The molecular evolution of arthropod aquaporins

Evolutionary studies of water channel proteins (aquaporins) have indicated that functionally distinct branches of water-selective (Aqp) and glycerol-transporting channels (aquaglyceroporins, Glp) arose deep in the prokaryotic domains of life. Within the hexapod lineage, however, phylogenetic and syntenic analyses suggest that Glps are absent in the megadiverse holometabolan insects (Endopterygota), yet several reports have shown that some aquaporins from this superorder can indeed transport glycerol. To resolve this paradox, we screened >130 arthropod genomes and phylogenetically reconstructed the available aquaporin superfamilies in Chelicerata, Myriapoda, Crustacea and Hexapoda. Our data reveal that arthropod genomes encode three major grades of aquaporins (1) classical type aquaporins including Big brain, Drip and Prip-like channels (2) aquaglyceroporins and (3) unorthodox aquaporins. Based upon functional studies, however, we found that a more effective form of glycerol transporter related to the human water-selective AQP4 channel specifically evolved and expanded in Hexapoda, resulting in the replacement of the ancestral branch of Glps in holometabolan insects. We termed these channels Entomoglyceroporins (Eglps). To recapitulate the evolutionary process, we generated specific mutants in distantly related insect aquaporins and human AQP4 and show that a single mutation in the selectivity filter converted a water-selective channel into an Eglp at the root of the crown clade of Hexapoda. Such an Aqp to Glp transition has not previously been observed in extant biota, despite billions of years of evolution. Integration of phanerozoic climate models suggests the positve selection and expansion of Eglps over Glps was associated with the emergence of complete metamorphosis and the unparalleled radiation of insects.