HOMEOBOXES IN SEA-ANEMONES (CNIDARIA, ANTHOZOA) - A PCR-BASED SURVEY OF NEMATOSTELLA-VECTENSIS AND METRIDIUM-SENILE

Homeobox genes belong to a phylogenetically widespread family of regul atory genes that play important roles in pattern formation and cell-fa te specification in several model systems(e.g., Drosophila, mouse, and C. elegans). Although the evolution of many classes of homeobox genes predates the diversification of the Bilateria, comparatively little i s known about homeobox genes in outgroups to the Bilateria, such as th e Cnidaria. We used the polymerase chain reaction to recover 12 partia l homeoboxes from 2 species of sea anemones, Metridium senile and Nema tostella vectensis (phylum Cnidaria; class Anthozoa). These homeoboxes appear to represent 9 distinct, mutually paralogous homeobox genes, 5 of which belong to previously identified cnidarian homeobox classes, and 4 of which appear to represent previously unidentified classes. Th e evolutionary relationships between the homeodomains of sea anemones and of bilaterian animals were assessed through database searches and phylogenetic analyses. As many as 5 of the anemone homeoboxes may belo ng to the Hox class, which suggests that the Hox gene complement of cn idarians is larger than previously expected. Homologs of the even-skip ped gene of Drosophila were also identified in both Metridium and Nema tostella.