This is the first report of QTLs for domestication-related traits in yardlong bean. The results provide a foundation for marker-assisted selection of domestication-related QTLs in yardlong bean and enhance understanding of domestication in the genus Vigna.

No abstract

Yardlong bean (Vigna unguiculata (L.) Walp. subsp. unguiculata Sesquipedalis Group) (2n = 2x = 22) is one of the most important vegetable legumes of Asia. The objectives of this study were to develop a genetic linkage map of yardlong bean using SSR makers from related Vigna species and to identify QTLs for pod length. The map was constructed from 226 simple sequence repeat (SSR) markers from cowpea (Vigna unguiculata (L.) Walp. subsp. unguiculata Unguiculata Group), azuki bean (Vigna angularis (Willd.) Ohwi & Ohashi), and mungbean (Vigna radiata (L.) Wilczek) in a BC(1)F(1) ((JP81610 × TVnu457) × JP81610) population derived from the cross between yardlong bean accession JP81610 and wild cowpea (Vigna unguiculata subsp. unguiculata var. spontanea) accession TVnu457. The markers were clustered into 11 linkage groups (LGs) spanning 852.4 cM in total length with a mean distance between adjacent markers of 3.96 cM. All markers on LG11 showed segregation distortion towards the homozygous yardlong bean JP81610 genotype. The markers on LG11 were also distorted in the rice bean (Vigna umbellata (Thunb.) Ohwi & Ohashi) map, suggesting the presence of common segregation distortion factors in Vigna species on this LG. One major and six minor QTLs were identified for pod length variation between yardlong bean and wild cowpea. Using flanking markers, six of the seven QTLs were confirmed in an F(2) population of JP81610 × TVnu457. The molecular linkage map developed and markers linked to pod length QTLs would be potentially useful for yardlong bean and cowpea breeding.

In this study, 520 cultivated and 14 wild accessions of black gram (Vigna mungo (L.) Hepper) were assessed for diversity using 22 SSR markers. Totally, 199 alleles were detected with a mean of 9.05 alleles per locus. Wild black gram showed higher gene diversity than cultivated black gram. Gene diversity of cultivated accessions among regions was comparable, while allelic richness of South Asia was higher than that of other regions. 78.67% of the wild gene diversity presented in cultivated accessions, indicating that the domestication bottleneck effect in black gram is relatively low. Genetic distance analysis revealed that cultivated black gram was more closely related to wild black gram from South Asia than that from Southeast Asia. STRUCTURE, principal coordinate and neighbor-joining analyses consistently revealed that 534 black gram accessions were grouped into three major subpopulations. The analyses also revealed that cultivated black gram from South Asia was genetically distinct from that from West Asia. Comparison by SSR analysis with other closely related Vigna species, including mungbean, azuki bean, and rice bean, revealed that level of gene diversity of black gram is comparable to that of mungbean and rice bean but lower than that of azuki bean.

Loss of pod shattering is one of the most important domestication-related traits in legume crops. The non-shattering phenotypes have been achieved either by disturbed formation of abscission layer between the valves, or by loss of helical tension in sclerenchyma of endocarp, that split open the pods to disperse the seeds. During domestication, azuki bean (Vigna angularis) and yard-long bean (Vigna unguiculata cv-gr. Sesquipedalis) have reduced or lost the sclerenchyma and thus the shattering behavior of seed pods. Here we performed fine-mapping with backcrossed populations and narrowed the candidate genomic region down to 4 kbp in azuki bean and 13 kbp in yard-long bean. Among the genes located in these regions, we found MYB26 genes encoded truncated proteins in azuki bean, yard-long bean, and even cowpea. As such, our findings indicate that independent domestication on the two legumes has selected the same locus for the same traits. We also argue that MYB26 could be a target gene for improving shattering phenotype in other legumes, such as soybean.