Carnivore Contact: A Species Fracture Zone Delineated Amongst Genetically Structured North American Marten Populations (Martes americana and Martes caurina)

Abstract

North American martens are forest dependent, influenced by human activity, and climate vulnerable. They have long been managed and harvested throughout their range as the American marten (Martes americana). Recent work has expanded evidence for the original description of two species in North America-M. americana and the Pacific Coast marten, M. caurina-but the geographic boundary between these groups has not been described in detail. From 2010 to 2016 we deployed 734 multi-taxa winter bait stations across a 53,474 km 2 study area spanning seven mountain ranges within the anticipated contact zone along the border of Canada and the United States. We collected marten hair samples and developed genotypes for 15 polymorphic microsatellite loci for 235 individuals, and 493 base-pair sequences of the mtDNA gene COI for 175 of those individuals. Both nuclear and mitochondrial genetic structure identified a sharp break across the Clark Fork Valley, United States with M. americana and M. caurina occurring north and south of the break, respectively. We estimated global effective population size (N e) for each mountain range, clinal genetic neighborhood sizes (NS), calculated observed (H o) and expected (H e) heterozygosity, fixation index (F ST) , and clinal measures of allelic richness (Ar), H o , and inbreeding coefficient (F IS). Despite substantial genetic structure, we detected hybridization along the fracture zone with both contemporary (nuclear DNA) and historic (mtDNA) gene flow. Marten populations in our study area are highly structured and the break across the fracture zone being the largest documented in North America (F ST range 0.21-0.34, mean = 0.27). With the exception of the Coeur d'Alene Mountains, marten were well distributed across higher elevation portions of our sampling area. Clinal NS values were variable suggesting substantial heterogeneity in marten density and movement. For both M. americana and

The role of the environment in the spatial dynamics of an extensive hybrid zone between two neotropical cats

“…Hybrid zones are formed in areas where species with weak reproductive barriers co‐occur (Barton & Hewitt, 1985). Often, these areas correspond to environmental gradients or ecotones that allow the secondary contact of related species adapted to different environments (Cullingham et al., 2012; Culumber et al., 2012; Kameyama et al., 2008; Lucid et al., 2020; Walsh et al., 2016). The genetic structure of hybrid zones is highly variable, and their maintenance in space and time is determined mainly by the type and strength of selection on hybrids (Barton, 2001).…”

Combined effects of climate and fire‐driven vegetation change constrain the distributions of forest vertebrates during the 21st century

“…americana ), which were studied and managed as one species, M . americana , before recent genomic studies distinguished them (Dawson et al, 2017; Lucid et al, 2020).…”

“…Martens occupy mesic subalpine conifer forests with complex vertical and horizontal structures, which provide habitat for prey and protected sites for breeding, denning and rearing (Buskirk & Ruggiero, 1994;Thompson et al, 2012). We here use Martes to refer to both species of the genus in North America (M. caurina and M. americana), which were studied and managed as one species, M. americana, before recent genomic studies distinguished them (Dawson et al, 2017;Lucid et al, 2020).…”

Whole‐genome resequencing reveals persistence of forest‐associated mammals in Late Pleistocene refugia along North America’s North Pacific Coast

“…A recent investigation into the Montana hybrid zone with expanded sampling (15 nuclear microsatellites for >200 individuals and mitochondrial COX1 sequences for 175 of those individuals) also found few admixed individuals and identified a single sample with approximately 50% ancestry from each species (Lucid et al, 2020). Detection of only early-generational hybrids and evidence of a geographically restricted (~5 km) strong phylogenetic break within the continental hybrid zone where few hybrids were detected, even with expanded sampling (Lucid et al, 2020), leads us to hypothesize that hybrid incompatibilities may impact the evolution of this system. In this case, F1 hybrids may experience a temporary elevation in fitness (heterosis) compared to later generational-stage hybrids (e.g., F2 and beyond), which may suffer outbreeding depression as a result of disrupted co-adapted gene complexes (Todesco et al, 2016).…”

“…Although sample sizes are small, the absence of late-generational hybrids is surprising, especially for the Montana zone which has persisted for many generations (Wright, 1953). A recent investigation into the Montana hybrid zone with expanded sampling (15 nuclear microsatellites for >200 individuals and mitochondrial COX1 sequences for 175 of those individuals) also found few admixed individuals and identified a single sample with approximately 50% ancestry from each species (Lucid et al, 2020). Detection of only early-generational hybrids and evidence of a geographically restricted (~5 km) strong phylogenetic break within the continental hybrid zone where few hybrids were detected, even with expanded sampling (Lucid et al, 2020), leads us to hypothesize that hybrid incompatibilities may impact the evolution of this system.…”