Inbreeding depression and consequences of mating system evolution in the genus Clarkia (Onagraceae)

The evolution of plant mating systems depends on a balance among selective forces favoring outcrossing versus selfing. Inbreeding depression is one of the selective forces that influence the evolutionary trajectory of mating system. Here we examine the effects of water availability on the magnitude of inbreeding depression within two predominantly outcrossing taxa in the genus Clarkia (Onagraceae). Under greenhouse conditions, we simulated a gradient of water availability across which phenotypic variation in plant size was similar to that found within natural populations. Maximum values of inbreeding depression are found in the mid-range of the gradient for C. unguiculata, while no significant changes are observed across the gradient for C. xantiana ssp. xantiana . We then compare two sets of outcrossing and selfing sister taxa, C. unguiculata versus C. exilis and C. xantiana ssp. xantiana versus C. xantiana ssp. parviflora, to see whether selfing taxa exhibit lower inbreeding depression than their outcrossing counterparts. The results indicate that purging of the genes causing inbreeding depression is likely to have occurred in one selfing taxon; evidence is less conclusive in the other although the pattern is similar. An analysis of genetic variation within family groups in the separate taxa suggests that self-fertilization in conjunction with other factors may reduce genetic diversity but does not necessarily rapidly purge the genetic load. We find additional evidence of inbreeding depression for both pollen production and for the proportion of viable pollen grains in C. unguiculata, but not in C. exilis. Here, our results indicate that the evolution of selfing in C. exilis is associated with a reduction of the genetic load for pollen viability. The proportion of inviable pollen grains is relatively constant across environmental conditions in C. unguiculata, suggesting that it may provide a reliable relative measure of the genetic load when compared among field populations or taxa. Finally, we evaluate two major hypothesis concerning species range size patterns by examining the relationship of plant range size to mating system and genomic structure (ploidy status) within the genus Clarkia. We argue that genetic diversity, colonization ability, or a combination of both factors may influence plant species' range sizes. Lastly, polyploid species appear to show significantly larger range sizes than diploid species irrespective of mating system.