68 March 2009 responses among invertebrates that fit the expectations of disease-specific immunity (Little and Kraaijeveld 2004, Little et al. 2005). This recent research differs from previous studies because they exploit well-defined host/parasite interactions that allow strictly controlled experiments to be undertaken with infectious microbes or parasites that are relevant to the host species being studied. These new studies have used vaccination to test the outcomes of host/ parasite interactions. Figure 7 shows the design for a classical vaccination experiment of the type that has been used to distinguish between diseasespecific immune systems and other non-specific types of immune response. In this experimental design, the immune system of host animals, oysters in this case, are primed by exposure to one type of infectious microbe. They are left to recover and then exposed to either the same species of microbe, or a different pathogen. If the host has a disease-specific immune system, enhanced responses will only occur when the same microbe is used in the second exposure, while non-specific responses do not discriminate between pathogens. In the past, this type of experiment has failed to provide useful information because the added involvement of non-specific pattern recognition responses clouded the results. The most recent trials have employed closely related microbes that cannot be distinguished by pattern recognition systems to overcome this problem. As a result, they have been able to test far more accurately and precisely for target specificity. In one of the recent studies, it was shown that infections of the copepod crustacean, Macrocyclops albidus, with the tapeworm, Schistocephalus solidus, were far less severe if the hosts had been primed with siblings of the worms used for the subsequent infections. This acquired protection was not evident if the tapeworms used in the initial and subsequent challenges were genetically distinct. Similar vaccination experiments in the shrimp, Penaeus monodon, identified discriminatory responses to virulent white spot virus. Injecting an envelope protein from the virus, VP28, provided substantial protection against white spot infection, whereas a closely related protein, VP19, did not. Protection against white spot syndrome in another species of prawn, Litopenaeus vannamei, can be elicited by injecting a special form of RNA, called double stranded RNA (dsRNA). Even though randomly generated dsRNA has some effect on reducing mortality, diseasespecific protection is induced only when dsRNA that mimicked the genetic code of white spot virus are used. In a final series of vaccination experiments, it was shown that induced, specific protection against pathogens in the water flea, Daphnia magna, can be transmitted from mother to offspring. What Lies Ahead? Future developments based on our recent research are still unpredictable, even though the potential benefits seem very large. The production of vaccines for specific disease agents and systems to deliver these vaccines to large numbers and a wide variety of commercially important invertebrates will be difficult, perhaps even impossible. Effective, economical large-scale delivery systems for vaccines simply may never become available. It is also possible that the hypervariable gene systems identified so far have evolved to control very specific host/parasite relationships, such as the interaction of FREPs with S. mansoni, and may have little relevance to the types of diseases that threaten aquaculture industries. Finally, it may prove feasible to induce disease-specific responses in some species only to find that the protective period provided by specific vaccinations is too short to make any real difference in the context of aquaculture management practices. Despite all of these considerations and potential obstacles, the only sensible way forward is to continue and scale up research in the area, simply because the potential benefits of positive outcomes far outweigh the initial investment in research. Think about it this way. An effective vaccine to control White Spot Virus Syndrome in shrimp would save the industry billions of dollars per year in lost production. Next to that, the cost of discovery is cheap. Notes 1Department of Biological Sciences, Macquarie University, North Ryde, NSW 2109, Australia, and the Sydney Harbour Institute of Marine Science. Email: draftos@rna.bio.mq.edu.au Acknowledgments Our work on sea urchins has been funded in part by a Discovery grant from the Australian Research Council and by the UN National Science Foundation. Our studies of Sydney rock oysters have been funded by a Linkage grant from the Australian Research Council in conjunction with the New South Wales Department of Primary Industries. The photographs of oyster blood cells shown here were taken in our laboratory by PhD students Daniel Butt and Saleem Aladaileh. References Bezemer, B., D.T. Butt , J.J. Nell, R. Adlard and D.A. Raftos. 2006. Breeding for QX disease resistance negatively selects one form of the defensive enzyme, phenoloxidase, in Sydney rock oysters. Fish and Shellfish Immunology 20:627-636. Butt D.T., K. Shaddick and D.A. Raftos. 2006. The effect of low salinity on phenoloxidase activity in Sydney Rock oysters. Aquaculture 251:159-166. Flajnik, M.F. and L. Du Pasquier. 2004. Evolution of innate and adaptive immunity. Trends in Immunology 25:640-644. Litman, G.W, J.P. Cannon and L.J. Dishaw. 2005. Reconstructing immune phylogeny. Nature Reviews Immunology 5:874-879 Little, T.J., D. Hultmark and A.F. Read. 2005. Invertebrate immunity and the limits of mechanistic immunology. Nature Immunology 6:651-654. Little, T.J. and A.R. Kraaijevel. 2004. 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