A HOX HOM HOMEOBOX GENE IN SPONGES

The hox/hom homeobox genes code for DNA-binding proteins that confer p ositional information during animal development; these genes have been found in a wide range of triploblasts and in cnidarians. We report he re the identification of a hox/hom gene and two other homeobox genes i n the genomes of sponges. This finding extends the detection of hox/ho m genes to the lowest metazoan phylum and suggests a monophyletic orig in of the kingdom Animalia. Because, in culture, sponge cells quickly reaggregate, differentiate and construct tissue after disaggregation, they can provide a useful model system for characterization of the bas ic roles of homeobox genes in the control of cellular differentiation.