Exploring the mating systems of wild rice Oryza rufipogon and O. nivara: implications for population genetic variation

Oryza rufipogon Griff. and O. nivara Sharma et Shastry are wild ancestors of Asian cultivated rice, harboring a wealth of genetic variation valuable for rice breeding. Mating systems play an important role in shaping genetic variation within species, and could shed light on germplasm conservation. Despite the importance, previous research on the mating systems of O. rufipogon and O. nivara has been hindered by limited sample sizes and the use of unsuitable genetic markers. In this study, by collecting a large number of maternal plants and seeds from representative populations of both species, we estimated their outcrossing rates using an extended mixed-mating model along with 12 simple sequence repeats markers. Our results revealed distinct mating strategies between the two species. Oryza rufipogon exhibited a mixed mating system with an average outcrossing rate of 0.478, while O. nivara was predominantly selfing, with an average outcrossing rate of 0.256. In addition, compared to O. nivara, O. rufipogon has a higher genetic diversity within populations and lower differentiation among populations. The contrasting genetic variation patterns between the two wild rice species could result from demographic bottlenecks and a shift from mixed mating to primarily selfing in O. nivara. Overall, through our geographically widespread sampling, we have obtained reliable outcrossing rate estimates for O. rufipogon and O. nivara, thus facilitating both in situ and ex situ conservation of their germplasm.