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Mechanisms of ion regulation in the euryhaline killifish Fundulus heteroclitus : molecular and evolutionary perspectives Scott, Graham R.

Abstract

A comprehensive approach was employed to study various aspects of ionoregulatory physiology in the euryhaline killifish Fundulus heteroclitus. I first characterized the changes in gene expression in the gills of individuals from a northern population of killifish after abrupt transfer from near-isosmotic brackish water to either freshwater or seawater. Many changes in response to seawater transfer were transient: increased mRNA expression occurred 1 day after transfer for Na⁺,K⁺-ATPase α[sub 1a] (3- fold), Na⁺,K⁺,2Cl⁻-cotransporter 1 (NKCC1) (3-fold), and glucocorticoid receptor (1.3- fold). In contrast, expression of the cystic fibrosis transmembrane conductance regulator (CFTR) Cl⁻ channel generally remained elevated (2-fold) in seawater. The responses to freshwater transfer were quite different than to seawater transfer. In particular, freshwater transfer caused a greater increase in Na⁺,K⁺-ATPase α[sub 1a] mRNA expression, but had no effect on V-type H⁺-ATPase expression. I also examined the ionoregulatory physiology of both northern and southern killifish populations after freshwater transfer to understand possible mechanisms of freshwater adaptation. Pronounced differences in freshwater tolerance existed between northern (2% mortality) and southern (19% mortality) killifish populations. Differences in Na+ regulation between each population were small and likely cannot account for this large difference in mortality, as plasma Na⁺,Na⁺,K⁺-ATPase mRNA expression and activity in the gills, and Na⁺ flux rates were all similar between populations after freshwater transfer. Large differences in Cl⁻ regulation existed between populations and likely contributed to the marked differences in mortality after freshwater transfer. Plasma Cl⁻ decreased rapidly and remained low following freshwater transfer in southerns, but not in northerns, corresponding to a higher rate of Cl⁻ loss from southerns after transfer. Elevated Cl⁻ loss from southern fish in freshwater was possibly due to a persistence of seawater gill morphology, as paracellular permeability and apical crypt density in the gills were both higher than in northern fish. Taken together, my data demonstrate that killifish use an array of regulatory strategies, including gene expression, to modulate ion transport in response to changing environmental salinity. Salinity tolerance is not uniform within the species, however, as intraspecific variation exists suggesting that northerns are better adapted to freshwater.

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