Evolutionary variation in salinity tolerance among species of killifishes (Fundulus spp.)

Abstract

Little is known about how salinity tolerance evolves in fishes. In this study I measured salinity tolerance, and candidate mechanisms underlying tolerance, in Common Killifish (Fundulus heteroclitus), Banded Killifish (Fundulus diaphanus) and clonal F. diaphanus x F. heteroclitus F1 hybrids. To quantify tolerance, killifish acclimated to near isosmotic water (10 ppt) were transferred to 0 ppt and 32 ppt and acclimated in a stepwise manner to 60 ppt. All fish could maintain homeostasis at 0 – 32 ppt, but F. heteroclitus have an increased tolerance to 60 ppt compared to F. diaphanus and F1 hybrids, which displayed an intermediate salinity tolerance compared to that of the parental species. I also found that F. heteroclitus upregulated candidate hypersaline saltwater tolerance genes (cftr, claudin-10c and claudin-10f) to a greater extent than the two less tolerant groups, suggesting that increased tolerance in F. heteroclitus is associated with their unique ability to upregulate these genes.