A complementary method for production of tetraploid Crassostrea gigas using crosses between diploids and tetraploids with cytochalasin b treatments.

We present a new method to produce tetraploid Crassostrea gigas by cytochalasin B inhibition of polar body 2 expulsion in diploid females crossed with tetraploid males. This offers a means of direct introgression of genetic characters from selected diploid to tetraploid lines, avoiding a triploid step. Offspring larval ploidy shifted over time and depended on size, with tetraploids more frequent among the smaller larvae and triploids among the large. Viable tetraploids were found at 4 and 6 months, indicating the technique was successful. The possibility that gynogenesis occurred was tested by microsatellite analysis to confirm the presence of paternally inherited alleles. These were present in all animals of the 2n x 4n + CB (female first) cross. However, a 4n x 2n + CB cross produced triploids, including some gynogens. Our method illustrates for the first time that diploid C. gigas eggs, if selected for large size, can give viable tetraploid offspring.

Detection of oyster herpesvirus DNA and proteins in asymptomatic Crassostrea gigas adults.

Since 1972, several herpes-like virus infections have been reported among different bivalve species around the world. Most of these reports involved larvae or juveniles presenting high mortalities. Two case reports of herpes-like viruses concerned adult oysters, Crassostrea virginica in USA and Ostrea angasi in Australia. Molecular techniques including PCR and in situ hybridization (ISH) have been recently developed to detect the oyster herpesvirus genome. In the present study, 30 Pacific oyster, Crassostrea gigas, adults have been analyzed using three different techniques: PCR, ISH and immunochemistry, in order to detect herpesviruses in asymptomatic individuals. PCR and ISH allowed detection of oyster herpesvirus DNA in 93.3 and 86.6%, respectively, of analyzed oysters while polyclonal antibodies allowed detection of viral proteins in 76.6% of analyzed adult oysters. These results suggest that oyster herpesvirus infects adult oysters with high prevalence and that the virus may persist in its host after primary infection. The detection of viral DNA and viral proteins in the gonad of several individuals supports the hypothesis of a possible vertical transmission of the infection. Lastly, concordance among the three techniques used in this study is discussed.

Influences of triploidy, parentage and genetic diversity on growth of the Pacific oyster Crassostrea gigas reared in contrasting natural environments.

An increasing number of hypotheses are being proposed to explain the faster growth potential of triploids in molluscs, including their partial sterility or their higher heterozygosity compared to diploids. Triploid advantage however, remains controversial for poorer sites, because of a potential trade-off with survival. These questions were addressed in Crassostrea gigas by deploying meiosis II triploids and their diploid siblings from a single mass spawning of three males and seven females, in two contrasting locations for their trophic resources. One hundred and fifty individuals were sampled at each site after nine months, measured for weight and biochemical composition, and genotyped using three microsatellite and seven allozyme loci. Higher performance was observed at the fast-growing site for all traits except shell weight, and triploids had greater weights and biochemical contents than diploids at harvest. Triploids also grew faster at the poorer site, and showed similar survival rates to diploids at both sites. Triploids had significantly higher average allozyme and microsatellite diversity. However, they performed better for a wide range of individual heterozygosity values, arguing for an advantage of the triploid state per se, that could be due to positive effects on growth of both sterility of triploids with subsequent resource re-allocation and possible faster transcription with three copies of each gene. Despite evidence of very low or no inbreeding in the diploid sample, positive associations between individual allozyme diversity and growth were detected, which explained little but significant amounts of phenotypic variation. These associations were interpreted as direct effects of allozymes, either alone or including epistatic interactions with other loci. In addition, measures of individual distance (mean-d2) specific to microsatellites, were negatively correlated with growth in diploids, indicating possible effects of outbreeding depression between more distant genomes of parents from distinct populations.