Worm Breeder's Gazette 9(3): 104
These abstracts should not be cited in bibliographies. Material contained herein should be treated as personal communication and should be cited as such only with the consent of the author.
Much of our understanding of the role of cell-cell communication in C. ent has been derived from studies of equivalence groups, sets of multipotential cells with fates determined by cell interactions. The best characterized is the vulval equivalence group, which consists of six ventral hypodermal cells, P(3-8).p. Each of these cells can express one of three fates, designated 1 , 2 , or 3 , which are determined by an inducing signal from the anchor cell of the gonad. We are interested in extending our knowledge of cell interactions to the cells of the male preanal equivalence group P(9-11) .p, which divide at the same stage and undergo about the same number of rounds of cell divisions as the cells of the vulval equivalence group. The progeny generated by P(9-11).p assume either hypodermal or neuronal cell fates; in particular, the cell P10.papp forms the hypodermal hook, which aids in male mating. The three fates of the cells of the male preanal equivalence group are also known as 1 , 2 and 3 , based upon the hierarchy of replacements seen after cell ablations: P11.p is 1 , P10.p is 2 and P9.p is 3 . In addition, in males, only the cell P9.p has been shown to be tripotential, as are P( 3-8).p in hermaphrodites. We have extended the laser ablation experiments of Sulston and White (Dev. Biol. 78: 577, 1980) to characterize further the control of cell fates within the male preanal equivalence group. First, we have confirmed that P(9-11).p determination occurs during the mid-L2 stage, one stage earlier than the time of P(3-8).p determination. We ablated either P10.p or P11.p at various times after their births in the mid- Ll stage through the early L3 stage and followed the cell lineages of the remaining cells to determine if replacement occurred. Replacement of cell fates was observed if ablations were performed before 20 hours of postembryonic development (mid-L2 stage). However, replacements did not occur if cells were ablated after 20 hours. Second, we have searched for a cell or cells that could be the source of a potential signal analogous to that of the anchor cell. Several cells and sets of cells have been ablated in the Ll stage. Candidates for ablation were selected by their positions relative to P( 9-11).p and/or by sex-specificity. A previous set of ablations performed by John Sulston included: the gonad; B; Y; U; P12.p; K cells; T cells; juvenile preanal ganglion; lumbar ganglion; anal depressor muscle; and tail body muscles (Sulston and White, Dev. Biol. 78: 577, 1980; J. Sulston, personal communication). We have extended and confirmed this set by ablating: the gonad; B; Y; B, U, F and Y; P12; M; VS, V6 and T (both sides); and P(1-12).a. None of these ablations resulted in the loss of 1 or 2 cell fates. Therefore, we have failed to identify a source of a signal. However, it must be kept in mind that we do not know how much of a cell, if any, remains following our laser ablations. It is possible that the remains of a signal-producing cell may still produce a signal. We have attempted to explore further the nature of the intercellular interactions. We tested a model analogous to the current model for vulval development (Sternberg and Horvitz, Cell 44: 761, 1986): a diffusible and spatially graded signal directly determines which of the three distinct fates each cell expresses. According to this model, it should be possible for a Pn.p cell to express a 2 cell fate in the absence of a cell that expresses a 1 cell fate. We have ablated P(10, ll).p in the Ll and observed the fate of P9.p in the L3. In 16/21 animals P9.p migrated posteriorly to occupy a position close to that normally occupied by P10.p. In 14 of those animals P9.p divided once only; in one animal P9.p divided and P9.pp went on to divide another two rounds; and in one animal P9.p expressed a 1 fate (normally expressed by P11.p). Significantly, in no animals was an isolated 2 cell fate observed (the 2 cell fate is normally expressed by P10.p), suggesting that there is no position within a hypothetical gradient that P9.p could assume that would cause it to express the 2 fate. Furthermore, after ablation of P10.p, P9.p efficiently assumed a 2 fate (3/3); after ablation of P11.p, P10.p efficiently assumed a 1 fate (5/5), and in 3/5 of these animals P9.p assumed a 2 fate. In no cases has any cell undergone a 2 fate without another cell assuming a 1 fate, suggesting that the expression of a 1 fate is required for a cell to undergo a 2 fate. Thus, there seems to be at least two differences between the specification of cell fates in the vulval and the preanal equivalence groups: l) the time of determination appears to be one stage earlier for the preanal equivalence group, and 2) there does not appear to be a cell outside the equivalence group producing a graded inducing signal. We currently are considering two alternative models for the specification of P(9-11).p cell fates. Both models assume that a cell with a 1 fate is necessary to induce a neighboring cell to express the 2 fate. In one model, P11.p is determined cell autonomously to express the 1 fate, and only P10.p and P9.p are actually multipotential. In the alternative model, each cell knows anterior from posterior and sends a signal to the cell in front of it, inhibiting that cell from expressing the 1 cell fate. Thus, only the posterior-most cell does not receive the signal and hence expresses the 1 cell fate.