Choanoflagellates are the closest known relatives of metazoans. To discover potential molecular mechanisms underlying the evolution of metazoan multicellularity, we sequenced and analysed the genome of the unicellular choanoflagellate Monosiga brevicollis. The genome contains approximately 9,200 intron-rich genes, including a number that encode cell adhesion and signalling protein domains that are otherwise restricted to metazoans. Here we show that the physical linkages among protein domains often differ between M. brevicollis and metazoans, suggesting that abundant domain shuffling followed the separation of the choanoflagellate and metazoan lineages. The completion of the M. brevicollis genome allows us to reconstruct with increasing resolution the genomic changes that accompanied the origin of metazoans. The genome sequence of the marine choanoflagellate Monosiga brevicollis has now been determined. Choanoflagellates are a mainly sessile group of protozoa resembling the 'feeding cells' of sponges, and are considered to be the closest living unicellular relatives of multicellular animals. Comparison of the M. brevicollis sequence with metazoan genomes suggests that the last unicellular ancestor of animals had intron-rich genes, some encoding protein domains characteristically associated with cell adhesion and the extracellular matrix in animals. This organism is strictly unicellular, but other choanoflagellates form colonies and may provide clues as to the origin of cell signalling and other systems in early metazoans.

The genome of the choanoflagellate Monosiga brevicollis and the origin of metazoans