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Koi herpesvirus disease (KHVD), caused by Cyprinid herpesvirus-3 (CyHV-3), is one of the most serious threats to carp farming. In the present study, we investigated the efficiency of a low-density (LD) SNP panel for estimating genetic parameters and breeding values to KHVD resistance in the Amur mirror carp (AMC). Two populations (Pop 1 and Pop 2) of AMC generated from unrelated parents were created using a partial factorial design. One-year old fish (Pop 1 = 1500 individuals.; Pop 2 = 1200 individuals) were challenged with CyHV-3 and phe-notyped to KHVD resistance. 218 SNPs originating from a medium genotyping platform previously applied to Pop 1 (15615 SNPs; denoted as MD panel) with the highest association to KHVD resistance were used as a LD panel to genotype individuals of Pop 2. Genetic parameters and estimated pedigree-based BLUP (EBV) and genomic-based GBLUP (GEBV_MD and GEBV_LD) breeding values were calculated and obtained for Pop 1 using either pedigree, MD or LD panel and for Pop 2 using either pedigree or the LD panel. The heritability estimates of KHVD resistance were very high for both populations ranging from 0.42 to 0.96. Selection for KHVD resistance in Pop 2 using the LD panel would have led to a relative increase of similar to 7% in prediction accuracy compared to the pedigree-based selection. Pearson correlation coefficients between pedigree-based and genomic-based estimated breeding values (EBV vs. GEBV_MD; EBV vs. GEBV_LD; GEBV_MD vs. GEBV_LD) showed a strong association for both populations (0.79 - 0.91). In addition, the concordance rate of individuals selected by pedigree-based (EBV) and genomic-based breeding values (GEBV_MD and GEBV_LD) within selection pressures of 5%, 10% and 20% were not statistically different in most cases. In conclusion, the low-density SNP panel could be useful for a selection program focused on the genetic improvement of KHVD resistance.

Arctic charr (Salvelinus alpinus) is a niche-market high-value species for Nordic aquaculture. Similar to other salmonids, both anadromous and landlocked populations are encountered. Whole-genome re-sequencing (22X coverage) was performed on two farmed populations of anadromous (Sigerfjord; n = 24) and landlocked (Arctic Superior; n = 24) origin from Norway and Sweden respectively. More than 5 million SNPs were used to study their genetic diversity and to scan for selection signatures. The two populations were clearly distinguished through principal component analysis, with the mean fixation index being similar to 0.12. Furthermore, the levels of genomic inbreeding estimated from runs of homozygosity were 6.23% and 8.66% for the Norwegian and the Swedish population respectively. Biological processes that could be linked to selection pressure associated primarily with the anadromous background and/or secondarily with domestication were suggested. Overall, our study provided insights regarding the genetic composition of two main strains of farmed Arctic charr from Scandinavia. At the same time, ample genomic resources were produced in the magnitude of millions of SNPs that could assist the transition of Nordic Arctic charr farming in the genomics era.

Routine implementation of genomic information for guiding selection decisions is not yet common in the majority of aquaculture species. Reduced representation sequencing approaches offer a cost-effective solution for obtaining genome-wide information in species with a limited availability of genomic resources. In the current study, we implemented double-digest restriction site-associated DNA sequencing (ddRAD-seq) on an Arctic charr strain with the longest known history of selection (approximately 40 years) aiming to improve selection decisions. In total, 1730 animals reared at four different farms in Sweden and spanning from year classes 2013-2017 were genotyped using ddRAD-seq. Approximately 5000 single nucleotide polymorphisms (SNPs) were identified, genetic diversity-related metrics were estimated, and genome-wide association studies (GWAS) for body length at different time points and age of sexual maturation were conducted. Low genetic differentiation amongst animals from the different farms was observed based on both the results from pairwise F-st values and principal component analysis (PCA). The existence of associations was investigated between the mean genome-wide heterozygosity of each full-sib family (year class 2017) and the corresponding inbreeding coefficient or survival to the eyed stage. A moderate correlation (-0.33) was estimated between the mean observed heterozygosity of each full-sib family and the corresponding inbreeding coefficient, while no linear association was obtained with the survival to the eyed stage. GWAS did not detect loci with major effect for any of the studied traits. However, genomic regions explaining more than 1% of the additive genetic variance for either studied traits were suggested across 14 different chromosomes. Overall, key insights valuable for future selection decisions of Arctic charr have been obtained, suggesting ddRAD as an attractive genotyping platform for obtaining genome-wide information in a cost-effective manner.