Heterostyly promotes disassortative pollination and reduces sexual interference in Darwin’s primroses: evidence from experimental studies

Different strategies to reduce selfing and promote outcrossing have evolved in hermaphroditic flowers. Heterostyly, a complex floral polymorphism that occurs in at least 27 families of angiosperms, is hypothesized to achieve both goals by optimizing cross-pollination (via disassortative pollen transfer) and restricting gamete wastage to autogamy (via the reduction of sexual interference between male and female organs). In heterostylous flowers, the reciprocal positioning of sexual organs in different morphs and the pollen incompatibility-system within flower or between flowers of the same morph are thought to optimize both male and female functions, reducing the conflicts inherent to the occurrence of both sexual organs in the same reproductive unit. Specific elements of the disassortative-pollination and sexual-interference hypotheses have been tested individually before. However, despite the long-standing interest in heterostyly – ever since Darwin's seminal work on primroses – the pr...