Reassessing embryogenesis in the Ctenophora: The inductive role of e₁ micromeres in organizing ctene row formation in the 'mosaic' embryo, Mnemiopsis leidyi

Ctenophores are a phylum of diploblastic marine animals displaying biradial symmetry organized along an oral-aboral axis. One of the apomorphic sets of adult structures in ctenophores are the eight external comb rows, which run along the oral-aboral axis. Comb rows consist of serial arrays of individual comb plates of cilia, which beat in a coordinated fashion for locomotory behavior. Classical cell lineage experiments using chalk particles indicated that comb rows are derived exclusively from the four e₁ micromeres at the 16-cell stage. This conclusion was also supported by the fact that no ctene rows (or their underlying endodermal canals) form when all four e₁ micromeres were deleted. We have used intracellular diI cell lineage tracing to determine that, in addition to e₁ micromeres, the four m₁ micromeres also make significant contributions to the ctene rows. Thus, e₁ micromere derivatives not only generate comb plates but are required for ctene row formation by m₁ derivatives. These results demonstrate that inductive interactions are an important component of early development in ctenophores and indicate that e₁ micromeres influence the development of adjacent cell lineages (both m₁ and endodermal lineages) during ctenophore 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 invite speculation as to the role of the cleavage program in embryonic patterning in the lower Metazoa.