The presence of a marker associated with Pacific oyster resistance to OsHV-1 does not affect susceptibility to Vibrio aestuarianus

The Pacific oyster, Crassostrea (Magallana) gigas, is an important species in aquaculture globally, but its production is threatened by pathogens including ostreid herpesvirus 1 (OsHV-1) and Vibrio aestuarianus. A genetic marker associated with resistance to OsHV-1 infection has been identified on chromosome 8 (Chr8) of the Pacific oyster genome. Marker-assisted selective breeding has used the Chr8 marker to produce oyster families with increased survivorship and resistance to OsHV-1 infection. The potential effect of the Chr8 marker on susceptibility to other pathogens remains largely unknown but was recently associated with increased resistance of spat to Vibrio coralliilyticus. Here we assess the effect of the presence and allelic dosage of the Chr8 marker on the susceptibility of selectively bred juvenile Pacific oysters to V. aestuarianus infection. Sixteen families were produced with various Chr8 marker genotypes, and a V. aestuarianus disease challenge was conducted. Challenged oysters were individually genotyped at the Chr8 marker, and Vibrio susceptibility was evaluated among genotypes within and between families. Prior to the challenge trial, the Chr8 marker did not occur at the expected Mendelian ratios in families with heterozygous parents; in multiple families the homozygous alternate genotype occurred at a lower frequency than was expected. Mortality rates in the Vibrio challenge differed between families, ranging from 47.9% to 85.4%, but no association was observed between the Chr8 marker and survival to V. aestuarianus exposure. Therefore, we observed no pleiotropic effect (positive or negative) of the Chr8 marker on survivorship to V. aestuarianus at the evaluated life stage. Reduced-representation sequencing was used to genotype the challenged oysters and a genome-wide association study for V. aestuarianus survivorship was performed, but no significant associations were found, suggesting polygenic architecture for the trait.