In previous Newsletters (WBG 9, No. 1, p. 70; WBG 9, No. 2, p. 97; WBG 9, No. 3, p. 52) and at the 1987 CSH Meeting (Abstracts, p.63) we have described the properties of
sup-26(
n1091 and
ct49) III mutations, which were isolated as EMS induced extragenic suppressors of the semidominant
her-1(
n695) mutation. Briefly,
sup-26 mutations can feminize both XX and XO animals and appear to act upstream of
tra-2 because: 1) the null
tra-2(
e1095) XX phenotype is not suppressed by
sup-26 and 2) a
sup-26 mutation can block the enhancement of the hypomorphic
tra-2(
n1106) phenotype by
sdc-1 (
n485), dramatically feminizing a
tra-2(
n1106); 5) XX strain. The latter result would not be expected if sup 26 mutations exerted only weak effects downstream of
tra-2. A
sup-26 mutation can substantially suppress the Egl (HSN-) phenotype exhibited by
tra-2(
n1106) XX animals, suggesting that
sup-26 mutations cause an elevation of
tra-2 activity in XX animals. Two possible mechanisms for this apparent elevation are: 1)
sup-26 mutations result in decreased
her-1 activity, which indirectly elevates
tra-2 activity or 2)
sup-26 mutations can cause direct effects on
tra-2 activity without necessarily affecting
her-1 activity. The former mechanism seems less likely because: 1)
sup-26 suppresses
tra2(n1106) in XX animals, where
her-1 activity is presumably OFF or LOW and, 2) neither of the loss-of-function mutations
her-1(
e1520) or
her-1(
y10) by itself suppresses
tra-2(
n1106) XX. [Preliminary experiments involving small numbers of animals (WBG 9, No. 3, p.52) suggested that
her-1(
e1520) might show weak suppression of
tra-2(
n1106) XX; however, examination of greater numbers of animals showed no significant suppression. ] To help us in thinking about models for the role of wild-type
sup-26 gene activity in normal sex determination, we wanted to determine whether
sup-26(
n1091 and
ct49) mutations represent gain or loss of gene function
sup-26 mutations are weak semidominant suppressors of
her-1(
n695), suggesting that they may represent gain of function [5- 10% of
sup-26/+; 5) XX animals are suppressed]. However, the existence of two apparently identical alleles, each isolated at approximately the frequency expected for EMS-induced nulls, suggests that they may represent loss-of-gene function. To address the question of gain or loss more directly, we wished to perform gene dosage experiments on
sup-26 using deficiencies or duplications. Attempts to isolate a deficiency of the
sup-26 locus were unsuccessful. Instead we have used the duplication mnDp37(III;f), which carries a wild-type
sup-26 allele. If
sup-26 mutations result in a gain of wild- type gene activity, or of a novel activity that does not compete with wild-type then mnDp37[
sup-26(+)1/sup-26/sup-26 animals should resemble
sup-26/sup-26 animals by exhibiting strong suppression of
her-1(
n695). If
sup-26 mutations result in a novel activity that competes with wild- type then mnDp37[
sup-26(+)1/sup26/sup-26 animals should exhibit stronger suppression than +/sup-26 animals. However, if
sup-26 mutations are loss-of-function alleles, then mnDp37[
sup-26(+)1/sup- 26/sup-26 animals should resemble +/sup-26 animals by showing only weak suppression of
her-1(
n695). We have found that mnDp37[
sup-26(+) 1/sup26
(n1091)/sup-26
(n1091);
her-1(
n695) XX animals show very weak suppression, suggesting that
sup-26(
n1091) is a loss of function mutation. As a control, we constructed mnDp37/+/+;
her-1(
n695) XX animals and found them to exhibit phenotypes indistinguishable from those of +/+;
her-1(
n695)XX animals; that is, the addition of mnDp37 by itself (which includes most of LG III) does not result in either dramatic enhancement or suppression of
her-1(
n695).Therefore, the gene dosage experiments suggest that loss of
sup-26 gene function is responsible for the apparent increase in
tra-2 activity caused by sup- 26 mutations. Consequently, wild-type
sup-26 activity may act to reduce
tra-2 activity in XO (and also XX) animals. If
sup-26 acts directly on
tra-2 without affecting
her-1 activity, there are at least two possible mechanisms for
sup-26 action: 1)
sup-26 activity could be controlled by
her-1 or 2)
sup-26, like
fog-2, could be controlled in some other fashion.