Genetic variation in infestation with a directly transmitted ectoparasite.

Genetic variation in levels of parasitism of hosts is an underlying assumption of studies of coevolution, but few such estimates are available from the field. We studied genetic variation in the abundance of the chewing louse Hirundoecus malleus on its barn swallow host Hirundo rustica. These parasites are directly transmitted and a test of genetic variation of parasite abundance would thus provide a particularly strong test. The prevalence and the abundance of the chewing lice did not differ significantly between adult male and female hosts. The resemblance in parasite intensity of H. malleus of offspring and their parents was positive and highly significant, and an analysis of extra-pair paternity in the host allowed partitioning of this resemblance between genetic and common environment effects. There was no significant resemblance in parasite intensity between extra-pair offspring and their foster parents, although the resemblance remained for within-pair offspring. This provides evidence for the abundance of directly transmitted parasites having an additive genetic component. We found no evidence of common environment effects as parents did not resemble each other with respect to lice abundance.