USFWS
Genetic Monitoring for Managers
Alaska

 

Type of GEM

Category 1b GeM Project Example: Hybridization

Spread of hybridization between native westslope cutthroat trout, Oncorhynchus clarki lewisi, and nonnative rainbow trout, Oncorhynchus mykiss.

Hitt et al. 2003

Hitt et al. (2003) provide an excellent example of using genetic information to monitor the spread of hybridization between native and nonnative fish species. Hybridization is one of the greatest threats to the persistence of westslope cutthroat trout (Oncorhynchus clarki lewisi; Allendorf and Leary 1998), a Species of Special Concern for the state of Montana. Although the species still occupies approximately 60% of its historic range, an unknown proportion of the populations have hybridized with other species, particularly the nonnative rainbow trout (Oncorhynchus mykiss).

hybridzatoin type bybrid styd area
(a) Study area. (b) Sampling sites. (c) Solid circles represent hybridized populations; open circles represent unhybridized populations. (d) Relavtive amount of introgression (black=rainbow trout; open=cutthroat; hash-marked=hybridized but no relative measure available. [from Hitt et al. 2003]

Hitt et al. (2003) examined changes in the spatial and temporal patterns of hybridization between westslope cutthroat trout and rainbow trout in streams of the Flathead River system in Montana, U.S.A. The Flathead river is recognized as a regional stronghold for westslope cutthroat trout, but historically, over two million rainbow trout were released into the Flathead River from the late 1800's to 1969, and hybrids were first noticed in the 1950s (Hitt et al. 2003).

The authors assessed hybridization status of cutthroat trout from 42 sites along the North Fork and and Middle Fork tributaries. They obtained genetic samples from a small portion of the anal fin, collected after single-pass electroshocking. Using combinations of primer pairs, they evaluated specific regions (fragments) of anonymous nuclear DNA from each species (6 from the rainbow trout and 7 from the westslope cutthroat trout). If individuals at a site had nonnative fragments at only a single locus, they concluded that the site showed native polymorphism. However, if nonnative fragments were present at more than one loci, the authors concluded that introgression had taken place.

As a next step, the authors investigated the timeframe of hybridization by evaluating the degree of nonrandom association of alleles between loci, a property known as linkage disequilibrium. A nonrandom distribution of parental nDNA fragments usually indicates recent introgression, whereas an equilibration of nDNA fragments indicates that hybridization occurred in previous generations.

hybridzation type hybrid fig hybridzation type bybrrid fig 3
Mantel correlogram (a) and percent genetic contribution (b) of cutthroat genetic material as a function of fluvial distance from Flathead Lake. [from Hitt et al. 2003]

The authors found that hybridization between westslope cutthroat trout and rainbow trout has increased rapidly in the Flathead River system over approximately five generations of cutthroat. Seven of 11 sample populations that were nonhybridized in 1984 showed new introgression during the 1998 - 2001 sampling period. Moreover, the spatial pattern of linkage disequilibrium indicated that hybridization was spreading in the upstream direction from the Flathead River mainstem.

hybridzation type bybid fig 4
Number of offspring per female as a function of the amount of nonnative species genetic admixture. [from Muhlfeld et al. 2009]

These results have important management implications for the persistence of westslope cutthroat trout. Muhlfeld et al. (2009) found that the degree of introgression by nonnative rainbow trout had a strong, negative relationship with the number of offspring produced by females cutthroat trout. However, some hybrids had very high levels of reproduction, and likely play a disproportionate role in propogating nonnative genes in this population.

Although the U.S.Fish and Wildlife Service has concluded that that headwater streams are "relatively secure from colonization by non-native fishes" (Anonymous 1999, p. 158), it is apparent that the headwaters are not at all secure, based on the current rate of hybridization and the lack of any ecological barriers between stream reaches. Mangement of hybrids constitutes one of the biggest challenges today. For example, at what degree of hybridization does a population no longer merit protected status (Allendorf et al. 2001). This question is relevant across all taxa, from plants (Ellstrand and Elam 1993) to mammals (e.g., red wolves, Adams et al. 2007).