Due to the insights gained from a decade’s worth of viral research, it’s not surprising that in the autumn of 2013, Penn State University offered a newly developed course in viral ecology. For a century, the word virus had been exclusively associated with certain human, animal, plant or computer diseases. Virus, which is rooted in the Latin word for a slimy, poisonous liquid, had nothing but negative connotations. Yet virologists have slowly come to realize that only a minority of viruses are virulent. Most are innocuous, some are definitely beneficial, and others are indispensable to their hosts. In light of this, as pointed out in a review of mutualistic viruses, a part of the old definition of a virus is incorrect:
intracellular parasites with nucleic acid,capable of directing their own replication, that do not serve any essential function for their host, have an extrachromosomal phase and are not cells.
Mutualism, as opposed to parasitism and commensalism, is a form of symbiosis that benefits two (or more) organisms. Given that viruses are not cells, it may seem odd that the term mutualistic is used to describe some of them, but some mutual symbioses between plant and fungi would not exist without the existence of a virus, and in other cases the living host and virus both directly benefit from their relationship.
Let’s survey some of the more colorful examples from a variety of life’s kingdoms.
Many bacteria integrate, within their own DNA, the entire nucleic acid sequence(shown in green in diagram below) of a virus(blue). These dormant (lysogenic) viruses protect bacteria from other forms (lytic) of viruses that could burst out and kill their hosts. Other bacteria(red) that do not carry viral genome are not protected from free viruses.
Several species of parasitic wasps lay their eggs in living hosts. It’s surprising that the immune system of the victim does not encapsulate and kill the foreign egg. In the lepitdopteran caterpillar, it is actually a mutualistic polydnavirus (class I virus: dsDNA) carried by the wasp which prevents encapsulation and keeps wasp eggs thriving within caterpillar hosts.
b) Aphids In the lab, rosy-appled aphids(A )that were free of virus did not develop wings.
Only those infected by dysaphis plantaginea densovirus, a class II ssDNA virus, (see B and C in two stages of development) grew wings. Interestingly, if the aphids were infected with rosy apple virus, they remained wingless. Flight helps the host and its viral guest move from one branch or tree to another.
3. Ménage à Trois Among Plants, Fungi and Viruses
In the hot spring environment of Yellowstone National Park, certain grasses (Dichanthelium lanuginosum) can withstand extremely hot soil. Using thermal soil simulators, researchers kept plants in soil at 65°C for 10 hours and
37°C for 14 hours per day for two weeks. The grasses only survived if they were colonized by the fungus Curvularia protuberata, which in turn had to serve as the host for a third mutualistic partner, a virus. In the diagram, Wt= wild type, An= virus-infected in lab, VF= virus-killed, and NS= non-symbiotic plant.
4. Animals (specifically mammals)
About 6 years ago, the media reported that in sheep, some retroviruses (class VI: ssRNA-RT) related to Jaagsiekte sheep retrovirus are critical during the early phase of pregnancy when the placenta begins to develop. But more generally, retroviruses may have played a key role in the evolution of the placenta.
The envelope (env) genes of retroviruses function to promote fusion of the viral membrane with the plasma membrane of a host cell. Syncytins are derived from envgenes and are expressed in the placenta, where they promote fusion of cytotrophoblasts with the syncytiotrophoblast. Thus far six syncytin genes have been discovered including two in the mouse and two in higher primates. These genes are not orthologous so each represents an independent capture from a retrovirus. Yet another example of convergent evolution! There is more. The envelope protein of retroviruses is immunosuppressive and endogenous env genes may contribute to immune tolerance by the mother of the fetal semi-allograft.
Whereas a rice plant is protected from drought when infected with cucumber mosaic virus, mice infected with lymphotropic viruses do not get type I diabetes. Investigators have no doubt hit upon the tip of a mind-boggling iceberg of viral relationships, which have even more profound implications in a world of genetic engineering.
- http://www.uam.es/personal_pdi/ciencias/genhum/bibliografiadesarrollo/Tema8/3waysymbioses.pdf (full article)
- Roossinck M. The good viruses: viral mutualistic symbioses. Nature Reviews Microbiology 9, 99-108 (February 2011) | doi:10.1038/nrmicro2491