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

Sib-mating avoidance is a pervasive behaviour that likely evolves in species subject to inbreeding depression. Laboratory studies have provided elegant demonstrations of sib-mating avoidance, but small-scale bioassays often minimize the costs associated with mate finding and choice, which could lead to spurious findings.

We used the hymenopteran parasitoid waspVenturia canescensas a model organism, because previous laboratory studies revealed that sib-mating led to a 25% decrease in fertile offspring, and that sib-mating was partially avoided.

Our study consisted of a mate choice experiment in laboratory cages to determine if kin discrimination occurs in this species. We further performed a field study in which 86 wild-caught males, 155 wild-caught females and their 226 daughters daughters were genotyped at eighteen microsatellite loci. With these field data, we reconstructed the genotype of each female’s mate and estimated the relatedness of each mating pair.

Mate choice experiments confirmed that females are capable of discriminating kin. Time to mating depended on the frequency of female encounters with related and unrelated males. Contrary to previously published results, however, no sib-mating avoidance was detected. In the field, the effective rate of sib-mating did not differ from the probability that sibs encounter one other at random, which corroborates the absence of sib-mating avoidance. We also detected a weak but significant male bias in dispersal, which could reduce encounters between sibs.

Our results suggest that, despite kin discrimination,V. canescenstolerates sib-mating in the field. The weak male-biased dispersal cannot explain entirely this pattern. This raises the question as to why kin discrimination is maintained in this species. It further calls into question the idea that inbreeding depression occurs in most species with single-locus complementary sex determination.