THE EVOLVING ROLE OF HOX GENES IN ARTHROPODS

Comparisons between Hox genes in different arthropods suggest that the diversity of Antennapedia-class homeotic genes present in modern inse cts had already arisen before the divergence of insects and crustacean s, probably during the Cambrian. Hox gene duplications are therefore u nlikely to have occurred concomitantly with trunk segment diversificat ion in the lineage leading to insects. Available data suggest that dom ains of homeotic gene expression are also generally conserved among in sects, but changes in Hox gene regulation may have played a significan t role in segment diversification. Differences that have been document ed alter specific aspects of Hox gene regulation within segments and c orrelate with alterations in segment morphology rather than overt home otic transformations. The Drosophila Hox cluster contains several home obox genes that are not homeotic genes - bicoid, fushi-tarazu and zen. The role of these genes during early development has been studied in some detail. It appears to be without parallel among the vertebrate Ho x genes. No well conserved homologues of these genes have been found i n other taxa, suggesting that they are evolving faster than the homeot ic genes. Relatively divergent Antp-class genes isolated from other in sects are probably homologues of fushi-tarazu, but these are almost un recognisable outside of their homeodomains, and have accumulated appro ximately 10 times as many changes in their homeodomains as have homeot ic genes in the same comparisons. They show conserved patterns of expr ession in the nervous system, but not during early development.