The role of genetic structure in the adaptive divergence of populations experiencing saltwater intrusion due to relative sea‐level rise

Abstract

Saltwater intrusion into estuaries creates stressful conditions for nektonic species. Previous studies have shown that Gambusia affinis populations with exposure to saline environments develop genetic adaptations for increased survival during salinity stress. Here, we evaluate the genetic structure of G. affinis populations, previously shown to have adaptations for increased salinity tolerance, and determine the impact of selection and gene flow on structure of these populations. We found that gene flow was higher between populations experiencing different salinity regimes within an estuary than between similar marsh types in different estuaries, suggesting the development of saline-tolerant phenotypes due to local adaptation. There was limited evidence of genetic structure along a salinity gradient, and only some of the genetic variation among sites was correlated with salinity. Our results suggest limited structure, combined with selection to saltwater intrusion, results in phenotypic divergence in spite of a lack of physical barriers to gene flow.

Genetic population structure in prickly sculpin (Cottus asper) reflects isolation-by-environment between two life-history ecotypes

“…Selective forces promoting genetic differentiation among tributaries and nearby estuarine/main channel sites could include selection against migrants and hybrids, differences in reproductive timing, and assortative mating (Räsänen & Hendry, ), as well as larval retention behaviour within the tributaries. Although we currently do not know the degree of spawning habitat overlap among ecotypes or the exact selective regimes favouring divergence among habitat types, differences in salinity tolerance and osmoregulation abilities among estuarine and freshwater tributary populations of C. asper have been shown before (Bohn & Hoar, ), suggesting a potential selective role in adaptive divergence similar to other fish species that are distributed across salinity gradients (Whitehead et al ., , ; Purcell et al ., ). Our results are in line with recent reviews suggesting that IBE is common and often may contribute more to genetic differentiation than IBD (Shafer & Wolf, ; Sexton, Hangartner & Hoffmann, ).…”

Salinity and hydrological barriers have little influence on genetic structure of the mosquitofish in a coastal landscape shaped by climate change

“…Coastal marsh provides good connectivity of habitats for small fishes such as G. affinis, but large bodies of open water may inhibit movement for marsh residents that are largely confined to shallow vegetated habitats (Hitch et al, 2011). Purcell et al (2012) found the evidence of high level of gene flow across the salinity gradient in the populations within coastal basin; however, there was considerably more genetic differentiation observed in fish from two marsh basins in Louisiana. The genetic divergence between the central and western portions of Louisiana coast was suspected due to Atchafalaya River restricting the gene flow (Purcell et al, 2012), but limited sampling over large spatial scales created the possibility that the observed differentiation was due to distance alone.…”

“…Similarly, the role of the coastal landscape in influencing the connectivity among the populations is not clear. For example, the effect of separation of habitats by fresh and saline water barriers has not been adequately evaluated, and it is unclear whether breaks formed by open water represent barriers for marsh-dwelling organisms (Purcell et al, 2012). Yet the presence of such potential barriers might influence adaptations to local conditions (Dobzhansky & Queal, 1938;Taylor, 1991;Fournier-Level et al, 2011).…”

“…This hypothesis would be rejected if there were differences in genetic diversity among populations or if measures of differentiation among populations were significantly greater than zero. Failure to reject this hypothesis would suggest populations are panmictic and selection would have to be quite strong to produce local adaptations to salinity (Purcell et al, 2012).…”

“…This hypothesis would be rejected if genetic differentiation was greater between populations separated by major rivers than those in more contiguous coastal marshes. Because mosquitofish are dependent on shallow, vegetated environments, major rivers have been hypothesized to reduce gene flow (Purcell et al, 2012).…”