Viruses infect all kinds of organisms, from amoebas to humans. Naturally, we hear little about most viruses, which do not have an immediate effect on our lives. Have you ever heard about White Spot Syndrome virus, which infects shrimps and causes $1000 Million loss each year in Asian shrimp industry? Or Sputnik virus, that is a virus that hitches a ride in other viruses to infect amoebas (sputnik means “fellow traveler” in Russian, although most people will probably relate it to the satellite)? Basically, name a species and Google will find a virus for it, as long as some scientist has bothered to look in that species. And so many interesting stories about strange viruses tend to go unnoticed in the press because very few of us really are interested in them. Therefore, I was very pleased this morning to hear Bob McDonal discussing a recent study on insect iridovirus on the Quirks & Quarks podcast.
Iridoviruses are DNA viruses that can infect insects. A team from Dalhousie University in Canada has initially noticed that there’s something strange with their cricket (Gryllus texensis) colony: the females in it stopped laying eggs. After dissection of several females it was noted that the fat body, an organ that has important metabolic and immune functions in insects, had a bluish shine to it. This blue shine apparently is a common phenotype of iridovirus infection. Using electron microscopy and PCR methods they then confirmed that the infection was indeed caused by iridovirus and in particular a cricket iridovirus variant 6. Figure below shows beautiful EM from the published paper in which the icosahedral particles are the iridoviruses in the fat body. Some viral proteins were also later identified in the cricket hemolymph, an insect equivalent of blood.
Next, the team investigated the effects of infection on various physiological and behaviour aspects of the crickets. They found that infected females had significantly reduced number of eggs and fewer developing follicles in the ovaries. By contrast, males produced the same amount of sperm, however it had reduced motility. In addition, animal’s immune system was also affected. Usually, infected animals, including insects, tend to exhibit a so called ‘sickness behaviour’. One aspect of sickness behaviour includes an illness-induced anorexia, meaning, that animal eats considerably less, which possibly reduces the nutrients available for the pathogen and prevents its replication. However, the study found that infected crickets did not show this type of sickness behaviour and in addition the production of antimicrobial compounds by the fat body was also decreased. To show that this is not just some weird cricket species that doesn’t have sickness behaviour scientists infected healthy crickets with heat killed bacteria and in that case crickets indeed showed sickness behaviour. In fact, when crickets with virus in them were given heat killed bacteria they also showed no sickness behaviour, meaning that virus somehow actively prevents induction of this behaviour.
It’s quite common that in insects pathogens are sexually transmitted and indeed, the fact that the virus has an effect on cricket reproductive tissues hints to the same idea. Not surprisingly, therefore, when scientist looked if an infected cricket can transmit the virus to an uninfected animal through mating they found that that indeed was the case. The uninfected cricket had 50% chance to get an infection after mating with infected partner.
Not only that but virus also seems to change cricket’s courting behaviour. Although infected and uninfected crickets were as likely to court, the infected males began their courtship singing sooner than the noninfected counterparts, so virus-containing insects tended to mate more readily. Sexual transmission of this virus also explains why the cricket shows no sickness behaviour. From an evolutionary perspective an animal that looks sick is less likely to get a mate, so naturally virus will “want” to reduce this behaviour for it to have more chance of being passed to another animal.
Now, how did the viruses get into the cricket colony in the lab in the first place is also an interesting story. As the main researcher discuss on the podcast, she had students in her lab who worked on a different project concerning predator and pray situation. The pray in this case was cricket and the predator was a bearded lizard. As it turns out the lizard was an asymptomatic carrier of the cricket iridovirus and possibly acquired the virus through being fed infected crickets from a pet store (of course, no one knew that the crickets were infected). The lizard itself shows no symptoms of infection and likely gets rid off the virus soon after eating the crickets, but for some time, it seems, it acted as a carrier of the virus and infected the colony of crickets in the lab.
That’s how serendipity in science works!
Here are the links to the original paper and the podcast:
(the paper is called “A viral aphrodisiac in the cricket Gryllus texensis“, though I’m not quite sure if ‘aphrodisiac’ is the right word to use :). Having said that probably just the title will attract more readers)