Parallel and non-parallel divergence within polymorphic populations of brook stickleback, Culaea inconstans (Actinopterygii: Gasterosteidae)

Studying parallel evolution allows us to draw conclusions about the repeatability of adaptive evolution. Whereas populations likely experience similar selective pressures in similar environments, it is not clear if this will always result in parallel divergence of ecologically relevant traits. Our study investigates the extent of parallelism associated with the evolution of pelvic spine reduction in brook stickleback populations. We find that populations with parallel divergence in pelvic spine morphology do not exhibit parallel divergence in head and body morphology but do exhibit parallel divergence in diet. In addition, we compare these patterns associated with pelvic reduction in brook stickleback to well-studied patterns of divergence between spined and unspined threespine stickleback. Whereas spine reduction is associated with littoral habitats and a benthic diet in threespine stickleback, spine reduction in brook stickleback is associated with a planktonic diet. Hence, we find that pelvic spine divergence is associated with largely non-parallel ecological consequences across species.

Persistence of pelvic spine polymorphism in a panmictic population of brook stickleback (Culaea inconstans) in Alberta, Canada

In the stickleback family (Gasterosteidae), divergence in pelvic morphology is typically associated with reproductive isolation and ecological niche specialization. In this study, we assessed whether reproductive isolation may be involved in the persistence of within-lake polymorphism in brook stickleback (Culaea inconstans (Kirtland, 1840)). We studied populations of brook stickleback in central Alberta, Canada, that are polymorphic for pelvic reduction (i.e., presence–absence of pelvic spines). We investigated the persistence of the pelvic polymorphisms over 40 generations in five lakes. We found that, of the five populations examined, the frequencies of pelvic phenotypes have remained unchanged in three populations, the polymorphism has persisted with significantly changed frequencies of pelvic phenotypes in one population, and the polymorphism has been lost in one population. Using 10 microsatellite loci, we observed low genetic divergence between individuals with and without pelvic spines in one lake, suggesting a lack of assortative mating among individuals with divergent pelvic phenotypes. This observation suggests that the pelvic polymorphism in brook stickleback persists due to balancing selection in the absence of reproductive isolation, although the selective forces were not evaluated. The significant changes in phenotypic frequencies in two populations are likely associated with environmental disturbances such as species introductions.

Effects of habitat and pulp and paper mill contamination on a population of brook stickleback (Culaea inconstans)

This study examined the responses of a population of brook stickleback (Culaea inconstans) exposed to pulp mill effluent at Jackfish Bay, Lake Superior, Canada, in May 2007 and May 2011. Brook stickleback were extirpated from the effluent-receiving site, presumably due to anoxia after this period. Females at the effluent-receiving site had significantly larger gonad sizes in 2007 and 2011. In 2011, effluent-exposed female gonadal development was significantly advanced when compared with reference sites; they were the second most mature when compared among three different reference sites. Analysis of 7-ethoxyresorufin-O-deethylase activity revealed that effluent-receiving site females had greater CYP1A induction in 2007 and significantly greater CYP1A induction in 2011. Effluent-receiving site males showed significantly reduced CYP1A induction in 2007 and significantly greater induction in 2011. Chemical evaluation of sediment from the receiving environment showed elevated levels of resin acids and the polycyclic aromatic hydrocarbon, retene. Higher condition factors and more mature gonads were consistent with higher winter and spring temperatures modified by effluent or by lake vs. stream environments. Overall, effects on effluent-exposed brook stickleback were not consistent with reported effects in white sucker exposed to the same effluent in previous studies. This article has been made Open Access thanks to the kind support of CAWQ/ACQE (https://www.cawq.ca).