Hremx, the ascidian homologue of ems/emx, is expressed in the anterior andposterior-lateral epidermis but not in the central nervous system during embryogenesis

Recent comparative studies on expression patterns of homeobox genes in the development between ascidians and vertebrates have come to suggest a possib ility that a common basic mechanism may exist in the patterning of the cent ral nervous system (CNS). The ems/emx genes have been demonstrated to be in volved in the formation and patterning of the anterior CNS in Drosophila an d vertebrate embryos. In the present study, we have isolated and analyzed e xpression of Hremx, the ascidian homologue of ems/emx with particular atten tion to whether it is expressed in the larval ascidian CNS. Expression of H remx was detected in the anterior trunk and lateral tail epidermis but not in the anterior CNS. The two expression domains of the epidermis responded in different ways upon treatment with retinoic acid: the anterior expressio n domain was unaltered, while the posterior expression domain extended to t he anterior. The present result suggests that Hremx may have a function in anterior patterning but not in the patterning of the CNS in the ascidian em bryo. We suggest the possibility that the function of ems/emx genes in the patterning of the anterior CNS in Drosophila and vertebrate embryos might h ave been acquired independently in the lineages to Drosophila and vertebrat es.