Origin of Dioscorea species

Sanjeet Kumar
sanjeet.biotech@gmail.com

The genus Dioscorea belongs to the family Dioscoreaceae, the most prominent within Dioscoreales (Burkil, 1960; Ayensu & Coursey, 1972, Danti et al., 1999, Tamiru , 2008). The family is believed to be one among the earliest angiosperms and probably originated in Southeast Asia (Coursey, 1976). The various Dioscorea species apparently followed a divergent evolutionary occurs in three continents separated by the formation of the Atlantic Ocean and desiccation of the Middle East (Hahn, 1995). As a result, this major species occur in three isolated centres: West Africa, Southeast Asia and Tropical America (Alexander and Coursey, 1969). These centres are also considered areas for independent yam domestication, and represent considerable diversity (Asiedu et al., 1997).  Among the selected Dioscorea species, the most economically important yam, D. alata originated in Southeast Asia, more specifically in tropical Myanmar and Thailand (Orkwor 1998; Tamiru, 2006), and currently it is the most diversified and extensively distributed species. The spread of Asiatic yams (D. alata) took place more than 2000 years ago, ago reaching Africa around 1000 AD (Coursey, 1967). D. alata was, than introduced into Tropical America from West Africa towards 16^th century by Portuguese and spanich travellers (Onwueme and Charles, 1994). Dioscorea pentaphylla is originated in India and D. puber is native to Indo-China region (Asideu et al., 1997).  The native range of D. bulbifera is vast, and includes much of Asia, tropical Africa and northern Australia (Coursey, 1967; Onwueme and Charles, 1994; Terauchi et al., 1991). It was first observed in the America in 1777 in Alabama (Bartram, 1998), and was later introduced into Florida in 1905 (Morton, 1976). The pathway of introduction into the United States is unknown, although Coursey (1967) speculated that it may have been introduced by slave ships arriving from West Africa. However, recent molecular evidences strongly suggest that D. bulbifera originated from Asia (Croxton et al. 2011). Of all the species of Dioscorea documented to exist in this region of the world, D. bulbifera is the only species believed to be native to both Asia and Africa (Martin, 1974; Wilkin, 2001). The native range of D. bulbifera in Africa includes: the east tropical Africa countries of Tanzania and Uganda; the southern African countries of Zambia, Zimbabwe, Malawi, Mozambique and Namibia; Cameroon in west-central tropical Africa; and, the west tropical Africa countries of Benin, Burkina Faso, Ivory Coast, Ghana, Guinea, Liberia, Nigeria, Senegal and Sierra Leone (Coursey, 1967; Wilkin, 2001). In Asia, D. bulbifera exists as a native species in two distinct regions referred to by the USDA, ARS, National Genetic Resources Program (GRIN) as Asia-Temperate (namely, China) and Asia- Tropical which is composed of the Indian subcontinent, Indo-China and Malesia. Countries of the Indian subcontinent in which D. bulbifera is native include: Bhutan, India, Nepal and Sri Lanka. D. bulbifera is native to the Indo-China countries of Cambodia, Laos, Myanmar, Thailand and Vietnam. The Malesia countries where D. bulbifera is native include Indonesia, Malaysia, Papua New Guinea and the Philippines. D. bulbifera is also indigenous to portions of northern coastline of Australia: Queensland, the Northern Territory and Western Australia. To date, in addition to the wide distribution of D. bulbifera in its native range, it is naturalized in Central and South America and the West Indies, and cultivated in Oceania and the West Indies (Schultz, 1993; Martin, 1974). Research conducted to date on D. bulbifera shows that there exists considerable intraspecific diversity. This diversity has allowed for a distinction between accessions of African and Asian origins (Ramser et al., 1996; Terauchi et al., 1991). Work conducted by Terauchi et al. (1991) showed that at the molecular level, air potato from Asia and Africa are quite different and can be readily distinguished by examining chloroplast DNA. Although preliminary examination of chloroplast DNA of Florida air potato suggested an African origin (Overholt et al., 2003), later more in-depth studies pointed to an Asian origin, with Florida air potato being most closely related to plants from China (Croxton et al. 2011). 

Source: From literature on Dioscorea species