The eastern oyster is the most valuable cultured mollusc in North America and the industry is increasingly reliant on seed produced in hatcheries. A critical step in hatchery production is the reproductive conditioning of broodstock. Throughout much of the species range, reproduction is seasonal, whereby germinal epithelia are resorbed during the fall months and develop anew in the summer. Environmental factors that promote reproduction (e.g. temperature and food availability) can be manipulated in hatcheries to accelerate and extend the spawning season but conditioning is a lengthy and expensive process. The lack of sexual dimorphism compounds costs because many more broodstock than necessary must be maintained to ensure the appropriate sex ratio for spawning is achieved. Therefore, the development of protocols that allow non-destructive sex identification early in the reproductive conditioning process would benefit the industry by reducing effort and cost.
Here we present the first step toward this goal by identifying genetic markers that differentiate female and male oysters. As with many marine invertebrates, little is known about the genes underlying sex differentiation in the eastern oyster. We compared global gene expression patterns in mature gonad tissue from 6 male and 6 female oysters. A total of 7,675 transcripts were differentially expressed between the sexes (3,936 and 3,739 up- regulated in males and females, respectively). Differentially expressed transcripts include those associated with sex in other invertebrate and vertebrate species such as Dmrt1, SOX-30, bindin, dpy-30, and histone H4 in males and foxl2, vitellogenin, and Bystin in females. GO terms associated with transcripts up-regulated in male gonad include protein modification, reproductive process, and cell projection organization, while RNA metabolic process and amino acid metabolic process were associated with transcripts up-regulated in females. Because gonad tissues are difficult to sample non-destructively, we also compared gene expression patterns in mantle tissues collected from the same 12 oysters. Far fewer transcripts were differentially expressed in this tissue; however, 41% of transcripts identified as differentially expressed between male and female mantle tissues were also differentially expressed between male and female gonads. This study represents the first characterization of differences between male and female eastern oyster transcriptomes and our results hold promise for the development of a non-destructive sex identification assay that can be applied to hatchery broodstock during the conditioning process.