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GENES from ancient viruses may be an essential component of mammalian
reproduction, helping the placenta to establish itself in the womb, suggest
biologists in California. Their research supports the idea that viruses鈥� talents
for ferrying DNA and genes into cells has played a key role in the evolution of
humans cells as well as those of other higher organisms.

鈥淰iruses are known primarily for causing disease,鈥� said Luis Villarreal of
the University of California at Irvine at the meeting. 鈥淏ut I think that鈥檚 a bum
谤补辫.鈥�

While many of the viruses that cause disease survive for only a short time in
the body during a brief burst of infection, other less virulent varieties stay
around for a long time, causing no symptoms and making it easy for the virus to
evolve along with its host. One group of viruses called endogenous retroviruses,
or ERVs, has developed an especially intimate relationship with mammalian
cells.

These ERVs are the remains of viruses that worked their way into mammalian
chromosomes a long time ago. Some biologists, including Villarreal, have
suggested that ERV genes may help the placenta function properly, because they
are switched on at high levels in this tissue
(New Scientist, 12 June 1999, p 26).

To test this idea, Villarreal used another virus that suppresses ERV genes in
mouse cells. When he and his colleagues put this virus into the cells of an
embryo, normal mice were born.

But when they put the virus into cells that form the placenta, these cells
would not implant in the mouse uterus, derailing the reproductive process right
at the start. This suggests that the ERV genes are either essential for
implantation itself or for preventing the mother鈥檚 immune system rejecting the
fetus.

Villarreal thinks that a viral transport system may also explain why
vertebrates seem to have inherited more than a hundred genes directly from
bacteria. Viruses could easily have shuttled those genes into our cells, he
says. The most recent work from his lab suggests that even the system of enzymes
that copies our DNA may have come from a virus (Journal of Virology,
vol 74, p 7079).

Villarreal points out that viruses have the ability to change rapidly. So
when it comes to inventing complex new biology, viruses may provide higher cells
with a quick fix. Even ERVs, which are now permanent fixtures in mammalian
chromosomes, evolve much faster than the surrounding genes. 鈥淰iruses can be a
great creative force in the evolution of their host,鈥� says Villarreal.