Gene structure and larval expression of cnox-2Am from the coral Acropora millepora

We have cloned a Hox-like gene, cnox-2Am, from a staghorn coral, Acropora m illepora, an anthozoan cnidarian, and characterised its embryonic and larva l expression. cnox-2Am and its orthologs in other cnidarians and Trichoplax most closely resemble the Gsx and, to a lesser extent, Hox 3/4 proteins. D evelopmental northern blots and in situ hybridisation are consistent in sho wing that cnox-2Am message appears in the planula larva shortly after the o ral/aboral axis is formed following gastrulation. Expression is localised i n scattered ectodermal cells with a restricted distribution along the oral/ aboral body axis. They are most abundant along the sides of the cylindrical larva, rare in the oral region and absent from the aboral region. These ce lls, which on morphological grounds we believe to be neurons, are of two ty pes; one tri-or multipolar near the basement membrane and a second extendin g projections in both directions from a mid-ectodermal nucleus. Anti-RFamid e staining reveals neurons with a similar morphology to the cnox-2Am-expres sing cells. However, RFamide-expressing neurons are more abundant, especial ly at the aboral end of the planula, where there is no cnox-2Am expression. The pattern of expression of cnox-2Am resembles that of Gsx orthologs in D rosophila and vertebrates in being expressed in a spatially restricted port ion of the nervous system.