I yam what I yam

Natural History, Oct, 2004 by Wolfgang J. Gruneberg, Marten Sorensen

"Supercrop," the title of Marten Sorensen's article (4/04), does not at present fit the neglected yam bean. The one supercrop is maize, the giant of the cereals. The yam bean could be the agronomist's and breeder's dream, however: all it would take is a small amount of "risky" investment. The potential profits--as well as the economic, ecological, and social dividends--could be enormous.

Many features of the yam bean have not even been described in published work. For example, some kinds of yam bean can be processed into a high-protein, easy-to-store product resembling gari, a flour that is made from cassava roots. And though yam beans are customarily sown, all yam beans can readily be propagated from cuttings; from one "good" genotype several hundred clones can be developed in one generation.

The various kinds of yam bean can have highly different properties. Although the current assumption is that there are three cultivated species, fertile and vigorous hybrids can be developed between all yam bean species. If the frequency of successful crossings within and between species is tested for equivalence--tests that have not been done to date--I suspect that all cultivated yam beans will turn out to be one species. Agronomists and breeders who experiment with other root crops or beans can only dream of working with such diversity and potential for hybridization.

Wolfgang J. Gruneberg

University of Gottingen

Gottingen, Germany

MARTEN SORENSEN REPLIES: Although the yam bean may not yet be a "supercrop," I do think it promises to be one, given its adaptability, yield potential, and many possible uses for food, fodder, and even nonfood products. Its sustainability is unmatched by any cereal, even maize, and for exactly that reason a number of traditional farming systems cultivate maize and yam bean together.

The taxonomic status of the yam bean is closely related to one's definition of species. Plant breeders and agronomists have, for practical reasons, tended to accept a fairly simple species concept, wherein all plants that produce fertile offspring belong to the same species. In contrast, botanists and taxonomists generally favor a more complex definition, not only weighing morphological and genetic differences, but also taking account of whether cross-fertilization is a naturally occurring possibility. (For example, if two variants do not overlap geographically, or if they flower at different times, that would be an argument for considering them different species.) In the end, however, whether the yam bean should be regarded as one species or as several that can be hybridized is not so important, as long as the different kinds can be manipulated to facilitate cross-pollination.

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