Evolution towards self-compatibility when mates are limited

Theory of plant mating system evolution predicts the spread of self-compatibility (SC) in a predominantly self-incompatible population when inbreeding depression (ID; the decline in fitness because of selfing) is small and when compatible mates are limited. I tested these two predictions by measuring the occurrence of SC in 13 natural populations of Ranunculus reptans L. that varied in ID and frequency of cross-incompatible mates. Enforced selfing experiments were conducted in 2 years. In the first year, self-pollination was applied at two flower ages to investigate the occurrence of delayed SC. I found that SC was not uncommon across all populations, but self-compatible plants usually produced few seeds. There was no evidence for delayed SC. The occurrence of SC was not associated with population-level ID, but populations with more limited availability of compatible mates had a significantly higher frequency of plants that were at least partially self-compatible. The results indicate that, in R. reptans, a shortage of available mates in small populations may cause the evolution of partial SC and mixed mating.