Floral traits underlying mating system differentiation in the wind-pollinated sister species <i>Oryza rufipogon</i> and <i>Oryza nivara</i>

The shift from outcrossing to predominantly selfing is one of the most common transitions in plant evolution. This evolutionary shift has received considerable attention from biologists; however, this work has almost exclusively been focused on animal-pollinated systems. Despite the seminal ecological and economic importance of wind-pollinated species, the mechanisms controlling the degree of outcrossing in wind-pollinated taxa remain poorly understood. As a first step toward addressing this issue, we have conducted a comparative study of floral biology between two recently diverged sister species, <i>Oryza rufipogon</i> and <i>Oryza nivara</i> (Poaceae), that are wind-pollinated and possess distinct mating systems with <i>O. rufipogon</i> being outcrossing and <i>O. nivara</i> highly self-fertilized Therefore, these species present an ideal system for exploring mating system evolution in wind-pollinated taxa. We have identified key floral traits that differ between populations of these species and that are associated with mating system divergence including anther length, anther basal pore size, stigma papillae density, panicle shape, panicle exsertion, pollen viability, and early anther dehiscence. Of these traits, large anther basal pore size and early anther dehiscence are hypothesized to confer reliable autogamous selfing in <i>O. nivara</i>. Manipulations of floret number were conducted to partition the role of geitonogamy and autogamy in conferring self-fertilization. This experiment revealed that selfing in <i>O. nivara</i> is consistent with autogamous selfing, whereas <i>O. rufipogon</i> achieves selfing through geitonogamy. This study serves as a model for understanding the floral mechanisms controlling the outcrossing rate in other wind-pollinated systems, most notably other grasses.