UC Santa Cruz

The germline has the amazing power and the tremendous responsibility to create anentirely new organism and control the information that gets transmitted to offspring. 2 unique properties grant the germline this power: totipotency and immortality. How the germline acquires and maintains these properties are burning questions. The primordial germ cells (PGCs) must launch and maintain a germline-appropriate gene expression program to acquire and protect germ fate. Chromatin regulation or DNA packaging is one mechanism that can tackle this challenge by allowing PGCs to selectively ‘turn on’ genes that encode germline-appropriate RNAs. How chromatin regulation impacts gene expression in PGCs is poorly understood. In C. elegans, the MES chromatin regulators are required maternally for germline development in offspring. MES-2, MES-3, and MES-6 form the worm version of the H3K27 histone methyltransferase (HMT) Polycomb Repressive Complex 2 (PRC2), and MES-4 is an H3K36 HMT. The essential role of PRC2 in ensuring germline survival and immortality in C. elegans is to repress genes on the X chromosome. MES-4’s role has not yet been determined and is far more puzzling. An attractive model is that MES-4 instructs PGCs to express germline genes by transmitting an epigenetic ‘memory of germline’ from parent germlines to offspring PGCs. The focus of my thesis was testing this model using a combination of genetics, transcriptomics, and microscopy. I found that neither MES-4 or its specific role in transmitting a memory of germline are required for PGCs to launch a germline program and develop into a healthy germline. Instead, MES-4’s critical role in protecting germline survival and immortality is repressing X genes, similar to PRC2’s role. This encouraged me to test whether MES- 4 and PRC2 cooperate to repress the same X genes in PGCs. I indeed found this to be the case. I also identified the THAP transcription factor LIN-15B as a major driver of X mis-expression and germline death in mes-4 mutants. Together my findings answered how the MES chromatin regulators protect germline immortality, and raised interesting follow-up questions, such as how do MES-4 and LIN-15B balance levels of X-chromosome expression in PGCs? My thesis work is one of the many testaments to the power of transcriptomics to study gene regulation at the whole-genome scale. However, many researchers who seek to embrace transcriptomics face the hurdle or ‘activation energy’ to get started. I led and published a major effort by the Strome lab to provide such researchers with a launchpad into transcriptomics; I discussed foundations, best practices, and commonly used strategies, and I summarized available resources.