GENE LOSS AND GAIN IN THE EVOLUTION OF THE VERTEBRATES

Homeobox cluster genes (Hox genes) are highly conserved and can be use fully employed to study phyletic relationships and the process of evol ution itself. A phylogenetic survey of Hox genes shows an increase in gene number in some more recently evolved forms, particularly in verte brates. The gene increase has occurred through a two-step process invo lving first, gene expansion to form a cluster, and second, cluster dup lication to form multiple clusters. We also describe data that suggest s that non-sox genes may be preferrentially associated with the Hox cl usters and raise the possibility that this association may have an ada ptive biological function. Hox gene loss may also play a role in evolu tion. Hox gene loss is well substantiated in the vertebrates, and we i dentify additional possible instances of gene loss in the echinoderms and urochordates based on PCR surveys. We point out the possible adapt ive role of gene loss in evolution, and urge the extension of gene map ping studies to relevant species as a means of its substantiation.