Hox gene duplication and deployment in the annelid leech Helobdella

The segmented leeches are members of the phylum Annelida within the Lophotr ochozoa. Here, we describe the isolation of a new Hox gene, Lox18, in the l eech Helobdella triserialis. Phylogenetic analysis indicates that Lox18 is a Deformed(Dfd) ortholog. H. triserialis has at least two Dfd orthologs, Lo x18 and the previously described Lox6 (Kourakis et al. 1997; Wong and Macag no 1998), indicating that these genes duplicated after the last common ance stor of annelids and arthropods. Although the temporal appearance of Lox18 message is similar to that of Lox6, the spatial pattern is different. Lox18 does not have a sharply defined anterior border of expression in the secon d neuromere of the subesophageal ganglion of the central nervous system (CN S) as does Lox6, but is expressed uniformly in a small subset of cells in t he longitudinal connectives and lateral roots in every segment of the CNS a long the entire anterior-posterior (AP) axis. Even though Lox18 shares grea ter sequence similarity within the homeodomain and flanking regions to Dros ophila Dfd than to the previously isolated Lox6, its expression pattern sug gests that its function has diverged from the ancestral Hox function. Previ ous sampling has indicated that the last common ancestor of protostomes and deuterostomes had as many as 10 clustered Hox genes representing distinct paralogy groups (Irvine et al. 1997; de Rosa et al. 1999); leech Hox genes may have undergone subsequent and independent cluster or genome-wide duplic ation. These results point to the need for total genome level understanding for key members of the Lophotrochozoa.