The O–P Equivalence Group

Another interesting result from a long series of experiments in Helobdella is that the distinct О and P lineages arise from what are initially equipotent blast cells, produced by two О/P teloblasts on each side of the embryo (Weisblat and Blair 1984; Zackson 1984; Shankland and Weisblat 1984; Ho and Weisblat 1987; Shankland 1987a, 1987b). The default condition for these equipotent о/p blast cells is to assume the ventrolateral (O lineage) fate, which is normally seen in the bandlet lying next to the ventral, n bandlet. In normal development, the other column of о/p blast cells is induced to follow the dorsolateral (P lineage) fate by signaling from the adjacent q blast cells (Huang and Weisblat 1996). In at least one Helobdella species, the P lineage can also be specified by a redundant signal from the M lineage (Kuo and Shankland 2004a). The Q-derived dorsalization of the P lineage involves BMP- mediated signaling, in common with dorsalization processes in other systems (Kuo and Weisblat 2011). The process in Helobdella differs significantly from that in commonly studied vertebrate and insect models, however: (1) BMP5-8 rather than BMP2/4 is the critical signaling ligand; (2) Gremlin, rather than Chordin, acts as the antagonist, and is expressed dorsolaterally rather than ventrally; (3) the signaling appears to be mediated by cell-cell contacts, rather than diffusion of the ligand (Kuo and Weisblat 2011; Tao et al. in preparation).

The anterior components of the О and P lineages also deviate from their midbody counterparts, but in an entirely different manner than seen for the M lineage. Intriguingly, the pair of equipotent О/P teloblasts on each side of the embryo arises as sister cells from the roughly equal division of an OP proteloblast. But before its equal division, the OP proteloblast itself undergoes four highly asymmetric teloblas- tic divisions (Sandig and Dohle 1988; Bissen and Weisblat 1989). The op blast cells contribute four segment’s worth of lateral ectoderm to segments R1 through R4. Lineage analysis has confirmed that these op blast cell clones contribute segmental sets of epidermal cells plus central and peripheral neurons that are roughly equivalent to the canonical О and P kinship group progeny of the midbody segments (Kuo and Shankland 2004b). Curiously, however, the initial divisions of the op blast cells do not immediately create progenitors of canonical О and P kinship group cells; even when the op clone comprises 4 cells, the anteriormost daughter cell in the clone (cell op.aa) is fated to contribute a mix of nominal О and P progeny. These observations support previous work showing that highly specific cellular phenotypes can arise by alternative pathways (Shankland 1987b). In addition, there is no evidence that BMP signaling plays a role in cell fate specification with the lateral ectoderm of the four rostral segments (Kuo et al. 2012).

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