A GENETIC BASIS OF REPRODUCTIVE PERFORMANCE IN SELECTIVELY BRED ARCTIC CHARR Salvelinus alpinus

Introduction

Arctic charr (Salvelinus alpinus) is an economically important farmed species in Sweden. However, the farming industry of Arctic charr is relatively small (global production totaled 5000 - 6000 t per year) and limited by the poor reproductive performance of hatchery-origin broodstock. An in-depth study of limiting factors in fertilization success and hatching rates is needed to expand the Arctic charr production.

This study aimed to gain extensive knowledge regarding factors that affect sperm quality in selectively bred Arctic charr. The main parameters of male gametes’ quality, including motility characteristics and velocity, were examined by a computer-assisted sperm analysis (CASA) system. Furthermore, we estimated inbreeding coefficients using available pedigree records and investigated for potential associations with the motility traits.

Materials and methods

Arctic charr from the national Swedish breeding program (2017-year class) were used in this study. Sperm samples were collected from about 499 males between October and November 2020. Males were separated into two experimental groups. The first experiment included a test group (group A) held with higher water flow and a control group with regular water flow. The second experiment comprised a test group (group B), which was exposed to water cooling (from July to November) at 3 lower temperature than in the control group with natural water temperature. The number of animals recorded per test group is specified in Table 1.

The evaluation of sperm motility and velocity, including average path velocity (VAP), curvilinear velocity (VCL), straight-line velocity (VSL), was performed by using a computer-assisted sperm analysis (CASA) system and SCA® Motility imaging software. Sperm density was measured using NucleoCounter® SP-100™ (Chemotech, Denmark). Basic statistics were computed for CASA-system parameters using statistical software packages in R (version .4.0.2).

Furthermore, the inbreeding coefficient for full-sibs was derived from the available pedigree recordings spanning since the 1980s using the INBUPGF90 v1.43 software from the BLUPF90 suite (Misztal et al., 2018). Single nucleotide polymorphisms (SNPs) were detected using double digest restriction-site associated DNA (ddRAD) sequencing. Thereafter, weighted single step genomic best linear unbiased prediction (WssGBLUP) analysis was performed using preGSf90 v1.21 and postGSf90 v1.70 from the BLUPF90 software suite.

Results

According to the results obtained, milt density ranged between 0.05 - 13.54 ×109 cells/ml (mean 3.27×109) for group A and 0.02 - 9.40 ×109 cells/ml (mean 3.02 ×109) for group B. In group A, 10 % higher density was observed in the test group exposed to higher water flow compared to the control group. However, this difference was not significant. Similarly, in group B, no significant difference of 14 % was found in sperm density between the control and test groups when applying water cooling.

The mean sperm motility of 74 % (range 5 % to 99 %) and 65 % (range 8 % to 99 %) was found for group A and B, respectively. The average VAP, VSL, and VCL for group A were equal to 52.9 µm/s, 38.5 µm/s, and 76.7 µm/s, respectively. The corresponding mean values of 47.9 µm/s (VAP), 33.3 µm/s (VSL), and 72.3 µm/s (VCL) were obtained for group B. A high positive correlation of 0.95 - 0.98 (p < 0.05) was observed between sperm velocity parameters (VAP, VSL, and VCL). The differences in the percentage of motility and sperm velocity between control and tested males in either group A or B were not significant.

The inbreeding coefficient amongst the studied animals ranged between 0.0 - 0.18, with a mean value of 0.07. A Pearson correlation coefficient close to zero was obtained between the inbreeding coefficient and sperm quality parameters. A heritability for sperm density was 0.18 (SE 0.08); for motility, it ranged between 0.18 - 0.24 (SE 0.08), while for the sperm-related velocities, it was 0.23 (SE 0.09). In addition, positive genetic correlations were observed among the sperm quality traits (range: 0.22 - 0.99).

Overall, 4113 SNPs that passed quality control step were used for genomic association studies. WssGBLUP did not identify loci with major effect for the sperm quality traits. Genomic regions explaining more than 1 % of the additive genetic variance for studied traits were identified across 21 chromosomes, where chromosomes 11 and 12 included both motility and sperm density traits.

Discussion and Conclusion

This study covered the males’ gamete quality evaluation related to milt density, motility, and velocity characteristics. No significant differences in the sperm quality parameters were found between the control and test animal groups kept under conditions with lower temperature or higher water flow.

The inbreeding coefficient of the studied animals was estimated based on the available pedigree records for animals from the 1986 - 2017 year classes, which did not exceed 0.18. Furthermore, the correlation estimated between the inbreeding coefficient and the milt quality parameters was close to zero.

Moderate heritability values for sperm related traits were estimated suggesting the potential for genetic improvement through selective breeding. Moreover, the positive genetic correlations between sperm parameters obtained in our study suggest that selection for one sperm trait should have a favorable effect on other traits. A genome-wide association study revealed genomic regions with a potential impact on male gamete quality. Overall, the obtained results disclose valuable insights into the genetic basis of sperm traits and could assist in designing breeding programs for improving reproductive performance in Arctic charr.

ACKNOWLEDGEMENTS

The authors acknowledge support from FORMAS under grant agreement 2018-00869 (NextGenCharr) and the Kolarctic funding body under the ARCTAQUA project (grant agreement 4/2018/095/KO4058).