UC Santa Cruz

The germ lineage is unique among cell types in that it must maintain its own cellular identity while simultaneously maintaining totipotency, the ability to generate all other cell types. To achieve this, germ cells package their genome into domains of transcriptionally active euchromatin and transcriptionally inactive heterochromatin to maintain expression of germline-appropriate genes and repression of germline-inappropriate genes. Germ cells must also differentiate between heterochromatin domains that are relatively permanent (constitutive heterochromatin) and heterochromatin domains that are temporary (facultative heterochromatin) and are activated at another time or in a different cell type. Precisely how constitutive and facultative heterochromatin promote germline development and function is not known. In C. elegans, the constitutive heterochromatin mark H3K9me and the facultative heterochromatin mark H3K27me are required for germline development in subsequent generations and have been shown to redundantly repress the single X chromosome in the male germline and promote germline development in offspring. Surprisingly, domains of H3K9me and H3K27me display significant overlap in C. elegans, suggesting that constitutive and facultative heterochromatin may be less distinct in C. elegans than in other organisms. Together, these observations suggest that H3K9me and H3K27me have at least some redundant roles in repressing genes to promote germline development in C. elegans. I determined the relative roles of H3K9me and H3K27me in gene repression in adult C. elegans germlines to promote germline development in offspring. I used RNA-sequencing to identify the gene targets of H3K9me alone, of H3K27me alone, and of the two marks redundantly in adult hermaphrodite and male germlines. I found that oogenesis genes on the X chromosome are a particular focus of repression in the germline and that repression of the X is primarily carried out by H3K27me alone in the hermaphrodite germline and primarily by H3K9me and H3K27me redundantly in the male germline. I utilized these differences in X repression by H3K9me and H3K27me between hermaphrodite and male germlines to identify a set of genes whose upregulation causes sterility in offspring. I found that the activity of the transcription factors LIN-15B, LIN-54, and possibly EFL-1 contribute to sterility in offspring from germlines that lack H3K9me and H3K27me. I synthesize my findings with those in the literature to present a model of X repression through germline development and how the roles of H3K9me and H3K27me in X repression differ between hermaphrodites and males.