Ets identified as a trans-regulatory factor of amphioxus Hox2 by transgenic analysis using ascidian embryos

Although the functions of Hox genes in anterior-posterior patterning and their clustered organization are well conserved among metazoans, some Hox genes have lost their original function, as exemplified by zen, ftz and bicoid in Drosophila. The Hox2 gene of amphioxus has also lost its original function and instead is expressed specifically in the preoral pit. As new cis-elements governing its expression in the preoral pit must have been essential for retention of AmphiHox2, we analyzed the transcriptional regulation of AmphiHox2. Although it is possible to make transgenic amphioxus, several technical limitations restrict their practical use; thus, we analyzed the cis-regulatory region surrounding AmphiHox2 in transgenic ascidians (Ciona intestinalis). We found that Ets binding sites of AmphiHox2 functioned in the ascidian embryo. As the amphioxus Ets1/2 homologue is expressed in the preoral pit, we concluded that AmphiHox2 is activated by Ets1/2 in the preoral pit. These analyses demonstrate the utility of Ciona embryos as a transgenic system for analyses of cis-elements from animals whose embryos are relatively inaccessible, such as amphioxus and hemichordates.