Meeting Abstract
Elongation via a posterior growth zone (PGZ) is a common feature of bilaterally symmetric embryos. In many taxa, the PGZ involves an ill-defined combination of cell movements and/or divisions. By contrast, the PGZ in clitellate annelid embryos, e.g., the leech Helobdella austinensis consists of five pairs of lineage-restricted stem cells (teloblasts). Typical stem cells undergo “parental interations”–the self-renewing cell generates the same type of differentiating progeny at each division. In leech PGZ, three pairs of teloblasts undergo such divisions. Remarkably, however, the teloblasts generating ventral and dorsal ectoderm (N and Q lineages, respectively) exhibit “grand-parental iterations”–that is, each N and Q teloblast gives rise to two different types of progeny in exact alternation (nf & ns; qf & qs blast cells, respectively). A priori, the differences between adjacent cells in the N and Q lineages could arise from a flip-flop mechanism in the teloblast, or by interactions among initially equipotent blast cells. We can distinguish the two types of cells in each grand-parental lineage by differences in the timing and symmetry of their first mitoses, roughly 50-60 hours after their birth from the teloblast. To distinguish between the two general mechanisms of blast cell differentiation, we seek to identify genes that are differentially expressed before the blast cells divide. The evolutionarily ancient Wnt signaling pathway is involved in the PGZ of many embryos. We have found differential expression of mRNAs encoding one Wnt ligand (Hau-wnt16a) and one putative Wnt modulator (Hau-sfrp2) of pre-mitotic f and s blast cells in the N lineage. Hau-sfrp2 but not Hau-wnt16a is differentially expressed between the f and s blast cells of the Q lineage.
