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.

A Mutation That May Transform the Phasmid Cell PHB into its Sister Cell the HSN

Gian Garriga and Bob Horvitz

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 
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.