From the standpoint of medical science, instability
is a highly undesirable feature of a genome as
it is very often a source of malignant alteration
of cells, spontaneous abortion, autoimmunity,
genetic diseases, emerging and even the reemerging
of new viruses. In contrast to this,
speaking in a Darwinian sense, the evolutionary
advantages of genome instability are probably
more important than potential and real negative
consequences of this phenomenon. In addition,
emitted DNA/RNA sequences and/or de novo created
viruses can operate as natural biological
weapons against predator and/or concurrent species.
This possibility could also be the source of
positive selective pressure supporting evolutionary
conservation of features such as genome
instability and its ability to emit its own sequences
[8–11].
Viruses apparently can, and obviously do, make
big jumps in hosts every now and then. It seems almost
certain, for example, that arthropods are the
original source for a number of virus families
infecting insects and mammals – such as the Flaviviridae
– and probably also of viruses infecting insects
and other animals and plants – such as the
Rhabdoviridae and Reoviridae. For example, picornaviruses
of mammals are very similar structurally
and genetically to a large number of small RNA
viruses of insects and to at least two plant viruses,
and – as the insect viruses are more diverse than
the mammalian viruses – probably had their origin
in some insect that adapted to feed on mammals
(or plants) at some distant point in evolutionary
time. The majority of existing viruses relevant to
humankind are zoonozis. In spite of the fact that
animals are the source of many viruses pathogenic
to humans, the most important factor in the dissemination of viruses is the fact that humans
live in a manner which increases the possibility of
transmission of new viruses from their endogen
hosts (animals) to humans. Rodents and arthropods
are also included in transmission of viruses from
one species to another, especially in an urban
milieu where their vector role is multi-amplified.
Other animals, especially primates, represent
important sources of viruses potentially pathogenic
for humans. In this context, we can mention a few
emerging or even re-emerging, new extremely
virulent and dangerous viruses which cause diseases
such as Ebola, Marburg and Congo-Crimean
haemorrhagic fever, Hantavirus lung syndrome,
Korean haemorrhagic disease, SARS-Co virus, and
of course, HIV1 and HIV2 [1,3,4,7].
HIV is important problem for humankind and
also a good example that can support our hypothesis
on viruses as natural biological weapons. It is
now generally accepted that HIV is a descendant
of simian immunodeficiency virus (SIV). Certain
simian immunodeficiency viruses bear a very close
resemblance to HIV-1 and HIV-2. For example,
HIV-2 corresponds to a simian immunodeficiency
virus found in the sooty mangabey monkey
(SIVsm), widely known as the green monkey,
which is indigenous to western Africa. The more
virulent strain of HIV, namely HIV-1, was, until
very recently, more difficult to place. The closest
counterpart that had been identified was the simian
immunodeficiency virus that was known to infect
chimpanzees (SIVcpz), but there were
significant differences between it and HIV. In
addition, it was reported that frozen tissue taken
from a chimpanzee carried a simian virus (SIVcpz)
which was almost identical to HIV-1. The chimpanzee
came from a sub-group of chimpanzees
known as Pan troglodytes troglodytes, which were
once common in west-central Africa. It is claimed
by some researchers that this shows that these
chimpanzees were the source of HIV-1, and that
the virus at some point crossed species from chimpanzees
to humans. However, it was not necessarily
clear that chimpanzees were the original
reservoir for HIV-1 because chimpanzees are only
rarely infected with SIVcpz. Also, there is opinion
that wild chimps became infected simultaneously
with two simian immunodeficiency viruses (SIVs)
that had ‘‘viral sex’’ to form a third virus capable
of infecting humans and causing AIDS [12–15].
Sharp et al. [16] discovered that the chimp virus
was an amalgam of the SIV infecting red-capped
mangabeys and the virus found in greater spotnosed
monkeys. The authors believe that the
hybridisation took place inside chimps that had
become infected with both strains of SIV after
hunting and killing the two smaller species of
monkey [16].
The hypothesis that HIV evolved from SIV is
based on the many similarities between these
two viruses, especially at the genetic level. The
two viruses are genetically very similar and are
transmitted in the same way. However, HIV only
causes AIDS in humans and SIV only causes AIDS
in monkeys. The SIV virus, like HIV, is found in
blood. This can provide support for the belief
that HIV entered man via monkey’s blood. For
this, possible routes include drinking the blood
of monkeys, eating raw monkeys or perhaps direct
exposure of humans to monkey blood
[13,14,16]. Finally, the possibility of interspecies
sexual transmission cannot yet be excluded.
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