Molecular evolution of mammalian aquaporin-2: further evidence that elephant shrew and aardvark join the paenungulate clade

A 328-bp sequence from exon 1 of the gene for aquaporin-2 (AQP2) wascompared in 12 mammalian species, representing as many eutherian orders.This sequence encodes the N-terminal half of this kidney- specific waterchannel protein. Most amino acid replacements, as well as an insertion,have occurred in extracellular loops connecting the transmembrane helices,in agreement with a lower functional importance of these loops.Phylogenetic analyses were performed with parsimony, distance, andmaximum-likelihood methods. The AQP2 data set, alone as well as incombination with previously published alpha A-crystallin protein sequences,strongly supports a clade consisting of elephant, hyrax, aardvark, andelephant shrew, reaching bootstrap values of 99%. This finding fully agreeswith the only other presently available sequence data sets that includethese taxa, those of von Willebrand factor and interphotoreceptorretinoid-binding protein, and suggests that this extended paenungulateclade is one of the most conspicuous superordinal groupings in eutherianphylogeny. Some support was obtained for an artiodactyl/perissodactylclade, while the grouping of pholidotes with edentates was contradicted.