Deficiency of either of the two homologs of poly(ADP-ribose) glycohydrolase (PARG), PARG-1 and PARG-2, in C. elegans lead to hypersensitivity to ionizing radiation (IR). In the germ cells of
parg-2 mutant worms, the dissipation of recombinase RAD-51 foci was slower than in wild-type cells, suggesting defects in DNA double-strand break (DSB) repair via homologous recombination (HR). Nevertheless, RPA-1, the large subunit of replication protein A, accumulated faster in
parg-2 worms and disappeared earlier than in wild-type worms. This accelerated RPA-1 accumulation may result from the enhanced expression of exonuclease-1 (EXO-1) after IR treatment. Accordingly, an
exo-1 mutation reduced IR sensitivity and accumulation of RPA-1 in
parg-2 worms. A mutation of
polq-1, encoding for a key factor in the alternative end-joining (Alt-EJ) pathway, suppressed the IR hypersensitivity phenotype of
parg-2 worms and normalized the kinetics of RAD-51 dissipation. This indicates that error-prone Alt-EJ may mediate DSB repair in
parg-2 worms, causing hypersensitivity to IR. In summary, PARG-2 deficiency in C. elegans causes hyperactive DSB end-resection likely through EXO-1 overproduction. DSBs with long single-stranded DNA ends in
parg-2 worms are thought to be repaired by Alt-EJ instead of HR, causing genomic instability.