A ONECUT Homeodomain Protein Communicates X Chromosome Dose to SpecifyCaenorhabditis elegansSexual Fate by Repressing a Sex Switch Gene

Abstract

Sex is determined in Caenorhabditis elegans through a dose-dependent signal that communicates the number of X chromosomes relative to the ploidy, the number of sets of autosomes. The sex switch gene xol-1 is the direct molecular target of this X:A signal and integrates both X and autosomal components to determine sexual fate. X chromosome number is relayed by X signal elements (XSEs) that act cumulatively to repress xol-1 in XX animals, thereby inducing hermaphrodite fate. Ploidy is relayed by autosomal signal elements (ASEs), which counteract the single dose of XSEs in XO animals to activate xol-1 and induce the male fate. Our goal was to identify and characterize new XSEs and further analyze known XSEs to understand the principles by which a small difference in the concentration of an intracellular signal is amplified to induce dramatically different developmental fates. We identified a new XSE, the ONECUT homeodomain protein CEH-39, and showed that it acts as a dose-dependent repressor of xol-1 transcript levels. Unexpectedly, most other XSEs also repress xol-1 predominantly, but not exclusively, at the transcript level. The twofold difference in X dose between XO and XX animals is translated into the male vs. hermaphrodite fate by the synergistic action of multiple, independent XSEs that render xol-1 active or inactive, primarily through transcriptional regulation.

Revisiting the X:A Signal That Specifies Caenorhabditis elegans Sexual Fate

“…The phenotypic discrepancy between the two strains means either that yDf20 removes other undefined XSEs or that yDf20 removes other factors essential for the viability of hermaphrodites. Evidence presented in Gladden and Meyer (2007) indicated that ceh-39 and fox-1 are not likely to be the only XSEs in this region of X, thus making the former possibility the most probable.…”

“…In all cases, three independent growths of the stains were used for the measurements. RNA was isolated as described in Gladden and Meyer (2007) except that sdc-2(y93, RNAi) was grown on NG-agar plates overlaid with HT115(DE3), and the following strains were grown on NG-agar plates overlaid with HB101 bacteria: dpy-27(y57), xol-1(y9), sex-1(y424), and xol-1(y9) sex-1(y424). For dpy-27(y57) and sex-1(y424) animals, plates were monitored, and young males were removed prior to reaching adulthood.…”

“…Both mechanisms of repression function together to ensure the fidelity of the X chromosome counting process. The predominant form of xol-1 regulation by XSEs appears, however, to be transcriptional (Gladden and Meyer 2007). The autosomal 1 component of the X:A signal is represented by at least two ASEs.…”

“…Recent studies (Gladden and Meyer 2007) have hinted that the earliest aspects of sex determination are controlled through a process more complex than first described (Carmi and Meyer 1999). These studies showed that individual XSEs make unequal contributions to the X:A signal but did not determine their relative contributions or assess whether the majority of XSEs had been defined.…”

A shared enhancer controls a temporal switch between promoters during Drosophila primary sex determination

“…We note that a logically similar, two-target control process operates in the primary sex determination of Caenorhabditis elegans. There, four XSE proteins exert primary control of the master regulator xol-1 at the level of transcription, and a fifth XSE acts posttranscriptionally to ensure the fidelity of X chromosome counting (32,33). The inclusion of multiple regulatory steps may prove to be a general mechanism for conferring robustness on dose-sensitive regulatory switches.…”