Contrasting genetic trajectories of endangered and expanding red fox populations in the western U.S

As anthropogenic disturbances continue to drive habitat loss and range contractions, the maintenance of evolutionary processes will increasingly require targeting measures to the population level, even for common and widespread species. Doing so requires detailed knowledge of population genetic structure, both to identify populations of conservation need and value, as well as to evaluate suitability of potential donor populations. We conducted a range-wide analysis of the genetic structure of red foxes in the contiguous western U.S., including a federally endangered distinct population segment of the Sierra Nevada subspecies, with the objectives of contextualizing field observations of relative scarcity in the Pacific mountains and increasing abundance in the cold desert basins of the Intermountain West. Using 31 autosomal microsatellites, along with mitochondrial and Y-chromosome markers, we found that populations of the Pacific mountains were isolated from one another and genetically depauperate (e.g., estimated Ne range = 3–9). In contrast, red foxes in the Intermountain regions showed relatively high connectivity and genetic diversity. Although most Intermountain red foxes carried indigenous western matrilines (78%) and patrilines (85%), the presence of nonindigenous haplotypes at lower elevations indicated admixture with fur-farm foxes and possibly expanding midcontinent populations as well. Our findings suggest that some Pacific mountain populations could likely benefit from increased connectivity (i.e., genetic rescue) but that nonnative admixture makes expanding populations in the Intermountain basins a non-ideal source. However, our results also suggest contact between Pacific mountain and Intermountain basin populations is likely to increase regardless, warranting consideration of risks and benefits of proactive measures to mitigate against unwanted effects of Intermountain gene flow.

Phylogenomic systematics of the spotted skunks (Carnivora, Mephitidae, Spilogale): Additional species diversity and Pleistocene climate change as a major driver of diversification

Four species of spotted skunks (Carnivora, Mephitidae, Spilogale) are currently recognized: Spilogale angustifrons, S. gracilis, S. putorius, and S. pygmaea. Understanding species boundaries within this group is critical for effective conservation given that regional populations or subspecies (e.g., S. p. interrupta) have experienced significant population declines. Further, there may be currently unrecognized diversity within this genus as some taxa (e.g., S. angustifrons) and geographic regions (e.g., Central America) never have been assessed using DNA sequence data. We analyzed species limits and diversification patterns in spotted skunks using multilocus nuclear (ultraconserved elements) and mitochondrial (whole mitogenomes and single gene analysis) data sets from broad geographic sampling representing all currently recognized species and subspecies. We found a high degree of genetic divergence among Spilogale that reflects seven distinct species and eight unique mitochondrial lineages. Initial divergence between S. pygmaea and all other Spilogale occurred in the Early Pliocene (∼ 5.0 million years ago). Subsequent diversification of the remaining Spilogale into an "eastern" and a "western" lineage occurred during the Early Pleistocene (∼1.5 million years ago). These two lineages experienced temporally coincident patterns of diversification at ∼0.66 and ∼0.35 million years ago into two and ultimately three distinct evolutionary units, respectively. Diversification was confined almost entirely within the Pleistocene during a timeframe characterized by alternating glacial-interglacial cycles, with the origin of this diversity occurring in northeastern Mexico and the southwestern United States of America. Mitochondrial-nuclear discordance was recovered across three lineages in geographic regions consistent with secondary contact, including a distinct mitochondrial lineage confined to the Sonoran Desert. Our results have direct consequences for conservation of threatened populations, or species, as well as for our understanding of the evolution of delayed implantation in this enigmatic group of small carnivores.

Noninvasive Sampling Reveals Short-Term Genetic Rescue in an Insular Red Fox Population

Genetic factors in the decline of small populations are extremely difficult to study in nature. We leveraged a natural experiment to investigate evidence of inbreeding depression and genetic rescue in a remnant population of subalpine-specialized Sierra Nevada red foxes (Vulpes vulpes necator) using noninvasive genetic monitoring during 2010-2017. Only 7 individuals were detected in the first 2 years. These individuals assigned genetically to the historical population and exhibited genetic hallmarks of inbreeding and no evidence of reproduction. Two years into the study, we detected 2 first-generation immigrant males from a recently expanding population of red foxes in the Great Basin Desert. Through annual resampling of individuals (634 red fox DNA samples, 41 individuals) and molecular reconstruction of pedigrees, we documented 1-3 litters/year for 5 years, all descended directly or indirectly from matings involving immigrant foxes. The observed heterozygosity and allelic richness of the population nearly doubled in 2 years. Abundance increased, indicative of a rapidly expanding population. Throughout the study, adult survival was high. Restoration of gene flow apparently improved the demographic trajectory of this population in the short term. Whether these benefits continue in the longer term could depend on numerous factors, such as maintenance of any locally adapted alleles. This study highlights the value of noninvasive genetic monitoring to assess rapidly shifting conditions in small populations. Uncertainties about the longer-term trajectory of this population underscore the need to continue monitoring and to research potential for both negative and positive aspects of continued genetic infusion.

The Red Fox Y-Chromosome in Comparative Context

While the number of mammalian genome assemblies has proliferated, Y-chromosome assemblies have lagged behind. This discrepancy is caused by biological features of the Y-chromosome, such as its high repeat content, that present challenges to assembly with short-read, next-generation sequencing technologies. Partial Y-chromosome assemblies have been developed for the cat (Feliscatus), dog (Canislupusfamiliaris), and grey wolf (Canislupuslupus), providing the opportunity to examine the red fox (Vulpesvulpes) Y-chromosome in the context of closely related species. Here we present a data-driven approach to identifying Y-chromosome sequence among the scaffolds that comprise the short-read assembled red fox genome. First, scaffolds containing genes found on the Y-chromosomes of cats, dogs, and wolves were identified. Next, analysis of the resequenced genomes of 15 male and 15 female foxes revealed scaffolds containing male-specific k-mers and patterns of inter-sex copy number variation consistent with the heterogametic chromosome. Analyzing variation across these two metrics revealed 171 scaffolds containing 3.37 Mbp of putative Y-chromosome sequence. The gene content of these scaffolds is consistent overall with that of the Y-chromosome in other carnivore species, though the red fox Y-chromosome carries more copies of BCORY2 and UBE1Y than has been reported in related species and fewer copies of SRY than in other canids. The assignment of these scaffolds to the Y-chromosome serves to further characterize the content of the red fox draft genome while providing resources for future analyses of canid Y-chromosome evolution.

Red Fox Ancestry and Connectivity Assessments Reveal Minimal Fur Farm Introgression in Greater Yellowstone Ecosystem

Rocky Mountain red foxes Vulpes vulpes macroura potentially encounter other red fox Vulpes vulpes lineages at lower elevations, which may include nonindigenous red foxes derived from fur farms. Introgression from nonindigenous red foxes could have negative evolutionary consequences for the rare Rocky Mountain red fox subspecies. Red foxes at high elevations in the Greater Yellowstone Ecosystem exhibit lighter coat colors than those at lower elevations, potentially indicating that they represent the indigenous subspecies and that gene flow across the elevational gradient is restricted. We collected tissue samples across a 1,750-m elevation range and examined mitochondrial DNA sequences and nuclear DNA microsatellite genotypes to assess the ancestry and genetic population structure of red foxes in the northern Greater Yellowstone Ecosystem. We also used reference samples from fur farm red foxes and indigenous red foxes of the western United States to assess the extent of nonindigenous introgression across the ecosystem. We found little overlap in the elevational distribution of maternally inherited mitochondrial DNA haplotypes: above 1,600 m, we only found indigenous Rocky Mountain haplotypes (n = 4), whereas below 1,600 m, we found haplotypes not indigenous to the Rocky Mountains (n = 5) that were associated with fur farms or indigenous to the Great Plains. In contrast, biparentally inherited microsatellite variation showed little population structure across the elevational gradient. Despite this evidence of nuclear gene flow across the elevational gradient, we found little fur farm introgression in the microsatellite genotypes. It is possible that long-standing nuclear (but apparently not mitochondrial) gene flow between Rocky Mountain red foxes and indigenous red foxes on the Great Plains explained the low nuclear differentiation of these populations. Importantly, our results suggested that high elevations of the northern Greater Yellowstone Ecosystem remained free of significant fur farm introgression. Mitonuclear discordance could reflect sex-biased dispersal, which we hypothesize could be the effect of elevational differences in reproductive phenology.

Phylogeography of a widespread small carnivore, the western spotted skunk (Spilogale gracilis) reveals temporally variable signatures of isolation across western North America

We analyzed phylogeographic patterns in the western spotted skunk, Spilogale gracilis Merriam, 1890 (Carnivora: Mephitidae) in relation to historical events associated with Pre-Pleistocene Divergence (PPD) and Quaternary climate change (QCC) using mitochondrial DNA from 97 individuals distributed across Western North America. Divergence times were generated using BEAST to estimate when isolation in putative refugia occurred. Patterns and timing of demographic expansion was performed using Bayesian skyline plot. Putative climatic refugia resulting from Quaternary climate change were identified using paleoecological niche modeling and divergence dates compared to major vicariant events associated with Pre-Pleistocene conditions. We recovered three major mitochondrial clades corresponding to western North America (California, Baja, and across the Great Basin), east-central North America (Texas, central Mexico, New Mexico), and southwestern Arizona/northwestern Mexico. Time to most recent common ancestor for S. gracilis occurred ~1.36 Ma. Divergence times for each major clade occurred between 0.25 and 0.12 Ma, with signature of population expansion occurring 0.15 and 0.10 Ma. Ecological niche models identified three potential climatic refugia during the Last Interglacial, (1) west coast of California and Oregon, (2) northwestern Mexico, and (3) southern Texas/northeastern Mexico as well as two refugia during the Last Glacial Maximum, (1) western USA and (2) southern Texas/northeastern Mexico. This study supports PPD in shaping species-level diversity compared to QCC-driven changes at the intraspecific level for Spilogale, similar to the patterns reported for other small mammals (e.g., rodents and bats). Phylogeographic patterns also appear to have been shaped by both habitat and river vicariance, especially across the desert southwest. Further, continuing climate change during the Holocene coupled with anthropogenic modifications during the Anthropocene appears to be removing both of these barriers to current dispersal of western spotted skunks.

Fate of the other redcoat: remnants of colonial British foxes in the eastern United States

Red foxes were absent or rare in the southeastern United States until the late 1800s. Their origins potentially include natural population increase/expansion, translocations from Europe, and, eventually, 20th century fur farming. Previous studies have found no European haplotypes in North America, but few samples were sourced from the Atlantic coastal plain, closer to the source of putative introductions. Through analysis of mitochondrial DNA in 584 red foxes from this region, we identified indigenous haplotypes in ≥ 35% of foxes, 1 of 2 European haplotypes in 17% of foxes and fur farm haplotypes in ≥ 13% of foxes; another 35% of foxes had haplotypes potentially indigenous or native. In contrast, only 3 of 135 (2%) male foxes carried a single European Y chromosome haplotype. Most European and fur farm haplotypes were found near the densely human-populated coastal plain and Hudson River lowlands; most red foxes of the Appalachians and Piedmont had native eastern haplotypes. Our findings suggest that the more remote, upland populations primarily reflect indigenous red fox matrilines, whereas urban-associated populations in and around the mid-Atlantic coastal plain and Hudson lowlands reflect an admixture of native and nonnative maternal sources. Autosomal markers are needed to further elucidate the extent of European and fur farm introgression in the Appalachians and further west.