Worm Breeder's Gazette 7(2): 40

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 Mutant lin-22 Alters the Fates of Cells in the Lateral Hypodermis

B. Fixsen, B. Horvitz

The mutant lin-22(n372)IV appears to be altered in the fates of 
certain cells in the lateral hypodermis.  lin-22(n372) was isolated 
during a screen for EMS-induced mutants abnormal in cellular anatomy 
as visualized after staining with the fluorescent nuclear stain DAPI.  
(Our DAPI-staining protocol is described in Vol.5, No.2 of this 
Newsletter.)  lin-22 appeared to have multiple, ectopic postdeirids.  
The postdeirids are lateral sensilla consisting of four cells with 
compact nuclei, one of which is a dopaminergic neuron.  In wild-type 
animals, the right and left V5 ectoblasts generate the right and left 
postdeirids, respectively.  In lin-22 animals, the right and left 
ectoblasts V2, V3, V4, V5 and more variably, V1, undergo a V5-like 
lineage, each generating a postdeirid.  Each postdeirid contains a 
single dopaminergic cell that is, at least superficially, 
indistinguishable in morphology from that of the wild-type postdeirid 
as visualized using formaldehyde-induced-fluorescence.  In lin-22 
males, further and more variable lineage abnormalities occur during 
the late L3 and early L4 stages.  Whereas in wild-type males, V5.pppap 
gives rise to a seam cell and V5.ppppp is a ray precursor cell, in lin-
22 males, the homologous cells derived from (V1), V2, V3, V4 and V5 (
ie.  Vn.pppap and Vn.ppppp) both are ray precursor cells.
Sulston and White (Devol.  Biol.  78:577, 1980) have shown that cell-
cell interactions in wild-type animals affect the fate of V5.  In both 
hermaphrodites and males, laser ablation of V6, but not V4, results in 
the absence of the postdeirid; in males, ablation of V6 also affects 
the number of rays generated by V5.  A similar effect is observed in 
lin-22 animals, but only for the V5 lineage and not for the V1-V4 
lineages, which appear identical to that of V5.  Ablation of V6 seems 
to have no effect on any of the lin-22 ectopic postdeirids or rays; 
furthermore, ablation of V5 has no effect on the ectopic postdeirid 
generated by V4, ablation of V4 has no effect on V3, etc.  
Occasionally, however, the generation of 'partial' postdeirids has 
been observed in lin-22 animals after laser ablation.  (Partial refers 
to when Vn.paa generates two compact nuclei and a cell that dies, as 
is normal, but Vn.pap generates a syncytial and a seam cell, instead 
of two compact nuclei.)  It is not clear whether this abnormal lineage 
reflects a difference in lin-22 regulation or is simply due to the 
fact that many more postdeirid-like lineages have been observed in lin-
22 animals after ablation than in wild-type.
By carefully timing our laser ablations, we have shown that ablation 
of V6 only affects the V5-generated postdeirid if the ablation occurs 
before the first division of V5 in the L1.  After this time, it 
appears that the postdeirid has already been determined.  If the 
'later' V6 ablations are made in a wild-type male, V5 not only 
generates a postdeirid, but it also generates two, rather than one, 
rays.  In fact, the resulting lineage appears to be exactly the same 
as that normally observed in lin-22 males for the V2-V5 ectoblasts.
As can be seen from the laser ablation studies, V6, or its 
descendants, appears to influence the development of the lateral 
hypodermis in at least two distinct ways: V6 is necessary early in 
development for the V5-generated postdeirid; later, the descendants of 
V6 suppress the generation of rays from, at least, V5.  The phenotype 
of lin-22 can mostly be described by postulating that the V1-V5 
ectoblasts have been transformed into a 'V5-like' ectoblast that is 
independent of these V6 influences.  However, V5 itself in lin-22 is 
still sensitive to the presence of V6 early in development.