Osmoregulation in freshwaters: Gene expression in the gills of a Neotropical cichlid in contrasting pH and ionic environments

Abstract

22 24 25 Abstract 26 27 Freshwater habitats of the Neotropics exhibit a gradient from relatively neutral, ion-rich 28 whitewater to acidic, ion-poor blackwater. Closely related species often show 29 complementary distributions among ionic habitats, suggesting that adaptation to divergent 30 osmoregulatory environments may be an important driver of Neotropical fish diversity. 31 However, little is known about the evolutionary tradeoffs involved in ionoregulation across 32 distinct freshwater environments. Here, we surveyed gill mRNA expression of Cichla 33 ocellaris var. monoculus, a Neotropical cichlid, to examine cellular and physiological 34 responses to experimental conditions mimicking whitewater and blackwater.35 Gene ontology enrichment of expressed genes indicated that the gills were remodeled 36 during both forms of environmental challenge, with changes biased towards the cellular 37 membrane. We observed expression of signaling pathways from both the acute and 38 extended response phases, including evidence that growth hormone (GH) may mediate 39 osmoregulation in whitewater through paracrine expression of insulin-like growth factor I 40 (IGF-I), but not through the GH receptor, which instead showed correlated up-regulation 41 with the prolactin receptor and insulin-like growth factor II in blackwater.42 Differential expression of genes related to paracellular tight junctions and transcellular ion 43 transport showed responses similar to euryhaline fishes in fresh versus seawater, with 44 some exceptions, suggesting that relaxed ion retention via the gills, possibly mediated by 45 the GH/IGF-I axis, is a strong candidate for evolutionary modification in whitewater and 46 blackwater endemic populations. In each osmoregulatory domain, we saw examples of 3 47 contrasting differential expression of paralogs of genes that are single copy in most 48 terrestrial vertebrates, indicating that adaption by fishes to diverse physicochemcial 49 environments has capitalized on diversification of osmoregulatory gene families. 50 51 53 54 Running title: gene expression across an Amazon ionic gradient 4 55 56 Introduction 57 58Gill surfaces of fishes must be thin in order to facilitate gas exchange, but this also 59 promotes rapid gain or loss of ions and water with the environment -a functional tradeoff 60 called the 'osmo-respiratory compromise' [1]. Management of osmotic and ionic stress is 61 therefore a major biological challenge for fishes and has been estimated to account for 2-62 20% of resting metabolic rate [2]. A great deal has been learned about mechanisms 63 involved in osmoregulation from studies of euryhaline fishes exposed to salinity gradients 64 [3]; however, these fishes represent a small fraction of fish diversity, as the vast majority 65 are stenohaline. Among these, freshwater fishes are challenged to maintain homeostasis in 66 the face of wide variation in solute concentration, pH, and other chemical factors, but are 67 less well-studied than their euryhaline counterparts [4]. Moreover, it is clear...