Low genetic variation reduces cross-compatibility and offspring fitness in populations of a narrow endemic plant with a self-incompatibility system

Genetic variation was shown earlier to bereduced in smaller populations of the narrowendemic putatively self-incompatible Cochlearia bavarica. To test whether thisnegatively affects plant fitness by reducedavailability of compatible mates and byinbreeding depression, we studied effects ofpopulation size and pollination treatments oncross-compatibility and offspring fitness in 16isolated populations of this plant. After openpollination, compatibility of crosses (i.e.,whether at least one fruit developed per markedflower), fruit set of compatible crosses, andcumulative fitness (number of plants permaternal ovule) after 14 months in a commongarden were lower for plants from smallerpopulations. Throughout the study, cumulativefitness was lower after hand pollination withpollen of one donor than after open pollination(finally 73.4% lower), suggesting that severalpollen donors or single pollen donors of higherquality are involved in open pollination.Moreover, cumulative fitness was lower afterhand selfing than after hand outcrossing(finally 69.4% lower), indicating bothinbreeding depression and reduced compatibilityafter selfing. High self-compatibility(40.6%), dry stigmas, and differences in thecompatibility of 11 of 33 experimentalreciprocal crosses between plant pairsconfirmed that C. bavarica has asporophytic self-incompatibility system, as iscommon in the Brassicaceae. Our studydemonstrates, that plants in smallerpopulations of species with a sporophyticself-incompatibility system can experiencetwofold fitness reductions associated withreduced genetic variability, i.e., twofoldgenetic Allee effects: via reducedcross-compatibility and via reduced offspringfitness.