Efficient Improvement in an Orphan Legume: Horsegram, Macrotyloma uniflorum (Lam.) Verdi, Using Conventional and Molecular Approaches

Horsegram (Macrotyloma uniflorum) is a legume species widely distributed in Indian subcontinent and an important supplement of dietary protein with medicinal effects. The genus Macrotyloma consists of 25 species, and most of them are wild except Macrotyloma uniflorum spp. uniflorum, which is cultivated in many parts of India. Horsegram is used as an important pulse crop since seeds are rich in protein and consumed in majority by poorest section of the society. Horsegram seeds are an excellent source of protein, carbohydrates, essential amino acids, energy, iron, molybdenum, phosphorus, iron and vitamin C. Its seeds are reported to be high in tannins and polyphenols as compared to other legumes and therefore considered as a good source of antioxidant. Despite nutritional advantages over other legumes, the crop is not grown commercially due to the presence of many undesirable traits in cultivated horsegram such as twining and indeterminate growth habit, long and thin stem and very late and non-synchronous maturity. Probable progenitor of cultivated horsegram is considered as M. axillare, which possesses many desirable traits such as higher number of seeds per plant and seed yield per plant and resistance against many pathogens, but due to non-compatibility with cultivated horsegram the traits cannot be transferred using conventional breeding strategies. The genome structure and organization of this crop are poorly understood, thereby limiting the effective use of gene resources for genetic improvement. The development of genomic resources in Macrotyloma uniflorum was started with the study on transferability of genomic SSR and COS markers from the related well-characterized legume species such as Medicago truncatula, Trifolium pratense and Cajanus cajan. In addition, the available sequence information in NCBI database was downloaded and used to design 63 SSR and 27 IPL markers. The first systematic attempt to develop genomic resources in horsegram was made by mining of transcriptome sequence data, and 3342 SSRs were developed and validated on the diverse panel of horsegram lines. Next-generation sequencing platform was used to develop draft genome sequence in this crop and reported the size of genome is 398 Mb. The whole genome sequence data was also used to develop genomic resources in horsegram, and 5755 genomic SSRs were designed and validated. All these genomic resources are being utilized for the diversity and structure analysis of germplasm, construction of linkage map and identification of genomic regions associated with different traits. The first framework linkage map of Macrotyloma uniflorum was constructed using 211bmolecular markers on 190 RILs progeny derived from a narrow cross between lines HPK4 and HPKM249. The linkage map showed ten linkage groups that spanned 1423.4cM with a mean marker interval of 9.6cM. Five QTLs related to drought-related parameters and seed yield were detected on five LGs with an LOD threshold of 4.0. The linkage and QTL analysis being carried in this crop can be useful for future research work pertaining to construction of highly enriched genetic maps as well as development of drought-resistant and high-yielding varieties in horsegram using marker-assisted selection.