Genetic variation of the American crow (Corvus brachyrhynchos) and the emergence of West Nile virus

The objective of this study was to assess the impact of the emergence of West Nile virus (WNV) on the genetic diversity of the American crow (Corvus brachyrhynchos). This avian species has been the most negatively affected host species resulting in estimated population decline up to 60% since the arrival of the virus. Thus, it is hypothesized that the genetic diversity and genetic structure of the crow has been modulated by WNV in a short timescale.;Changes on mortality rate and seroprevalence to WNV in avian species were extensively reviewed as an indication of heterogeneity of host susceptibility across time, geographic areas, and host species. A consistent increment of the seroprevalence was detected in eight species, including C. brachyrhynchos , although changes on time were at different magnitudes. C. brachyrhynchos showed also a consistent decline in the mortality rate.;Thirty two polymorphic microsatellite markers distributed in six multiplex panels were developed and characterized for genetic analysis. The temporal analysis detected a significantly lower allelic richness and heterozygosity after four years of WNV arrival. Reduction of allelic diversity was faster than heterozygosity, a measure of recent population bottleneck. The genetic diversity was rapidly recovered by the year 2002 showing evidence of a population expansion by immigrants. Finally, a consistent pattern of higher heterozygosity levels towards resistance to WNV was detected. However, these differences in infection status were not significant, neither by a local effect nor by overall genetic diversity.;This study provides molecular evidence for a strong genetic impact induced by severe mortality events attributable to WNV since 1999 in the American crow. These changes occurred in a short time after the introduction of the virus. Relatively slow changes in the seroprevalence and mortality rates by WNV may indicate the slow process of adaptation of the American crow to this virulent pathogen. Thus, WNV may be exerting considerable selection force to drive genetic evolution of the crow population but in an ecologically relevant time scale. These results suggest a lack of specific genetic adaptation of the American crow towards resistance to WNV may be due to a constant immigration of susceptible individuals.