Interspecific divergence of ionoregulatory physiology in killifish: Insight into adaptation and speciation

Adaptation to salinity is potentially a critical driving force of speciation in fishes. Here, we tested for differences in ion/osmoregulatory gene expression between two species of killifish Lucania goodei and L.parva that differ in salinity tolerance. Expression patterns of several genes encoding ion transport proteins were quantified for animals taken directly from populations that varied in salinity, as well as animals from a salinity transfer experiment. We found that L.parva, a euryhaline species, expressed higher levels of the genes involved in saltwater ion/osmoregulatory regulation than its stenohaline counterpart L.goodei (Na ⁺/K ⁺-ATPase 1a and 1b, Na ⁺-K ⁺-2Cl ^- cotransporter 1 and glucocorticoid receptor) when exposed to a change in salinity in the laboratory. However, both species expressed similar levels for two of the three genes involved in freshwater osmoregulation (14-3-3a and V-type H ⁺-ATPase). Surprisingly, we found little evidence for differential plasticity between L.parva and L.goodei in our salinity transfer experiment. Lucania parva expressed high levels of genes involved in both freshwater and saltwater ion/osmoregulation, while L.goodei only expressed high levels of genes involved in freshwater osmoregulation. These results indicate that L.parva may increase their transcript levels of osmoregulatory genes when faced with any type of salinity challenge. Thus, changes in ion/osmoregulatory physiology may be occurring post-transcriptionally via differential RNA processing or enzyme activity. These findings provide unique insight into the ion/osmoregulatory physiology that underlies species and population differences in salinity tolerance.