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Sex is determined in Caenorhabditis elegans by the ratio of X chromosomes to the sets of autosomes, the X:A signal. A set of genes called X signal elements (XSEs) communicates X-chromosome dose by repressing the masculinizing sex determination switch gene xol-1 (XO lethal) in a dose-dependent manner. xol-1 is active in 1X:2A embryos (males) but repressed in 2X:2A embryos (hermaphrodites). Here we showed that the autosome dose is communicated by a set of autosomal signal elements (ASEs) that act in a cumulative, dose-dependent manner to counter XSEs by stimulating xol-1 transcription. We identified new ASEs and explored the biochemical basis by which ASEs antagonize XSEs to determine sex. Multiple antagonistic molecular interactions carried out on a single promoter explain how different X:A values elicit different sexual fates. XSEs (nuclear receptors and homeodomain proteins) and ASEs (T-box and zinc finger proteins) bind directly to several sites on xol-1 to counteract each other's activities and thereby regulate xol-1 transcription. Disrupting ASE- and XSE-binding sites in vivo recapitulated the misregulation of xol-1 transcription caused by disrupting cognate signal element genes. XSE- and ASE-binding sites are distinct and nonoverlapping, suggesting that direct competition for xol-1 binding is not how XSEs counter ASEs. Instead, XSEs likely antagonize ASEs by recruiting cofactors with reciprocal activities that induce opposite transcriptional states. Most ASE- and XSE-binding sites overlap xol-1's -1 nucleosome, which carries activating chromatin marks only when xol-1 is turned on. Coactivators and corepressors tethered by proteins similar to ASEs and XSEs are known to deposit and remove such marks. The concept of a sex signal comprising competing XSEs and ASEs arose as a theory for fruit flies a century ago. Ironically, while the recent work of others showed that the fly sex signal does not fit this simple paradigm, our work shows that the worm signal does.

Molecular antagonism between X-chromosome and autosome signals determines nematode sex