Populations of a cyprinid fish are self-sustaining despite widespread feminization of males

Abstract

BackgroundTreated effluents from wastewater treatment works can comprise a large proportion of the flow of rivers in the developed world. Exposure to these effluents, or the steroidal estrogens they contain, feminizes wild male fish and can reduce their reproductive fitness. Long-term experimental exposures have resulted in skewed sex ratios, reproductive failures in breeding colonies, and population collapse. This suggests that environmental estrogens could threaten the sustainability of wild fish populations.ResultsHere we tested this hypothesis by examining population genetic structures and effective population sizes (Ne) of wild roach (Rutilus rutilus L.) living in English rivers contaminated with estrogenic effluents. Ne was estimated from DNA microsatellite genotypes using approximate Bayesian computation and sibling assignment methods. We found no significant negative correlation between Ne and the predicted estrogen exposure at 28 sample sites. Furthermore, examination of the population genetic structure of roach in the region showed that some populations have been confined to stretches of river with a high proportion of estrogenic effluent for multiple generations and have survived, apparently without reliance on immigration of fish from less polluted sites.ConclusionsThese results demonstrate that roach populations living in some effluent-contaminated river stretches, where feminization is widespread, are self-sustaining. Although we found no evidence to suggest that exposure to estrogenic effluents is a significant driving factor in determining the size of roach breeding populations, a reduction in Ne of up to 65% is still possible for the most contaminated sites because of the wide confidence intervals associated with the statistical model.

Isolation by distance and non-identical patterns of gene flow within two river populations of the freshwater fish Rutilus rutilus (L. 1758)

“…2) and are consonant with estimates observed in this species in the UK, exhibiting significant overlap between mean and variances in heterozygosity and allelic richness across all surveyed Thames sub-populations (Hamilton et al 2014). Concordance of diversities is also to be found in European roach (Demandt 2010) and in European coarse fish generally (e.g., Dehais et al 2010).…”

“…Hierarchical structuring across all sub-populations from both rivers is low but significant, although most of the apportioning of genetic variance was found to be among and within individuals (F IS = 0.144, 14 % of the variation, p = 0.001; F IT = 0.186, 81 % of the variation, p = 0.001). The degree of genetic diversity apportioned between sub-populations within rivers and between rivers accounts for 5 % of the entire total variation (F SC = 0.033, 3 % of the variation, p = 0.001; F CT = 0.017, 2 % of the variation) of a similar scale to that previously observed for UK roach (Hamilton et al 2014). …”

“…Furthermore, if there is significant obstruction to migration in the Stour, we would predict strong IBD over short distances but at larger spatial scales the signal would be lost through allelic differences at independent loci accruing through genetic drift (Type IV IBD sensu Hutchinson and Templeton 1999). We also predict an increase in population inbreeding in the Stour relative to that found in the Thames, manifesting in lower effective sub-population sizes, seeking to corroborate the low sub-population effective population sizes (N e s) reported in this species (Hamilton et al 2014). Finally, we seek to observe whether there exists a spike in genetic divergence at a site within the Thames that has a history of large-scale stocking.…”

“…We investigated patterns of genetic structuring and gene flow in a very common sport fish with demonstrably high within-population genetic variability and low levels of intraspecific differentiation (Bouvet et al 1991(Bouvet et al , 1995Baranyi et al 1997;Wolter 1999;Hänfling et al 2004;Demandt 2010;Hamilton et al 2014). Whilst a strong signal of IBD was only detected in the Stour, both rivers possessed idiosyncratic patterns of divergence.…”

“…Its abundance, motility and potential for natal philopatry during spawning (Goldspink 1977;L'Abée-Lund and Vøllestad 1985), thereby exacerbating genetic structuring (Massicotte et al 2008), make it an excellent candidate model species to investigate patterns of gene flow. Whilst Hamilton et al (2014) found evidence for IBD in roach in the Thames, it was only on a system-wide scale; they did not specifically focus on the issue of connectivity within linear stretches of river. By contrast, we investigate the distribution of microsatellite variation along the Rivers Stour and Thames to determine the effects of these river's traits upon levels of IBD and gene flow.…”