Abstract
Yams (Dioscorea spp.) constitute a staple food crop for over 100 million people in the humid and subhumid tropics. They are polyploid and vegetatively propagated. The Guinea yams, Dioscorea rotundata and D. cayenensis, are the most important yams in West and Central Africa where they are indigenous, while D. alata (referred to as water yam) is the most widely distributed species globally. The genetics of yams is least understood among the major staple food crops due to several biological constraints and research neglect. Research to unravel the apparent complexity of the yam genome will have far-reaching implications for genetic improvement of this important tuber crop. Some progress has been made in recent years in germplasm characterization and the development of molecular markers for genome analysis. A genetic linkage map based on amplified fragment length polymorphism (AFLP) markers has been constructed for Guinea and water yams. These linkage maps were used to scan the genome for quantitative trait loci (QTL) associated with genes conferring resistance to Yam Mosaic Virus (YMV) in D. rotundata and anthracnose (Colletotrichum gloeosporioides) inD. alata. In addition, candidate random amplified polymorphic DNA (RAPD) markers associated with major genes controlling resistance to YMV and anthracnose have been identified that could be used for selection and pyramiding of YMV and anthracnose resistance genes in yam improvement. Also, molecular markers such as RAPDs, AFLPs, and microsatellites or simple sequence repeats (SSRs) have been developed for yam genome analysis. An initial c-DNA library has been constructed in order to develop expressed sequence tags (ESTs) for gene discovery and as a source of additional molecular markers. This paper will review the advances made, discuss the implications for yam genetic improvement and germplasm conservation, and outline the direction for future research. Key words: Genetic mapping, genome analysis, molecular breeding, PCR-based markers, QTLs, resistance genes, yam. 
Highlights
Yam (Dioscorea spp.) is a multi-species, polyploid and vegetatively propagated tuber crop that is cultivated widely in the tropics and subtropics
Guinea yams were domesticated in West Africa, while water yam is believed to be a true cultigen that might have been domesticated in Indo-China from Dioscorea hamiltoni and D. persimilis (Barrau, 1965)
The results showed that each of the three marker-systems could unequivocally identify each cultivar, but that markersystems differed in the mean number of profiles generated per primer per cultivar, referred to as genotype index (GI)
Summary
Yam (Dioscorea spp.) is a multi-species, polyploid and vegetatively propagated tuber crop that is cultivated widely in the tropics and subtropics. Progress in breeding for increased yield, adaptability to marginal environments, improved tuber quality, and pest and disease resistance has, been painfully slow due to biological constraints that impede the elucidation of the genetics of important traits in yam These include a long growth cycle, dioecy, poor to non-flowering, polyploidy, vegetative propagation, a heterozygous genetic background and poor knowledge of the organization of crop diversity. The advent of molecular markers, genome studies, and plant genetic transformation has opened avenues for circumventing breeding obstacles in long growth cycle and heterozygous crops like yam In spite of these opportunities to build on the success in classical breeding, huge challenges had to be overcome before yam scientists could develop modern technologies towards assisting breeding programs boost yam production and keep up with the ever-increasing demand. This review will cover the progress made in molecular marker development, characterization of genetic diversity, linkage mapping, QTL analysis and development of genomic resources
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