Paul Spruell

The problems with hybrids: setting conservation guidelines

Abstract Rates of hybridization and introgression are increasing dramatically worldwide because of translocations of organisms and habitat modifications by humans. Hybridization has contributed to the extinction of many species through direct and indirect means. However, recent studies have found that natural hybridization has played an important role in the evolution of many plant and animal taxa. Determining whether hybridization is natural or anthropogenic is crucial for conservation, but is often difficult to achieve. Controversy has surrounded the setting of appropriate conservation policies to deal with hybridization and introgression. Any policy that deals with hybrids must be flexible and must recognize that nearly every situation involving hybridization is different enough that general rules are not likely to be effective. We provide a categorization of hybridization to help guide management decisions

Fragmentation of riverine systems: the genetic effects of dams on bull trout (Salvelinus confluentus) in the Clark Fork River system

Migratory bull trout (Salvelinus confluentus) historically spawned in tributaries of the Clark Fork River, Montana and inhabited Lake Pend Oreille as subadult and adult fish. However, in 1952 Cabinet Gorge Dam was constructed without fish passage facilities disrupting the connectivity of this system. Since the construction of this dam, bull trout populations in upstream tributaries have been in decline. Each year adult bull trout return to the base of Cabinet Gorge Dam when most migratory bull trout begin their spawning migration. However, the origin of these fish is uncertain. We used eight microsatellite loci to compare bull trout collected at the base of Cabinet Gorge Dam to fish sampled from both above and further downstream from the dam. Our data indicate that Cabinet Gorge bull trout are most likely individuals that hatched in above‐dam tributaries, reared in Lake Pend Oreille, and could not return to their natal tributaries to spawn. This suggests that the risk of outbreeding depression associated with passing adults over dams in the Clark Fork system is minimal compared to the potential genetic and demographic benefits to populations located above the dams.

INTROGRESSIVE HYBRIDIZATION BETWEEN NATIVE CUTTHROAT TROUT AND INTRODUCED RAINBOW TROUT

Introgressive hybridization threatens the persistence of several species of native salmonids in the western United States, but little is known about the factors influencing the establishment and maintenance of introgressed populations. We examined the occurrence of introgressive hybridization in westslope cutthroat (Oncorhynchus clarki lewisi) and rainbow trout (O. mykiss) populations in relation to physical characteristics of streams, trout density estimates, and the distance from stocking source. Trout were sampled from 80 stream sites in the Clearwater River Basin, Idaho, USA, and tissues from individual trout were analyzed to detect hybridization using noncoding sequences of nuclear DNA. We found a broad zone of hybridization detected at 64% of the sampled sites. The presence and degree of introgression was negatively related to elevation and positively related to stream width in our logistic regression model. Stream elevation and size likely influence hydrologic and thermal regimes. An interaction between the life history characteristics of the native and nonnative trout with these hydrologic and thermal stream gradients could explain the invasion success of rainbow trout and hence, the extent of the hybrid zone. Alternatively, the influence of elevation and stream width could be the result of habitat selection by the parental species, thereby reducing the opportunity for hybridization. Understanding the relationship between abiotic factors and introgressive hybridization will assist fisheries managers when evaluating the potential threat of introgression in different stream habitats and applying the necessary management actions to conserve the native cutthroat trout genotypes across broad landscapes. Corresponding Editor: K. D. Fausch

Ecological and life history characteristics predict population genetic divergence of two salmonids in the same landscape

Ecological and life history characteristics such as population size, dispersal pattern, and mating system mediate the influence of genetic drift and gene flow on population subdivision. Bull trout (Salvelinus confluentus) and mountain whitefish (Prosopium williamsoni) differ markedly in spawning location, population size and mating system. Based on these differences, we predicted that bull trout would have reduced genetic variation within and greater differentiation among populations compared with mountain whitefish. To test this hypothesis, we used microsatellite markers to determine patterns of genetic divergence for each species in the Clark Fork River, Montana, USA. As predicted, bull trout had a much greater proportion of genetic variation partitioned among populations than mountain whitefish. Among all sites, FST was seven times greater for bull trout (FST = 0.304 for bull trout, 0.042 for mountain whitefish. After removing genetically differentiated high mountain lake sites for each species FST, was 10 times greater for bull trout (FST = 0.176 for bull trout; FST = 0.018 for mountain whitefish). The same characteristics that affect dispersal patterns in these species also lead to predictions about the amount and scale of adaptive divergence among populations. We provide a theoretical framework that incorporates variation in ecological and life history factors, neutral divergence, and adaptive divergence to interpret how neutral and adaptive divergence might be correlates of ecological and life history factors.

Can common species provide valuable information for conservation

To demonstrate the importance of genetic data for multispecies conservation approaches, we examined the distribution of genetic variation across the range of the mountain whitefish (Prosopium williamsoni) at microsatellite and allozyme loci. The mountain whitefish is a common species that is particularly well suited for accurately revealing historical patterns of genetic structure and differs markedly from previously studied species in habitat requirements and life history characteristics. As such, comparing the population genetic structure of other native fishes to similar data from mountain whitefish could inform management and conservation strategies. Genetic variation for mountain whitefish was hierarchically distributed for both allozymes and microsatellites. We found evidence for a total of five major genetically differentiated assemblages and we observed subdivision among populations within assemblages that generally corresponded to major river basins. We observed little genetic differentiation within major river basins. Geographic patterns of genetic differentiation for mountain whitefish were concordant with other native species in several circumstances, providing information for the designation of conservation units that reflect concordant genetic differentiation of multiple species. Differences in genetic patterns between mountain whitefish and other native fishes reflect either differences in evolutionary histories of the species considered or differences in aspects of their ecology and life history. In addition, mountain whitefish populations appear to exchange genes over a much larger geographic scale than co‐occurring salmonids and are likely to be affected differently by disturbances such as habitat fragmentation.

A Model Using Phenotypic Characteristics to Detect Introgressive Hybridization in Wild Westslope Cutthroat Trout and Rainbow Trout

Abstract Introgressive hybridization is a substantial threat to native populations of cutthroat trout Oncorhynchus clarki ssp. To assess the status of native cutthroat trout and protect existing pure (nonhybridized) populations, fisheries managers need to identify introgressive hybridization in wild populations. Genetic techniques are the most reliable methods for detecting introgression but are typically expensive and time-consuming. Phenotypic characteristics are generally easy to measure in the field and have been investigated for their value in identifying hybrids in several genera of fish. We developed a practical quantitative tool for detecting introgressive hybridization in westslope cutthroat trout O. c. lewisi by fitting a classification tree model to the phenotypic characteristics of known pure and hybrid individuals. We then tested it as a means of making site-level assessments of the level of introgression. The genotypes of individuals were determined using noncoding sequences of nuclear DNA. ...

Molecular Genetic Markers Identifying Hybridization between the Colorado River-Greenback Cutthroat Trout Complex and Yellowstone Cutthroat Trout or Rainbow Trout

Abstract We used polymerase chain reaction (PCR) primers that are complementary to interspersed nuclear DNA elements to identify genetic markers capable of detecting hybridization between native Colorado River cutthroat trout Oncorhynchus clarki pleuriticus (CRCT) or greenback cutthroat trout O. c. stomias (GCT) and introduced Yellowstone cutthroat trout O. c. bouvieri (YCT) or rainbow trout O. mykiss (RT). Using four different pair combinations of five PCR primers, we detected 6 genetic markers that distinguish CRCT from YCT (3 that are characteristic of CRCT and 3 that are characteristic of YCT) and 14 markers that distinguish CRCT from RT (9 characteristic of CRCT and 5 of RT). Likewise, we detected 5 genetic markers that distinguish GCT from YCT (2 characteristic of GCT and 3 of YCT) and 14 that distinguish GCT from RT (8 characteristic of GCT and 6 of RT). We did not, however, find markers that distinguish CRCT from GCT. This molecular genetic technique will thus be effective as a nonlethal means of ...

Fine-Scale Genetic Structure of Bull Trout at the Southern Limit of Their Distribution

Abstract We used six polymorphic microsatellite loci to analyze the population genetic structure of bull trout Salvelinus confluentus in the Boise River, Idaho, and we compared our results with previous data from similarly sized river systems in western North America. Within the Boise River, we found low genetic variation within and significant differentiation among sample sites. Two cohesive groups of populations were associated with the two major subbasins in this system, which we attributed to long-term reduction of gene flow or distinct sources of colonization at this scale. We observed a significant pattern of isolation by distance in one subbasin and not in the other; this result suggests that the relative influences of gene flow and drift have differed between the two subbasins. Ecologically defined patches of suitable habitat were not good predictors of genetic variation among samples. Dams and other anthropogenic barriers have recently changed the potential for gene flow and genetic drift but wer...

Detection of Hybrids between Bull Trout (Salvelinus confluentus) and Brook Trout (Salvelinus fontinalis) Using PCR Primers Complementary to Interspersed Nuclear Elements

Abstract We used four pairs of polymerase chain reaction primers complementary to the ends of interspersed nuclear elements to amplify markers that differentiate bull trout (Salvelinus confluentus) and brook trout (S. fontinalis) and allow the identification of hybrids between these species. We identified 21 DNA fragments in bull trout that are absent from brook trout and 13 DNA fragments in brook trout that are absent from bull trout. This PCR-based test gave results that are consistent with results from allozyme loci known to be diagnostic for these species. Similar interspersed elements are found in all eukaryotes, making this a potentially useful technique for species identification in many taxa.