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The distinction of the two cupped oysters Crassostrea gigas (Thunberg, 1793) and Crassostrea angulata (Lamark, 1819) into two species was chiefly due to their differing geographical distributions, C. gigas being present in Asia and C. angulata in Europe. Today it is commonly accepted that C. angulata and C. gigas are a single species according to morphological, genetic and F1 hybridization data. However, the demonstration of the fertility of their hybrids and the absence of any reproductive isolation remained to be investigated. Consequently, we studied the fertility of hybrids and sperm competition by performing three different experiments and producing G1 and G2 hybrid progenies between wild populations of C. angulata and C. gigas. Progenies showed very close developmental yields, at 24 hours after fertilization, according to dam taxa suggesting a strong maternal transmission of oocyte quality, but no reproductive isolation was observed between the two taxa. Significant decreases of developmental yields were noticed in C. angulata females with sperm competition, most probably due to early larval mortality. The fertility of hybrids C. angulata × C. gigas was demonstrated, which is further evidence that they are the same species. To definitively state the precise taxonomic classification of C. angulata and C. gigas, further studies are needed to (i) identify geographical zones where these taxa are in contact and (ii) assess their level of hybridization in these zones.

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.

Abstracl. A nove! mcthod using 6-dimcthylaminopurinc (6-0MAP) has rcccntly bccn dcmonstrated to bc a good triploid inductor for bivalvc molluscs. Thc cffccts of 6-0MAP concentration, timing of trcatment aftcr fertilization. and treatment duration on survival and triploid induclion of Crassastrea gigas (Thunberg) embryos wcre examined. A.n influcnce of sperm concentration was also detected and discussed. Survival loO-stage was invcrscly re!ated to 6-0MAP concentration and the percentage of triploids was shown to bc 6-0MAP dose dependenl. Two optimal moments for treatment application were delcrmined. cor rcsponding to extrusions of the firsl and sceond polar bodies. Optimal treatment of 450 pmolll 6-0MAP bcginning 15 min after fertilization over a lOmin period yiclded mcan survival to O-stagc of 64% with a mean of 85% triploid production. As a comparison, a sample trcated with 1 mg/l of cytoehalasin B (CB). 20 min aftcr fertilization over a 15 min pelriod, yieldcd a me an survival to O-stage of only 36% with a mean of 95% triploid production. The advantages of 6-0MAP ovcr CB arc clearly identified: this chemical is noll carcinogcnic, cheapcr than cytochalasin Band much casier to use as it is water soluble.