Posted Tue, Feb, 14,2017
The following is a personal account of Dr. Roy Hall, Australian Infectious Diseases Research Centre, School of Chemistry and Molecular Biosciences, The University of Queensland, St Lucia, QLD, Australia, during a research expedition to Darwin, in search of the source and proliferation of mosquito-borne viruses.
In 2010, I traveled to the city of Darwin to search for new viruses in Australian mosquitoes. Darwin, named after the British evolutionist Charles Darwin, is a tropical city in northern Australia with a typical monsoon season and is an ideal place to hunt for mosquito-borne viruses.
I have been studying mosquito-borne viruses for more than 30 years focusing on the viral pathogens that cause diseases in humans and animals including West Nile virus and Dengue. Prior to my sabbatical in Darwin, I became interested in a new group of viruses that are genetically related to West Nile virus and Dengue virus, but are so highly adapted to their mosquito hosts that they are unable to infect vertebrates such as mammals and birds.
These “mosquito-specific” viruses have particular significance because they may represent ancestral relics of the evolution of modern day mosquito-borne pathogens. The current theory is that modern viruses like Dengue and West Nile, which can infect vertebrates, are descended from an ancient virus that only replicated in mosquitoes. It is thought that, at some point in the past, this ancestral virus was introduced into a vertebrate species as the mosquito fed on the animal. According to our theory this virus, which used RNA for its genome instead of DNA, had a high rate of mutation. This accelerated the process of natural selection resulting in a family of new viruses containing just the right set of random mutations that enabled their adaption to the new cellular environment and allowed replication in vertebrate hosts. We think that some of these viruses also retained the ability to grow in mosquitoes, and would cycle back and forth between the insect and a vertebrate - as do Dengue and West Nile in the present day.
Unfortunately, RNA viruses leave no fossils, so we have no direct evidence of the “missing links” in the evolution of mosquito-borne viruses to prove this theory. In lieu of this, our theories are all based on studies of viruses isolated within the last 100 years. To find additional evidence to support current theories, we need to detect and isolate a range of new viruses to understand their biodiversity. This includes analyzing the genetic relationship between mosquito-borne viruses and determining their various traits and characteristics such as which host cells they can infect and how they are transmitted.
Our studies on these mosquito-specific viruses will provide new insights about the evolution of mosquito-borne viruses and allow us to build a picture of how the mosquito-borne viruses that cause disease in man and animals have evolved from harmless mosquito-specific viruses. This knowledge is particularly relevant for predicting whether new viruses that we discover have the potential to become emerging pathogens in the future.
My sabbatical in Darwin turned out to be very productive. I discovered three new viruses that represented the first mosquito-specific viruses to be found in Australia. These discoveries spawned a fascinating study in my lab spanning the last seven years and resulting in the detection and characterization of more than 20 new mosquito-specific viruses. These new viruses have revealed interesting phenotypes; some display a broad host range amongst mosquitoes of different genera while others are highly adapted to a single mosquito species. Using a range of molecular and immunological techniques, we are beginning to understand why these viruses cannot infect vertebrates and how they are transmitted between mosquitoes in nature.
Another important thing that the members of my lab have discovered during the last 7 years, is that finding new viruses and learning how they fit into the larger picture of evolution is a highly addictive and rewarding experience.
Professor Roy Hall is the author of the recently published paper Commensal Viruses of Mosquitoes: Host Restriction, Transmission, and Interaction with Arboviral Pathogens, available for download now in Evolutionary Bioinformatics.
Hall RA, Bielefeldt-Ohmann H, McLean BJ, O'Brien CA, Colmant AM, Piyasena TB, Harrison JJ, Newton ND, Barnard RT, Prow NA, Deerain JM, Mah MG, Hobson-Peters J. (2016) Commensal Viruses of Mosquitoes: Host Restriction, Transmission, and Interaction with Arboviral Pathogens. Evolutionary Bioinformatics, 12(S2): 35-44.
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