Evolutionary developmental biology: Ghost locus appears.

Abstract

The sequences of two sponge genomes provide evidence that the ParaHox developmental genes are older than previously thought. This has implications for animal taxonomy and for developmental and evolutionary biology. See Letter p.620 The simple body plan of sponges belies the complexity of their genomes, which contain a variety of transcription factors involved in body patterning in more complex creatures, such as Antennapedia class genes that in bilaterians (animals with distinct anterior and posterior ends) include Hox, ParaHox and NK. The genome of the demosponge Amphimedon queenslandica contains a tight cluster of NK genes, but no Hox or ParaHox genes. This led to the hypothesis that Hox and ParaHox genes evolved after sponges diverged from the lineage leading to cnidaria (jellyfishes, sea anemones and others) and bilateria. Study of the overall structure of the sponge genome suggested, however, that Hox and ParaHox might once have been present and later lost — the 'ghost locus' hypothesis. Maja Adamska and colleagues have studied the genome of two calcisponges, a distinct lineage from demosponges, and show that whereas the NK genes are not clustered, there is at least one ParaHox gene (Cdx). This adds weight to the ghost locus hypothesis and highlights the need to examine the genomes of many different sponge lineages in order to obtain a more complete picture of the ancestral animal complement of Antennapedia genes.