Insects and incest: sib-mating tolerance in natural populations of a parasitoid wasp

Sib-mating avoidance is a pervasive behaviour, which likely evolves in species that are subject to inbreeding depression. Laboratory studies have provided elegant demonstrations, but small-scale bioassays often minimize the costs associated with mate finding and mate-choice and may for this reason produce spurious findings. We inferred the mating behaviour of the parasitoid wasp Venturia canescens from lab experiment and genetic analyses of natural populations. We used V. canescens as a model organism because in this species laboratory experiments have shown that sib-mating yields a 25% decrease in fertile offspring, and as a consequence, sib-mating is partially avoided. Our study consisted of a mate choice experiment in laboratory cages and a field study based on the genotyping of 86 wild-caught males, 155 wild-caught females and their 226 daughters at eighteen microsatellite loci. With these field data, we reconstructed the genotype of each female9s mate and estimated the relatedness of each mating pair. The mate choice experiment confirmed that kin discrimination occurs in this species. Time to mating depended on the frequency of female encounters with related and unrelated males. However, contrary to previously published results, no sib-mating avoidance was detected during these experiments. In the field, we found that the effective rate of sib-mating did not differ from the probability that sibs encounter at random, which corroborates the absence of sib-mating avoidance. We also detected a weak but significant male bias in dispersal, a pattern which could reduce encounters between sibs. These results suggest that despite kin discrimination, Venturia canescens tolerates sib-mating in the field. The weak male-biased dispersal cannot explain entirely this pattern. The observed sib-mating tolerance raises the question as to why kin discrimination is maintained in this species. It also calls into question common beliefs on inbreeding depression in species with single-locus complementary sex determination.