T-box and homeobox genes from the ctenophore Pleurobrachia pileus: Comparison of Brachyury, Tbx2/3 and Tlx in basal metazoans and bilaterians

Abstract

Most animals are classified as Bilateria and only four phyla are still extant as outgroups, namely Porifera, Placozoa, Cnidaria and Ctenophora. These non-bilaterians were not considered to have a mesoderm and hence mesoderm-specific genes. However, the T-box gene Brachyury could be isolated from sponges, placozoans and cnidarians. Here, we describe the first Brachyury and a Tbx2/3 homologue from a ctenophore. In addition, analysing T-box and homeobox genes under comparable conditions in all four basal phyla lead to the discovery of novel T-box genes in sponges and cnidarians and a Tlx homeobox gene in the ctenophore Pleurobrachia pileus . The conservation of the T-box and the homeobox genes suggest that distinct subfamilies with different roles in bilaterians were already split in non-bilaterians.