Migration inflation: Effects of continuous immigration on the population structure of biting midges

The amount and frequency at which populations exchange genes has an enormous impact on the population structure of each. A lack of gene flow can lead to increased divergence between populations and higher levels of inbreeding whereas increased gene flow has the opposite effect. In Culicoides biting midges, however, we have observed a paradoxical pattern in which there are high levels of inbreeding in the face of high levels of gene flow. It has been hypothesized that null alleles or sub-structuring within populations (i.e., the Wahlund effect) were responsible for this phenomenon but a lack of empirical data has prevented a definitive answer. To investigate this further, we tracked the genetic diversity of a single population of C. sonorensis in California over a three-year period. By genotyping individuals through time, we were able to measure changes in allelic frequency and diversity. Surprisingly, this population remained relatively unchanged despite significant seasonal fluctuations. Additionally, each microsatellite contained a high number of alleles with an average of 33 alleles per marker. Most of these alleles though were present only at relatively low frequencies and were likely introduced from other populations via wind-transported migrants. It appears that this is artificially inflating estimates of the expected heterozygosity (H_e) and by extension increasing the inbreeding coefficient (F_IS). Regardless, the consistent patterns of genetic diversity over time means only a single sampling is enough to fully characterize the structure of C. sonorensis populations.