Developmental Regulators of Sex Determination and Dosage Compensation in C. Elegans

Abstract

Several developmental regulators are thought to be responsible for initiating the activation of sex determination during embryogenesis, and for the establishment and maintenance of dosage compensation throughout embryonic and larval development in the nematode C. elegans. Two of the crucial players in this process are the X-signal element sex-1, involved in communicating X chromosome dosage in embryos, and its direct transcriptional target xol-1, the master sex-switch gene that toggles the activation of the appropriate sex development pathway during embryogenesis.

I characterized novel roles for both these regulators in mediating the pathways of sex determination and dosage compensation in XX early embryos. xol-1 promotes XO male-specific development, and its expression was previously thought to only have roles in the development of male embryos. Using imaging techniques and transcriptomic analysis, I found that low-level expression of xol-1 is important for the development of XX embryos as well. Loss of xol-1 leads to an acceleration of embryonic development, precocious accumulation of the dosage compensation complex onto the X chromosomes, and misregulation in sex-biased transcriptional pathways. The X-signal element and transcription factor sex-1 has previously been shown to have uncharacterized downstream roles beyond the transcriptional repression of xol-1. I used computational analysis to characterize the pathways regulated by the xol-1-independent function of sex-1. My work suggests that sex-1 exerts multi-level transcriptional control over the xol-1 pathway through direct transcriptional regulation of targets. Using experimental methods, I demonstrated xol-1-independent sex-1-mediated transcriptional regulation of the male developmental gene her-1. I characterized an additional role of sex-1 in the process of X chromosome dosage compensation in hermaphrodites and identified dpy-21 as a target of sex-1 in this pathway. My work demonstrates that sex-1 regulates dpy-21-mediated enrichment of H4K20me1 on the X chromosomes, which is known to promote repression of gene expression. However, this evidence also suggests that disruption of H4K20me1 enrichment is not the driver of sex-1-mediated lethality in XX embryos.

In addition to these embryonic regulators of dosage compensation, I explored the contribution of the dosage compensation complex component DPY-27 in the maintenance of X chromosome repression in post-embryonic tissues during larval development using computational analysis. My work suggests that the presence DPY-27, in addition to being crucial for the establishment of dosage compensation, is also continuously required to maintain gene repression from the X chromosomes in post-embryonic cells. I also showed that the loss of dosage compensation components such as DPY-27 and DPY-21, in addition to the loss of the tethering pathway mediated by CEC-4 responsible for the localization of the X chromosomes to the nuclear periphery, results in a synergistic loss of X chromosome gene repression. Using transcriptomic analysis of combinatorial disruptions in both these pathways, this work documents the highest level of X derepression demonstrated so far in C. elegan XX animals. Taken together, my work expands the known functions of the embryonic regulators xol-1 and sex-1 in sex determination and dosage compensation, and characterizes the novel function of DPY-27 in regulating dosage compensation during post-embryonic larval development.