THE EVOLUTION OF FLOWERING STRATEGIES IN US WEEDY RICE

Premise of the study: Local adaptation in plants often involves changes in flowering time in response to day length and temperature. Many crops have been selected for uniformity in flowering time. In contrast, variable flowering may be important for increased competitiveness in weed species invading the agricultural environment. Given the shared species designation of cultivated rice (Oryza sativa) and its the invasive conspecific weed, weedy rice, we assessed the extent to which flowering time differed between these groups. We further assessed whether genes affecting flowering time variation in rice could play a role in the evolution of weedy rice in the United States. Methods: We quantified flowering time under day-neutral conditions in weedy, cultivated, and wild Oryza groups. We also sequenced two candidate gene regions: Hd1, a locus involved in promotion of flowering under short days, and the promoter of Hd3a, a locus encoding the mobile signal that induces flowering. Key results: We found that flowering time has diverged between two distinct weedy rice groups, such that straw-hull weeds tend to flower earlier and black-hull awned weeds tend to flower later than cultivated rice. These differences are consistent with weed Hd1 alleles. At both loci, weeds share haplotypes with their cultivated progenitors, despite significantly different flowering times. Conclusions: Our phenotypic data indicate the existence of multiple flowering strategies in weedy rice. Flowering differences between weeds and ancestors suggest this trait has evolved rapidly. From a weed management standpoint, there is the potential for overlap in flowering of black-hull awned weeds and crops in the United States, permitting hybridization and the potential escape of genes from crops.