By Nina Hårdnes Tremoen
Boar fertility has a major impact on overall pig reproductive efficiency, and good semen quality is essential for successful fertilization and proper embryo development. Thus finding semen characteristics that can predict fertility traits like pregnancy rate and total number of piglets born (TNB) is of great importance. However, the results of pregnancy rate and TNB are not available until the boars have been used in production for a period, and determination of phenotypes with effects on fertility at an earlier stage in the boars’ lives would be beneficial for estimating breeding values. To find methods to evaluate the boar’s sperm quality in relation to TNB and thus the boar’s fertility is of great importance also for the semen produced for sale. Identifying gene variants affecting these traits is of equal importance.
We found significant breed differences in the motility characters comparing ejaculates from Norwegian Landrace (NL) and Norwegian Duroc (ND). The percentage of hyperactivated sperm cells increased significantly upon storage in NL. In ND a larger portion of sperm cells with a hyperactive swimming pattern were detected at day 0, and the size of this population decreased upon storage. A significant decrease in the ATP level (p<0.0001) was also found in both breeds during storage. The motility characters linearity and wobble showed an effect on TNB in NL, at the day of collection and after storage, respectively. For ND, the percentage of motile cells, curvilinear velocity and amplitude of lateral lateral head displacement at the day of collection and linearity after storage showed an effect on TNB.
A significant negative effect on TNB was found for boars with contrasting DNA fragmentation index (DFI). This might explain some of the variation in TNB caused by the sperm quality of the individual boars, although the effect was moderate.
Transcriptome profiling by RNA sequencing (RNAseq) of testis tissue from NL and ND boars showed that 308 and 374 genes displayed significant different expression between high and low DFI boars, respectively. Of these genes, 71 were differentially expressed in both breeds. Gene ontology analysis revealed that significant terms in common for the two breeds included extracellular matrix, extracellular region and calcium ion binding capacity.
Two SNPs in BMPR1 and one SNP in COX-2 in NL were found significantly associated with an estimated breeding value for TNB. In ND, two SNPs in PLCz, one SNP in VWF and one SNP in ZP3 were found significantly associated to TNB. These SNPs explained between 0.27% and 1.18% of the genetic variance, which is quite low and not interesting for direct selection in breeding programs. However, the associated variants can be of interest in SNP-panels used for genomic selection.
Based on the associations found between motility parameters and TNB, and between DNA fragmentation and TNB, this thesis shows that male fertility is an important part of the total fertility in pigs. In addition, this thesis have contributed on the knowledge on the genetics of male fertility and DNA fragmentation in sperm cells in pigs.