EXPRESSION OF THE HOX GENE-COMPLEX IN THE INDIRECT DEVELOPMENT OF A SEA-URCHIN

Hox complex genes control spatial patterning mechanisms in the develop ment of arthropod and vertebrate body plans. Hox genes are all express ed during embryogenesis in these groups, which are all directly develo ping organisms in that embryogenesis leads at once to formation of maj or elements of the respective adult body plans. In the maximally indir ect development of a large variety of invertebrates, the process of em bryogenesis leads only to a free-living, bilaterally organized feeding larva. Maximal indirect development is exemplified in sea urchins. Th e 5-fold radially symmetric adult body plan of the sea urchin is gener ated long after embryogenesis is complete, by a separate process occur ring within imaginal tissues set aside in the larva. The single Hox ge ne complex of Strongylocentrotus purpuratus contains 10 genes, and exp ression of eight of these genes was measured by quantitative methods d uring both embryonic and larval developmental stages and also in adult tissues. Only two of these genes are used significantly during the en tire process of embryogenesis per se, although all are copiously expre ssed during the stages when the adult body plan is forming in the imag inal rudiment. They are also all expressed in various combinations in adult tissues, Thus, development of a microscopic, free-living organis m of bilaterian grade, the larva, does not appear to require expressio n of the Hox gene cluster as such, whereas development of the adult bo dy plan does. observations reflect on mechanisms by which bilaterian m etazoans might have arisen in Precambrian evolution.