Spatial expression of Hox cluster genes in the ontogeny of a sea urchin

The Hox cluster of the sea urchin Strongylocentrous purpuratus contains ten genes in a 500 kb span of the genome. Only two of these genes are expresse d during embryogenesis, while all of eight genes tested are expressed durin g development of the adult body plan in the larval stage. We report the spa tial expression during larval development of the five 'posterior' genes of the cluster: SpHox7, SpHox8, SpHox9/10, SpHox11/13a and SpHox11/13b, The fi ve genes exhibit a dynamic, largely mesodermal program of expression. Only SpHox7 displays extensive expression within the pentameral rudiment itself. A spatially sequential and colinear arrangement of expression domains is f ound in the somatocoels, the paired posterior mesodermal structures that wi ll become the adult perivisceral coeloms, No such sequential expression pat tern is observed in endodermal, epidermal or neural tissues of either the l arva or the presumptive juvenile sea urchin. The spatial expression pattern s of the Hox genes illuminate the evolutionary process by which the pentame ral echinoderm body plan emerged from a bilateral ancestor.