Defining Dermo resistance phenotypes in an eastern oyster breeding population

Abstract

Perkinsus marinus, the causative agent of Dermo disease, is responsible for mass mortalities and negatively impacts aquaculture production of the eastern oyster, Crassostrea virginica. Selective breeding is a viable option for Dermo disease management; however, fluctuations in natural selection pressure and environmental noise hinder accumulation of genetic gains acquired through field performance trials. The purpose of this study was to adapt and apply laboratory disease challenge methods to eastern oysters, better characterize resistance‐specific traits and assess the potential for genetic variation in Dermo resistance among distinct families within a breeding population. Two challenge experiments were conducted, one in 2014 and the other in 2015. Significant treatment (control vs. challenged) and family effects on survival (measured as per cent survival and days to death) were detected in the 2014 challenge, while overall high survival precluded the detection of a significant family effect in the 2015 challenge. An alternate measure of resistance, parasite elimination rate, was also measured in the 2015 challenge, and this varied significantly among families. Thus, both survival and the change in parasite concentration in oyster tissues over time represent Dermo resistance phenotypes that can be measured accurately with the adapted laboratory disease challenge protocol described here. The obvious next step is to incorporate the challenge protocol in eastern oyster breeding programmes to assess whether well‐defined, accurately measured, Dermo‐resistant phenotypes result in enhanced genetic improvement for this commercially important trait.

Understanding Crassostrea virginica tolerance of Perkinsus marinus through global gene expression analysis

“…To estimate infection levels in each sampled oyster, 1 µL diluted DNA was added to a P. marinus -specific quantitative polymerase chain reaction (qPCR) assay ( De Faveri et al, 2009 ). Parasite load was initially measured as the number of P. marinus DNA copies in 100 ng of total DNA and subsequently converted to (log) spores g −1 wet tissue weight as described in Proestou et al (2019) . Mantle tissue is recommended when using PCR-based methods to diagnose and quantify the intensity of dermo disease by the World Organization of Animal Health (2022) .…”

“…We restricted our RNAseq and expression analyses to individuals censored at Day 7 post-injection. These oysters were the focus because previous work identified peak infection intensities and significant transcriptomic responses to the parasite at this time point ( Proestou et al, 2019 ; Proestou and Sullivan, 2020 ). Specific animals were selected based on the median (log) spores g −1 wet weight for each group to minimize within group variation in transcriptomic response to the parasite.…”

“…Significant differences in parasite avoidance behavior (particle clearance rate and time spent feeding in the presence of P. marinus ) were detected among genetically distinct groups of eastern oysters ( Ben-Horin et al, 2018a ). A 6-week laboratory dermo challenge that tracked survival and parasite concentrations in eastern oyster tissues over time also identified differences in resistance (measured as the slope of the line depicting the relationship between parasite load and time) among multiple families ( Proestou et al, 2019 ). Furthermore, differential gene expression analysis of P. marinus -injected and uninfected control oysters from one resistant and one sensitive eastern oyster family indicated that resistant oysters mounted an immediate response to the parasite that involved regulation of proteolysis, peptidase inhibitor activity, and oxidoreductase activity.…”

Potential of genomic technologies to improve disease resistance in molluscan aquaculture

“…The resulting lines have been experimentally tested across the range of eastern oyster production, and survivorship was generally superior compared to wild oysters, although oysters had greater survival in areas where they were selectively bred making the comparison more difficult [61,62]. Laboratory challenge of oysters with P. marinus revealed that there was a 54.1% difference in survival between two families in 2014, but no significant family effect was observed when the challenge was repeated in 2015 [63]. This demonstrates that while breeding programmes based on mass selection can achieve significant results, it is more difficult to develop an understanding of disease resistance as environmental factors are highly variable.…”