Improving the Yam

Yams are an important food crop in tropical and sub-tropical regions. They are dioecious herbaceous vines cultivated for their starchy tubers. Yams are exclusively propagated by vegetative multiplication by means of small tubers or small pieces of tubers. New combinations can be obtained via sexual reproduction, and breeding new cultivars has proven to be an efficient method for genetic improvement. Dioscorea alata is a monocot that belongs to the family Dioscoreaceae. This genus includes >600 species of which the three main cultivated species are D. rotundata, D. alata and D. trifida.

Dioscorea alata is a polyploid species with diploid (2n = 40), triploid (2n = 60) and tetraploid (2n = 80) cytotypes. The origin of this species is still a matter of debate because it has not yet been clearly identified in its wild state in nature, whereas wild forms have been recorded. Ploidy increase is correlated with growth vigour, higher and more stable tuber yield and increased tolerance to abiotic and biotic stress. Diversity studies have shown that the most common forms are diploids, followed by triploids, and that tetraploids are rare and only exist in diversification centres in Asia and the South Pacific.

A new paper in Annals of Botany uses flow cytometry and microsatellite markers to understand the origin of D. alata spontaneous triploids and tetraploids. This work is the first study reported in Dioscoreacea where seeds were desiccated to allow separate ploidy analysis on endosperm and embryos. However, it appeared that although endosperm analysis is efficient for plump seeds, it is not easy to apply to shrivelled seeds. Because polyploids were obtained at a low rate, this could lead to doubts about the sample size of the progeny used to obtain accurate estimates of non-reduced gamete formation.