Species Identification of Two Suspected Hybrid Salmonids in the North Fork Touchet River in Southeastern Washington State

Hybridization is the process in which two genetically distinct species breed and produce an offspring that is commonly known as a hybrid. This phenomenon has been observed in nature on many instances and with a variety of plants and animals. Some common examples of hybrids include the mule (cross between a horse and a donkey) and peppermint (a hybrid between spearmint and water mint). What is the significance of hybridization? Some scientists contend that natural hybridization has lead to the production of relatively fit hybrids that possess novel genetic variation or found new evolutionary lineages (Arnold and Hodges 1995). Additionally, studies have demonstrated that some hybrid species are more fit than either parental species. This increased fitness, due to the hybrids mixed genome complexes, may allow hybrids to develop new niches and adapt more readily to change (Cruzan and Arnold 1993). Conversely, others argue that a majority of hybrids are less fit than their parental species because they are sterile and unable to propagate their genetic information into future generations. This sterility, therefore, makes hybridization of little evolutionary significance (Dobzhansky 1970). Yet, fertile hybrids have been observed in nature, thereby providing a mechanism for gene flow between species. Additionally, if the gene flow is sufficiently large, hybridization may lead to the merging of two formally distinct species.

Fish hybridization is observed more frequently in the wild than any other group of vertebrates (Ryman & Utter 1987). Examples include natural hybridization between Dolly Varden and bull trout (Baxter et al. 1997) and crosses between steelhead trout and costal cutthroat trout (Campton & Utter 1985). Several factors contribute to the increased rate of fish hybridization:

(1) external fertilization;

(2) weak ethological isolating mechanisms;

(3) unequal abundance of two parent species;

(4) competition for spawning habitat; and

(5) susceptibility to secondary contact between recently evolved forms (Campton 1987).

Introduction of non-native fish species into freshwater habitats has also resulted in hybridization. In the western United States, the introduction of the rainbow trout into nonnative regions has resulted in the introgression of rainbow genes into the indigenous cutthroat population that is morphologically undetectable (Leary et al. 1984). This induced gene flow is a concern because it may impact the integrity of native gene pools and ultimately result in the extinction of several freshwater species through introgression (Rubidge & Taylor 2005).

The present study was precipitated by an instance of possible hybridization, when a Washington Department of Fish and Wildlife (WDFW) biologist collected two salmonids while conducting fieldwork in southeastern Washington State (North Fork Touchet River) that could not be identified to species (Figure 1). When keyed, these fish had several characteristics of rainbow trout, and were identified as likely rainbow trout, except their coloration was unusual (i.e. dirty brown) and they lacked parr marks (Glen Mendel, pers. comm.). The coloration exhibited was darker then that observed for any indigenous salmonid species suggesting possible hybridization. Indigenous salmonids include steelhead/redband trout, bull trout, and Chinook salmon. Nonnative rainbow trout have been introduced, as have brown trout. Coho salmon reintroductions have occurred elsewhere in the Columbia River Basin. It is possible that Atlantic salmon have escaped from aquaculture facilities and dispersed into the area where these unknowns were found. The primary objective of this project was to use genetic analyses to determine if the unknown salmonids were of hybrid origin. Mitochondrial DNA was used to establish the species of the maternal lineage. Microsatellite analysis was used as an additional method of species identification and as a way to detect possible hybridization (i.e. the possession of a mixture of alleles from both parental species).