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 is 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.