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Introduction
Yams (Dioscorea spp.) are the world's fourth most economically important edible tuber crop after potatoes, cassava and sweet potatoes (Srivastava et al., 2012). They are grown in tropical and sub-tropical regions including West Africa, Asia, Far East, Oceania and tropical America. Tubers of yams are indigenous food and cash crop, predominantly cultivated in the coastal savanna and humid forest agroecologies of western Cameroon, Nigeria, Benin, Togo, Ghana and Cote d'Ivoire. This region, known as the 'yam belt', accounts for 96% of the total world yam production (FAOSTAT, 2013). The crop is a dominant source of food and income for up to 60 million smallholder farmers in West Africa, contributing >200 dietary calories per person each day (Degras, 1993). Globally, the genus Dioscorea comprises about 450 species (Govaerts et al., 2007). However, only 10 of these species are mainly cultivated for food (Lebot, 2009).
The Genetic Resources Centre (GRC) of the International Institute of Tropical Agriculture (IITA) maintains the largest international collection of yams representing six species, comprising more than 3000 accessions mainly from central and West Africa. These include Dioscorea rotundata Poir., Dioscorea cayenensis Lam., Dioscorea alata L., Dioscorea bulbifera L., Dioscorea dumetorum (Kunth) Pax. and Dioscorea esculenta (Lour.) Burkill. GRC also maintains other wild relatives such as Dioscorea hirtiflora Benth., Dioscorea togoensis R. Knuth, Dioscorea preussi Pax., Dioscorea praehensilis Benth., Dioscorea burkilliana J. Miège and Dioscorea mangenotiana J. Miège. Facilitating the utilization of accessions with desirable traits for cultivar improvement through characterization and classification of collections is one of the user oriented activities of genebanks. This helps in broadening the genetic base of breeders' collection and reduces the challenges of utilizing large collections for evaluation in breeding programs. However, it is always not feasible, mainly for clonally propagated crops, to conduct replicated multi-location characterization/evaluation of such large collections to generate detailed information on every trait of interest of the breeders. This limits the use of germplasm and a large gap still exists between availability and actual use of germplasm collections in breeding programs (Peeters and Galwey, 1988; Wright, 1997; Upadhyaya et al., 2007). To overcome these challenges and facilitate easy access to germplasm collections, Frankel and Brown (1984) introduced the concept of core collection representing about 10% of...