Evaluation of DNA damage in rainbow trout (Oncorhynchus mykiss) and gilthead sea bream (Sparus aurata) cryopreserved sperm

Cryopreservation causes several types of damage to spermatozoa, such as loss of plasma membrane integrity and functionality, loss of motility, and ATP content, resulting in decrease of fertility rates. This spermatozoal damage has been widely investigated for several marine and freshwater fish species. However, not much attention has been paid to the nuclear DNA. The objective of this study was to determine the degree to which cryopreservation induces spermatozoal DNA damage in two commercially cultured species, rainbow trout (Oncorhynchus mykiss) and gilthead sea bream (Sparus aurata), both of which could benefit from the development of cryopreservation strategies on a large scale. We have used the single-cell gel electrophoresis, commonly known as Comet assay to detect strand breaks in DNA. This technique was performed on fresh and cryopreserved sperm from both species. In rainbow trout there was a significant increase in the averages of fragmented DNA and Olive tail moment after cryopreservation (11.19-30.29% tail DNA and 13.4-53.48% Olive tail moment in fresh and cryopreserved sperm, respectively), as well as in the proportion of cells with a high percentage of DNA fragmentation. For gilthead sea bream there were no significant differences in the percentage of tail DNA between the control samples and sperm diluted 1:6 and cryopreserved (28.23 and 31.3% DNA(t), respectively). However, an increase in the sperm dilution rate produced an increase in the percentage of DNA fragmentation (41.4%). Our study demonstrates that cryopreservation can induce DNA damage in these species, and that this fact should be taken into account in the evaluation of freezing/thawing protocols, especially when sperm cryopreservation will be used for gene bank purposes.