Meeting Abstract
Freshwater habitats of the Neotropics exhibit a gradient from neutral, relatively ion-rich whitewater to acidic, ion-poor blackwater. Closely related species often show complementary distributions among ionic habitats, suggesting that adaptation to divergent ionic environments may be an important driver of Neotropical fish diversity. However, little is known about the evolutionary tradeoffs involved in osmoregulation across distinct freshwater environments. First, we surveyed gill mRNA expression of a Neotropical cichlid that inhabits both environments, Cichla ocellaris var. monoculus, in laboratory conditions mimicking whitewater and blackwater. Gene ontology enrichment indicated that the gills were remodeled during both forms of environmental challenge, with changes biased towards the cellular membrane. Differential expression of genes related to paracellular tight junctions and transcellular ion transport showed responses similar to euryhaline fishes in fresh versus seawater. Second, we performed a population genomic survey using ddRAD markers of populations in different habitat types. These data confirmed previous results from microsatellite markers: strong population structure not strictly correlated with habitat type, indicating recent gene flow or co-ancestry across water chemistry in the central Amazon. Finally, using the Nile tilapia genome, we estimated the physical proximity of loci strongly divergent across habitats and differentially expressed genes to identify loci most critical in facilitating osmoregulatory adaptation. We discuss why these approaches may emphasize different genomic regions.
