Inductive interactions and embryonic equivalence groups in a basal metazoan, the ctenophore Mnemiopsis leidyi

Ctenophores undergo locomotion via the metachronal beating of eight longitudinally arrayed rows of comb plate cilia. These cilia are normally derived from two embryonic lineages, which include both daughters of the four e₁ micromeres (e₁₁ and e₁₂) and a single daughter of the four m₁ micromeres (the m₁₂ micromeres). Although the e₁ lineage is established autonomously, the m ₁ lineage requires an inductive interaction from the e₁ lineage to contribute to comb plate formation. Successive removal of the e ₁ progeny at later stages of development indicates that this interaction takes place after the 32-cell stage and likely proceeds over a prolonged period of development. Normally, the e₁₁ cell lies in closest proximity to the m₁₂ cell that generates comb plate cilia; however, either of the e₁ daughters (e₁₁ or e ₁₂) is capable of emitting the signal required for m₁ descendants to form comb plates. Previous cell lineage analyses indicate that the two e₁ daughters generate the same suite of cell fates. On the other hand, the m₁ daughters (m₁₁ and m₁₂) normally give rise to different cell fates. Reciprocal m₁ daughter deletions show that in the absence of one daughter, the other cell can generate all the cell types normally formed by the missing cell. Together, these findings demonstrate that the two m₁ daughters (m₁₁ and m₁₂) represent an embryonic equivalence group or field and that differences in the fates of the two m₁ daughters are normally controlled by cell-cell interactions. These combined properties of ctenophore development, including the utilization of deterministic cleavage divisions, inductive interactions, and the establishment of embryonic fields or equivalence groups, are remarkably similar to those present in the development of various bilaterian metazoans.