Hey, some traffic!! Wow!
Now seriously: Paul Keese (now of National Univ of Singapore) stood
up at the last Int Soc Plant Mol Biol meeting in Singapore (21-27
Sept) with a very interesting theory / proposition to account for
why most plant viruses have ssRNA genomes (and mammals have all
sorts). He maintains (or so my understanding goes) that organisms
tend to have the viruses that they have defence mechanisms for
(presumably because otherwise they would be dead). Thus bacteria
have mainly dsDNA viruses as they have restriction systems; mammals
have all sorts as the selection is on the basis of the virion outer
surface (immune system) - and plants have ssRNA viruses because they
have evolved mechanisms like gene silencing to shut off excessive
production of mRNA. Fungi have a prepondeance of dsRNA viruses
because they have learned to live with them (vertically transmitted,
etc.). VERY interesting...
However, Thomas Hohn (who asked the question) and I independently
realised that precisely the opposite may be true: that what we see in
an organism is what it CANNOT handle; that is, that classes of
organisms may be "genetically immune" to classes of viruses, and what
we see in any organism as an infection is what the organism is
learning to handle.
There is also an evolutionary angle to the whole question: plants
were probably the first organims (with bacteria) to colonise dry
land; the next organisms to crawl out of the water en masse were
invertebrates / arthropods, which of course immediately preyed / fed
on the plants. Long after that, the first vertebrates crawled out,
to eat plants / each other / insects, and to be preyed on by insects.
Thus, the viruses present on land are (i) those that came out with
bacteria (which may be discounted from here on in, in that there is
no hard evidence of any bacterial virus infecting anything else);
(ii) viruses that crawled out with plants (which, at that stage of
evolution may well have only been ssRNA viruses); (iii) viruses that
came out with arthropods / insects (which had had a long while to
develop along their own paths); and (iv) viruses that came out in
vertebrates. Fungi also wandered out onto land at some point,
bringing their own evolutionarily-isolated collection of viruses with
them as well. Of course, as fungi colonised land plants, and insects
preyed on both plants and vertebrates, viruses were exchanged /
adapted to new hosts: thus we have the dsRNA cryptoviruses inplantrs,
which almost certainly derive from fungi; similarly, there are
picorna-like viruses in plants and insects that appear more diverged
than anything in mammals, indicating an origin in either plants or
insects (probably the latter). Poxviruses may well have originated
in insects and become adapted to vertebrates; the same is possibly
true of many arboviruses.
There are still, however, collections of viruses which
characteristically have only one kind of host - and could be assumed
to have evolved in that host: for example, adeno / herpesviruses
appear vertebrate-limited; many of the ssRNA viruses of land plants
have no counterparts in other phyla.
Seeing as we are so terrestrially oriented in our search for /
knowledge of viruses, this collection represents most of what we know
about viruses: however, as has been amply demonstrated in recent
years, there are a vast number of viruses in every litre of sea
water, most of which have never been described - and most virus
diversity may in fact still be out there, waiyting for
characterisation. Why do algae have such big dsDNA viruses, while
land plants do not? A case of the one that stayed behind developing
a new kind of virus? Viral "founder effects"?
Ed Rybicki, PhD
Dept Microbiology | ed at molbiol.uct.ac.za
University of Cape Town | rybicki at uctvms.uct.ac.za
Private Bag, Rondebosch | phone: x27-21-650-3265
7700, South Africa | fax: x27-21-689 7573
WWW URL: http://www.uct.ac.za/microbiology/ed.html
"Every year is getting shorter, never seem to find the time..."