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) was c ompared in 12 mammalian species, representing as many eutherian orders . This sequence encodes the N-terminal half of this kidney-specific wa ter channel protein. Most amino acid replacements, as well as an inser tion, have occurred in extracellular loops connecting the transmembran e helices, in agreement with a lower functional importance of these lo ops. Phylogenetic analyses were performed with parsimony, distance, an d maximum-likelihood methods. The AQP2 data set, alone as well as in c ombination with previously published alpha A-crystallin protein sequen ces, strongly supports a clade consisting of elephant, hyrax, aardvark , and elephant shrew, reaching bootstrap values of 99%. This finding f ully agrees with the only other presently available sequence data sets that include these taxa, those of von Willebrand factor and interphot oreceptor retinoid-binding protein, and suggests that this extended pa enungulate clade is one of the most conspicuous superordinal groupings in eutherian phylogeny. Some support was obtained for an artiodactyl/ perissodactyl clade, while the grouping of pholidotes with edentates w as contradicted.