Animals mix it up too: the distribution of self-fertilization among hermaphroditic animals

Excluding insects, hermaphroditism occurs in about one-third of animal species, providing numerous opportunities for the evolution of selfing. Here we provide an overview of reproductive traits in hermaphroditic animal species, review the distribution of selfing rates in animals, and test for ecological correlates of selfing. Our dataset (1342 selfing-rate estimates for 142 species) is 97% based on estimates derived from the analysis of population structure (F(IS)-estimates) using genetic markers. The distribution of selfing is slightly U-shaped and differs significantly from the more strongly U-shaped plant distribution with 47% of animal t-estimates being intermediate (falling between 0.2 and 0.8) compared to 42% for plants. The influence of several factors on the distribution of selfing rates was explored (e.g., number of populations studied per species, habitat, coloniality, sessility, or fertilization type), none of which significantly affect the distribution. Our results suggest that genetic forces might contribute to the evolution of self-fertilization to the same extent in animals and plants, although the high proportion of intermediate outcrossing suggests a significant role of ecological factors (e.g., reproductive assurance) in animals. However, we caution that the distribution of selfing rates in animals is affected by various factors that might bias F(IS)-estimates, including phylogenetic underrepresentation of highly selfing and outcrossing species, various genotyping errors (e.g., null alleles) and inbreeding depression. This highlights the necessity of obtaining better estimates of selfing for hermaphroditic animals, such as genotyping progeny arrays, as in plants.