The germline is the ancient cell lineage that gives rise to the gametes. It is responsible for the propagation and continuity of life. The germline directly connects animal species across evolutionary history and is also thus known as the immortal lineage. Germline immortality is of paramount importance for life. This immortality comes at a cost; the necessity to establish, maintain, protect, and repair the germline. In mammals, the acquisition of the germline from the soma provides the germline with an essential challenge, the necessity to erase and reset genomic methylation. The loss of DNA methylation unleashes transposon expression with the potential to annihilate the germline. The PIWI-interacting pathway (piRNA) pathway provides anti-transposon immunity. At the core of this pathway are the piRNAs and the PIWI proteins. piRNAs are short non-coding RNAs that are bound to PIWI proteins, members of the Argonaute family. Through base complementarity, piRNAs act as guides to recruit the PIWI proteins to cellular RNAs. In the cytoplasm, piRNAs guide PIWI-mediated cleavage of transposon transcripts through an RNAi mechanism to suppress transposition. In the nucleus, the piRNA pathway mediates transcriptional silencing of transposons through DNA methylation.
Antisense transposon-derived piRNAs guide the nuclear PIWI protein, MIWI2, to instruct transposon DNA methylation. piRNAs tether MIWI2 to the active transposon loci by base pairing to nascent transcripts. We have found the first effectors of nuclear MIWI2 function: SPOCD1, C19ORF84, SPIN1, and TEX15. SPOCD1 is a molecular scaffold and together with the adaptor protein C19ORF84 connects MIWI2 to the de novo methylation machinery (Nature2020 and Mol Cell 2024). We found that active LINE1 copies are marked with a unique set of chromatin modifications that recruit SPOCD1 through the chromatin reader SPIN1. This is the foundation for two-factor authentication process that prevents off-targeting or autoimmunity (Nature2024).
The piRNA pathway is found throughout metazoan life. The cytoplasmic pathway is ancient. The nuclear pathway is less conserved and has evolved several times during metazoan evolution. The nuclear piRNA pathway that mediates transposon DNA re-methylation in mice is believed to be bespoke to mammals. However, we found that Anolis SPOCD1 can interact with SPIN1 through the same mechanism as the mouse SPOCD1-SPIN1 interaction (Nature2024). This hints to an origin for the piRNA-directed DNA methylation early in tetrapod evolution.
The goal of this project is twofold:
- To further explore the mechanism by which SPOCD1 mediates piRNA-directed DNA methylation through biochemical and genetic means.
- To understand the origin the mammalian nuclear piRNA pathway through determining the conservation of piRNA factors’ interactions across tetrapod evolution.
For more information on the project, eligibility and how to apply for the School's PhD programme, please click on the 'Visit Institutional Website' link.
Applicants should apply to the School's Biological Sciences PhD programme via the University’s admissions portal (EUCLID) with a start date of 01 October 2026.
In the EUCLID application, applicants should state the project “The mechanism and evolution of piRNA-directed transposon methylation”, the research supervisor (Prof Donal O'Carroll) and their anticipated funding source (e.g. Darwin Trust).
