How To Evolve a Weed: Examining Convergent Evolution In Weedy Red Rice

Convergent phenotypic evolution may or may not be associated with convergent genotypic evolution. Agricultural weeds have repeatedly been selected for weed-adaptive traits such as rapid growth, increased seed dispersal and dormancy, thus providing an ideal system for the study of convergent evolution. Weedy red rice (Oryza sativa), a conspecific weed of cultivated rice, infests rice fields worldwide, and is believed to have multiple evolutionary origins. We are assessing the extent of convergent evolution at the genetic and phenotypic level in two US weedy rice populations with separate origins from distinct cultivated ancestors. While convergence is evident for some weedy traits, such as seed shattering, other traits are divergent between the two weed groups. Using crop x weed crosses, we have mapped QTL for four quantitative traits differentiating weed groups from cultivated rice. We identified QTL on nine of the twelve rice chromosomes, yet most QTL locations do not overlap between the two populations. Preliminary genome scans also suggest limited sharing of genes divergent from cultivated ancestors among weeds. Our results suggest that, despite a close evolutionary relationship, US weedy rice groups have adapted to the same agricultural environment through different genetic mechanisms. The relationship of US weedy rice groups with worldwide weedy rice populations will be discussed.