Mating consequences of polyploid evolution in flowering plants: Current trends and insights from synthetic polyploids

The evolutionary transition from diploidy to polyploidy is prevalent in flowering plants and may result in correlated changes in mating system (outcrossing rate). Most theory predicts a shift toward self-fertilization (decrease in outcrossing) in polyploids, but empirical evidence for this pattern and its underlying mechanisms is inconclusive or restricted to a few cases. In an analysis of variation in outcrossing rates among diploid-polyploid species pairs from the literature, polyploids had lower outcrossing rates (higher selfing; t = 0: 23, SE 0: 09) than diploids (t = 0: 52, SE 0: 12). Among polyploids, however, allopolyploids were predominantly selfing (t = 0: 20, SE 0: 099), whereas autopolyploids had significantly higher outcrossing rates (t = 0: 64, SE 0: 087), raising the question of what limits the evolution of selfing in autopolyploids. To address this, we examined the magnitude of inbreeding depression in synthetic polyploids of the plant Chamerion angustifolium. The intrinsic cost of selfing in newly formed polyploids was negligible compared with extant polyploids, thus promoting the spread of selfing. However, there was weak evidence that inbreeding depression increases with history of inbreeding, suggesting that the rise in selfing may be ephemeral and that selection ultimately favors mixed or outcrossed mating systems in autopolyploids. Such constraints on selfing have some theoretical support, but additional research on patterns of variation and genetic mechanisms governing polyploid mating systems are needed.