ABSTRACT
We have isolated a novel ascidian homeobox gene, designated AHoxl, by screening the genomic DNA of Halocynthia roretzi with the Bombyx mori Antennapedia type homeobox as a probe. The AHoxl gene encodes a protein that consists of 741 amino acids. The homeobox of AHoxl is interrupted by 2 introns each of which is about 300 bp in length and it shows about 70 % similarity at a deduced amino acid level to that of Drosophila H2.0. This suggests that AHoxl is one of the most diverged homeobox genes so far characterized. Northern blot hybridization with an AHoxl probe showed the presence of single transcripts approximately 2.8 kb in length in larvae, juveniles and some adult tissues. The expression of AHoxl is scarcely detected during the course of early development but it increases to a moderate level at the larval stage. After metamorphosis, the level of AHoxl expression increases as development proceeds. In situ hybridization to the juvenile 7 days after metamorphosis showed that the site of AHoxl expression is the epithelium of digestive tract. Among the adult tissues examined, digestive tract, digestive gland and coelomic cells were the major sites of the expression of AHoxl. In gonad, body wall muscle and pharyngeal epithelium, the expression of AHoxl is relatively weak. These results suggest that AHoxl is primarily expressed in the tissues of endodermal origin and that the gene expression may be associated with differentiation of the endodermal tissues.