Triploid oysters are considered sterile and preferred by oyster farmers because of their fast growth and premium meat quality during the summer seasons. However, recent reports revealed the phenomenon of ‘triploid mortality’ and found that triploid eastern oysters are more susceptible to environmental stressors than diploids even though their gonadal development was delayed and/or diminished. These results highlight the need for alternative sterilization technologies to preserve the market advantages of growing sterile oysters without altering their chromosome sets. We have developed a technology to produce reproductively sterile fish using dnd-MO-Vivo to transiently silence dead end, an essential gene responsible for fish primordial germ cell (PGC) development, by administering Morpholino oligomer through a molecular transporter, Vivo.
To apply this bath-immersion technology to produce reproductively sterile diploid eastern oyster (Crassostrea virginica), genes that are indispensable for oyster PGC development need to be identified since no dnd ortholog was found in oyster. Several genes in the invertebrate model organism Drosophila have been shown to be crucial for PGC development, including germ plasm and germ cell formation, PGC specification and PGC migration. Loss of function mutants of these genes are reproductively sterile. The orthologs of these candidate genes have been identified and cloned, and immersion treatments targeting these genes by Vivo conjugated Morpholino have been established in eastern oyster. In addition, to observe the uptake of Morpholino, fluorescent labelling was employed. Immersion of oyster embryos in fluorescent labelling Morpholino compound leads to the delivery of Morpholino shown by the fluorescent signal (Fig. 1). The fluorescent intensity inside oyster embryos increases with the increasing concentration of Morpholino. Unfortunately, none of the treatments produce sterile diploid oyster in these trials. The possible reasons and future perspectives are discussed. Nevertheless, the immersion-based approach provides a new accessible molecular tool to conduct gene function studies in oysters.