Going, going, gone: evidence for loss of an endemic species pair of threespine sticklebacks (Gasterosteus aculeatus) with implications for protection under species-at-risk legislation
Genomic extinction occurs when the unique combination of genetic traits that characterize distinct phenotypes are eliminated by introgressive hybridization even if population size is greater than zero. Benthic and limnetic threespine sticklebacks (Gasterosteus aculeatus) constitute reproductively isolated undescribed biological species that have evolved independently in several lakes in southwestern British Columbia, Canada (known as “species pairs” in each lake). Here we investigated whether the two species that comprise the pair from Enos Lake, southeastern Vancouver Island, remain as two distinct gene pools. Multi-season samples (>1200 fish) obtained over two years from throughout the lake and assayed for variation in morphological traits characteristic of the two species (i.e., body depth, dorsal spine count, gill raker counts) and at 12 microsatellite DNA loci consistently indicated the existence of only a single group of sticklebacks. There was no consistent evidence of two groups in any morphological trait, and mean gill raker counts were consistently intermediate (20–21) to those of known benthics (~18) and limnetics (~24) which together comprised strikingly bimodal distributions in historical samples. Genetic analyses employing model-based clustering also consistently indicated the presence of only a single genetic group of sticklebacks. Compared to historical samples and to benthics and limnetics from other lakes, no Enos Lake fish could be identified confidently as a pure benthic or limnetic. Our results provide the strongest evidence yet that the Enos Lake sticklebacks now consist of a single morphological and genetic population of sticklebacks, that the unique combination of genetic and morphological traits that characterized benthic and limnetic sticklebacks no longer exist, and that their current status under Canada’s Species-at Risk Act as Endangered should be re-evaluated.
KeywordsGenomic extinction Microsatellites Morphology Threespine sticklebacks Species-at-Risk Act Species pairs
The cooperation of the BC Ministry of Environment, Fisheries and Oceans Canada (Pacific Region), and Dave Scott of Fairwinds Real Estate Management, Inc. is greatly appreciated. We thank Elias Elhaimer for assistance with morphological measurements and Sara Miller and Dolph Schluter for provision of samples from Murdo Fraser Pond. The manuscript benefitted from helpful commentary by two anonymous reviewers and C. Garza.
Funding for this project was provided by the Natural Sciences and Engineering Research Council of Canada (Discovery and Equipment Grant Programs) and the BC Ministry of Environment in the form of grants awarded to EBT.
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