Genetic diversity of weedy red rice (Oryza sativa) in Arkansas, USA

Weedy red rice (Oryza sativa) is a problematic weed in cultivated rice. About 50% of US rice is produced in Arkansas and 60% of these fields have some red rice infestation. Red rice populations are morphologically and phenologically diverse. We hypothesise that red rice in Arkansas has high genetic diversity, which underlies its wide phenotypic diversity, and that some alleles from cultivated rice have been introgressed into red rice during more than a century of coexistence. We tested 137 red rice accessions from four ecological zones in Arkansas and 36 cultivars that have been grown in Arkansas in the past century. Twenty-seven rice microsatellite primers, distributed across 12 chromosomes, were used to generate molecular markers. The overall Nei's genetic distance (GD) of red rice accessions was 0.70. Rice grown in the last century had an overall GD of 0.26. The awnless strawhull red rice was genetically distant from blackhull (GD = 0.55) and brownhull (GD = 0.60) red rice types. Nei's GD between blackhull and brownhull red rice was 0.42. Brownhull and blackhull formed one genotypic cluster, whereas the majority of strawhull red rice formed another cluster. Within blackhull red rice, the GD was 0.76, whereas for awnless strawhull it was 0.68, 0.75 for awned strawhull and 0.80 for brownhull types. The gene diversity of blackhull and strawhull correlated with zone of origin. A quarter of the red rice accessions share common alleles with cultivated rice. A diverse complex of weedy populations has evolved in a region devoid of other weedy and wild Oryza species.