EVOLUTION OF ANTENNAPEDIA-CLASS HOMEOBOX GENES

Antennapedia (AntP)-class homeobox genes are involved in the determina tion of pattern formation along the anterior-posterior axis of the ani mal embryo. A phylogenetic analysis of Antp-class homeodomains of the nematode, Drosophila, amphioxus, mouse, and human indicates that the 1 3 cognate group genes of this gene family can be divided into two majo r groups, i.e., groups I and II. Group I genes can further be divided into subgroups A (cognate groups 1-2), B (cognate group 3), and C (cog nate groups 4-8), and group II genes can be divided into subgroups D ( cognate groups 9-10) and E (cognate groups 11-13), though this classif ication is somewhat ambiguous. Evolutionary distances among different amino acid sequences suggest that the divergence between group I and g roup II genes occurred similar to 1000 million years (Mn) ago, and the five different subgroups were formed by similar to 600 MY ago, probab ly before the divergence of Pseudocoelomates (e.g., nematodes) and Coe lomates (e.g., insects and chordates). Our results show that the genes that are phylogenetically close are also closely located in the chrom osome, suggesting that the colinearity between the gene expression and gene arrangement was generated by successive tandem gene duplications and that the gene arrangement has been maintained by some sort of sel ection.