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 3 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 that could not be identified to species. These salmonids exhibited
darker coloration then any of the observed indigenous species suggesting
possible hybridization. Indigenous salmonids include steelhead/redband
trout, cutthroat trout, bull trout, and Chinook salmon. Non-native
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).
