Posts Tagged ‘virus evolution’

White death: A diabolical pact between an insect and two viruses

22 August, 2012

See on Scoop.itVirology News

This is actually an article in The Economist from 2007 – forwarded to me by a Professor of Philosophy, as it happens, and which has mouldered on my desk lo, these past five years.  Thanks David Benatar!

“Whiteflies are pests in every continent that they are found in—and they are found in every continent except Antarctica. They cause damage directly, by consuming plant juices, and indirectly, by spreading viral diseases. But Liu Shusheng, of Zhejiang University, in Hangzhou, and his colleagues have found a strain of the species that delivers a double whammy. Not only does it spread diseases, but it is also vastly more successful when it lives on plants infected with the diseases in question [tomato yellow leafcurl and tobacco curly shoot begomoviruses, both ssDNA geminiviruses]  than when it subsists on healthy plants.”

This is a fascinating example of just why it is that certain vector-virus-host combinations can lead to success of the vector, and increased spread of the virus.  Basically,

“…type B insects lived six times longer on infected plants than uninfected ones, and their population per infected plant might rise as high as 13 times that on an uninfected one”

This means that geminivirus infection of host plants actually gives a survival advantage to the insects which transmit them.  Simple if unfortunate!!

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Goodbye, Mimi – we got Mega!

11 October, 2011

Through the unlikely medium of a local online version of a local daily paper, comes the following:

“A virus found in the sea off Chile is the biggest in the world, harbouring more than 1,000 genes, surprised scientists reported on Monday. The genome of Megavirus chilensis is 6.5 percent bigger than the DNA code of the previous virus record-holder, Mimivirus, isolated in 2003. “

The relevant article is from the group led by Jean-Michel Claverie, of the Institut de Microbiologie de la Méditerranée, in Marseilles, and appears in the October 10th online issue of PNAS.

From the abstract:

An electron micrograph of Megavirus: thanks to Jean-Michel Claverie

Here, we present Megavirus chilensis, a giant virus isolated off the coast of Chile, but capable of replicating in fresh water acanthamoeba. Its 1,259,197-bp genome is the largest viral genome fully sequenced so far. It encodes 1,120 putative proteins, of which 258 (23%) have no Mimivirus homologs. The 594 Megavirus/Mimivirus orthologs share an average of 50% of identical residues. Despite this divergence, Megavirus retained all of the

genomic features characteristic of Mimivirus, including its cellular-like genes. Moreover, Megavirus exhibits three additional aminoacyl-tRNA synthetase genes (IleRS, TrpRS, and AsnRS) adding strong support to the previous suggestion that the Mimivirus/Megavirus lineage evolved from an ancestral cellular genome by reductive evolution. The main differences in gene content between Mimivirus and Megavirus genomes are due to (i) lineages specific gains or losses of genes, (ii) lineage specific gene family expansion or deletion, and (iii) the insertion/migration of mobile elements (intron, intein).

I could argue with the choice of name as it does not conform to ICTV rules, as far as I can see – but then, neither did Mimivirus.  The important fact about the discovery – apart from the fact that it is a discovery, and therefore not amenable to hypothesising, which I rather like – is that it shows how very diverse these viruses are, and how long they must have been evolving.  For example, despite their morphological similarity, Mimi- and Megavirus genomes do not share nearly 25% of their ORFs – and sequence identities of  predicted homologous proteins are as low as 50%.

I have blogged earlier on Mimivirus structure and evolution – see “Mimivirus unveiled” – and it is nice to see that an important speculation from those earlier papers appears to be borne out here.  Namely, and quite important when considering both viral and cellular origins, is further evidence that very large viral genomes do not seem to have evolved by extensive horizontal gene transfer from cells, and in fact, the reverse may be true.  The authors state in their conclusion, in discussion of opposing views of the origin of these viruses:

“The potential origin of giant mimivirus-like genomes has been hotly debated, basically opposing two views. One is depicting Mimivirus as an extremely efficient gene “pickpocket,” explain- ing its large genome as the result of considerable HGTs from its host, bacteria, or other viruses. This scenario has been criticized in detail elsewhere [see paper for refs]. The opposite view claims that the level of HGT remained marginal (10%) and that most of the Mimivirus genes originated from an even more complex viral ancestor, itself eventually derived from an ancestral cellular genome.”

I have fond memories of an essay I won a school prize with, in about 1970, entitled “The Sea, and All that Therein Is”.  I should update it to “The Sea, and All the Viruses that Therein Are”…B-)