In C. elegans, X-signal elements (XSEs) act in a cumulative, dose-dependent and synergistic manner to repress the master sex switch gene
xol-1. In XX animals, the dose of XSEs is sufficient to represses
xol-1, promoting the hermaphrodite fate and initiating dosage compensation. In XO animals, with half the dose of XSEs,
xol-1 is robustly expressed, promoting male fate. Thus far, two XSEs,
sex-1 and
fox-1, have been characterized.
sex-1 acts transcriptionally, and
fox-1 acts post-transcriptionally to repress
xol-1. The mutation,
y323, may also define an XSE. An RNAi based screen for suppression of XO-specific lethality caused by XSE duplication revealed
ceh-39 to be a possible XSE. This suppression is comparable to that seen with RNAi of
sex-1 or
fox-1.
ceh-39 encodes a protein similar to the ONECUT class of homeodomain proteins. This class of proteins is involved in transcriptional regulation in vertebrates and invertebrates.Synergy is a defining characteristic of XSEs. For example, while
fox-1 XX animals have no phenotype and 35% of
sex-1 XX animals dead, virtually all
fox-1 sex-1 XX animals are dead.
fox-1 also synergies with
y323 in XX animals, resulting in mildly Dpy but fully viable animals.
ceh-39 RNAi causes 90% synergistic XX-specific lethality with
sex-1 but does not appear to synergise with
fox-1 or
y323. The fact that no synergy was seen in combination with
fox-1 or
y323 indicates that
ceh-39 may be a weak XSE. Epistasis analysis with
xol-1 will further pin down where
ceh-39 functions. Since XSEs act in a cumulative manner, increasing the dose of an XSE should 1) cause XO lethality and 2) compensate for decreasing the dose of another XSE.
ceh-39 fits both these criteria. First, extrachromosomal arrays overexpressing
ceh-39 decrease the XO viability in animals with two extra copies of XSEs from 38% to 1%. Second,
ceh-39 overexpression can rescue the Sdc phenotypes associated with a
sex-1 mutation. For example, overexpression of
ceh-39 reduces the penetrance of Dpy from 77% to 7% in
sex-1 animals. If
ceh-39 is an XSE, it should act to repress
xol-1 expression. To test this, animals overexpressing
ceh-39 will be assayed for their ability to repress
xol-1 reporters in XO and
sex-1 XX animals. Loss of
xol-1 expression in these backgrounds would indicate that
ceh-39 is an XSE. CEH-39 is expressed from the 20 to 300 cell stage of embryogenesis, consistent with the temporal expression of other XSEs. It is expressed throughout the cell cycle and appears to be associated with DNA during mitosis. Though
ceh-39 shares many of the characteristic of an X-signal element the mechanism of its role in sex determination has yet to be elucidated.