The onset of C. elegans dosage compensation is linked to the loss of developmental plasticity

Abstract

Dosage compensation (DC) equalizes X-linked gene expression between sexes. In Caenorhabditis elegans, the dosage compensation complex (DCC) localizes to both X chromosomes in hermaphrodites and downregulates gene expression two-fold. The DCC first localizes to hermaphrodite X chromosomes at the 30-cell stage, coincident with a developmental transition from plasticity to differentiation. To test whether DC onset is linked to loss of developmental plasticity, we established a timeline for the accumulation of DC-mediated chromatin features on X (depletion of histone H4 lysine 16 acetylation (H4K16ac) and enrichment of H4K20 monomethylation (H4K20me1)) in both wild type and developmentally delayed embryos. Surprisingly, we found that H4K16ac is depleted from the X even before the 30-cell stage in a DCC-independent manner. This depletion requires the activities of MES-2, MES-3, and MES-6 (a complex similar to the Polycomb Repressive Complex 2), and MES-4. By contrast, H4K20me1 becomes enriched on X chromosomes several cell cycles after DCC localization to the X, suggesting that it is a late mark in DC. MES-2 also promotes differentiation, and mes-2 mutant embryos exhibit prolonged developmental plasticity. Consistent with the hypothesis that the onset of DC is linked to differentiation, DCC localization and H4K20me1 accumulation on the X chromosomes are delayed in mes mutant hermaphrodite embryos. Furthermore, the onset of hermaphrodite-specific transcription of sdc-2 (which triggers DC) is delayed in mes-2 mutants. We propose that as embryonic blastomeres lose their developmental plasticity, hermaphrodite X chromosomes transition from a MES protein-regulated state to DCC-mediated repression.

Balancing up and downregulation of the C. elegans X chromosomes

“…Interestingly, there is no evidence of X-upregulation temporarily preceeding X-downregulation, and thus representing a “first” step prior to the “second” downregulation step. In fact, the X in early embryos, both male and hermaphrodite, appears underacetylated (suggesting that it is hypotranscribed) compared to autosomes [*44]. The X chromosome is enriched for genes which are not turned on until the 40–60 cell stage in development [45], the same time when the DCC assembles on the X chromosomes [21].…”

“…Instead, the two processes may be initiated simultaneously. Consistently, the X in male embryos appear to transition from a hypoacetylated state to a presumably hypertranscribed state at about this time in development, although the exact timing is hard to pinpoint without better molecular markers [*44]. …”

“…Timing of this transition from plasticity to differentiation is under the control of the Polycomb-like complex made up of MES-2, MES-3 and MES-6 [53]. In mes mutants, both the transition from plasticity to differentiation [53] and the onset of dosage compensation are delayed [*44]. The onset of dosage compensation is linked to differentiation in mammals as well [54], indicating that this link might be universal.…”

“…In C. elegans , the DCC assembles on the X chromosomes at about the 30-cell stage [21], but H4K20me1 enrichment does not appear until much later, around the comma stage [*29,*44]. By this time, the C. elegans embryo consists of about 600 cells, and most cells have stopped cycling [46].…”

“…Using temperature sensitive alleles of DCC components, the critical window of DCC action was defined as a 4–5 hour time window centered around the comma stage [11]—significantly, the time window when DCC mediated chromatin marks are being established [*29,*44]. The observation that temperature shifts after this time period have less of an effect on viability suggests that once dosage compensation is established, the DCC becomes dispensable.…”