The evolution of flowering strategies in US weedy rice

Authors: THURBER, CARRIE - University Of Illinois; REAGON, MICHAEL - The Ohio State University; OLSEN, KENNETH - Washington University; Jia, Yulin; CAICEDO, ANA - University Of Massachusetts

Submitted to: American Journal of Botany
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/21/2014
Publication Date: 10/1/2014
Publication URL: http://handle.nal.usda.gov/10113/60692
Citation: Thurber, C.S., Reagon, M., Olsen, K.M., Jia, Y., Caicedo, A.L. 2014. The evolution of flowering strategies in US weedy rice. American Journal of Botany. 101(10):1737-1747. doi:10.3732/ajb.1400154.

Interpretive Summary: The timing of when plants initiate flowering has a large impact on mating success and on seed production and viability. Multiple environmental variables, such as temperature and day length, can act as cues for flowering time in different species. Many crop varieties have been selected for uniformity in flowering time. In contrast, variable flowering may be important for increased competitiveness of weed species invading the agricultural environment. Given their shared species designation, we assessed the extent to which flowering time differed between cultivated rice and its invasive conspecific weed, weedy rice. We further assessed whether genes affecting flowering time variation could play a role in the evolution of flowering time in US weedy rice. We found that flowering time has diverged between two distinct weedy rice groups, such that straw-hulled (SH) weeds flower earlier and black-hulled awned (BHA) weeds flower later than cultivated rice. Our phenotypic data indicate that there is no single optimal weed flowering strategy. Flowering differences between weedy rice groups and their ancestors suggest this trait has evolved rapidly. From a weed management standpoint, there is the potential for overlap in flowering of SH and BHA weeds and US rice cultivars. This could potentially permit gene flow between herbicide resistant rice and weedy rice.

Technical Abstract: Local adaptation in plants often involves changes in flowering time in response to day length and temperature differences. Many crop varieties 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 their shared species designation, we assessed the extent to which flowering time differed between cultivated rice and its invasive conspecific weed, known as weedy rice. We further assessed if genes affecting flowering time variation could play a role in the evolution of flowering time in US weedy rice. We quantified flowering time under day neutral conditions in weedy and cultivated forms of Oryza sativa as well as other wild related species of rice. In all groups, we 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. We found that flowering time has diverged between two distinct weedy rice groups, straw-hulled (SH) and black-hulled, awned (BHA), such that SH weeds flower earlier and BHA weeds flower later than cultivated rice. These differences are consistent with weedy rice Hd1 alleles. At both the HD1 and Hd3a loci, weeds share haplotypes with their cultivated forms, despite significantly different flowering times. This implies that flowering time is also influenced by other genes. Our phenotypic data indicate that there is no single optimal weed flowering strategy for fitness. Flowering differences between weeds and wild related species of rice suggest this trait has evolved rapidly. From a crop management standpoint, there is the potential for overlap in flowering of SH and BHA weedy rice and US rice cultivars, permitting hybridization and the potential escape of transgenes and other genes into this weed.