Wednesday, 4 November 2009

Rice genetics watch: many sweet-smelling genes

Although it is now a couple of months old, the paper by Kovach et al. (2009) in PNAS in August on the "Origin and Evolution of Fragrance in Rice" is an important contribution on the cultural history of rice. It is a clear example of selection by cultural preferences for rice that cooks a certain way, in this case with sweet or 'jasmine' aroma. Clearly many people from many cultural traditions have preferred their aromatic rices, whether Indian Basmati or Thai Jasmine rice, and this trait has been selected just as surely as ecological or domestication traits . But equally some people prefer otherwise, for their rice to smell of rice, which is true through Central China and much of east Asia.

While the responsible gene, BADH2, has been known for a while what is of interest here is that mutations in the sequence that produce fragrant rice have been distinguished and these have been tested over a very large diverse geographical sample. A single mutation is most widespread throughout regions with fragrant rice. It is clear that this originally from an early japonica lineage, presumably East Asian. And my inclination would be to see this evolving en route as Neolithic rice dispersed from S. China to SE Asia, (although it could be a later wave). But there are 9 additional more minor alternative mutations to the same gene to the same effect. They show clear geographical patterning, and the implication of that is this geographically separate groups have recurrently developed a preference for and selected for aromatic rices. It remains to be determined. Whether some of these were developed truly independently, with local fragrant mutations being kept or pushed to low frequency as the dominant BADH2 came in, or else these local aromatic variants were selected to mimick a preference developed after fragrant rice arrives. Of course the story may be different in different cases...

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