Are differences in claudin 10c isoform structure and expression associated with variation in salinity tolerance among species of killifish?

Abstract

To survive, fish must maintain ionoregulatory and osmoregulatory homeostasis during changes in environmental salinity. In hypersaline water, fish must actively excrete salts and drink water to counterbalance the passive influx of salts and efflux of water. Excess sodium ions are excreted through paracellular pathways, and the claudin proteins are core tight junction components that influence paracellular ion permeabilities. It is predicted that claudin isoform switching in osmoregulatory epithelia is critical for fish salinity acclimation. However, it is not known if the evolution of claudin structure or expression leads to differences in salinity tolerance among species of fishes. To study this question, we attempted to compare claudin isoform structure and expression in the opercular epithelia of killifishes. The Common Killifish (Fundulus heteroclitus) can tolerate salinities far exceeding that of full strength seawater and expresses a unique subset of claudin 10 isoforms in hypersaline conditions. The Common Killifish interbreeds with sympatric Banded Killifish (Fundulus diaphanus), a species that is less tolerant to high salinities, forming viable hybrid offspring, which are also less salinity tolerant. In this thesis, I aimed to test if hyper-salinity tolerance is associated with changes in claudin 10c isoform structure or expression in the operculum. In studying the expression of claudin 10c isoforms from opercular tissue, I found low concentrations of RNA, hindering my ability to study claudin expression. I was able to use gill mRNA to test for sequence variation in claudin 10c among species, and found no mutations in amino acids at functionally important sites between the Common and Banded Killifish. These results suggest that inter-specific differences in claudin 10c permeability are not the cause of differences observed in salinity tolerance between the Common Killifish and Banded Killifish.