Abstract
Freshwater dreissenid mussels evolved from marine ancestors during the Miocene ∼30 million years ago and today include some of the most successful and destructive invasive species of freshwater environments. Here, we sequenced the genome of the quagga mussel Dreissena rostriformis to identify adaptations involved in embryonic osmoregulation. We provide evidence that a lophotrochozoan-specific aquaporin water channel, a vacuolar ATPase subunit and a sodium/hydrogen exchanger are involved in osmoregulation throughout early cleavage, during which time large intercellular fluid-filled 'cleavage cavities' repeatedly form, coalesce and collapse, expelling excess water to the exterior. Independent expansions of aquaporins coinciding with at least five freshwater colonization events confirm their role in freshwater adaptation. Repeated aquaporin expansions and the evolution of membrane-bound fluid-filled osmoregulatory structures in diverse freshwater taxa point to a fundamental principle guiding the evolution of freshwater tolerance and provide a framework for future species control efforts.