PROJECT SUMMARY We propose a cross-species approach to understand the pathways that maintain genome integrity in germ cells. The perpetuation of the species requires that the genome is protected from endogenous and exogenous threats. Nowhere is this more critical than in the germ cells, the precursors of egg and sperm. Defects in the vigilance pathways lead to loss of genome integrity and subsequent deterioration of offspring vigor and/or miscarriage. Detailed studies of a newly identified, highly conserved genetic factor, GCNA (Germ Cell Nuclear Acidic Peptidase), will provide novel insights into key determinants of genome integrity and fertility. Our preliminary data show that loss of GCNA results genomic instability in both Drosophila and C. elegans, indicating conservation of function. The experiments outlined here will provide some of the first insights into the shared mechanisms by which conserved GCNA family members promote genomic integrity and fertility across species. We find that GCNA both interacts with Topoisomerase 2 and prevents the formation of DNA:protein crosslinks (DPCs). GCNA mutant animals display increased chromosome loss, mutation accumulation, and cell cycle defects, placing GCNA at the hub of multiple genome integrity processes. To explore the mechanisms underlying gcna functions in germ cell maintenance, we propose three specific aims: 1) to analyze the etiology of chromosome loss, and to determine the contribution of gcna to cell cycle regulation, 2) to characterize the interaction with topoisomerase 2, identify conserved binding partners, and illuminate the nature and etiology of DPCs, and 3) to understand the domain structure of GCNA and its functional relevance to human fertility.