Refinement of a classification system for gonad development in the triploid oyster Crassostrea gigas

Breakthroughs in polyploidy technologies have enabled the scaling-up of the commercial utilization of phenotypically-sterile triploid Crassostrea gigas. Compared with diploid oysters, gonad development in triploids is retarded, but not absent. Although histological methods of characterizing gonadal development in triploid C. gigas has been reported, the selection of sampling sites and densities might lead to bias when taking seasonal variations into consideration. Moreover, previously-employed classification systems were found to be incompatible with our long-term observations. In this study, by sampling Pacific oysters at different seasons, over several years and from different locations, we were able to establish a new classification system for gonadal development in triploid Pacific oyster based on histological analysis. Based on the types of germ cells, triploids were grouped into female (divided into female α and female β), male and hermaphrodites during the reproductive season. Surprisingly, female α was partially sterile, showing active gametogenesis and few abnormal germ cells, which were defined as β gonia. Unlike female α, triploid female β had the most severely retarded gonad along with abnormal morphological features in the gonia. We inferred that β gonia were correlated with female triploid sterility. Despite the presence of numerous gametes in triploid male gonads, the fecundity of triploid males was still severely reduced compared with that of diploids. In addition, maturation arrest of spermatogenesis in triploid males was detected histologically. Based on the types of germ cells and the presence of β gonia, hermaphrodites were further divided into hermaphrodite I and hermaphrodite II. An almost male-like triploid was redefined as a hermaphrodite triploid. The gonadal development of triploids from different sites was mostly similar. As shown via annual histological analysis, the increase in the percentage of female α and the decrease in the percentage of female β matched the seasonal development of the oyster gonad. Hence, we speculated that gonad development in female triploid oysters was delayed.