We are interested in understanding the primary sex determination signal of C. elegans (the X/A ratio) and how it regulates its likely target
xol-1.
xol-1 is expressed at high levels in XO males in response to a low X/A ratio and at lower levels in XX hermaphrodites in response to a high X/A ratio. To identify upstream regulators of
xol-1, and possibly components of the primary signal, JK screened for mutations that increase the level of expression of a
xol-1::lacZ translational fusion in XX embryos (IWM abstract, 1993 and 1995).
y263 was one mutation isolated in this screen, and we consequently obtained a second allele of the same gene,
gm41. We are provisionally calling the gene defined by these mutations
rox-1 (regulator of xol).
rox-1 is X-linked, and in addition to increasing the level of expression of a
xol-1::lacZ transcriptional fusion in XX embryos, it also causes XX-specific lethality. Escapers of this lethality are Dpy, Egl and masculinized, revealing defects in dosage compensation and sex determination, as might be expected for a mutation that acts upstream of
xol-1 in the sex determination and dosage compensation pathways.
rox-1 mutations appear to be recessive.
rox-1/+ hermaphrodites are wild type, and a duplication of the region, stDp2, rescues
rox-1 hermaphrodites. In addition, a deficiency of the region, nDf19, behaves like the
rox-1 mutant in that it increases the expression of the
xol-1::lacZ reporter gene in XX embryos. The
rox-1 phenotype is fully suppressed by a
xol-1(
y9) loss-of-function mutation, providing further evidence that
rox-1acts upstream of
xol-1. We are conducting a genetic analysis of
rox-1 to determine if it is a counted element that forms part of the X/A ratio, or instead a signal transduction gene that acts between the X/A ratio and
xol-1. If
rox-1 is a signal element, we expect that varying its dose would change the perceived X/A ratio: increasing the dose of a signal element should increase the perceived X/A ratio and favor hermaphrodite development, while decreasing its dose would favor male development. In addition, since the primary signal is multigenic, the effects of a signal elements may be additive. As indicated by the experiments described below, the dose sensitivity of
rox-1 and its interactions with other components of the signal are consistent with its being a signal component. Since loss-of-function mutations in
rox-1 can kill hermaphrodites, we tested if the converse is also true. That is, can extra copies of
rox-1 cause XO-specific lethality? We found that two copies of stDp2 causes XO-specific lethality. To determine if this lethality is due to dosage compensation defects, we tested whether mutations in
sdc-2, a downstream gene in the dosage compensation pathway, can suppress this lethality and found that the males are rescued. Subsequently, we found that
rox-1 mutations suppress the XO-specific lethality caused by stDp2. This result demonstrates that the lethality is due in part to the presence of extra copies of
rox-1. Since dose sensitivity is one characteristic of signal elements, we tested whether
rox-1 has other properties of a counted element. We already know of at least three signal elements at the left end of X and therefore we asked if
rox-1 acts additively with these elements to determine the X/A ratio. First, we tested whether mutations in
rox-1 can counteract the effect of increasing the dose of other signal elements. yDp14 and yDp13 are duplications of signal elements at the left end of X. These duplications increase the perceived X/A ratio and hence cause XO-specific lethality.
rox-1 mutations partially suppress this male lethality. Conversely, increasing the dose of other signal elements can partially suppress the phenotype of
rox-1 hermaphrodites. The duplications yDp14 and mnDp66 can suppress the XX masculinization caused by
rox-1 mutations. In addition, the effects of
rox-1 mutations are synergistic with those of other signal elements. yDf20 is a deficiency that uncovers at least two signal elements at the left end of X. While yDf20/+ and
rox-1/+ XX hermaphrodites are wild type,
rox-1/yDf20 hermaphrodites are Dpy, Egl and masculinized. Similarly, mutations in the signal element
fox-1 cause no detectable phenotype in XX animals, but are completely lethal to hermaphrodites in combination with
rox-1 mutations. We have initiated a molecular characterization of
rox-1. Based on genetic mapping and RFLP analysis, we mapped
rox-1 to a narrow region on the X chromosome near
unc-115. Using transformation rescue experiments, we identified two cosmids from this region, F44A6 and CO6F4, that rescue the
rox-1 mutant phenotype. According to the C. elegans genome sequencing project, the region of overlap between these two cosmids contains four open reading frames. We have tested individual subclones containing each of these open reading frames for their ability to rescue
rox-1 mutants. One of the subclones, a 10Kb fragment encoding a predicted nuclear hormone receptor homologue of the retinoic acid receptor type rescues
rox-1 hermaphrodites. We are pursuing experiments to verify that this open reading frame is responsible for rescue.