Heterostyly, a term referred to plant species comprising either two (distyly) or three (tristyly) self-incompatible floral morphs that differ reciprocally in the positions of stigmas and anthers (i.e. reciprocal herkogamy), is considered as one of the most effective mechanisms to avoid selfing and promote outcrossing. This complex floral syndrome, which occurs in at least 28 families of flowering plants, represents a remarkable example of convergent evolution. Therefore, explaining the origin, function and genetic basis of heterostyly has attracted considerable attention ever since Darwin’s classic book on polymorphic sexual systems in plants. Self and intramorph incompatibility is generally considered to play an important role in the maintenance of distylous floral syndromes, yet variation in the expression of self-incompatibility within a species has seldom been assessed.
In a recent study published in AoBP, Shao et al. study the intraspecific variation of self-incompatibility that exists in the distylous herb Primula merrilliana. As P. merrilliana is a fast growing plant that can be easily grown in the glasshouse, the authors used manual pollination experiments and population genetic analyses with nuclear microsatellite (simple sequence repeat, nSSR) markers to investigate variation in this trait. They discovered that the strength of self-incompatibility varied extensively among individuals and populations. Indeed, each distylous population included self-incompatible, partly self-compatible and self-compatible individuals, but the proportions of self-incompatible individuals were higher in central than in peripheral populations. The pronounced variation of self-incompatibility documented in this species provides a more complex and nuanced understanding of the role of self-incompatibility in heterostyly. Importantly, these results show that estimates of self-incompatibility drawn from a few individuals in one or two populations should be regarded with extreme caution.
Jianwen Shao is a professor of the College of Life Sciences in Anhui Normal University of Wuhu City in China. He is interested in the conservation genetics of rare and endangered plants in China, and the adaptation and evolution of plants (particularly those with heterostyly).