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First published online 26 November 2003
doi: 10.1242/dev.00919

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A distinct patterning mechanism of O and P cell fates in the development of the rostral segments of the leech Helobdella robusta: implications for the evolutionary dissociation of developmental pathway and morphological outcome

Graduate Program in Zoology and Section of Molecular Cell and Developmental Biology, Institute of Cellular and Molecular Biology, University of Texas at Austin, Austin, Texas 78712, USA

^* Author for correspondence (e-mail: dhkuo{at}mail.utexas.edu)

Accepted 1 October 2003

Despite a high degree of homonomy in the segmental organization of the ectoderm, the body plan of the leech is divided into two zones based on the distinct cell lineage patterns that give rise to the O/P portion of the segmental ectoderm. In the midbody and caudal segments, each segmental repeat of ectoderm arises in part from one o' blast cell and one `p' blast cell. These two blast cells are positionally specified to distinct O and P fates, and give rise to differentiated descendant cells called O and P pattern elements, respectively. In the rostral segments, each segmental repeat of O and P pattern elements arises from a single `op' blast cell. Based on their developmental fates and their responses to the ablation of neighboring cells, the granddaughters of the primary op blast cell are categorized into two O-type cells and two P-type cells. The O-type cells do not require the presence of the rest of the op blast cell clone for their normal development. By contrast, normal development of the P-type cells depends upon interactions with the other OP sublineages. Additional experiments showed that the O-type cells are the source of a repressive signal involved in the normal fate specification of the P-type cells. Our data suggest that the cell interactions involved in fate specification differ substantially in the rostral and midbody segments, even though the set of differentiated descendants produced by the rostral OP pathway and the midbody O and P pathways are very similar.

Key words: Cell lineage, Cell interaction, Rostral segment, Leech, Helobdella robusta