Patterns of hybridization among cutthroat trout and rainbow trout in northern Rocky Mountain streams

Abstract

Introgressive hybridization between native and introduced species is a growing conservation concern. For native cutthroat trout and introduced rainbow trout in western North America, this process is thought to lead to the formation of hybrid swarms and the loss of monophyletic evolutionary lineages. Previous studies of this phenomenon, however, indicated that hybrid swarms were rare except when native and introduced forms of cutthroat trout co‐occurred. We used a panel of 86 diagnostic, single nucleotide polymorphisms to evaluate the genetic composition of 3865 fish captured in 188 locations on 129 streams distributed across western Montana and northern Idaho. Although introgression was common and only 37% of the sites were occupied solely by parental westslope cutthroat trout, levels of hybridization were generally low. Of the 188 sites sampled, 73% contained ≤5% rainbow trout alleles and 58% had ≤1% rainbow trout alleles. Overall, 72% of specimens were nonadmixed westslope cutthroat trout, and an additional 3.5% were nonadmixed rainbow trout. Samples from seven sites met our criteria for hybrid swarms, that is, an absence of nonadmixed individuals and a random distribution of alleles within the sample; most (6/7) were associated with introgression by Yellowstone cutthroat trout. In streams with multiple sites, upstream locations exhibited less introgression than downstream locations. We conclude that although the widespread introduction of nonnative trout within the historical range of westslope cutthroat trout has increased the incidence of introgression, sites containing nonadmixed populations of this taxon are common and broadly distributed.

Climate, Demography, and Zoogeography Predict Introgression Thresholds in Salmonid Hybrid Zones in Rocky Mountain Streams

“…Errors for some of the genotyping methods used in our analysis can exceed 1% (cf. [18,90]), which could lead to incorrect assignment of sites above or below a threshold. Likewise, some populations of westslope cutthroat trout may have natural polymorphisms such that they have alleles otherwise diagnostic for rainbow trout [46], which can lead to overestimates of introgression.…”

“…Likewise, some populations of westslope cutthroat trout may have natural polymorphisms such that they have alleles otherwise diagnostic for rainbow trout [46], which can lead to overestimates of introgression. And because rainbow trout alleles are not randomly distributed among individuals in sites with admixture [18], the relatively small samples of fish from each site could yield imprecise estimates of introgression. Because 307 (61%) of the sites with estimates of PRTA were within 1% of the lowest threshold (0–2% rainbow trout introgression), performance of the logistic regression models for the lowest threshold would be particularly sensitive to these issues.…”

“…In both circumstances, genetically intact populations of cutthroat trout remain common in headwater streams [15,16]. Moreover, many populations purportedly representing hybrid swarms contain substantial numbers of nonadmixed individuals of either or both parental species and often feature a non-random distribution of alleles among individuals [17,18]. Both conditions imply resistance to runaway introgression, and hint at the potential for ecological segregation between parental taxa to mediate the degree of hybridization and the location of hybrid zones [19–21].…”

Slow climate velocities of mountain streams portend their role as refugia for cold-water biodiversity

“…As a result, a common pattern is that warmer, low-elevation stream communities are broadly and repeatedly invaded (14, 17) whereas colder, high-elevation streams are less altered and often serve as refugia for native species tolerant of cold temperatures (18,19). The same is true regarding more subtle genetic invasions and introgressive hybridization, because many genetically pure populations exist in headwater streams where climatic conditions limit the upstream expansion of hybrid zones (20,21).The ubiquity of isolated populations of many species in mountain headwaters (14, 18, 19) suggests they have existed for extended periods, but climate-induced stream warming (22, 23) raises legitimate concerns about their future persistence. Concerns are heightened by evidence of faster air temperature increases at high elevations (24), and by model predictions that streams in snowmelt-dominated landscapes will be highly sensitive (i.e., large stream Δ°C/air Δ°C) to air temperature increases (25)(26)(27).…”

“…As a result, a common pattern is that warmer, low-elevation stream communities are broadly and repeatedly invaded (14, 17) whereas colder, high-elevation streams are less altered and often serve as refugia for native species tolerant of cold temperatures (18,19). The same is true regarding more subtle genetic invasions and introgressive hybridization, because many genetically pure populations exist in headwater streams where climatic conditions limit the upstream expansion of hybrid zones (20,21).…”