Identification of Planarian Homeobox Sequences Indicates the Antiquity of Most Hox/Homeotic Gene Subclasses

• Published in: Proceedings of the National Academy of Sciences - PNAS Vol. 92; no. 16; pp. 7227 - 7231
• Main Authors: Balavoine, Guillaume, Telford, Maximilian J.
• Format: Journal Article
• Language: English
• Published: United States National Academy of Sciences of the United States of America 01-08-1995; National Acad Sciences; National Academy of Sciences
• Subjects: Amino Acid Sequence; Animals; Base Sequence; Biochemistry; Biological Evolution; Complementary DNA; DNA; DNA - genetics; Drosophila; Evolution; Gene Amplification; Genes; Genes, Homeobox; Genomics; Homeodomain Proteins; Homeodomain Proteins - genetics; Humans; Life Sciences; Molecular Sequence Data; Multigene Family; Parsimony; Phagocata; Phylogenetics; Phylogeny; Planarians; Planarians - classification; Planarians - genetics; Polycelis nigra; Polymerase Chain Reaction; Sequence Homology, Amino Acid; Species Specificity; Vertebrates; Worms
• ISSN: 0027-8424; 1091-6490
• DOI: 10.1073/pnas.92.16.7227

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