Background DNA damage is higher in summer than winter in both free-living and captive birds.

Although stress can cause overall damage to the genome, it is currently unknown whether normal background damage to DNA varies throughout the annual cycle. If DNA damage did vary seasonally, it would have major implications on environmental-genomic interactions. We measured background DNA double-stranded breaks using the neutral comet assay in five tissues (nucleated red blood cells, abdominal fat, hippocampus, hypothalamus, and liver) in four cohorts of house sparrows (Passer domesticus): free-living summer, captives on a summer light cycle, free-living winter, and captives on a winter light cycle. The experiment was designed to answer three questions: (1) Is red blood cell DNA damage representative of other tissues? (2) Is DNA damage in captive birds representative of DNA damage in free-living birds? (3) Does DNA damage show seasonality? We found that (1) blood is a representative tissue, (2) captive animals are representative of free-living animals, and (3) DNA damage is higher in the summer than in the winter. These data indicate that red blood cells can be an index of DNA damage throughout the body and that background levels of DNA damage show substantial seasonal variation. The latter result suggests the possibility that underlying molecular mechanisms of DNA damage and/or repair also change seasonally.