Conservation of Hox/ParaHox-related genes in the early development of a cnidarian

To clarify the relationship between axial patterning in cnidarians and bila terians, we have investigated the embryonic development of the hydrozoan Po docoryne carnea. The expression of Hox-like homeobox genes was analyzed by RT-PCR and in situ hybridization. Cnox1-Pc, an anterior Hox gene, is a mate rnal message. It is present throughout larval development, first weakly in all blastomeres and later restricted mostly to the anterior pole of the pla nula. Gsx, an anterior ParaHox gene, is first seen in the anterior endoderm but also extends into posterior regions. Cnox4-Pc, an orphan Hox-like gene , is expressed in the egg as a ring-shaped cloud around the germinal vesicl e. After fertilization, the message remains in most animal blastomeres. Whe n the embryo elongates in late blastula, staining is restricted to a few ce lls at the posterior pole where gastrulation will start. However, once gast rulation starts, the Cnox4-Pc signal disappears and is absent in later stag es of larval development. Phylogenetic analysis shows that not all enidaria n Hox-like genes have recognizable orthologues in bilaterian groups. Howeve r, the expression analysis of Cnox1-Pc and Gsx correlates to some extent wi th the expression pattern of cognate genes of bilaterians, confirming the c onservation of genes involved in organizing animal body plans and their put ative common ancestral origin. (C) 2001 Academic Press.