Concordant developmental expression profiles of orthologues in highly divergent Bilateria

Bilateria are the predominant clade of animals on earth. Despite having evolved a large variety of body-plans and developmental modes, they are characterized by common morphological traits. However, it is not clear if clade-specific genes can be linked to these traits, distinguishing bilaterians from non-bilaterians, with their less complex body morphology. Comparing proteomes of bilaterian and non-bilaterian species in an elaborate computational pipeline we aimed to find and define a set of of bilaterian-specific genes. Finding no high-confidence set of such genes, we nevertheless detected an evolutionary signal possibly uniting the highly diverse bilaterian taxa. Using a novel multi-species GO-enrichment method, we determined the functional repertoire of genes that are widely conserved among Bilateria. We found that these genes contribute to morphogenesis, neuronal-system and muscle development, processes that have been described as different between bilaterians and non-bilaterians. Analyzing gene expression profiles in three very distantly related bilaterina species, we find characteristic peaks at comparable stages of development and a delayed onset of expression in embryos. In particular, the expression of the conserved genes appears to peak at the phylotypic stage of different bilaterian phyla. In summary, our data underpin the orthologue conjecture and illustrate how development connects distantly related Bilateria after millions of years of divergence, pointing to processes potentially separating them from non-bilaterians.