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

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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.