Abstract
The incidence of hybridization between coastal cutthroat (Oncorhynchus clarki clarki) and rainbow trout (Oncorhynchus mykiss) varies widely among populations. The breakdown of reproductive isolation is of concern to managers, and raises the question: how have the two species retained their genetic and morphological divergence? Using a combination of mitochondrial DNA and nuclear DNA markers coupled with watershed attribute and disturbance data, we determined the distribution and frequency of trout hybridization on Vancouver Island, BC and the environmental factors associated with the hybridization. We found 284 hybrids (among 1004 fish) in 29 of 36 sampled populations. High variation in levels of hybridization was observed among populations, and no single environmental factor was found to dominate in determining hybridization levels. However, logging activity, urban infrastructure development, and stocking of hatchery rainbow trout played significant roles in determining hybridization levels, and populations in small watersheds are more at risk of reproductive barrier breakdown. This study illustrates that cutthroat–rainbow trout reproductive barrier breakdown is widespread on Vancouver Island and that anthropogenic disturbance plays a role in the process. As similar environmental disturbance is common in much of coastal trout habitat, large-scale hybridization may be occurring elsewhere and thus may represent a critical management issue for Pacific trout species.
Highlights
Many forms of reproductive barriers have been postulated to contribute to maintaining species integrity
Those six populations were excluded from all further analyses because field identification and genetic analyses did not identify a sympatric relationship between the trout species, no gene flow between the species was possible
Seven populations showed evidence for departures from Hardy– Weinberg equilibrium. Those seven populations departed from Hardy–Weinberg equilibrium at three to seven of the marker loci (Table 3), and in all cases the deviations were due to deficiencies in heterozygote genotypes
Summary
Many forms of reproductive barriers have been postulated to contribute to maintaining species integrity. Generalizations are difficult to make, anthropogenic change, or disturbance, appear as Factors affecting trout hybridization common themes in studies examining the causes of hybridization (e.g., Docker et al 2003; Lamont et al 2003; Schwarz and McPheron 2007; Keller et al 2008). The form of the environmental disturbance varies; in plants, physical disturbance such as roadways or building sites may foster hybridization (e.g., Lamont et al 2003), while in animals the introduction of non-native species is often implicated (Grosholz 2002; Rubidge and Taylor 2005; Schwarz and McPheron 2007). Few studies have attempted to partition the relative contribution of various factors that contribute to the erosion of reproductive isolation between sympatric species
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