​Comparative Study of Crossability Behavior in Intra-specific and Inter-specific Crosses of Vigna radiata and Vigna mungo

Mungbean [Vigna radiata (L.) Wilczek; 2n=2x=22] is a short duration pulse crop with low input requirements having high worldwide demand. It is also an outstanding rotation crop in rice-based agricultural system (Bhanu et al., 2018). However, the total production and productivity of mungbean is affected by a number of biotic and abiotic factors. Among, biotic factors, mungbean yellow mosaic virus (MYMV) disease  leads to significant yield losses of upto 85% (Haq et al., 2010). The other cultivated Vigna species, urdbean [Vigna mungo (L.) Hepper; 2n=2x =22], which is also a self-pollinating pulse crop. It possess non shattering pods with synchronous maturity, more clusters per plant, pods with large seeds and comparatively more durable resistance to MYMV (Singh, 1990). Breeding for resistant varieties is one of the most suitable strategies against biotic stresses (Karthikeyan et al., 2012).

The gene pool of a cultivated species could be widened through intra-specific and inter-specific hybridization in order to produce such crop varieties that integrate high yield with disease and pest resistance. Effectiveness of distant hybridization and subsequent foreign gene transfer is mostly determined by pollination behaviour, frequency of pollination, ploidy status of the species involved in the cross and the employment of efficient techniques. Many wild Vigna species are not crossable with their cultivated equivalents due to pre- and post-fertilization obstacles and, therefore, are useless for crop improvement (Tyagi and Chawla, 1999). When such hybridization is attempted, crossing barriers are often encountered. Keeping these aspects in consideration, current investigation was carried out to know the crossability barriers existing among the two Vigna species, belonging to mungbean and urdbean, through inter-specific and intra-specific hybridization with the aim to combine the yield and yield attributing traits along with the MYMV resistance. 

Plant material for present experiment consisted of nine mungbean and five urdbean genotypes. Initially, 42 different crosses were made comprising of 13 mungbean × mungbean, four urdbean × urdbean and 25 mungbean × urdbean crosses (Supplementary Table 1). All crosses were attempted during Kharif 2019 and evaluated in the summer of 2020. The experiment was conducted at the Agricultural Research Farm, Banaras Hindu University, Varanasi, Uttar Pradesh, India. On the basis of higher crossability percent and number of cross pod produced (Supplementary Table 1), six cross combinations were further evaluated. Details of the parents involved in these six crosses of mungbean and urdbean have been presented in Table 1.





Observations were made on the number of buds emasculated, flower drop, per cent emasculated flower drop, number of buds pollinated, number of flowers with stigma breakdown, per cent of flowers with stigma breakdown, number of successful crosses, number of cross pod produced and crossability per cent (Supplementary Table 1). At physiological maturity, crosses were harvested and the crossability per cent obtained from the formulae i.e.,
 
 
 
The pollen fertility testing was done on the parents and their hybrids during blooming using acetocarmine staining methodology.
 
  
 
  
 
During summer 2020, parents and F₁ s were sown in two rows with one row of male and female parents with spacing of 45 × 30 cm in two replications under randomized block design (RBD). Five plants were randomly chosen from each of the parents and F₁  to record data on ten quantitative traits, viz., days to 50 per cent flowering, days to maturity, plant height (cm), number of primary branches per plant, number of clusters per plant, number of pods per plant, pod length (cm), number of seeds per pod, 100-seed weight (g) and seed yield per plant (g).

Inter-specific hybridization has been limited in many crops due to presence of certain pre- and post-fertilization barriers, but in pulse crops such as mungbean and urdbean, interspecific hybridization has proved successful (Pandiyan et al., 2010). The current investigation sought to obtain better recombinants amongst six intra-specific and inter-specific crosses of V. radiata and V. mungo in terms of yield and MYMV resistance.

Crossability percent of intra-specific and inter-specific crosses

The higher crossability attained in intra-specific crosses in comparison to inter-specific crosses depict absence of external barriers impeding cross pollination (Supplementary Table 1). Though, the possible causes of hybrid failure and breakdown of interspecific crosses in Vigna are poorly understood (Chen et al., 1983), several factors including specific cross combination, genetic divergence and environment factors can be the major reasons which influence the hybridization process leading to reduced success rate in case of interspecific crosses. However, the crossability barriers are pre-dominant, but still there is possibility to recover interspecific hybrids. Bhanu et al., (2018) reported higher crossability per cent in crosses between V. radiata × V. umbellata and V. mungo × V. umbellata. Moreover, crossability is genotype dependent, which leads to cross-incompatibility in some particular combinations while, other cross-combinations may be comparatively more successful (Rashid et al., 1988).
 
Germination and pollen fertility of parents and F1 hybrids (inter-specific and intra-specific crosses)
 
Parents and the six promising crosses were utilized to determine number of seeds per parent, number of seeds germinated, seed germination per cent, number of seedlings matured, hybrid lethality per cent, hybrid in-viability, hybrid breakdown per cent and pollen fertility per cent (Table 2). In the intra-specific crosses of mungbean (HUM 2 × IPM 02-3 and HUM 26 × IPM 02-3), moderate pollen fertility (42.21%-39.12%) coupled with moderate seed germination percent (85-80%) were observed. Whereas, the two urdbean crosses (NDU 1 × Mash 338 and NDU 1 × R3/12) exhibited moderate pollen fertility (38.22-38.12%) and low germination (60-50%). These two urdbean × urdbean crosses revealed 8.33% and 10% hybrid breakdown coupled with, 33.33% and 40% hybrid lethality, as well. Similarly, the two inter-specific crosses (SKAUM 365 × R3/28 and HUM 2 × R3/28) revealed low hybrid pollen fertility of 37.24% and 31.29% as well as, moderate and low seed germination percent of 75% and 40%, respectively (Table 2).



Present study revealed minimum occurrence of fertilization barriers in the above mentioned crosses which might have resulted in the production of higher number of successful cross pods (Supplementary Table 1). Results have been similarly interpreted by Basavaraj et al., (2019) who produced a cross between V. radiata and V. umbellata and discovered certain post and pre-fertilization obstacle, prohibiting cross pollination based on crossability percent, pod set per cent, seed germination per cent and hybrid lethality. On the contrary, Mahalingam and Manivannan (2021) discovered the presence of many pre and post zygotic crossability impediments between wild and cultivated species.
 
Yield performance of parents and F₁ hybrids (inter-specific and intra-specific crosses)
 
Results on yield and its component traits in parents and the six crosses have been presented in Table 3. Compared to the parents, hybrid showed high per se performance in the urdbean × urdbean intra-specific cross, NDU 1 × R3/12 manifested by the traits, viz., number of clusters per plant (19), number of pods per plant (69) and seed yield per plant (14.12 g) while, in the mungbean ×  mungbean intra-specific cross, HUM 26 × IPM 02-3, recorded as many as 45 pods per plant with highest yield (17.95 g) among the crosses attempted. In the mungbean × urdbean inter-specific cross, SKAUM 365 × R3/28, have recorded 16  clusters per plant, 46 pods per plant and 14.95 g of seed yield per plant. It was observed from Table 3 that different quantitative traits were contributing differently in inter and intra-specific crosses. Successful inter-specific hybridization between mungbean and urdbean to obtain transgressive segregants for different yield traits have been reported by many workers (Singh et al., 1996; Pathak et al., 2015; Singh et al., 2020).

In the present study, 42 crosses have been made and despite crossability constraints which were prevalent, a few inter-specific and intra-specific hybrids with intermediate characters of both the parents along with MYMV resistance were obtained. On the basis of pollen fertility, germination per cent, hybrid lethality, hybrid in-viability and hybrid breakdown per cent, three crosses viz., HUM 2 × IPM 02-3, SKAU M 365 × R3/28 and NDU 1 × R3/12 showed higher crossability. Among the crosses, two M × M crosses (HUM 26 × IPM 02-3 and HUM 2 × IPM 02-3)  have recorded highest seed yield per plant whereas, one inter-specific cross between M × U i.e., HUM 2 × R3/28 is third highest yielder. These three crosses have been recommended to be used in Vigna crop improvement program. Since, these crosses have positive contribution of many desirable yield and yield contributing traits along with MYMV resistance.

The first author thankfully acknowledges the financial assistance in form of UGC, Research fellowship during her Ph.D. Program. Infrastructural support of BHU is gratefully acknowledged as well.