Worm Breeder's Gazette 11(1): 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.

Terminal Phenotypes and Genetic Mosaic Analysis of New Embryonic Lethal Mutations

Bruce Wightman and Gary Ruvkun

To identify and study genes that control embryogenesis in C.  
elegans, we have generated (so far) 18 independent EMS-induced 
embryonic lethal mutations in strain GR1000 (ncl-1 
unc-36/ncl-1 unc-36: sDp3+++]).
The mutations map to the region complemented by the free duplication 
and were identified in an F2 screen for absence of Dpy-17 
ny and presence of dead eggs.  No 
complementation tests or further mapping have been done.
All 18 mutants have been characterized by examining the terminal 
phenotypes of arrested embryos.  Seven mutants arrest development as 
'pretzels' with no obvious defects, and generally show some L1 
escapers (these animals usually arrest as L1's).  One mutant arrests 
at various stages throughout morphogenesis.  Eight mutants (including 
mg21) proceed to the three-fold stage and then retract to one-and-a-
half-fold near the time of cuticle synthesis.  These mutants arrest 
with a morphology similar to that of sqt-3 embryos.  Many of these 
mutations may be in genes required for cuticle structure or formation. 
Two mutants, mg29 and mg35, arrest earlier, with serious defects in 
elongation.  At the 'lima bean' stage, development begins to slow and 
the embryos frequently take on a somewhat 'ragged' appearance, 
especially along the ventral surface.  Most mg29 and mg35 embryos do 
not progress past the 'comma' or two-fold stage in terms of overall 
morphology.  Many of these embryos subsequently collapse into a 
twitching mass of differentiated, but disorganized cells.  Some 
embryos will maintain a two-fold shape for several hours, but suffer 
from interior cells extruding out from the anterior and posterior ends.
Because hypodermal cells appear to control embryonic elongation, we 
it is possible that these mutations may be in a gene or genes required 
for the generation, migration, or function of hypodermal cells.  We 
are currently examining hypodermal cell fates in these mutants by 
staining embryos with various cell-specific antibodies.
We have also been able to determine which of these mutations are in 
genes required in particular lineages or cell types by mosaic analysis.
Wild-type dpy-17 gene product is required in AB and P2, while wild-
type unc-36 gene product is required only in ABp (C.  Kenyon.  J.  
Yuan).  Mosaic animals were identified at the dissecting microscope as 
Dpy non-unc or Unc semi-Dpy animals.  resulting from loss of the free 
duplication sDp3  in particular lineages.  Both Dpy non-Unc and Unc 
semi-Dpy animals were recovered from the 'pretzel'-arrest mutant mg21, 
most of which matured to sterile adults.  These results show this 
mutation is in a gene which is either non cell-autonomous or is 
required only in specific cells for embryonic viability.  Five Dpy non-
Unc mosaic animals from elongation mutant mg35 were isolated.  Most of 
these animals progressed to adulthood, although many were sterile.  
Since ncl-1 is cell autonomous (E.  Hedgecock) and results in enlarged 
nucleoli in most cells, it provides us with a marker for which 
lineages in particular have lost the duplication in a mosaic animal.  
These animals were Ncl- in either the ABa lineage, C lineage or both.  
While analysis of more mosaic animals is needed, these results 
indicate that the mutation is in a gene which is not required in the 
ABa or C lineage for embryonic viability, but could be required for 
fertility in these lineages.  No Unc semi-Dpy animals from this mutant 
have been identified yet.
Finally, some of the Dpy non-Unc mosaic animals from these strains 
will be germ-line mosaics.  Such animals would be detected if the 
duplication were lost from the P1 or P2 lineages, generating an animal 
which lacked the duplication in gene tissue and produced no wild-type 
progeny.  This class of mosaics are useful to assess if maternal 
contribution of the embryonic lethal gene product ameliorates the 
lethal effect of no or little zygotic contribution.  One mg35 Dpy non-
unc mosaic animal has been identified which appeared to be a germ-line 
mosaic.  This animal produced about 20 eggs, none of which progressed 
past about 200 cells.  At various stages up to that point, embryos 
became necrotic.  Unfortunately, much of the parent's somatic tissue 
was also degenerating, so it was not possible to assess the parental 
Ncl phenotype and verify that it was a mosaic animal.  However, these 
data suggest that the gene identified by mg35 may also have a maternal 
We intend to isolate lots more mosaics from all these mutants.