Transcriptomic responses of diploid and triploid oysters to stress

Triploid oysters are commonly used as the basis for production in the aquaculture of eastern oysters along the U.S East and Gulf of Mexico coasts. While they are valued for their rapid growth, incidents of triploid mortality during summer months have been well documented in eastern oysters, especially at low salinity sites. We compared global transcriptomic responses of diploid and triploid oysters bred from the same three maternal source populations at two different hatcheries and outplanted to a high and low salinity site. Oysters were sampled for gene expression at the onset of a mortality event in summer of 2021 to identify triploid-specific gene expression patterns associated with low salinity sites, which ultimately experienced greater triploid mortality. We also examined chromosome-specific gene expression to test for instances of aneuploidy in experimental triploid oyster lines, another possible contributor to elevated mortality in triploids. We observed a strong effect of hatchery conditions (cohort) on triploid-specific mortality (field data) and a strong interactive effects of hatchery, ploidy and outplant site on gene expression. At the low salinity site where triploid oysters experienced high mortality, we observed downregulation of transcripts related to calcium signaling (Calmodulin-a, Histidine-rich calcium binding protein (HRC), and cadherin EGF LAG seven-pass G-type receptor 1(CELSR1)), ciliary activity (axonemal and cytoplasmic dyneins) , and cell cycle checkpoints (CDK1, HAUS augmin-like complex subunit 3, and MAPKKK, MCM7, SMCs, RTEL1) in triploids relative to diploids. These transcripts suggest dampening of the salinity stress response and problems during cell division as key cellular processes associated with elevated mortality risk in triploid oysters . No instances of aneuploidy were detected in our triploid oyster lines. Our results suggest that triploid oysters may be fundamentally less tolerant of rapid decreases in salinity, indicating that oyster farmers may need to limit the use of triploid oysters to sites with more stable salinity conditions.