The prediction of male fertility remains today a major goal for the promotion of animal insemination (AI) in the bovine industry. Current predictions obtained by combining several markers of sperm quality provide promising results but remain still unsatisfactory for a routine use in AI centers. Data acquired in the last years now emphasize the crucial role of sperm epigenetics in sperm functions and fertilization efficiency. A better understanding of epigenetic mechanisms involved in spermatogenesis and sperm fertility may highlight new key criteria that could improve prediction models.

The objective of this study was to evaluate the relationship between CpG methylation status of the imprinted Gnas locus and the field fertility of 25 Holstein bulls. A total of 56 CpG positions have been analyzed by bisulfite conversion and pyrosequencing. The fertility values (FV; mean=-2.5±4.7; min/max=[-16.4/+4.1]) was expressed as non-return rates at 28 days from 582±77 inseminations, taking environmental effects into account. Bulls were grouped into three classes of fertility according to their FV (sub-fertile N=11 [-16.4/-4]; normal FV N=9 [-2.7/+1.2]; fertile N=5 [+2.2/+4.1]).

Sub-fertile bulls showed more methylation (for instance, 27.2% for one CpG position) than normal or fertile bulls (22.9%) on several CpG sites (non parametric permutation test; p<0.05). Significant negatives correlations were observed between methylation percentage and FV (R=-0.39 to -0.54 p<0.05 according to the CpG position).

In conclusion, our study provides evidences that DNA methylation of Gnas is correlated to the field fertility of bulls. Methylation quantification, used as a new marker, alone or in combination with other sperm quality assessment parameters, may thus represent an important tool for the bull fertility prediction.  

This work was supported by ANR (Labcom SeQuaMol).