mgu FY2015 Annual Report 3.1.1 fig1

Figure 1. Conservation of a pharyngeal gene cluster across deuterostomes. a, Linkage and order of six genes including the four genes encoding transcription factors Nkx2.1, Nkx2.2, Pax1/9 and FoxA, and two genes encoding non-transcription factors Slc25A21 (solute transporter) and Mipol1 (mirror-image polydactyly 1 protein), which are putative ‘bystander’ genes containing regulatory elements of pax1/9 and foxA, respectively. The pairings of slc25A21 with pax1/9 and of mipol1 with foxA occur also in protostomes, indicating bilaterian ancestry. The cluster is not present in protostomes such as Lottia (Lophotrochozoa), Drosophila melanogaster, Caenorhabditis elegans (Ecdysozoa), or in the cnidarian, Nematostella. SLC25A6 (the slc25A21 paralogue on human chromosome 20) is a potential pseudogene. The dots marking A2 and A4 indicate two conserved non-coding sequences first recognized in vertebrates and amphioxus³⁶, also present in S. kowalevskii and, partially, in P. flava and A. planci. b, The four transcription factor genes of the cluster are expressed in the pharyngeal/foregut endoderm of the Saccoglossus juvenile: nkx2.1 is expressed in a band of endoderm at the level of the forming gill pore, especially ventral and posterior to it (arrow), and in a separate ectodermal domain in the proboscis. It is also known as thyroid transcription factor 1 due to its expression in the pharyngeal thyroid rudiment in vertebrates. The nkx2.2 gene is expressed in pharyngeal endoderm just ventral to the forming gill pore, shown in side view (arrow indicates gill pore) and ventral view; and pax1/9 is expressed in the gill pore rudiment itself. In S. kowalevskii, this is its only expression domain, whereas in vertebrates it is also expressed in axial mesoderm. The foxA gene is expressed widely in endoderm but is repressed at the site of gill pore formation (arrow). An external view of gill pores is shown; up to 100 bilateral pairs are present in adults, indicative of the large size of the pharynx.