Late specification of veg1 lineages to endodermal fate in the sea urchin embryo

Single blastomeres of the sixth-cleavage veg(1) and veg(2) tiers of Strongylocentrotus purpuratus embryos were labeled with DiI lineage tracer, and the disposition of the progeny was followed through the blastula and gastrula stages in order to determine their respective endodermal and ectodermal contributions. In the endoderm of postgastrula embryos, veg(1)-derived cells constituted nearly all of the prospective hindgut and about half of the prospective midgut, while veg(2)-derived cells made up the prospective foregut and half the midgut. Oral veg(1) clones consistently contributed more cells to endoderm than aboral veg(1) clones. Oral veg(1) clones extended along the archenteron up to the foregut region, while aboral veg(1) clones contributed only small numbers of hindgut cells but large patches of ectoderm cells that extended Out to the prospective larval vertex, The oral/aboral asymmetry in veg(1) allocations was also demonstrated using chimeric embryos, the animal halves of which were labeled with a rhodamine-dextran. Lineages expressing the vegetal plate marker Endo16 were more precisely determined by combining lineage tracer injection with whole-mount in situ hybridization. Endo16 expression was found in all cells that are going to participate in gastrulation. Recruitment of new cells to the Endo16 domain occurs in advance of the actual invagination of those cells. During the blastula stages Endo16 expression expands radially until all cells in the veg(2) lineages express this gene, The first phase of gastrulation, including the normal buckling of the vegetal plate and primary invagination of the archenteron, involves only the Endo16-expressing cells of the veg(2) lineages. As the archenteron begins to elongate, marking the onset of the second phase of gastrulation, there is an asymmetric expansion of Endo16 into the veg(1)-derived cells that will contribute to the hindgut and midgut in accordance with lineage tracing observations. The results indicate a relatively late specification of veg(1)-derived cells, resulting in late recruitment to the periphery of the vegetal plate territory as gastrulation proceeds. Differential recruitment of veg(1)-derived cells on the oral side of the embryo introduces an oral bias to gastrulation by disproportionately increasing the number of cells on the oral side that are competent to participate in gastrulation. (C) 1998 Academic Press.