I wrote a post (and published it on the blog on 25th October 2013) describing evolutionary arms race between bacteria and their phages. In that context, there are also similar interactions between invertebrates and their pathogens. Red Queen Hypothesis proposes that pathogens are continuously trying to evade immune system of their hosts and on the other side immune system of the host is trying to improve its defence system against the pathogens. And perhaps, there is equilibrium between host’s defence and virulence of pathogens; when this equilibrium moves to one side, the result is either disease epidemic or disease resistance shown by the host.
Viral diseases are the major problem in shrimp farming worldwide with White Spot Syndrome Virus (WSSV) being the most serious pathogen infecting farmed shrimps as well as wild crustaceans. Like diseases of corals, under stressful conditions, opportunistic microbes can become pathogenic. This could lead to emergence of new diseases.
Considering the economic impact of shrimp diseases, research into understanding host-microbe interaction of shrimp diseases has been encouraged. This study explored transcriptomic profile of shrimps during experimentally-induced viral (WSSV) and fungal (Fusarium solani) infection and mortality events. Gene expression profiles in hemocytes from diseased shrimps were studied. High throughput microfluidic RT-qPCR analyses were used for identifying pathogen-specific gene expression signatures.
Analysis of transcriptome-wide expression revealed mortality-related gene expression signatures in shrimps dying either because of viral or fungal infection. This paper grabbed my attention especially because it also demonstrates RNAi mechanisms acting in WSSV infected cells. LvDcr2 which is involved in RNA interference pathway was found to be up-regulated, two days after the infection of WSSV whereas fungal infection did not alter LvDcr2 expression at all. Authors reported that most of the analysed antiviral genes stimulate RNAi mechanisms against viral infection. Similarly, authors also found up-regulation of other anti-viral genes (not related with RNAi) which could lead to other defence mechanisms against viruses. All in all, as one can expect, WSSV-specific gene expression profile contained up-regulation of many anti-viral genes. Nevertheless, all these up-regulated antiviral defence mechanisms could not rescue shrimp during lethal viral infection. I am wondering if animals naturally have such an elaborate mechanisms of fighting viral infections (RNAi in particular) although, why we see so many incidents of viral outbreaks, not only in aquaculture but also in other animals, including humans? Does the viral infection-induced death mean failure of these mechanisms or mutated better viruses or both? Perhaps, infection of mutated viruses is essential to update these antiviral mechanisms.
Shrimps dying because of lethal fungal infections showed characteristic drop in transcription. Expression of more genes (68%) was halted in shrimps dying from fungal infection compared to viral infection (WSSV) which halted only 26% of infection-modulated genes. This means fungal infection had stronger effect on suppression of analyzed genes. Expression of some of the genes was not at all affected by either of the infection. The authors also noted that some transcriptomic response was similar for both viral and fungal infection. For example, both the infections affected expression of protease inhibitor and prophenoloxidase (proPO) both of which have been shown to be associated with diseases. Authors reported that defence mechanisms related to proPO cascade and melanin production might be impaired in viral and fungal -infected cells. Thus, these pathogens could evade immune system of shrimp by interfering with proPO related defence mechanisms of shrimp.
In summary, this is the first study demonstrating immune related transcriptomic response of shrimps which are about to die from the infection. Such gene expression signatures could be used for diagnostic purposes. As mentioned before, this paper has reinforced RNAi related pathways against viral infection. This is noteworthy in relation to the talk by Dr Leigh Owens (on 10th January 2014) and his work on using RNAi in crustacean aquaculture.
Goncalves, P., Guertler, C., Bachère, E., de Souza, C. R., Rosa, R. D., & Perazzolo, L. M. (2014). Molecular signatures at imminent death: Hemocyte gene expression profiling of shrimp succumbing to viral and fungal infections. Developmental & Comparative Immunology, 42(2), 294-301.