Worm Breeder's Gazette 10(2): 48
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.
Egg laying in C. elegans is driven by the HSNs, a pair of serotonergic motorneurons that innervate the vulval muscles. During embryogenesis, the HSNs are generated in the tail and migrate to their final positions near the gonad. So far, 27 mutations in 12 genes have been identified that lead to defects in HSN migration. We would like to determine how these mutations perturb HSN migration. Here we present evidence suggesting that in the HSN migration mutant n1438, the segregation of fate between the HSN and its sister, the phasmid cell PHB, is defective. Unlike its sister, PHB does not migrate but remains in the tail. A defect in the segregation of fate between the HSN and the PHB could lead to an abnormal HSN that is defective in migration. The mutation n1438 was isolated in a screen for egg-laying defective mutants by Michael Stern. The mutation maps between dpy-20 and unc-30 on IV, and is uncovered by the deficiency sDf22. Where as wild-type animals have two HSN neurons, n1438 animals have zero to four serotonergic HSN-like cells. In n1438 animals with three or four serotonergic cells, the extra cells appear to be HSNs by several criteria. First, these cells, which are usually found in the tail, have processes that run in the ventral cord as do processes from misplaced HSNs of other HSN migration mutants. Second, rare n1438 animals contain three serotonergic cells that are located along the HSN migratory route, indicating that, like the HSNs, the extra cells can migrate. Third, also like the HSNs, the extra HSN-like cells begin to synthesize detectable serotonin in young adults. Serotonergic cells other than the HSNs contain serotonin at hatching. Studies of double mutants also indicate that the extra cells in n1438 are HSN-like. (1) In egl-1 hermaphrodites, the HSN undergoes programmed cell death shortly after starting to migrate. For example, in egl-1(n986) animals, 0/121 animals analyzed had HSNs based on staining with antisera to serotonin. Similarly, only 7/190 n1438; n986 animals had serotonin-positive HSN-like cells. Since 40% of n1438 animals contain three or four HSN-like cells, almost none of the extra HSN-like cells present in n1438 animals can be detected by staining in the n1438; n986 double mutant. Presumably these cells undergo programmed cell death in the egl-1 background because they are HSN-like. (2) egl-5 mutations block both migration of the HSNs and expression of HSN serotonin. In egl-5(n945); n1438 double mutant animals, the extra HSN-like cells also fail to express serotonin. (3) In egl-45 and unc-86 mutants, the HSNs degenerate during the L4 stage. In both egl-45(n999); n1438 and unc-86(e1416); n1438 adults, no extra cells are seen after staining with antisera to serotonin; presumably, like the HSNs, the extra cells have degenerated. Where do the extra HSN-like cells come from? The sister to the HSN is the phasmid neuron PHB. In wild-type animals, the phasmid cells PHA and PHB can be visualized by fluorescence microscopy after incubation with the fluorescent dye FITC (Hedgecock et al., 1985, Dev. Biol. 111: 158-170). In contrast, in most n1438 animals incubated with FITC, only the PHAs stain. Specifically, of the 52 n1438 animals analyzed, only 16 of the potential 104 PHBs were stained with FITC. This observation suggests that the PHBs are abnormal or missing in n1438 animals. Thus, in n1438 animals, there are extra HSN-like cells and the HSN sister is abnormal. These results suggest that PHB may be transformed to express the fate of its sister cell, the HSN. On the other hand, the HSN-like cells are also abnormal in n1438 animals; these cells are migration-defective and often no or one HSN-like cell is detectable by staining for serotonin. In addition, a few HSN-like cells escape programmed cell death in the double mutant n1438; egl-1 (n986) (see above). These results suggest that both the HSNs and the PHBs are variably transformed to an intermediate fate in n1438 animals. One possibility is that the n1438 gene product is required for the segregation of fate between the HSN and the PHB.