Abstract
Recent genetic analyses indicated that the Sacramento Valley (SV) red fox (Vulpes vulpes) is native to its current range, occurs at low abundance, and is the closest evolutionary relative of the endangered and California state-listed Sierra Nevada red fox. Although the SV red fox is reproductively isolated from other native populations in the West, a new population of nonnative red foxes of evolutionarily divergent descent currently comes into contact with the SV population at its southern boundary. Encroachment by these nonnative red foxes could threaten the native population through competition for resources, potentially leading to displacement of natives, and hybridization, potentially causing loss of locally adapted alleles or disruption of coadapted gene complexes.
This research used genetic approaches to investigate these potential threats. We used mitochondrial Cytochrome b and microsatellite markers to assess displacement and nuclear genetic introgression between native and non-native populations. I developed 33 red-fox sensitive microsatellite markers with the aid of the dog genome. Markers were combined into 5 multiplex assays that were used to genotype red fox samples collected from native and nonnative ranges (n = 296 total). Statistical assignment and coalescent-based approaches were used to estimate the proportion of each individual's ancestry with respect to native and nonnative populations and the migration rates between populations. Mitochondrial sequences indicated distinct populations in the Sacramento Valley and San Joaquin Valley. Microsatellite analysis did not indicate displacement, however, six individuals sampled within the Sacramento Valley were identified as hybrids. The level of nonnative genetic introgression in hybrid individuals declined with distance from the contact zone, consistent with introgressive hybridization. Structure analysis indicated substructure within the nonnative population but not in the SV population. Independent analyses based on Bayesian and Maximum Likelihood methods indicated asymmetrical gene flow, mainly from the nonnative to the native population. Nevertheless, the SV and nonnative population were genetically differentiated (Fst = 0.17). The existence of the distinct and native SV red fox which is closely related to the state listed Sierra Nevada red fox is threatened by encroachment and genetic dilution of its alleles from hybridization with nonnative red foxes. This situation is similar to genetic introgression between domestic dogs and Ethiopian wolves leading to the dilution of the endangered red wolves genome.