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Since 2012, aquacultured eastern oysters Crassostrea virginica have been reported by oyster farmers to display mortality approaching 30%, and in some cases 85%. in areas of the lower Chesapeake Bay, VA. Based on accounts from industry, this mortality has typically affected 1-y-old oysters between May and early July, and has tended to occur in triploid oysters, which represent the vast bulk of production in the area. During this period, samples submitted for pathology have not revealed the presence of major pathogens as a cause. In 2015, to gain deeper insight into this mortality and determine whether specific sites, ploidy condition, or genetic lines were affected, oyster seed commercially produced in early 2014 were obtained from four lines, one diploid (2N DEBY) and three triploid (3N DEBY, 3N hANA, and 3N Northern). These lines were deployed in July 2014 at aquaculture farms at five Chesapeake Bay locations: Locklies Creek and Milford Haven on the western shore, and Pungoteague Creek. Nassawadox Creek, and Cherrystone Creek on the Eastern Shore. During this study, mortality was observed to peak in June at most sites, reaching a mean mortality across all tested lines of 17.0% and a cumulative mortality for the study period of 32.0% at Nassawadox Creek, the site most severely affected by mortality that followed the expected early summer mortality pattern. Interval mortality at all sites decreased to under 5% after June, but cumulative levels for the study period reached from 8.8% to 18.6% even at the sites least affected by mortality. This represents a high level of mortality given the documented absence of material involvement by major oyster pathogens such as Hapolo.sporidium nelsoni and Perkinsus marinus. Infiltration of gill tissues by hemocytes, observed in up to 33% of individuals at Nassawadox Creek coincident with the increase in mortality, was the only pathology observed. Harmful algal blooms were not associated with the mortality, nor were abnormal temperatures or salinities. There was no clear relationship of mortality to oyster genetic heritage, although there was variability in susceptibility among oyster lines and interactions between lines and specific sites. At some locations and in comparison with diploids, triploid oysters appeared to be more susceptible to mortality. Mortality in triploids was coincident with the timing of peak gametogenic development in diploids. Given the lack of involvement by major pathogens and the possible association of mortality with oyster gametogenesis, future work should seek to better understand the suite of environmental stressors potentially impacting cultured oysters in these systems and their interactions with the physiology and energetics of these animals. KEY WORDS: Crassostrea virginica, mollusc health, marine disease, summer mortality, aquaculture-environment interactions, polyploidy, INTRODUCTION The eastern oyster Crassostrea virginica has been plagued by two major pathogens through much of its North American distribution, the protozoan parasites Perkinsus marinus (Mackin et al. 1950) and [...]

Introduction Polyploids, organisms having more than two chromosome sets, possess some advantages compared to diploids (Comai 2005). One of the advantages, hybrid vigor, can be obtained in organisms with more [...], For commercial oyster aquaculture, triploidy has significant advantages. To produce triploids, the principal technology uses diploid X tetraploid crosses. The development of tetraploid brood stock for this purpose has been successful, but as more is understood about tetraploids, it seems clear that chromosome instability is a principal feature in oysters. This paper is a continuation of work to investigate chromosome instability in polyploid Crassostrea virginica. We established families between tetraploids--apparently stable (non-mosaic) and unstable (mosaic)--and normal reference diploids, creating triploid groups, as well as tetraploids between mosaic and non-mosaic tetraploids. Chromosome loss was about the same for triploid juveniles produced from either mosaic or non-mosaic tetraploids or from either male or female tetraploids. However, there was a statistically significant difference in chromosome loss in tetraploid juveniles produced from mosaic versus non-mosaic parents, with mosaics producing more unstable progeny. These results confirm that chromosome instability, as manifested in mosaic tetraploids, is of little concern for producing triploids, but it is clearly problematic for tetraploid breeding. Concordance between the results from cytogenetics and flow cytometry was also tested for the first time in oysters, by assessing the ploidy of individuals using both techniques. Results between the two were non-concordant. Key words: Crassostrea virginica, aneuploidy, polyploidy, mitotic instability, cytogenetics, flow cytometry. En ostreiculture commerciale, la triploidie presente des avantages significatifs. Pour produire des triploides, la principale technologie consiste a effectuer des croisements diploide x tetraploide. Le developpement de geniteurs tetraploides pour ces fins a connu du succes, mais au fur et a mesure que les connaissances sur les tetraploides s'accroissent, il est devenu clair que l'instabilite chromosomique est une caracteristique importante chez les huitres. Ce travail constitue la suite d'une investigation sur l'instabilite chromosomique chez le Crassostrea virginica polyploide. Les auteurs ont produit des descendances en croisant des tetraploides--en apparence stables (sans mosaicisme) et instables (avec mosaicisme)--avec des diploides de reference, creant ainsi des triploides, ainsi qu'en croisant des tetraploides avec et sans mosaicisme. La perte de chromosomes etait semblable chez des juveniles triploides issus de croisements impliquant des tetraploides avec ou sans mosaicisme, de meme qu'entre ceux issus de croisements ou le tetraploide etait employe comme parent male ou femelle. Cependant, une difference statistiquement significative a ete obtenue en matiere de perte de chromosomes chez des juveniles tetraploides produits a partir de parents avec ou sans mosaicisme, ceux avec mosaicisme presentant une plus grande instabilite. Ces resultats confirment que l'instabilite chromosomique, tel qu'observee chez des tetraploides avec mosaicisme, ne presente pas de problemes pour la production de triploides mais est clairement problematique dans le contexte de l'amelioration genetique des tetraploides. La concordance entre les resultats d'analyses cytogenetiques et de cytometrie en flux a ete examinee pour la premiere fois chez les huitres en mesurant la ploidie d'individus au moyen des deux techniques. Les resultats de ces deux approches ne concordaient pas. [Traduit par la Redaction] Mots-cles : Crassostrea virginica, aneuploidie, polyploidie, instabilite mitotique, cytogenetique, cytometrie en flux.