As arguably the simplest free-living animals, placozoans may represent a primitive metazoan form, yet their biology is poorly understood. Here we report the sequencing and analysis of the ∼98 million base pair nuclear genome of the placozoan Trichoplax adhaerens. Whole-genome phylogenetic analysis suggests that placozoans belong to a ‘eumetazoan’ clade that includes cnidarians and bilaterians, with sponges as the earliest diverging animals. The compact genome shows conserved gene content, gene structure and synteny in relation to the human and other complex eumetazoan genomes. Despite the apparent cellular and organismal simplicity of Trichoplax, its genome encodes a rich array of transcription factor and signalling pathway genes that are typically associated with diverse cell types and developmental processes in eumetazoans, motivating further searches for cryptic cellular complexity and/or as yet unobserved life history stages. Often touted as the 'simplest' free-living animal alive, Trichoplax is central to the debate on early animal origins. It is a disk-shaped placozoan (meaning a 'flat animal') about a millimetre across and was first observed growing on the walls of an aquarium. Now the genome of Trichoplax adhaerens has been sequenced and analysed. The organism retains many features of the last common ancestor with cnidarians and bilaterians, calculated to have existed more than 600 million years ago. But Trichoplax also contains genes for developmental patterns and cell types which have never been seen in this animal, suggesting that there may be stages in its life cycle that have not yet been observed. The genome sequence for Trichoplax adhaerens is analysed, and it is reported that the organism retains many features of the last common ancestor with cnidarians and bilaterians, estimated to be over 600 million years ago. However, T. adhaerens also contains genes for developmental patterns and cell types which have never been seen in this animal, suggesting that we might still not know the full story.

The Trichoplax genome and the nature of placozoans