REASSESSING EMBRYOGENESIS IN THE CTENOPHORA - THE INDUCTIVE ROLE OF E(1) MICROMERES IN ORGANIZING CTENE ROW FORMATION IN THE MOSAIC EMBRYO,MNEMIOPSIS-LEIDYI

Ctenophores are a phylum of diploblastic marine animals displaying bir adial symmetry organized along an oral-aboral axis, One of the apomorp hic sets of adult structures in ctenophores are the eight external com b rows, which run along the oral-aboral axis, Comb rows consist of ser ial arrays of individual comb plates of cilia, which beat in a coordin ated fashion for locomotory behavior, Classical cell lineage experimen ts using chalk particles indicated that comb rows are derived exclusiv ely from the four e(1) micromeres at the 16-cell stage, This conclusio n was also supported by the fact that no ctene rows (or their underlyi ng endodermal canals) form when all four e(l) micromeres were deleted. We have used intracellular diI cell lineage tracing to determine that , in addition to e(l) micromeres, the four m(1) micromeres also make s ignificant contributions to the ctene rows, Thus, e(l) micromere deriv atives not only generate comb plates but are required for ctene row fo rmation by m(1) derivatives, These results demonstrate that inductive interactions are an important component of early development in ctenop hores and indicate that e(l) micromeres influence the development of a djacent cell lineages (both m(1) and endodermal lineages) during cteno phore embryogenesis, In addition, intracellular labeling has revealed that there are subtle variations in the composition of clones derived from identified embryonic blastomeres. Together these findings reveal a picture of ctenophore embryogenesis, which is in marked contrast to the former rigid 'mosaic' reputation of ctenophore development, and in vite speculation as to the role of the cleavage program in embryonic p atterning in the lower Metazoa.