ALBERT

Description: The resistance gene Pi-ta has been effectively used to control rice blast disease, but some populations of cultivated and wild rice have evolved resistance. Insights into the evolutionary processes that led to this resistance during crop domestication may be inferred from the population history of domesticated and wild rice strains. In this study, we applied a recently developed statistical method, time-dependent Poisson random field model, to examine the evolution of the Pi-ta gene in cultivated and weedy rice. Our study suggests that the Pi-ta gene may have more recently introgressed into cultivated rice, indica and japonica , and U.S. weedy rice from the wild species, O. rufipogon . In addition, the Pi-ta gene is under positive selection in japonica , tropical japonica , U.S. cultivars and U.S. weedy rice . We also found that sequences of two domains of the Pi-ta gene, the nucleotide binding site and leucine-rich repeat domain, are highly conserved among all rice accessions examined. Our results provide a valuable analytical tool for understanding the evolution of disease resistance genes in crop plants.

Description: Weedy rice ( Oryza spp.) is a problematic weed of cultivated rice ( O. sativa ) around the world. Recent studies have established multiple independent evolutionary origins of weedy rice, raising questions about the traits and genes that are essential for the evolution of this weed. Among world regions, South Asia stands out due to the heterogeneity of its weedy rice populations, which can be traced to at least three origins: two through de-domestication from distinct cultivated rice varieties, and one from local wild rice ( O. rufipogon / O. nivara ). Here we examine five traits considered typical of or advantageous to weedy rice in weedy, cultivated and wild rice samples from South Asia. We establish that convergence among all three weed groups occurs for easy seed shattering, red pericarp color, and compact plant architecture, suggesting that these traits are essential for weed success in the South Asian agricultural environment. A high degree of convergence for black hull color is also seen among weeds with wild ancestors and weeds evolved from the aus cultivated rice group. We also examine polymorphism in five known domestication candidate genes, and find that Rc and Bh4 are associated with weed seed pericarp color and hull color, respectively, and weedy alleles segregate in the ancestral populations, as do alleles for the seed dormancy-linked gene Sdr4 . The presence of a domestication related allele at the seed shattering locus, sh4 , in weedy rice populations with cultivated ancestry supports a de-domestication origin for these weedy groups, and raises questions about the reacquisition of the shattering trait in these weedy populations. Our characterization of weedy rice phenotypes in South Asia and their associated candidate genes contribute to the emerging understanding of the mechanisms by which weedy rice evolves worldwide, suggesting that standing ancestral variation is often the source of weedy traits in independently evolved groups, and highlighting the reservoir of genetic variation that is present in cultivated varieties as well as in wild rice, and its potential for phenotypic evolution.

Description: Weedy rice is a conspecific form of cultivated rice ( Oryza sativa L.) that infests rice fields and results in severe crop losses. Weed strains in different world regions appear to have originated multiple times from different domesticated and/or wild rice progenitors. In the case of Malaysian weedy rice, a multiple-origin model has been proposed based on neutral markers and analyses of domestication genes for hull color and seed shattering. Here, we examined variation in pericarp (bran) color and its molecular basis to address how this trait evolved in Malaysian weeds and its possible role in weed adaptation. Functional alleles of the Rc gene confer proanthocyanidin pigmentation of the pericarp, a trait found in most wild and weedy Oryzas and associated with seed dormancy; nonfunctional rc alleles were strongly favored during rice domestication, and most cultivated varieties have nonpigmented pericarps. Phenotypic characterizations of 52 Malaysian weeds revealed that most strains are characterized by the pigmented pericarp; however, some weeds have white pericarps, suggesting close relationships to cultivated rice. Phylogenetic analyses indicate that the Rc haplotypes present in Malaysian weeds likely have at least three distinct origins: wild O. rufipogon , white-pericarp cultivated rice, and red-pericarp cultivated rice. These diverse origins contribute to high Rc nucleotide diversity in the Malaysian weeds. Comparison of Rc allelic distributions with other rice domestication genes suggests that functional Rc alleles may confer particular fitness benefits in weedy rice populations, for example, by conferring seed dormancy. This may promote functional Rc introgression from local wild Oryza populations.