IDENTIFICATION OF PLANARIAN HOMEOBOX SEQUENCES INDICATES THE ANTIQUITY OF MOST HOX HOMEOTIC GENE SUBCLASSES

The homeotic gene complex (HOM-C) is a cluster of genes involved in th e anteroposterior axial patterning of animal embryos. It is composed o f homeobox genes belonging to the Hox/HOM superclass. Originally disco vered in Drosophila, Hox/HOM genes have been identified in organisms a s distantly related as arthropods, vertebrates, nematodes, and cnidari ans. Data obtained in parallel from the organization of the complex, t he domains of gene expression during embryogenesis, and phylogenetic r elationships allow the subdivision of the Hox/HOM superclass into five classes (lab, pb/Hox3, Dfd, Antp, and Abd-B) that appeared early duri ng metazoan evolution, We describe a search for homologues of these ge nes in platyhelminths, triploblast metazoans emerging as an outgroup t o the great coelomate ensemble. A degenerate PCR screening for Hox/HOM homeoboxes in three species of triclad planarians has revealed 10 typ es of Antennapedia-like genes. The homeobox-containing sequences of th ese PCR fragments allowed the amplification of the homeobox-coding exo ns for five of these genes in the species Polycelis nigra. A phylogene tic analysis shows that two genes are clear orthologues of Drosophila labial, four others are members of a Dfd/Antp superclass, and a sevent h gene, although more difficult to classify with certainty, may be rel ated to the pb/Hox3 class, Together with previously identified Hox/HOM genes in other flatworms, our analyses demonstrate the existence of a n elaborate family of Hox/HOM genes in the ancestor of all triploblast animals.