Worm Breeder's Gazette 10(3): 147
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.
The VC motoneurons branch from the ventral nerve cord to innervate the vulval muscles with a complex tuft of processes or arborization. We have been examining the assembly of this egg-laying circuit. We have previously reported that the formation of this arborization (as assayed immunocytochemically with an anti-FMRFamide antibody) is not dependent on neuron-target muscle or neuron-neuron (HSN) interactions, but is dependent on the presence of vulval cells. Vulval cells, irrespective of their position or their lineage, are capable of inducing VC branching into the vulval region. Only vulval cells derived from a primary lineage, however, are necessary to induce the branching and form the arborization. The presence of vulval cells alone, however, is not sufficient for the formation of the arborization. We have found that the somatic gonad also plays a role in the formation of the arborization. Because the anchor cell normally induces the vulva (1), gonadal ablations were performed in lin-15(n309) Multivulva mutants in which vulval lineages can be expressed independent of the gonad (2). Ablation of Z1 and Z4, the somatic gonad precursors, in lin-15 results in animals in which VC cell branches are present at the vulva and in pseudo vulvae, but the arborization is absent (n=12). By contrast, ablation of Z2 and Z3, the germline precursors, has no effect on the arborization (n=4). Induction of VC branching, therefore, can occur independently of the gonad, but the presence of the gonad is required for the formation of the arborization. We have begun to examine the question of how the vulval muscles are organized into their final positions around the vulva. Previous work suggested that the final position of the sex myoblasts, the vulval muscle precursors, is regulated by the somatic gonad (3, P. Sternberg, pers. comm.). We examined whether the vulval muscles were under the same instructive cues as the VC neurons. Vulval muscles were visualized immunocytochemically with an antimyosin antibody (kind gift from D. Miller), and were scored for their position around the vulva and/or between VC4 and VC5 and for their orientation. Our results suggest that the position of the sex myoblasts largely determines the position of the vulval muscles. The final position of the muscles, however, is dependent on gonadal and vulval cues. When the somatic gonad is ablated in wild-type animals, the sex myoblasts migrate anteriorly to a variable extent (3) but usually to between P6. pp (about VC4) and P7.pp (about VC5) (M. Stern and H.R. Horvitz, pers. comm.). In similarly ablated animals (n=9), the muscles are observed scattered throughout the length of the animal, as far anteriorly as VC2 and as far posteriorly as halfway between VC6 and the anus. In one animal muscles were detected dorsally. These muscles do not appear to be body wall muscle and look like vulval muscles, and will be referred to as vulval-like muscles. In Vulvaless mutants, the sex myoblasts migrate to their normal position (P. Sternberg, pers. comm). In these animals vulval-like muscles develop in the correct position between VC4 and VC5, but are randomly oriented. The muscles, therefore, can appear in their correct position with cues from the gonad and without input from the vulval cells. Whether vulval muscle orientation is lost due to the loss of the vulval attachment point or whether vulval cues are also important for orientation can not be distinguished from these experiments. The vulval cells, however, can also play a role in the positioning of the vulval muscles. Ablation of the somatic gonad precursors in lin-15(n309) Multivulva animals results in a protruding vulva. In such ablated animals (n=14) vulval-like muscles in their correct orientation are detected at the protruding vulva and at 2-3/4 pseudo vulvae. No vulval-like muscles are detected except at the pseudo vulvae. The vulval-like muscles are derived from the M lineage. Ablation of Z1, Z4, and M in lin-15 animals (n=2) results in the absence of any vulval or vulval-like muscles. lin-15 animals in which no ablations have been performed or in which the germline precursors were ablated (n=3) have correctly positioned vulval muscles and no ectopic vulval-like muscles. These results suggest that in the absence of gonadal cues, the vulval cells are capable of directing the position and orientation of the vulval muscles. Nematode muscle differs from muscle in other animals since it sends muscle arms to nerve cells rather than nerve processes seeking the muscle. The mispositioning of the muscles in these experiments could be caused by muscles projecting to ectopic VC branches in the pseudo vulvae. We think this possibility unlikely, however, because vulval- like muscles can be present at pseudo vulvae in the absence of any ectopic VC branches, and, conversely, ectopic VC branches are seen projecting into pseudo vulvae with no vulval-like muscles. These results are consistent with the hypothesis that the VC cells do not provide cues to the vulval muscles. We propose that the components of this neural circuit, the VC cells and the vulval muscles, are organized by the somatic gonad and the vulva. Preliminary experiments suggest that the HSNs may be under similar control.