HOMOLOGY OF HOX GENES AND THE ZOOTYPE CONCEPT IN EARLY METAZOAN EVOLUTION

The correct identification of homologous Hox genes within and between diplo- and triploblastic animals is of crucial importance for recent h ypotheses on the anagenetic evolution of animal bauplans. While the ho mology discussion in general has reached new heights, we apply traditi onal homology criteria to assign homology to Hox genes from diploblast ic animals. Comparison of the Hox gene from the presumably most basal metazoan animal, the placozoan Trichoplax adhaerens, to other Hox gene s suggests the presence of unambiguous homologs in Hydrozoa and Scypho zoa and the absence of any specific homolog in triploblasts. Furthermo re, the comparisons provide support for the idea that Hox genes - at l east in diploblastic animals - are composed of functional subunits (mo dules), which to some degree have undergone independent evolution. The findings are not readily compatible with the existence of the ''zooty pe'' in diploblastic animals. (C) 1998 Academic Press.