The pattern of genetic diversity of Guinea-race Sorghum bicolor (L.) Moench landraces as revealed with SSR markers

The Guinea-race of sorghum [Sorghum bicolor (L.) Moench] is a predominantly inbreeding, diploid cereal crop. It originated from West Africa and appears to have spread throughout Africa and South Asia, where it is now the dominant sorghum race, via ancient trade routes. To elucidate the genetic diversity and differentiation among Guinea-race sorghum landraces, we selected 100 accessions from the ICRISAT sorghum Guinea-race Core Collection and genotyped these using 21 simple sequence repeat (SSR) markers. The 21 SSR markers revealed a total of 123 alleles with an average Dice similarity coefficient of 0.37 across 4,950 pairs of accessions, with nearly 50% of the alleles being rare among the accessions analysed. Stratification of the accessions into 11 countries and five eco-regional groups confirmed earlier reports on the spread of Guinea-race sorghum across Africa and South Asia: most of the variation was found among the accessions from semi-arid and Sahelian Africa and the least among accessions from South Asia. In addition, accessions from South Asia most closely resembled those from southern and eastern Africa, supporting earlier suggestions that sorghum germplasm might have reached South Asia via ancient trade routes along the Arabian Sea coasts of eastern Africa, Arabia and South Asia. Stratification of the accessions according to their Snowden classification indicated clear genetic variation between margeritiferum, conspicuum and Roxburghii accessions, whereas the gambicum and guineënse accessions were genetically similar. The implications of these findings for sorghum Guinea-race plant breeding activities are discussed.