| Summary: | Osmoregulation has played an important role in the evolution, dispersal, and diversification of vertebrates. Using an approach that integrates genetics, genomics and transcriptomics, I characterized the genetic architecture of salinity tolerance capacity in salmonid fishes. Genome-scans and Quantitative Trait Loci (QTL) analyses revealed that variation in salinity tolerance capacity has a genetic basis in Arctic charr (Salvelinus alpinus) and Atlantic salmon (Salmo salar). The detection of QTL in homologous chromosomal regions indicates that salinity tolerance in salmonids is controlled by some of the same loci. Homeologous QTL suggest that regions duplicated in the salmonid ancestor contribute to the hypo-osmoregulation of modern species. In addition, genetic variation at candidate gene loci from the seawater mitochondrion-rich cell complex may affect salinity tolerance capacity for multiple species. Comparisons between species revealed that Atlantic salmon contain unique candidate gene combinations produced by chromosome arm fusions in their ancestor. I hypothesize that the superior salinity tolerance capacity of Atlantic salmon could be related to these clusters. Comparisons of gene expression profiles in gill from Arctic charr exhibiting divergent salinity tolerance QTL genotypes revealed that intraspecific variation in salinity tolerance capacity is correlated with differential expression of immune response genes. Some genes formed clusters along salmonid linkage groups that are conserved on chromosomes in other teleost species. I postulate that such clusters were preserved via purifying selection. Co-localization of QTL with differentially expressed genes suggests that polymorphisms in cis-regulatory elements comprised a majority of QTL. Expression profiles between two Arctic charr families exposed to freshwater and seawater revealed that several genes exhibited striking congruence in direction and magnitude of expression, suggesting they are tightly regulated and possibly integral for hypo-osmoregulation. ...
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