Expression of Raldh2, Cyp26 and Hox-1 in normal and retinoic acid-treated Ciona intestinalis embryos.

Spatially regulated synthesis and degradation of retinoic acid (RA) organize embryonic pattern formation in vertebrate embryos. Here, we show expression pattern of genes encoding Ciona intestinalis homologs of the retinaldehyde dehydrogenase, RALDH2, and the cytochrome P450 RA-degrading enzyme, CYP26, in normal and RA-treated embryos. The Ciona homolog of Raldh2, Ci-Raldh2, was expressed in a few muscle-lineage blastomeres in the middle gastrula. Strong expression was then restricted to the anterior-most three muscle cells on each side of the tailbud embryo. The Ciona homolog of Cyp26, Ci-Cyp26, was expressed in the presumptive brain cells in the middle gastrula. The expression was then upregulated in the neck region. The posterior end of the tail was also weakly stained. Non-overlapping expression domains of Ci-Raldh2 and Ci-Cyp26 look similar to those in vertebrates, although the expression of both genes was restricted to a small number of cells in Ciona embryos. RA upregulated Ci-Cyp26 expression and slightly downregulated Ci-Raldh2 expression in the tailbud embryo. We also show expression pattern of a Hox-1 ortholog (CiHox-1) in the Ciona embryo. CiHox-1 was expressed in two separated regions of the nerve cord and neck epidermis at the neurula stage. Expression pattern of these three genes are essentially similar to that in vertebrates.