Intercrosses and the U.S. Endangered Species Act: Should Hybridized Populations be Included as Westslope Cutthroat Trout?

Abstract

There are currently no policy guidelines for treating hybrids under the U.S. Endangered Species Act (ESA). We considered the scientific basis for determining whether hybridized populations should be included as part of the westslope cutthroat trout (Oncorhynchus clarki lewisi) unit considered for listing under the ESA. Westslope cutthroat trout are threatened by genomic extinction because of widespread introgressive hybridization with introduced rainbow trout (O. mykiss) and Yellowstone cutthroat trout (O. c. bouvieri). Experimental results suggest that first-generation hybrids between westslope cutthroat trout and rainbow trout have reduced fitness. However, hybridization may spread even when hybrids have severely reduced fitness because the production of hybrids is unidirectional-that is, all the progeny of a hybrid will be hybrids. In addition, heterosis resulting from the sheltering of deleterious recessive alleles in early-generation hybrids may increase the effective rate of introgression. However, such short-term increases in fitness may disrupt important long-term adaptations of native populations. The loss of these adaptations will be difficult to detect because some local adaptations might only be apparent during periodic episodes of extreme environmental conditions, such as winter storms, drought, or fire. Thus, rapid spread of hybridization could result in the loss of local adaptations in native populations of westslope cutthroat trout and decrease their probability of long-term persistence. Protection of populations with some admixture would protect sources of spreading hybridization. Treatment of hybrids in conservation planning depends primarily on the amount of evolutionary divergence between the hybridizing taxa and the geographical extent of introgression. We recommend that only nonhybridized populations be included as westslope cutthroat trout in the unit to be considered for listing. Populations of unknown status should be protected until more information about these populations becomes available. Entrecruzas y el Acta de Especies en Peligro de E. U. A.: Inclusión de Poblaciones Híbridas de Oncorhynchus clarki lewisi Resumen: Actualmente no hay lineamientos políticos para el tratamiento de híbridos bajo el Acta de Especies en Peligro. Consideramos las bases científicas para determinar si las poblaciones híbridas deben ser incluidas como parte de la unidad de Oncorhynchus clarki lewisi considerada para ser enlistada en el Acta de Especies en Peligro. Oncorhynchus clarki lewisi está amenazada de extinction genómica debido a hibridación introgresiva generalizada con la trucha arco iris introducida (O. mykiss) y O. c. bouvieri. Resultados experimentales sugieren que los híbridos de primera generación entre O. clarki lewisi y O. mykiss tienen adaptabilidad reducida. Sin embargo, la hibridación puede extenderse aun cuando los híbridos tienen adaptabilidad severamente reducida, porque la producción de híbridos es unidireccional (esto es, toda la progenie de un híbrido serán híbridos). Adicionalmente,...

Rapid spread of invasive genes into a threatened native species

“…Other studies of hybridization between native and introduced species have documented replacement of native gene pools by admixed or even predominantly introduced genotypes (4,12,(37)(38)(39)(40). Demographic imbalances between domestic and wild populations might create asymmetrical gene flow and deterministic spread of selectively neutral or even deleterious alleles (41).…”

“…Hybrid invasions can be difficult to detect without molecular data, but their impacts can be severe, including both conventional ecological effects and effects that depend specifically on genetic mixture (5,6). These uniquely genetic issues include (i) the question of whether introgression of introduced alleles should be considered a minor evolutionary change (7)(8)(9) or a "genomic extinction" (4,10), (ii) the problem of how to define the legal status of introgressed populations (11)(12)(13), and (iii) the possibility that recombinant genotypes might express novel phenotypes with novel ecological consequences (14,15).…”

Genetic monitoring as a promising tool for conservation and management

“…Genetic monitoring programs have already been initiated for several species; for example, native westslope cutthroat trout Oncorhynchus clarki lewisi in the Rocky Mountains are threatened with genomic extinction owing to widespread hybridization with introduced rainbow trout O. mykiss [27,28]. Diagnostic molecular assays applied to trout from the Flathead River system of Northwest Montana have demonstrated an increase in the proportion of streams with westslope cutthroat-rainbow trout hybrids between 1984 and 2001 [27].…”

Assignment methods: matching biological questions with appropriate techniques

“…Identification of hybrid individuals is often a necessary first step in the implementation of management strategies, such as breeding or translocation programs for threatened species, [42,43], and the standard AT has been applied to this problem [44]. A hypothetical hybrid taxon is created by combining randomly sampled alleles from the two parental taxa, and each individual is then assigned to one of three potential sources (two parental taxa and the hybrid taxon).…”