Tolerance thresholds underlie responses to DNA damage during germline development

Selfish DNA modules like transposable elements (TEs) are particularly active in the germline, the lineage that passes genetic information across generations. New TE insertions can disrupt genes and impair the functionality and viability of germ cells. Using P-M hybrid dysgenesis in Drosophila, a sterility syndrome triggered by the P-element DNA transposon, we find that, unexpectedly, dysgenic germ cells contain few new TE insertions despite accumulating DNA double-strand breaks (DSBs) and inducing cell cycle arrest. Using an engineered CRISPR-Cas9 system, we show that generating DSBs at silenced P-elements or other non-coding sequences is sufficient to induce germ cell loss. Indeed, we demonstrate that developing and mitotic adult germ cells are sensitive to DSBs, in a dosage-dependent manner. Following the mitotic-to-meiotic transition, however, germ cells become more tolerant to DSBs, completing oogenesis despite accumulated genome damage. Our findings establish DNA damage tolerance thresholds as crucial determinants of safeguarding genome integrity during germline development.