvest index (%) and protein content (%), indicating substantial amount of genetic variation existed in the studied material (Table 2). Hence, this genetic variability can be exploited through recombination breeding. Both the testers showed considerable differences as they had extreme high v/s low relation with the population and would give an estimates of additive and dominance variation with equal accuracy (Datt et al., 2011).
 

 
Detection of epistasis
 
The significant influence of non-allelic interaction was observed for all the traits under study indicating the presence of epistasis for these traits. Identifying epistatic interactions in genetic association studies can help to understand better genetic manner of multifaceted traits (Crawford et al., 2017). Epistasis was identified as an essential part of genetic system for all the 12 traits studied (Table 3). Nehvi et al., (2007) also detected the presence of epistasis for majority of the traits in the present set of materials underlined the importance of additive and dominance components of variance which would have been biased if procedure assuming no epistasis had been employed. Similarly, significant mean square due to epistasis for most of the yield contributing characters has also been reported by Sinha et al., (2020). Further partitioning of epistasis into additive × additive (i type) and additive × dominance + dominance × dominance (j+l type) revealed the existence of both i, j and l types of epistasis for all of the traits except plant height (cm), pod length (cm), seeds yield per plant (g) and protein content (%) where only j and l type of epistasis was observed. Similarly, Moreto et al., (2012) also observed j+l type of epistasis for number of pods per plant and number of grains per plant in comman bean. Hence, epistasis is found to be an integral component of genetic variation and ignorance of the presence of epistasis would lead to misleading of breeding procedures. The predominant effect of i type epistasis than j and l type was observed for all traits except plant height (cm), pod length (cm), number of pods per plant and protein content. In a self-pollinated crop like urdbean i type epistasis may have a special significance, where a linear directional and fixable component (i type epistasis) of genetic variation can be most easily exploited compared with unfixable component (j and l type epistasis) and contributes to the superiority of elite lines. The i type epistasis has also been found important in wheat (Singh and Singh 1976), mungbean (Khattak et al., 2001) and rice (Saleem et al., 2005).
 

 
Additive and dominance components
 
The significance of mean square due to sums and differences provide a direct test of significance of additive and dominance components of variation. The significance of mean squares due to sums and differences for all the traits (Table 4), viz., days to 50% flowering, days to 75% maturity, plant height (cm), number of branches per plant, pod length (cm), number of pods per plant, number of seeds per pod, seed yield per plant (g), biological yield per plant (g), 100 seed weight (%), harvest index (%) and protein content (%), indicates the importance of both additive and dominance gene effects, controlling the expression of these traits. Keerthi et al., (2015) also recorded significant values of both D and H for most of the traits in Lablab bean. Both additive and dominance gene action were observed to be equally important for all of the traits (Table 5). The magnitude of additive variance (D) was higher for plant height (cm), number of branches per plant, number of pods per plant, number of seeds per pod, biological yield per plant (g), seed yield per plant (g), harvest index (%) and protein content (%), indicates improvement in these traits by pedigree method of selection procedure, whereas all other traits exhibited higher dominance. Generally the presence of common alleles in the testers increase the magnitude of additive component by adding the dominance effect of common alleles in testers along with the cross product effects of dominance and additive effects for the common alleles. Moreover, the dominance effect will be estimated for non-common loci only, reducing the magnitude of dominance variance. Senthamizhselvi et al., (2019) also reported additive gene action for plant height, primary branches per plant, pods per plant, 100- seed weight and yield per plant. Similarly, other scientists also reported higher magnitude of additive gene action viz., Sharma et al., (2008) in pea; Kumar et al., (2011) in lentil; Rialch and Sharma (2020) in soybean.
 

 

       
Dominace gene action was found predominant for number of pods per plant and seed weight per plant in mungbean (Khajudparn et al., 2019). Since both additive and non-additive gene actions are important for the traits under study, simple selection procedures in the immediate progenies will not be so effective for achieving improvement in these characters. Thus, use of recurrent selections or biparental intermating may be suggested to improve the characters for exploiting both types of genetic variances in lentil (Kumar et al., 2011).
       
The average degree of dominance was in the range of over-dominance [(H/D)^1/2>1] for days to 50% flowering, days to 75% maturity, pod length (cm) and 100 seed weight (g) highlighting the relative importance of non-additive gene action for these traits, whereas, rest of the traits showed partial dominance showing importance of additive type of gene action (Table 5). The preponderance of the partial dominance is in consistent with the studies of Singh et al., (2006) for seed yield and protein content in pea and Singh et al., (2011) for majority of the traits and over-dominance for number of pods per plant and seed yield per plant in pea. Similarly, Patial et al., (2022) revealed over-dominance for days to 50% flowering, days to 75% maturity, plant height (cm), number of branches per plant, number of pods per plant, pod length (cm), number of seeds per pod, 100 seed weight (g), biological yield per plant (g), seed yield per plant (g), harvest index (%) and crude protein content (%). All traits showed non-significant correlation (r) indicating that these traits did not supply any evidence for directional dominance in urdbean for these traits and alleles having increasing and decreasing effects appear to be dominant and recessive to the same extent.

From the present study results showed that epistatis contributed majorly in genetic variations for all the traits. Hence, epistasis cannot be ignored while formulating breeding plan(s). Use of recurrent selections, will be fruitful to bring improvement in these traits, since both additive and dominance gene effects played an important role among all of the traits. Therefore, selection in the early segregating generations has to be avoided and should be exercised in F₆ or F₈ generations where most of the loci are in homozygous state for yield and yield contributing traits.