A phylogenomic investigation into the origin of metazoa

The evolution of multicellular animals (Metazoa) from theirunicellular ancestors was a key transition that was accompaniedby the emergence and diversification of gene families associatedwith multicellularity. To clarify the timing and order of specificevents in this transition, we conducted expressed sequence tagsurveys on 4 putative protistan relatives of Metazoa includingthe choanoflagellate Monosiga ovata, the ichthyosporeans Sphaeroformaarctica and Amoebidium parasiticum, and the amoeba Capsasporaowczarzaki, and 2 members of Amoebozoa, Acanthamoeba castellaniiand Mastigamoeba balamuthi. We find that homologs of genes involvedin metazoan multicellularity exist in several of these unicellularorganisms, including 1 encoding a membrane-associated guanylatekinase with an inverted arrangement of protein-protein interactiondomains (MAGI) in Capsaspora. In Metazoa, MAGI regulates tightjunctions involved in cell-cell communication. By phylogenomicanalyses of genes encoded in nuclear and mitochondrial genomes,we show that the choanoflagellates are the closest relativesof the Metazoa, followed by the Capsaspora and Ichthyosporealineages, although the branching order between the latter 2groups remains unclear. Understanding the function of "metazoan-specific"proteins we have identified in these protists will clarify theevolutionary steps that led to the emergence of the Metazoa.