Kamran Shalchian-Tabrizi; Marianne A. Minge; Mari Espelund; Russell Orr; Torgeir Ruden; Kjetill S. Jakobsen; Thomas Cavalier-Smith

“ As in previous studies [24] , [27] many of our gene sequences are incomplete, so in some cases we can only be sure that domains homologous with those in related animal genes are present in Ministeria (for details, see Supporting Table S2) . Nonetheless, there is a remarkably high diversity of domains involved in animal early embryonic development – undoubtedly many more precursors of key animal genes could be found in Ministeria if a full genome were sequenced. ”

“ Of these, nucleariid filose amoebae [7] are probably the closest relatives of fungi [8] , [9] . However, the closest relatives of animals were not previously firmly established. Traditionally, one choanozoan lineage, the choanoflagellates, was regarded as most closely related to animals [4] – [10] , [11] because some of their tentacles (with a rigid internal skeleton of bundled actin as in animal intestinal microvilli) [12] are aggregated as a collar surrounding the cilium ( = flagellum) in the choanocytes of sponges [13] . ”

“ Thus, in order to identify precursors of genes and domains involved morphogenesis of animals, it is necessary to apply several approaches that can detect homologies even if sequences lack animal-specific domains [24] – [25] , [27] . By using a combination of sequence similarity searches (i.e. BLASTx against NCBInr, NCBIest and Gene ontology databases) and structure analyses against Pfam, InterProScan and Conserved Domain Database, we have annotated several sequences from Ministeria and other Choanozoa as signaling and cell adhesion components (Table 1) . ”

“ Abstract Animals are evolutionarily related to fungi and to the predominantly unicellular protozoan phylum Choanozoa, together known as opisthokonts. To establish the sequence of events when animals evolved from unicellular ancestors, and understand those key evolutionary transitions, we need to establish which choanozoans are most closely related to animals and also the evolutionary position of each choanozoan group within the opisthokont phylogenetic tree. ”

“ The Ministeria polypeptide has a structure only containing the Hint domain and the conserved cleavage motive. Thus, the typical hedgehog domain combination of higher animal genes seems to be absent in Choanozoa and sponges. This also implies that hedgehog domains predate animals and choanoflagellates, and that several successive innovations occurred in this gene family before and after the actual origin of these lineages [3] , [17] , [26] , [28] . ”

“ Interpretation of molecular phylogeny, morphological and biochemical features suggest that the vast majority of the eukaryote diversity may belong to only six supergroups for review see ref [1] . One of these supergroups, the opisthokonts, is composed of animals, fungi and several smaller groups of unicellular eukaryotes belonging to the phylum Choanozoa [2] – [3] . Recent molecular phylogenetic evidence indicates that animals and fungi evolved independently from different unicellular protozoan choanozoan ancestors [2] – [3] . ”

“ A recent multigene analysis showed that choanoflagellates are more closely related to animals than are ichthyosporean Choanozoa [14] . Single gene phylogenies of a broader diversity of Choanozoa raised the possibility that the recently discovered minute marine protozoan Ministeria vibrans [5] might be closer still to animals [10] , [15] . ”

“ A fusion between ubiquitin and ribosomal small subunit S30 protein once thought animal-specific [19] is here identified in all included choanozoan lineages, thus unifying the holozoa (filozoa plus Ichthyosporea) [20] . Our present analysis leaves undetermined the position of one key free-living choanozoan lineage (Corallochytrium, whose cell wall hints at a relationship to Ichthyosoporea, though some trees put it nearer choanoflagellates) . ”

“ Other molecular innovations will only become apparent with full genome sequences of each choanozoan lineage, but can eventually be mapped onto this phylogeny. As Fig. 3 indicates, the origin of slender filose projections, which apparently occurred during the origin of opisthokonts when their ancestor diverged from Amoebozoa (which typically have broad pseudopodia) was probably a key enabling innovation for opisthokont evolution. ”

“ Godt, D & Winklbauer, R (2002) , “Cell sorting in animal development: signalling and adhesive mechanisms in the formation of tissue boundaries.”, Curr Opin Genet Dev 12: 572–582, 12200163, pmid:12200163 1 2 3 4 Nichols, SA ”