WEISBLAT, D.A.; RIVERA, A.S.; GONSALVES, F.C.; SONG, M.H.; ZHANG, S.O.; Univ. of Calif., Berkeley; Univ. of Calif., Berkeley; Univ. of Calif., Berkeley; NIDDK, NIH; Univ. of Calif., Berkeley: Segmentation in Helobdella robusta, a glossiphoniid leech

Leeches are a clade of hermaphroditic, segmented worms, classically regarded as one of three classes within Annelida. Compared to most other annelids, segmentation in leeches is highly determinate. Exactly 32 segments arise embryonically, and there is no post-embryonic segmentation, regeneration of segments after bisection, or vegetative reproduction. In leeches, segments arise in anteroposterior progression from a posterior growth zone composed of 5 pairs of segmentation stem cells (teloblasts), each of which contributes a particular, segmentally iterated set of definitive progeny (a kinship group) to the adult. In glossiphoniid leeches such as Helobdella, the teloblasts are relatively large cells, amenable to microinjection of lineage tracers and, more recently synthetic mRNAs. Teloblasts undergo repeated asymmetric divisions, forming columns of segmental founder cells (blast cells). Blast cells undergo stereotyped patterns of further asymmetric cell division, migration and differentiation to generate kinship groups. Thus, the segmentation clock in leech is strictly linked with the cell cycle clock of the teloblasts. For molecular comparisons, we have employed the candidate gene approach, characterizing the homologs of Drosophila segmentation genes in Helobdella. We also find that Notch– and hes-related genes are expressed during segmentation. We do not interpret our results as providing evidence of homology between segmentation in leech and arthropods or vertebrates, however. Instead, we suggest that segmentation arose independently in the three clades, perhaps from stem cell processes associated with terminal growth in the last common ancestor.