The quagga mussel genome and the evolution of freshwater tolerance

European freshwater dreissenid mussels evolved from marine ancestors during the Miocene approximately 30 million years ago and today include some of the most successful and destructive invasive invertebrate species of temperate freshwater environments. Here we sequenced the genome of the quagga mussel Dreissena rostriformis to identify evolutionary adaptations involved in embryonic osmoregulation. We found high gene expression levels of a novel subfamily of lophotrochozoan-specific aquaporin water channel, a vacuolar ATPase and a sodium/hydrogen exchanger during early cleavage, a period defined by the formation of inter-cellular fluid-filled ‘cleavage cavities’. Independent expansions of the lophotrochoaquaporin clade that coincide with at least five independent colonisation events of freshwater environments confirm their central role in freshwater adaptation. The pattern of repeated aquaporin expansion and the evolution of membrane-bound fluid-filled osmoregulatory structures in diverse taxa points to a fundamental principle guiding the evolution of freshwater tolerance that may provide a framework for future efforts towards invasive species control.