degraded preferentially (30,39,43). Selective autophagy of mitochondria or mitophagy under certain conditions is mediated by accumulation of PINK1, a serine/threo- nine–protein kinase on dysfunctional mitochondria and subsequent recruitment of Parkin, an E3 ubiquitin ligase to those mitochondria (44–46). However, the necessity and sufﬁciency of these events to induce mitophagy in vivo is unclear (47,83). UNC51 is a serine/threonine kinase with a role in autophagy induction; BEC1, a class III phosphatidy- linositol 3-kinase (PI3K), is involved in recruiting other autophagy proteins to pre-autophagosomal structures; and ATG18, which binds phosphoinositol-3-phosphate, is required for autophagosomes formation; mutation or knockdown of these genes results in abnormal autophagy in C. elegans (71). We found that RNAi knockdown of autophagy/mitophagy genes bec-1, unc-51 and pink-1 inhibited removal of UVC-induced mtDNA damage, suggesting that autophagy is an important mechanism of degradation of damaged mtDNA. The lack of an effect of knockdown of atg-18 may result from insufﬁcient knockdown or a degradation mechanism independent of atg-18. We noted that knockdown of autophagy proteins bec-1 and unc-51 prevented the trend of an age-dependent decrease in mtDNA copy number; however, atg-18 knockdown did not prevent this decrease. This is consist- ent with incomplete knockdown of atg-18, which might permit residual mitochondrial turnover and explain why knockdown did not inhibit damage removal. Interestingly, knockdown of pink-1 but not pdr-1, the C. elegans Parkin homolog, inhibited mtDNA damage removal. Parkin is thought to act downstream of PINK1 and in other model systems compromised function of either reduces mitophagy (44,45). In C. elegans, few studies have investigated the role of these genes in relation to mito- chondrial function. pdr-1 mutants have reduced oxygen consumption and are sensitive to mitochondrial complex I inhibitors (84), pink-1 mutants have reduced cristae length and are sensitive to oxidative stress (85) and expres- sion of wild-type pink-1 and pdr-1 rescued a-synuclein induced mitochondrial fragmentation (86). These reports are consistent with those in other models suggesting conserved function of these proteins between C. elegans and mammals (85). Further research is needed to better understand mitophagy in C. elegans and the role of pink-1 and pdr-1 in that process. Knockdown of fusion genes fzo-1 and eat-3 and ﬁssion gene drp-1 also inhibited removal of UVC-induced mtDNA damage. The role of mitochondrial morphology and dynamics in mediating mitophagy is unclear. Inhibition of ﬁssion via manipulation of ﬁssion genes Fis1 and Drp1 results in reduced mitophagy, whereas overexpression of ﬁssion proteins induces mitophagy and decreases mitochondrial mass (87). Consistent with these studies, our results indicate the Drp-1 is required for removal of damaged mtDNAs. It may be tempting then to assume that knockdown of fusion genes would enhance mitophagy and removal of mtDNAs. Certainly OPA1 deﬁciency results in a general increase in autophagy, and mitochondria targeted for degradation exhibit OPA1 depletion (39,88). However, loss of mitochondrial mass Nucleic Acids Research, 2012, Vol. 40, No. 16 7925 resulting from nicotinamide treatment is partially inhibited by fusion knockdown rather than enhanced as might be predicted (89), and studies that inhibit fusion without compromising MP or mitochondrial function show that ﬁssion alone without accompanying mito- chondrial dysfunction does not result in increased mitophagy (43,90). Consistent with our data, human ﬁbroblasts harboring a missense mutation in the fusion gene MFN2 exhibit signiﬁcantly reduced repair efﬁ- ciency of oxidative mtDNA damage and DNA instability (91). Our data suggest that fusion is required for removal of mtDNA damage; whether this is by facilitating autophagy or another mechanism requires further research. Persistent mtDNA damage induces autophagy without detectable changes in mitochondrial morphology We considered that slow removal of UVC-induced mtDNA damage may reﬂect the natural turnover rate of mitochondria and not an induction in autophagy. We therefore evaluated the level of autophagy following UVC exposure in LGG-1::GFP expressing C. elegans. We observed a mild increase in the number of autophagosomes in the UVC-exposed nematodes 24 h after treatment, suggesting that UVC exposure does induce autophagy. Whether this increase in autophagy signiﬁes a greater mitochondrial clearance requires further research. Mitochondrial dynamics are responsive to mitochon- drial function. Loss of MP inhibits fusion and results in an overall fragmented morphology and treatment with pro-oxidants or mitochondrial respiratory chain uncouplers triggers fragmentation (92–94). On the other hand, mitochondrial hyperfusion has been observed following treatment with DNA intercalators or UVC (94,95) highlighting the complex responses of mitochon- drial dynamics to mitochondrial stressors. We evaluated the effect of UVC exposure on mitochondrial morph- ology. We did not observe a difference in mitochondrial interconnectivity or elongation with UVC treatment at 24 or 48 h after treatment. There remains the possibility that changes in morphology occurred between the time points analyzed; however, given the persistence of this damage we would expect any morphological changes induced by this damage would also be persistent. Thus, while our genetic data demonstrate the importance of mitochondrial dynamics in responding to persistent mtDNA damage, such damage did not lead to changes in mitochondrial morphology that were easily observable by microscopy. Recovery from UVC-induced larval arrest requires mtDNA replication and mitochondrial fusion and is aided by removal of damaged mtDNA via autophagy Mitochondria contain multiple copies of DNA; thus recovery from helix-distorting mtDNA damage could be achieved by both removal of the damaged copies and dilution of damaged copies with undamaged genomes. In adult glp-1 C. elegans raised at 25 C [(52,67) and Figure 1d] and 15 C (Supplementary Figure S1),