Genetic evidence for population expansion in Hydrotaea irritans (Fallèn) (Diptera: Muscidae)

Allozyme variation in the sheep headfly Hydrotaea irritans was studied on two spatial scales. Geographic variation among seven Danish and one Dutch population revealed significant but rather low genetic differentiation with F _ST = 0.01 over all loci. The Dutch population was on average not more different from the Danish populations than the Danish populations from each other. Allele frequencies were very skewed with the most common allele always exceeding 0.85 and usually 0.9 in frequency, but with many rare alleles at some loci. Tests for neutrality of the variation at the nine polymorphic loci revealed highly significant deviations from expected homozygosity in this species, which was not found in a comparative analysis of allozyme variation at similar loci of seven other Hydrotaea species. To explain the peculiar observed pattern of allozyme variation in H. irritans , it is suggested that this species has successfully expanded its range and spread through northern and central Europe in the recent past. Alternatively, H. irritans may have recently invaded a new niche, resulting in increased abundance of the species and subsequent dispersal to former areas of the species distribution.