The Trichoplax genome and the nature of placozoans

Placozoans are arguably the simplest free-living animals, possibly evoking an early stage in metazoan evolution, yet their biology is poorly understood. Here we report the sequencing and analysis of the {approx}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 exhibits conserved gene content, gene structure, and synteny relative 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 signaling 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.