This notion nevertheless contrasts with experimental studies showing that some aquaporins from different insect species can transport glycerol 14, 15, 16, 17, 18, 19. While maximum glycerol accumulation and cold tolerance predominantly occurs in the pupal stage 8, 9, 10, recent phylogenetic studies of insect aquaporins have suggested that members of the aquaporin gene superfamily known as aquaglyceroporin (Glp) channels, which typically transport water and glycerol, are absent in holometabolan insects 11, 12, 13. As in other insects, holometabolan species can accumulate high levels of colligative polyols, such as glycerol or sorbitol, as an adaptive response to dessication and freezing temperatures 5, 6, 7. In terms of numbers of species, the holometabolan insects (Holometabola or superorder Endopterygota) that undergo complete metamorphosis mediated by a unique pupal stage are the most successful clade of organisms in the history of life, with current estimates accounting for more than half of the world’s eukaryotic biodiversity 4.
Whether real world examples of such functional co-option and gene replacement have occurred through natural selection remains to be established. The potential for such a rewired evolutionary pathway has been observed in engineered immotile strains of bacteria that can recover their lost flagella in just a few mutational steps 3. Due to the modular and interrelated nature of multigene families, however, an alternative non-linear pathway of gene evolution could occur if a chance mutation in a duplicated gene lead to the gain-of-function of a distantly related member of the superfamily. Integration of phanerozoic climate models suggests that these events were associated with the emergence of complete metamorphosis and the unparalleled radiation of insects.Įstablished models suggest that evolutionary novelty typically arises from gene duplication followed by gradual neofunctionalization or subfunctionalization 1, 2. To recapitulate this evolutionary process, we generate 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 a glycerol transporter at the root of the crown clade of hexapod insects. By combining phylogenetic and functional studies, here we show that a more efficient form of glycerol transporter related to the water-selective channel AQP4 specifically evolved and multiplied in the insect lineage, resulting in the replacement of the ancestral branch of aquaglyceroporins in holometabolan insects. It thus remains unknown how glycerol transport evolved in the Holometabola.
In holometabolan insects however, aquaglyceroporins are absent, yet several species possess polyol permeable aquaporins. Transmembrane glycerol transport is typically facilitated by aquaglyceroporins in Prokaryota and Eukaryota.
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