WEISBLAT, D.A.; REN , X.*; GONSALVES, F.C.; Univ. of California, Berkeley; Univ. of California, Berkeley; Univ. of California, Berkeley: Paradoxical aspects, and their possible evolutionary implications, of cell fate determination and cell signaling in the early embryo of Helobdella robusta, a glossiphoniid leech

Though highly derived, leeches are among the best studied representatives of the proposed clade Lophotrochozoa at the cellular level. In particular, Hirudo and Helobdella are used for studies in neurobiology and early development, respectively. Glossiphoniid species are useful for studying early development, as their embryos are relatively large and hardy, so that their blastomeres are amenable to experimental manipulation. Various species of Helobdella can be grown in the lab, and have been subject to developmental analyses since the 1970s. Several studies have shown that, despite a highly determinate, unequal first cleavage, the 2-cell H. robusta embryo exhibits dynamic activation of two canonical signalling pathways (the Notch and Wnt pathways), including a stochastic phase in which either cell AB or CD shows Notch- or Wnt pathway activity, while the other cell does not. More recently, a highly inbred strain of H. robusta has been produced, for use in preparation of a BAC library (with NSF support) and for genome sequencing (at the Joint Genome Institute, Walnut Creek, CA). With the experimental techniques already available, these developments should make H.robusta a useful starting point for comparisons with other lophotrochozoans, despite its derived condition. In further support of that effort, we also report on a life cycle- and reproductive history-analysis of lab-reared H.robusta that shows significant differences with respect to a similar study of H. triserialis (Wedeen et al., 1990).