Analysis of ciliary band formation in the mollusc Ilyanassa obsoleta.

Two primary ciliary bands, the prototroch and metatroch, are required for locomotion and in the feeding larvae of many spiralians. The metatroch has been reported to have different cellular origins in the molluscs Crepidula fornicata and Ilyanassa obsoleta, as well as in the annelid Polygordius lacteus, consistent with multiple independent origins of the spiralian metatroch. Here, we describe in further detail the cell lineage of the ciliary bands in the gastropod mollusc I. obsoleta using intracellular lineage tracing and the expression of an acetylated tubulin antigen that serves as a marker for ciliated cells. We find that the I. obsoleta metatroch is formed primarily by third quartet derivatives as well as a small number of second quartet derivatives. These results differ from the described metatrochal lineage in the mollusc C. fornicata that derives solely from the second quartet or the metatrochal lineage in the annelid P. lacteus that derives solely from the third quartet. The present study adds to a growing body of literature concerning the evolution of the metatroch and the plasticity of cell fates in homologous micromeres in spiralian embryos.