PRIMITIVE pond-dwelling algae are helping to answer
one of biology's ultimate questions — why did sex evolve? It seems that
combining genes with your pond-mates is all about repairing DNA when the
going gets tough.
Sex is puzzling because many species, including
some plants and reptiles, manage to reproduce perfectly well without it.
Instead of combining their genes with another individual they simply churn
out carbon copies of themselves.
Richard Michod and his colleagues from the
University of Arizona in Tucson argue that sex started out as a way of
repairing damage to DNA. A version of sex in bacteria backs up his
hypothesis. They sometimes exchange bits of DNA via tube-like structures
that form between cells. "If you look at bacteria, the sex genes are
intimately coupled with the DNA repair system. It seems obvious that
bacterial sex originated for repair," says Michod. But conventional sex
involving the fusion of two sex cells is very different and some experts
doubted that the same explanation would hold.
However, sex can help repair DNA: if a section is
damaged in one individual, and the same site is intact in a partner, then
recombination during sex can discard the damaged portion and fill the gap
with the correct sequence from the mate.
Now Michod's team has demonstrated that sex and DNA
repair are closely linked in the multicellular green alga Volvox carteri.
Most of the time, these algae clone themselves to reproduce. But when the
temperature rises to 42.5 °C, DNA damage triggered by heat stress causes
them to resort to sex. Females release eggs and males release sperm, which
float off into the water and combine.
Heat stress causes the alga's metabolic machinery
to produce highly reactive forms of oxygen, including hydrogen peroxide.
Collectively termed reactive oxygen species (ROS), these chemicals are
known to damage DNA. When ROS levels cross a threshold, the expression of
two sex-associated genes goes up and the alga switches to sexual
reproduction. But when the team added antioxidants to mop up ROS, the alga
stayed asexual, showing that the switch-over only happens in response to
ROS (Proceedings of the Royal Society B, DOI: 10.1098/rspb.2004.2747).
Evolutionary biologist Neil Blackstone of the
Northern Illinois University in DeKalb agrees that the study supports
Michod's DNA-damage hypothesis. "But this is just one organism. There will
have to be corroborative work on other species."
Michod says that there is preliminary evidence of a
similar link between stress and sex in the related single-celled alga,
Chlamydomonas reinhardtii. In this case it is nitrogen depletion rather
that heat that stresses the organism and leads to sexual reproduction.
Michod is confident that similar mechanisms will be found in all organisms
that can choose between sexual and asexual reproduction.
"When the temperature rises to 42.5°C, DNA damage
triggered by heat stress causes the algae to resort to sex."
PHOTO (COLOR): For Volvox carteri, heat and sex go
together
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By Anil Ananthaswamy