Differential resistance of mammalian sperm chromatin to oxidative stress as assessed by a two-tailed comet assay

Maria EncisoA,C, Stephen D. JohnstonB and Jaime GosaalvezA

AUnidad de Genetica, Edificio de Biologia, Universidad Autonoma de Madrid, C/Darwin n° 2, 28049 Madrid, Spain.
BSchool of Agriculture and Food Science, University of Queensland, Gatton, Qld 4343, Australia.
CCorresponding author. Email:

Protamines of eutherian species are cysteine-rich molecules that become cross-linked by disulfide bonds during epididymal transit, whereas the protamines of most marsupial species lack cysteine residuals. The present study made use of the differences in protamine structure between eutherian and metatherian mammal spermatozoa to examine the comparative resistance of sperm DNA to oxidative damage in three eutherian species (Mus musculus, Homo sapiens, Sus domesticus) and three metatherian species (Vombatus ursinus, Phascolarctos cinereus, Macropus giganteus). Sperm
DNA fragmentation of samples exposed to increasing concentrations of hydrogen peroxide was assessed by means of the two-tailed comet assay. The sperm DNA of the marsupial species studied were significantly more sensitive to oxidative stress than the spermatozoa of eutherian species. Such susceptibility is consistent with the lack of disulfide cross-linking in marsupial sperm chromatin and suggests that the oxidation of thiols to disulfides for chromatin condensation during epididymal transit in eutherian mammals is likely to be important in order to provide stability and protect these cells from the genotoxic effects of adverse environments.