Worm Breeder's Gazette 9(1): 78

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.

egl-16: A Gene Which Unites The Sex Determination and Dosage Compensation Pathways

A. Villeneuve and B. Meyer

At the recent CSH worm meeting we reported that mutations in the 
gene egl-16 1) can promote male development in XX animals, and 2) have 
phenotypes which are sensitive to X-chromosome dosage, affecting XX 
but not XO animals.  The former assertion was based on the 
observations that a) egl-16(n485.Y4)/Df XX animals display an 
incompletely penetrant Tra phenotype, and b) in double mutant strains, 
egl-16(n485) enhances the sexual transformation phenotypes of weakly 
masculinizing mutations in the genes tra-2 and egl-41.  The latter 
assertion was based on the fact that her-1(e1520);egl-16(n485) XX 
hermaphrodites are Egl and slightly short, while XO hermaphrodites are 
essentially wild-type.  Likewise, tra-1(e1099.e1488); 
85) XX animals are short pseudomales with abnormal 
bursae, while XO males are wild-type in length and have normal bursae.
Since the meeting, we have extended our understanding of egl-16 in 
The sexual transformation phenotype of egl-16 is effected through 
the major sex determination pathway.
XX animals homozygous for the weak allele tra-2(n1106) are not 
visibly transformed but are merely HSN Egl hermaphrodites.  The vast 
majority of animals of the genotype tra-2(n1106)unc-4(e120);egl-16(
n485), however, are either visibly sexually transformed or sterile; a 
homozygous strain of this genotype cannot be maintained.
To test whether this enhancement of sexual transformation caused by 
the egl-16 mutation occurs through the major sex determination pathway,
we constructed the triple mutant her-1(e1520);tra-2(n1106)unc-4(e120);
85).  The her-1 mutation blocks sexual 
transformation in this self-fertile Egl hermaphrodite strain, 
indicating that the egl-16 mutation promotes masculinization directly 
through the major sex determination pathway rather than via some 
peripheral mechanism.  Arguing from epistasis, egl-16 acts upstream of 
her-1, perhaps by modulating her-1 activity.
egl-16(n485) disrupts dosage compensation in XX animals.
The fact that egl-16(n485) affects XX but not XO animals regardless 
of sexual phenotype suggested that this mutation may disrupt dosage 
compensation, producing an elevated level of X-linked gene expression 
in XX animals.
This hypothesis was tested using both the biochemical assay for X-
specific mRNA levels (Meyer and Casson) and the morphological assay 
for lin-14 expression (Plenefisch and De Long) described in this issue.

We have measured the levels of myo-2(X) transcripts present in egl-
16(n485) and N2 hermaphrodite RNA preparations.  When the preps are 
normalized to myo-1(I) transcript levels, we find that the level of 
myo-2 mRNA in the egl-16 prep is indeed substantially increased over 
that seen in the N2 prep (quantitation is in progress).  We are 
currently preparing to measure transcript levels in him-5(e1490);egl-
16(n485) males to determine whether the observed increase is specific 
for XX animals.
egl-16(n485) also causes phenotypic suppression of the hypomorphic 
mutation lin-14(n179) in the double mutant.  This result is consistent 
with an increase in the level of X-linked gene expression caused by 
egl-16.  No such suppression is observed in XO animals, suggesting 
that the effect is limited to XX animals.
Thus, both biochemical and genetic evidence indicate that egl-16(
n485) disrupts dosage compensation in XX animals.
egl-16(n485) differs from previously characterized sex determination 
and dosage compensation mutations in that it identifies a common 
function which unites the two pathways.  Our data are consistent with 
a model in which the wild-type egl-16 activity is involved in 
transmitting information about the X:A ratio to both sex determination 
and dosage compensation pathways.  By this model, a mutation in egl-16 
results in an apparent underassessment of the X:A ratio in XX animals, 
shifting both sex determination and dosage compensation mechanisms 
toward their XO modes.
We have begun to examine interactions between egl-16 and other 
components of the dosage compensation mechanism.  A few results are 
summarized briefly for 
1.  dpy-21(e428);egl-16(n485) is a viable Dpy 
odite strain.
2.  him-5(e1490);egl-16(n485) XXX animals are inviable, or at least 
greatly reduced in viability.
3.  dpy-28(y1);egl-16(n485) is inviable as a strain, even at 15 C (
permissive temperature for Y1); Y1/y1;n485 progeny from Y1/+;n485 
mothers are sometimes self-fertile, but produce broods comprised 
entirely of arrested larvae and occasional wild-type males.