Origins and functional diversification of salinity-responsive Na+, K+ ATPase α1 paralogs in salmonids

The Salmoniform whole-genome duplication is hypothesized to have facilitated the evolution of anadromy, but little is known about the contribution of paralogs from this event to the physiological performance traits required for anadromy, such as salinity tolerance. Here, we determined when two candidate, salinity-responsive paralogs of the Na+, K+ ATPase a subunit (alpha 1a and alpha 1b) evolved and studied their evolutionary trajectories and tissue-specific expression patterns. We found that these paralogs arose during a small-scale duplication event prior to the Salmoniform, but after the teleost, whole-genome duplication. The 'freshwater paralog' (alpha 1a) is primarily expressed in the gills of Salmoniformes and an unduplicated freshwater sister species (Esox lucius) and experienced positive selection in the freshwater ancestor of Salmoniformes and Esociformes. Contrary to our predictions, the 'saltwater paralog' (alpha 1b), which is more widely expressed than alpha 1a, did not experience positive selection during the evolution of anadromy in the Coregoninae and Salmonine. To determine whether parallel mutations in Na+, K+ ATPase alpha 1 may contribute to salinity tolerance in other fishes, we studied independently evolved salinity-responsive Na+, K+ ATPase alpha 1 paralogs in Anabas testudineus and Oreochromis mossambicus. We found that a quarter of the mutations occurring between salmonid alpha 1a and alpha 1b in functionally important sites also evolved in parallel in at least one of these species. Together, these data argue that paralogs contributing to salinity tolerance evolved prior to the Salmoniform whole-genome duplication and that strong selection and/or functional constraints have led to parallel evolution in salinity-responsive Na+, K+ ATPase alpha 1 paralogs in fishes.