LATE SPECIFICATION OF VEG(1) LINEAGES TO ENDODERMAL FATE IN THE SEA-URCHIN EMBRYO

Single blastomeres of the sixth-cleavage veg(1) and veg(2) tiers of St rongylocentrotus purpuratus embryos were labeled with DiI lineage trac er, and the disposition of the progeny was followed through the blastu la and gastrula stages in order to determine their respective endoderm al and ectodermal contributions. In the endoderm of postgastrula embry os, veg(1)-derived cells constituted nearly all of the prospective hin dgut and about half of the prospective midgut, while veg(2)-derived ce lls made up the prospective foregut and half the midgut. Oral veg(1) c lones 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 num bers of hindgut cells but large patches of ectoderm cells that extende d Out to the prospective larval vertex, The oral/aboral asymmetry in v eg(1) allocations was also demonstrated using chimeric embryos, the an imal halves of which were labeled with a rhodamine-dextran. Lineages e xpressing the vegetal plate marker Endo16 were more precisely determin ed by combining lineage tracer injection with whole-mount in situ hybr idization. Endo16 expression was found in all cells that are going to participate in gastrulation. Recruitment of new cells to the Endo16 do main occurs in advance of the actual invagination of those cells. Duri ng the blastula stages Endo16 expression expands radially until all ce lls in the veg(2) lineages express this gene, The first phase of gastr ulation, including the normal buckling of the vegetal plate and primar y 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 asy mmetric expansion of Endo16 into the veg(1)-derived cells that will co ntribute 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 o f the vegetal plate territory as gastrulation proceeds. Differential r ecruitment of veg(1)-derived cells on the oral side of the embryo intr oduces an oral bias to gastrulation by disproportionately increasing t he number of cells on the oral side that are competent to participate in gastrulation. (C) 1998 Academic Press.