Worm Breeder's Gazette 10(1): 115

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.

Sex Myoblast Migration and Sex Determination

M.J. Stern and B. Horvitz

As described previously (C.  elegans CSH Meeting Abstracts, 1987, p .
187), mutations in two genes , egl-15 and egl-17, cause defective 
sex myoblast (SM) migrations in the hermaphrodite as a result of the 
repulsion of the SMs by somatic cells in the gonad.  The migration of 
the SMs is sexually dimorphic: hermaphrodite SMs migrate anteriorly, 
while male SMs migrate posteriorly.  The egl-15 and egl-17 mutations 
appear not to affect the male SM migrations.  One possibility was that 
the altered SM migration is a consequence of a confused sexual 
identity of the SM-gonad interaction.  In support of this hypothesis, 
we have found that the masculinizing, dominant allele of the her-1 
gene, n695, can cause the SMs to be located in positions similar to 
those found in egl-15 and egl-17 mutants.
The observed repulsion in egl-15 and egl-17 mutants could be a 
subtle masculinization of the direction of the SM migration if the 
normal male gonad propels the SMs posteriorly.  However, laser 
ablation of the gonad primordium in N2 males does not obviously alter 
the migration of the SMs.  The possibility remains that repulsion of 
the SMs by the gonad in males is a secondary mechanism involved in the 
correct positioning of these cells and their descendants.  Precedent 
for this type of backup mechanism for positioning the SMs is found in 
the hermaphrodite (Thomas and Horvitz, WBG 9(2):59-60).
We have taken two approaches to determine if the effects of egl-15 
and egl-17 mutations are due to sexual transformation.  First, we have 
tested these mutations for interactions with sex determination genes.  
Second, we have sought to isolate new alleles by non-complementation 
screens to see if they more broadly affect sex determination.  We 
suspected that the null phenotypes of egl-15 and egl-17 might not be 
merely Egl, since Egl mutations in these two genes arise at 
frequencies at least five times lower than the typical null frequency. 
One possibility was that the alleles isolated to date specifically 
affect the sexually dimorphic migration of the SMs, while null alleles 
might more generally affect sex determination.
The weak tra-2 allele n1106 causes hermaphrodites to be Egl as a 
result of the male-like programmed cell death of the HSN neurons that 
drive egg-laying (Desai and Horvitz, C.  elegans CSH Meeting Abstracts,
1985, p. 112).  These animals can be more fully masculinized in 
combination with other mutations that cause subtle masculinization (
Miller and Meyer, C.  elegans CSH Meeting Abstracts, 1987, p. 61).  We 
have found that tra-2 (n1106); egl-17(e1313) animals are infertile 
pseudomales while tra-2(n1106); 84) animals portray 
no detectable interaction.  Similarly, sdc-1(n485) shows pronounced 
masculinization by egl-17(e1313) but no strong interaction with egl-15(
n484) .  Thus, it seemed likely that the altered SM migration in egl-
17(e1313) animals could be due to partial sexual transformation.  
However, while a recessive allele of her-1, e1520, prevents the 
masculinization of tra-2(n1106) by egl-17(e1313) (Chad Nusbaum, 
personal communication), it does not relieve the altered SM 
positioning.  This result suggests that egl-17 interacts with the sex 
determination gene tra-2 in a her-1-dependent manner, but that the 
altered SM positioning in egl-17(e1313) animals is independent of, or 
at least requires a much lower level of, the her-1(+) gene product.  
One possible model is that the altered SM migration in both egl-15 and 
egl-17 mutants is not the result of partial sexual transformation of 
the SM-gonad interaction, and that egl-17 has a second, independent 
function in determining sexual identity.
To determine if we could expect to isolate null alleles by 
noncomplementation screens using the existing alleles of egl-15 and 
egl-17, we checked the phenotypes of eT2(I)/+; egl-17(e1313)/eT2(X) 
and egl-15(n484)/nDf19 (eT2 is missing egl-17 and the left tip of LGX, 
and nDf19 deletes egl-15).  Both are Egl, although +/eT2(I); egl-17(
e1313)/eT2(X) animals are unhealthy.  We screened 36,000 EMS-
mutagenized haploid genomes for mutations that failed to complement 
egl-17(e1313) and found four new alleles that are similar to the two 
previously known ones.  The low frequency of these alleles suggests 
that they are not null alleles and that egl-17(e1313)/egl-17(null) 
animals may be too sick to isolate easily.
We have also screened 32,000 EMS-mutagenized haploid genomes in a 
non-complementation screen for egl-15 alleles, yielding eleven new 
mutations that fail to complement egl-15(n484).  Three are similar to 
egl-15(n484), while the other eight are associated with a linked 
larval arrest phenotype with no obvious cause of the arrest.  One of 
these, n1477 is Egl and scrawny at 20 C and arrested at 25 C.  n1477 
fails to complement all of the other arrested alleles at the 
restrictive temperature.  Another allele, n1454, maps to egl-15 (
between dpy-6 and lin-14, 6% right of dpy-6).  These data, taken 
together, suggest that the null phenotype of egl-15 is larval arrest.
Results from these non-complementation screens suggest the 
involvement of these genes in other functions, although not 
specifically in sex determination.  We plan to study the new egl-15 
and egl-17 alleles further in the hope of better understanding the 
roles of these genes.