Quantitative assessment of Hox complex expression in the indirect development of the polychaete annelid Chaetopterus sp.

A prediction from the set-aside theory of bilaterian origins is that patter n formation processes such as those controlled by the Hox cluster genes are required specifically for adult body plan formation. This prediction can b e tested in animals that use maximal indirect development, in which the emb ryonic formation of the larva and the postembryonic: formation of the adult body plan are temporally and spatially distinct. To this end, we quantitat ively measured the amount of transcripts for five Hox genes in embryos of a lophotrochozoan. the polychaete annelid Chaetopterus sp. The polychaete Ho x complex is shown not to be expressed during embryogenesis, but transcript s of all measured Hox complex genes are detected at significant levels duri ng the initial stages of adult body plan formation. Temporal colinearity in the sequence of their activation is observed, so that activation follows t he 3'-5' arrangement of the genes. Moreover, Hox gene expression is spatial ly localized to the region of teloblastic set-aside cells of the later-stag e embryos. This study shows that an indirectly developing lophotrochozoan s hares with an indirectly developing deuterostome, the sea urchin, a common mode of Hox complex utilization: construction of the larva, whether a troch ophore or dipleurula, does not involve Hox cluster expression, but in both forms the complex is expressed in the set-aside cells from which the adult body plan derives.