Worm Breeder's Gazette 8(3): 47

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More about HSN-Defective Mutants

C. Desai, R. Horovitz

Figure 1

We have isolated 45 C.  elegans egg-laying defective mutants that 
appear to have functionally impaired HSNs (see Newsletter Vol.  8 No.  
1).  (The HSNs are two motorneurons that innervate the vulval muscles.)
These mutants define at least 14 genes, of which we have now mapped 
11 (see Map on the next page).  In the last Newsletter (Vol.  8 No.  2)
we reported that in the wild type the HSNs are stained by anti-
serotonin antisera, and we described the phenotypes of some of these 
HSN-defective mutants as observed after staining with anti-serotonin 
antisera.  We have now examined the appearance of the HSNs in mutants 
representing all 14 of these genes using indirect immunofluorescence 
to visualize serotonin and Nomarski optics to visualize cell bodies (
see Table on the next page; an * in the table indicates a mutant 
allele studied by microscopy).
These HSN-defective mutants can be divided into five categories.  
Mutants in Category A lack HSNs by both criteria; mutations in three 
of the five genes of this category appear to cause HSNs to undergo 
programmed cell death, as these mutations are suppressed by mutations 
in the genes ced-3 or ced-4 (which block the onset of programmed cell 
deaths; see Ellis et al., this Newsletter).  Mutants in Category B 
appear to generate abnormal HSNs, which can be absent or variably 
mispositioned and which when present invariably lack serotonin.  The 
mutant egl-46(n1127), which defines Category C, has HSN cell bodies in 
almost normal positions but lacks serotonin in the HSNs.  (This mutant 
contains serotonin in the other normally serotonin-positive neurons 
that are visualized after staining with the anti-serotonin anti-sera.) 
It has not been unambiguously demonstrated that the cells considered 
to be misplaced HSNs in Categories B and C mutants are indeed HSNs.
The mutant egl-43(n997), which defines Category D, appears to be 
defective in the migration of the HSNs.  (HSN migration is a normal 
aspect of embryonic development.) As seen with Nomarski optics, egl-43 
animals lack HSNs in their normal positions; serotonin-staining 
reveals ectopic serotonin-positive cells variably located along the 
path of HSN migration (from the postanal region to near the position 
of the vulva).  A double mutant containing egl-43(n997) and the 
Category A mutation egl-41(n1069) lacks these ectopic serotonin-
positive cells, indicating that they are mispositioned HSNs.  The 
misplaced putative HSNs of egl-43 animals have aberrant processes: 
some run abnormally long distances along the ventral cord into the 
nerve ring (normal HSNs run from the region of the vulva along the 
ventral cord into the nerve ring), others are bipolar, and others send 
a process posteriorly to the tip of the tail or anteriorly via a 
sublateral route.  The putative HSNs of egl-43 animals seem to migrate 
slightly farther on average at 20 C or 25 C an they do at 15 C.
Category E mutants have HSNs that appear normal as visualized with 
both Nomarski and fluorescence microscopy.
[See figure 

Figure 1