AUTONOMOUS AND NONAUTONOMOUS DIFFERENTIATION OF ECTODERM IN DIFFERENTSEA-URCHIN SPECIES

During early embryogenesis, the highly regulative sea urchin embryo re lies extensively on cell-cell interactions for cellular specification. Here, the role of cellular interactions in the temporal and spatial e xpression of markers for oral and aboral ectoderm in Strongylocentrotu s purpuratus and Lytechinus pictus was investigated. When pairs of mes omeres or animal caps, which are fated to give rise to ectoderm, were isolated and cultured they developed into ciliated embryoids that were morphologically polarized. In animal explants from S. purpuratus, the aboral ectodermspecific Spec1 gene was activated at the same time as in control embryos and at relatively high levels. The Spec1 protein wa s restricted to the squamous epithelial cells in the embryoids suggest ing that an oral-aboral axis formed and aboral ectoderm differentiatio n occurred correctly. However, the Ecto V protein, a marker for oral e ctoderm differentiation, was detected throughout the embryoid and no s tomodeum or ciliary band formed. These results indicated that animal e xplants from S. purpuratus were autonomous in their ability to form an oral-aboral axis and to differentiate aboral ectoderm, but other aspe cts of ectoderm differentiation require interaction with vegetal blast omeres. In contrast to S. purpuratus, aboral ectodermspecific genes we re not expressed in animal explants from L. pictus even though the res ulting embryoids were morphologically very similar to those of S. purp uratus. Recombination of the explants with vegetal blastomeres or expo sure to the vegetalizing agent LiCl restored activity of aboral ectode rm-specific genes, suggesting the requirement of a vegetal induction f or differentiation of aboral ectoderm cells. These results demonstrate that differences exist in aboral ectoderm differentiation between S. purpuratus and L. pictus and suggest that the formation of a cell type may occur by alternative mechanisms in two related species.