Evolutionary origins and population genetics of red rice in the southern United States
Kuntz, Eleanor Johanna
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Domestication has generally been used as a powerful exemplar of the power of selection to create novel phenotypes in short amounts of time. Reversion of cultivated species to an ancestral state is a less well-understood phenomenon and has been suggested as being the process responsible for the evolution of red rice in the southern United States. Field-collected red rice populations are highly morphologically and genetically diverse, forming a distinct taxonomic group. Contrary to findings suggesting red rice seed morphology is indicative of evolutionary history, we find no association with seen phenotype and genetic assignment, as well as evidence for mixed ancestry, with four distinct genetic groups co-occurring across the region. Our results provide strong support for multiple hybrid origins of red rice in Asia with subsequent dispersal to the United States and no support for a “de-domestication” event. Understanding the introduction history, including source population localities and the dynamics of invasion is important in order to generate hypotheses regarding the environmental and evolutionary factors responsible for the successful establishment and persistence of invading organisms. A complex evolutionary history places the origin red rice in Asia, arising as the result of crop to weed hybridization between cultivated rice (Oryza sativa) and wild rice (Oryza rufipogon). A long and well documented history of weed and invasive plant movement between the United States and China, in addition to the reported existence of hybrid derived red rice populations in China lead us to investigate populations of Chinese red rice in an attempt to locate the source population(s) of US red rice. Our results provide no evidence for a Chinese origin in the population of red rice sampled. Direct comparisons between populations of red rice in the United States and China surprisingly show no genetic overlap. These comparisons also indicate that US populations are more diverse then their Chinese counterparts. Gene flow is a general term used to describe mechanisms which move genetic information between individuals in the same population and among populations. Occurring through the movement of seeds and pollen, gene flow has the potential to act as a genetic bridge between individuals, populations and species. The balance between selfing and outcrossing, largely characterized by mating system, has a direct effect on the ability individuals to incorporate genetic variability into their populations. Red rice persists in sympatry with large numbers of conspecific cultivated congeners. Genetic admixture gained as the result of seed introduction and long flowering periods in red rice provide ample opportunity genetic exchange. Inter-specific gene flow in this system additionally opens the prospect of introgression of cultivated genetic material in the red rice, potentially creating novel, highly fit genotypes. We find direct evidence for crop to weed gene flow, suggesting that the introgression of cultivated alleles into weedy rice populations may be playing a role in the weeds persistence and survival. Out crossing rates comparable to other wild Oryza taxa imply that selection for the maintenance of a mixed-mating system is acting to increase diversity in red rice populations. Collectively, these results indicate that red rice has a complex evolutionary history that has lead to the generation of a diverse and dynamic weed complex.