Unexpected complexity of the Aquaporin gene family in the moss Physcomitrella patens

Background: Aquaporins, also called major intrinsic proteins (MIPs), constitute an ancient superfamily of channel proteins that facilitate the transport of water and small solutes across cell membranes. MIPs are found in almost all living organisms and are particularly abundant in plants where they form a divergent group of proteins able to transport a wide selection of substrates. Results: Analyses of the whole genome of Physcomitrella patens resulted in the identification of 23 MIPs, belonging to seven different subfamilies, of which only five have been previously described. Of the newly discovered subfamilies one was only identified in P. patens (Hybrid Intrinsic Protein, HIP) whereas the other was found to be present in a wide variety of dicotyledonous plants and forms a major previously unrecognized MIP subfamily (X Intrinsic Proteins, XIPs). Surprisingly also some specific groups within subfamilies present in Arabidopsis thaliana and Zea mays could be identified in P. patens. Conclusion: Our results suggest an early diversification of MIPs resulting in a large number of subfamilies already in primitive terrestrial plants. During the evolution of higher plants some of these subfamilies were subsequently lost while the remaining subfamilies expanded and in some cases diversified, resulting in the formation of more specialized groups within these subfamilies.