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Pacific oysters, Crassostrea gigas, are one of the most productive aquaculture species in the world. However, they are threatened by the spread of Ostreid herpesvirus-1 (OsHV-1) and its microvariants (collectively &ldquo, &micro, vars&rdquo, ), which cause mass mortalities in all life stages of Pacific oysters globally. Breeding programs have been successful in reducing mortality due to OsHV-1 variants following viral outbreaks, however, an OsHV-1-resistant oyster line does not yet exist in the United States (US), and it is unknown how OsHV-1 &micro, vars will affect US oyster populations compared to the current variant, which is similar to the OsHV-1 reference, found in Tomales Bay, CA. The goals of this study were to investigate the resistance of C. gigas juveniles produced by the Molluscan Broodstock Program (MBP) to three variants of OsHV-1: a California reference OsHV-1, an Australian &micro, var, and a French &micro, var. This is the first study to directly compare OsHV-1 &micro, vars to a non-&micro, var. The survival probability of oysters exposed to the French (FRA) or Australian (AUS) &micro, var was significantly lower (43% and 71%, respectively) than to the reference variant and controls (96%). No oyster family demonstrated resistance to all three OsHV-1 variants, and many surviving oysters contained high copy numbers of viral DNA (mean ~3.53 &times, 108). These results indicate that the introduction of OsHV-1 &micro, vars could have substantial effects on US Pacific oyster aquaculture if truly resistant lines are not achieved, and highlight the need to consider resistance to infection in addition to survival as traits in breeding programs to reduce the risk of the spread of OsHV-1 variants.

Yearly mass mortalities of Pacific oysters in a major oyster-growing estuary on the West Coast of the United States (Tomales Bay, California) present a constraint to shellfish aquaculture. These mortalities have been associated with the presence of Ostreid herpesvirus 1 (OsHV-1; Tomales Bay strain) and increased seawater temperature in the estuary. In order to develop oyster families that are more tolerant to OsHV-1 than current oyster stocks, one cohort was planted in Tomales Bay per year over a three-year period, two of which were selected based on survival data collected from previous cohorts. Analyzing survival data from 23,590 oysters from 191 biparental families planted in Tomales Bay over three years, we estimated the heritability of survival on the observed and underlying liability scales to be 0.30 and 0.47, respectively. Survival breeding values increased 10.3 and 21.2 percentage points after one and two cycles of selection, respectively, suggesting that genetic selection was highly effective at producing oysters that were more tolerant of OsHV-1. The oyster families developed in this breeding program are the first in the United States specifically bred to be tolerant to OsHV-1 under elevated temperatures and will be valuable to oyster growers as well as researchers studying the genetic basis of OsHV-1 tolerance in Pacific oysters.

Background Variants of the Ostreid herpesvirus 1 (OsHV-1) cause high losses of Pacific oysters globally, including in Tomales Bay, California, USA. A suite of new variants, the OsHV-1 microvariants (μvars), cause very high mortalities of Pacific oysters in major oyster-growing regions outside of the United States. There are currently no known Pacific oysters in the United States that are resistant to OsHV-1 as resistance has yet to be evaluated in these oysters. As part of an effort to begin genetic selection for resistance to OsHV-1, 71 families from the Molluscan Broodstock Program, a US West Coast Pacific oyster breeding program, were screened for survival after exposure to OsHV-1 in Tomales Bay. They were also tested in a quarantine laboratory in France where they were exposed to a French OsHV-1 microvariant using a plate assay, with survival recorded from three to seven days post-infection. Results Significant heritability for survival were found for all time points in the plate assay and in the survival phenotype from a single mortality count in Tomales Bay. Genetic correlations between survival against the French OsHV-1 μvar in the plate assay and the Tomales Bay variant in the field trait were weak or non-significant. Conclusions Future breeding efforts will seek to validate the potential of genetic improvement for survival to OsHV-1 through selection using the Molluscan Broodstock Program oysters. The lack of a strong correlation in survival between OsHV-1 variants under this study’s exposure conditions may require independent selection pressure for survival to each variant in order to make simultaneous genetic gains in resistance.

We describe the introduction, taxonomic identification and genetic parameters for farm traits of the Midori strain of the Pacific oyster Crassostrea gigas - a possible alternative to the widely farmed Miyagi strain on the US West Coast. Broodstock (G0) oysters collected from the mouth of the Midori river and near Iwajima Island, Ariake Bay, Japan were identified as C. gigas based on the nuclear internal transcribed spacer (ITS) region and 28S gene as well as a mitochondrial (COI) marker. G0 and G1 generations and G2 larvae and spat were maintained in quarantine conditions and subjected to numerous histological exams and PCR tests for detection of pathogens of concern. After the G1 generation and G2 early life stages were found to be free of detectable diseases, G2 spat were planted at farm test sites and adults were used to produce a G3 generation. This G3 generation was cultured under non-quarantine conditions and families were planted at sub-tidal and inter-tidal test sites, in Oregon and California, respectively. Field performance traits were measured to determine the potential for improvement through selection. Overall, Midori oysters showed significantly lower values for harvest yield, survival, individual average weight and dry meat weight compared with oysters from a selected stock of the Miyagi strain of Pacific oyster; however, shell shape index (a measure of shell depth) was significantly greater in Midori oysters. Heritability estimates for group traits at harvest (survival, yield and individual average harvest weight) were high, whereas heritability estimates for individual traits (individual weight, wet meat weight and dry meat weight) and for shell dimension measurements (shell length, shell width and shell depth) were moderate-to-high. Family-by-site (GxE) correlations were low-to-moderate for all measured traits. Genetic correlations were positive and moderate-to-high among group traits and shell dimensions. In contrast, correlations between shell shape and other traits had small-to-moderate negative values but these were not statistically significant. Estimated potential genetic gain for dry meat weight was high (46%) whereas estimated gains for yield, individual average weight, individual weight were high-to-moderate (approximately 10%). In contrast, shell shape showed very little potential for genetic gain through selection. Overall, the Midori strain of the Pacific oyster shows promise as a deep-cupped oyster for the half-shell trade. Statement of relevance We describe the introduction of a new strain of Pacific oyster (Midori) collected from southern Japan to the US West Coast. Rigorous quarantine protocols and disease tests were applied to ensure that the risks of introducing diseases were minimized. The Midori strain was found to have a deeper cup than the commonly farmed Miyagi strain but the potential estimated gain for improving this trait through selection was small.

Several Pacific oysters breeding programs selecting for improved aquaculture traits have been described in the literature; however, there are differences in the approaches and traits under selection. Here we discuss the approach and results of the Molluscan Broodstock Program (MBP) to improve traits of interest for US West Coast oyster farmers. In particular, we compare selection methods based on individual versus group traits measured per grow-out container. A total of 442 group records (per container) and 2197 individual records for field traits of up to 55 families were used for statistical analyses. Heritability estimates for group traits at harvest for survival, yield and individual average harvest weight were moderate-to-high and the latter estimate was not statistically significantly different from harvest weight based on individual measurements. Family-by-site (G × E) correlations were moderately negative between shell length and shell shape and strongly positive between yield and individual average weight. Genetic correlations were all positive among group traits, ranging from moderately positive between individual average weight and survival to highly positive between individual average weight and yield. Genetic correlations between individual average weight (a group trait) and individual weight (an individual trait) ranged from 0.95 to 0.99. In contrast, genetic correlations between shell shape and group traits (yield, individual average weight and survival) ranged from −0.63 to −0.22. The potential genetic gains were higher for group traits (12.1%, 8.4% and 6.9% for yield, individual average weight and survival, respectively) than for individual weight (5.6%). The BLUP accuracies were similar between group traits and individual weight. High genetic correlations between individual average weight and individual weight at harvest suggest that gains in individual weight can be achieved by selection for either individual average weight or individual weight; however, our results indicated that selecting families for shell shape will cause an unfavorable decrease of other commercially important traits. Statement of relevance The Pacific oyster, Crassostrea gigas, is a globally cultivated species that is grown under a wide range of environmental and husbandry conditions. In this study, we describe genetic parameters for desirable traits of selected Pacific oysters planted at commercial farms on the West Coast, US, as well as a comparison of selection methods based on individual versus group traits measured per grow-out container. Our results indicate that group traits and individual traits are genetically associated and have moderate-to-high heritabilities. Furthermore, we found a very high genetic correlation between individual average weight per container (a group trait) and weights of individual oysters sampled per container (an individual trait) at harvest, suggesting that individual average weight is a valid and less costly alternative to measurements of sampled individual oysters in breeding programs designed to improve oyster harvest weight. The results will be of interest to other breeding programs for Pacific oysters.