Revealing developmental networks by comparative transcriptomics.

Metazoan development relies upon the precise control of the genome's expression. This enables different cells in the animal to have different properties, despite having the same genetic material, and different animals to have different morphologies despite sharing developmental genes. However, near-identical organisms may have different overall genomic content, suggesting that the mechanisms by which evolution of the phenotype proceeds on a global level are not well understood. We review here recent works that have discovered a tremendous amount of variation between the developmental transcriptomes of both closely and distantly related organisms. It is evident that the evolution of regulatory programs occurs at a rapid rate comparable to that of other genomic processes. Distinguishing the selective pressures on each regulatory element will thus be crucial towards understanding its functional relevance. We propose that such a comparative approach is most suited to the identification of unifying principles in cell fate specification and differentiation in the animal embryo.