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June 2004

Feature

 

Extinct? Think again. Valuable virus reappears

Cheryl Walter, Rhodes University

Nudaurelia larvae in the first instar (stage of development)A South African student has rediscovered a valuable insect virus long thought to have been extinct for the past twenty years. The rediscovery of the Nudaurelia Beta virus means good news for saving pine trees from damaging insect larvae. It could show promise in biotechnology as a biopesticide.

Biopesticide

Insect viruses have long been a subject of interest since they have the potential to act as a biological control agents, which have fewer detrimental effects on ecosystems than the commonly used chemical pesticides. What's more, you do not have to repeatedly dose as one would with certain chemical pesticides, the virus does that for you: it simply replicates from one insect to another. But even better, insect viruses can be host specific, whereas most chemical pesticides are not. This means that the insect virus will only destroy its target.

A Nudaurelia larva in its final instar (before pupation)Small insect RNA viruses have been successfully used on a number of occasions. For example, the Darna trima virus, was used successfully against a moth pest of oil palms in Malaysia, and a picorna-like virus, the Gonometa virus, has been used to control a moth pest of pines in Uganda.  Furthermore, news of trials in Australia has shown that Helicoverpa armigera stunt virus (HaSV) is highly effective in the control of the cotton bollworm.

The applications of Nudaurelia Beta virus a biological insecticide are immense as its one and only host is widely spread throughout the world. Specifically, this virus only infects the larvae of the Pine Emperor Moth, (Nudaurelia cytheria capensis) which sows destruction on pine trees. The larvae of the pine emperor moth can feed on a mixture of indigenous and exotic plants. The indigenous plants are preferred, but if found intergrowing in exotic Pinus radiata plantations, egg-laying adults prefer the exotic plants. Food shortages force the larvae to attempt feeding on unsuitable plants.

Transmission electron micrograph of Nudaurelia Beta Virus isolated from Nudaurelia capensis larvae in StellenboschThe Nudaurelia Beta virus and other members in the Tetraviridae genus are small insect viruses which are extremely host specific, often infecting only a particular species of insect. The Tetraviridae family of viruses are so named because they have a T=4 symmetry (T refers to the triangulation number). These viruses were named according to their hosts (Nudaurelia cytherea capensis) and according to the order in which they where first identified, using the Greek alphabet.

Destruction of pine trees

The Tetraviridae first attracted attention in South Africa in the early part of the 20th century when larvae of the Pine Emperor Moth reached extremely high densities completely denuding large plantation areas of introduced pine trees (Pinus radiata). But it was only in the 1940's that scientists noted the annual occurrence of a disease that was so severe and widespread that dead larvae literally formed a carpet beneath the tree. Their investigations revealed that the diseased insects were infected by five small RNA viruses which were termed Nudaurelia Gamma, Delta, Alpha, Epsilon and Beta viruses in order of increasing prevalence.

Much later, in 1985, another Nudaurelia virus was discovered and named the Nudaurelia Omega virus, so named because it was thought to be the last of the viruses to be discovered in this genus and hence given the last character of the Greek alphabet, omega.

Soon after, no trace of Nudaurelia Beta virus could be found. It was thought to have become extinct along with Nudaurelia Alpha, Gamma, Delta and Epsilon virus. Seemingly, the Nudaurelia Beta virus had disappeared from larvae of the Pine Emperor Moth which became preferentially infected with the Nudaurelia Omega virus.

Now almost 20 years later, MSc student Cheryl Walter from Rhodes University in South Africa has excited scientists with the news of the re-emergence of Nudaurelia Beta virus.

Larva killing virus reappears

Collecting samples of Nudaurelia capsensis larvae in the Stellenbosch pine plantations.In the winter of 2002, an alert Forester with the Mountain to Ocean group in South Africa, Mr Mark Wilmot, alerted microbiologists Profs Don Hendry and Rosemary Dorrington of Rhodes University to an outbreak of Nudaurelia capensis larvae which had infested a pine plantation of the Boland District in the Western Cape. In the spring of 2003, it became evident that as quickly as the numbers of Nudaurelia capensis larvae increased, they were dying off. Something had infected the larvae.

Symptoms of insects infected with a Tetravirus include discolouration of the larvae, flaccidity, diarrhoea, vomiting and finally, liquefication of the internal organs. The symptoms were present, so armed with containers, Ziploc bags and lots of enthusiasm, Dorrington and students flew to Cape Town and headed for the pine plantations high up on the mountain slopes. They were greeted with the extraordinary sight of denuded pine trees and a forest floor covered in dead and dying larvae. 

Back in the lab, Cheryl Walter had the task of extracting and identifying the viruses from the larvae.

It is known that these viruses contain RNA molecules containing no less than two genes encoding two proteins. The central dogma in molecular biology is DNA - RNA - protein. Here, the DNA step is left out. Nudaurelia Beta virus contains only one strand of RNA containing the following genes: the replicase gene which encodes for a RNA-dependent RNA polymerase which reads and makes copies of the RNA, and the capsid gene which encodes for the protein which makes the shell of the virus - the outer coat.

Tetraviruses with their two genes, are incredibly simple. Yet they are able to effectively convert their host with a genome 25 000 times larger consisting of approximately 14 000 genes into a highly productive "virus factory". When they were first discovered 36 years ago, using simple yet logical experiments, scientists could discover what theses particles looked like, how big these viruses were, and how many genes and what type of genomic material they were comprised of. Molecular biology at the time was still in its first exploratory stages. Techniques such as the polymerase chain reaction (PCR), DNA sequencing, comparing and characterising a gene were then still just hopeful dreams. Now all of this is possible and achievable in the space of a few hours.

Research now continues apace in this laboratory to begin to understand the biology of the Nudaurelia Beta virus and other tetraviruses, and to answer the question of whether this virus can be used to safely control Nudaurelia capensis in commercial pine forests. And then of course there is the question of why do the tetraviruses remain dormant for so long? What triggers them to emerge with such devastating effect on their tiny hosts? Only time and research will tell. And who knows, perhaps next season, may see the return of Nudaurelia Epsilon, Delta or Alpha virus

- Article by By Cheryl Walter, Don Hendry, Val Hodgson, Mark Wilmot and Rosemary Dorrington, Rhodes University Department of Biochemistry, Microbiology and Biotechnology.


Related articles:

  Natural born killers - insect viruses. 

  Green Muscle: an environmentally friendly weapon against locust and grasshopper plagues

More information:

Rhodes University Department of Biochemistry, Microbiology and Biotechnology

Sources:

1. Moore, N.F. 1991a. The Nudaurelia Beta family of insect viruses. In Viruses of Invertebrates. Ed. E. Kurstak. New York: Marcel Dekker. Pages 277-285.
2. Geertsma, H. Die demnepouoogmot, Nudaurelia cytheria (Fabri.) (Lepidoptera Saturniidae) P{lamflet 273. Plae en siektes van Suid-Afrikaanse bosse en hout.
3. Olson, N.H., T.S. Baker, J.E. Johnson, and D.A. Hendry. 1990. The three-dimensional structure of frozen-hydrated Nudaurelia capensis Beta virus, a T=4 insect virus. J. Struct. Biol. 105: 111-122.
4. Tooke, F.G.C. and C.S. Hubbard. 1941. The pine tree emperor moth Nudaurelia cytherea capensis Stoll. Dept. of Agric. And Forrestry (Ent. Ser). Sc. Bulletin. 210: 1-57.
5. Hendry, D.A., M.F. Bekker, and M.H.V. van Regenmortel. 1968. A nonoccluded virus of the pine emperor moth Nudaurelia cytherea capensis Stoll. S.A. Medical Journal 42: 117-117
6. Hendry, D.A., V. Hodgson, R. Clark, and J. Newman. 1985. Small RNA viruses coinfecting the Pine Emperor Moth (Nudaurelia cytherea capensis). J. Gen. Virol. 66: 627-632.
7. Juckes, I.R.M., Longworth, J.F. & Reinganum, C. 1973. A serological comparison of some nonoccluded insect viruses. J. Inverterbr. Path. 21: 119-120.


Public Understanding of Biotechnology                                                  Department of science and technology, South Africa.

 

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