Adaptive divergence despite strong genetic drift: genomic analysis of the evolutionary mechanisms causing genetic differentiation in the island fox (Urocyon littoralis).

The evolutionary mechanisms generating the tremendous biodiversity of islands have long fascinated evolutionary biologists. Genetic drift and divergent selection are predicted to be strong on islands and both could drive population divergence and speciation. Alternatively, strong genetic drift may preclude adaptation. We conducted a genomic analysis to test the roles of genetic drift and divergent selection in causing genetic differentiation among populations of the island fox (Urocyon littoralis). This species consists of six subspecies, each of which occupies a different California Channel Island. Analysis of 5293 SNP loci generated using Restriction-site Associated DNA (RAD) sequencing found support for genetic drift as the dominant evolutionary mechanism driving population divergence among island fox populations. In particular, populations had exceptionally low genetic variation, small Ne (range = 2.1-89.7; median = 19.4), and significant genetic signatures of bottlenecks. Moreover, islands with the lowest genetic variation (and, by inference, the strongest historical genetic drift) were most genetically differentiated from mainland grey foxes, and vice versa, indicating genetic drift drives genome-wide divergence. Nonetheless, outlier tests identified 3.6-6.6% of loci as high FST outliers, suggesting that despite strong genetic drift, divergent selection contributes to population divergence. Patterns of similarity among populations based on high FST outliers mirrored patterns based on morphology, providing additional evidence that outliers reflect adaptive divergence. Extremely low genetic variation and small Ne in some island fox populations, particularly on San Nicolas Island, suggest that they may be vulnerable to fixation of deleterious alleles, decreased fitness and reduced adaptive potential.

Prevalence of shiga toxin-producing Escherichia coli and Salmonella enterica in rock pigeons captured in Fort Collins, Colorado.

The potential role of rock pigeons (Columba livia) in the epidemiology of shiga toxin-producing Escherichia coli (STEC) and Salmonella enterica is unclear. Our objective was to determine the prevalence of STEC and S. enterica in pigeons at urban and dairy settings as a function of season. Prevalence of STEC and S. enterica was estimated by bacteriologic culture of cloacal swabs collected from pigeons trapped at urban and dairy locations in and around Fort Collins, Colorado from January to November 2003. Presumptive E. coli isolates were tested for the presence of virulence genes SLT-1, SLT-2, eae, hlyA, K1, CNF-1, CNF-2, and LT using polymerase chain reaction. Shiga toxins were not isolated from any of 406 samples from pigeons, but virulence genes typically associated with disease in humans were identified in isolates from 7.9% (95% CI: 5.5% to 10.9%) of captured pigeons. S. enterica were detected in 3.2% of 277 samples from pigeons, with all positive samples originating from dairy locations (nine of 106 [8.5%]; 95% CI: 4.0-15.5%). The results suggest that although pigeons may acquire S. enterica from cattle and play a role in recirculation and persistence of the microorganism at dairies, pigeons are not important carriers of STEC.