Now, Thomas W. Cline, professor of molecular and cell biology
at UC Berkeley, and graduate student Diana J. Starr, have
discovered that the bacterium Wolbachia pipientis allows
a sterile female fruit fly to lay eggs, circumventing a
genetic mutation in a gene that is the key to determining
the sex of offspring.
"It's stunning that we've created in the lab, without intending
to, the same situation that people have found in the wild,"
Cline said. "This is very exciting, because now we can study
how the bacterium interacts with the genes of a model organism
we know a lot about, the fruit fly. This will help us find
out what determines the ratio of males to females, and how
host-parasite interactions drive evolution."
A natural parallel is a parasitic wasp, Asobara tabida,
that depends on Wolbachia infection to reproduce. When "cured"
of its parasites, researchers reported last year, it is
unable to reproduce.
"The remarkable thing in this case is that infection with
the parasite makes a bad fly mutation more benign - but
at the cost of potentially addicting the species to the
parasite," Cline said.
Starr and Cline publish their findings in the July
4 issue of Nature.
Various species of Wolbachia have been found to infect
from 25 to 75 percent of all insect species in the wild,
as well as spiders, nematodes and some crustaceans. The
bacteria pass from one host to another only through eggs
- which means only through the female line - and have developed
a very aggressive strategy to improve their chances of success.
In mosquitoes, infected males can mate successfully only
with females infected with the same strain of the bacteria,
thereby ensuring that the number of infected mosquitoes
increases with each generation. Similar techniques exploiting
"cytoplasmic incompatibility" have been found in beetles,
wasps, moths and fruit flies.
In some wasps, the bacteria allow females to reproduce
without the assistance of males. And in several species,
the bacteria kill males during early development.
Though Wolbachia apparently don't infect vertebrates, it
can have an impact on human health. The nematode that causes
river blindness in tropical countries is able to lay eggs
and reproduce in humans only thanks to the Wolbachia bacteria
infecting it. Wolbachia, not the nematode itself, trigger
the immune response that damages the eye.
No one knows how Wolbachia achieve this, but now that they
have been shown to affect a specific fruit fly gene, called
sex-lethal (Sxl), Cline said researchers have their first
handle on solving the problem.
"The fact that the interacting gene in this case has been
studied so extensively and belongs to a model experimental
organism can be exploited to yield further insights into
the mechanism by which this parasite takes advantage of
its various arthropod hosts," the authors write in their
Cline has studied the genetics of sex determination in
fruit flies for 30 years, and discovered the genes that
set the sex ratio - what percentage of offspring is male
and what percentage is female. These genes work by controlling
sex-lethal, and Cline has mapped out the molecular details
of this control.
"Interestingly, the genetic mechanism that regulates sex
ratio changes dramatically in very closely related organisms,"
said Cline. "Sex ratio seems so fundamental, though, it
raises a big question: Why does the mechanism change so
fast across species lines?"
Hence his interest in a parasitic bacterium that changes
the sex ratio in fruit flies. The existence of such a parasite
suggests the important role that host-parasite interactions
have in evolution, including the rise of new species.
"There's a very real possibility that species often get
in a bind with a parasite that changes the sex ratio in
a way that doesn't benefit the species," he said. "To get
out of the bind, the species has to evolve further. Most
of life is a race between host and parasite, plus change
resulting from interaction between the sexes."
Cline's focus is on the sex-lethal gene, which he named
because mutations in the gene kill one sex or the other.
Over many years of research, he has established that Sxl
is the master gene determining an offspring's sex. The activity
of Sxl is determined by the number of X-chromosomes allotted
to an egg: females have two X chromosomes, which switch
on Sxl and generate all the characteristics that make females
female; males have one X, which leaves Sxl silent.
The gene also plays a role in the physical process of egg
development. While some mutations in sex-lethal knock out
the entire gene and are lethal to female offspring, a few
point mutations affect only the egg-production machinery,
creating viable but sterile offspring.
The fruit flies observed in this experiment had a point
mutation in both copies of their sex-lethal gene, which
made the females sterile. (Some flies had a mutation - an
altered base pair - in one area of the gene, while others
had a single base-pair change at a second, nearby site.
Both mutations had the same effect.) While preparing to
irradiate some infertile flies to try to create mutations
that reverse the infertility, a researcher in Cline's lab
found an unusual group of flies that already appeared to
have lost this trait. In this group of flies, the females
produced large numbers of eggs, though few offspring.
Starr found an inherited difference between the fertile
flies and the supposedly identical and still infertile mutants,
but showed that this genetic factor was not located on any
fly chromosome. Cline immediately suspected Wolbachia infection.
Starr searched with the polymerase chain reaction and found
evidence of the bacteria. She subsequently crossed a Wolbachia-free
strain of mutant fly with a wild fly infected with Wolbachia,
and found most of the resulting females fertile. The clincher
was that treating these flies with the antibiotic tetracycline
to kill Wolbachia made them sterile again.
The strange results have already told them that Wolbachia
probably produce some protein that interacts with the protein
encoded by the sex-lethal gene. The bacteria apparently
do not affect expression of the gene or bypass the protein
in the pathway leading to egg production. In other insects,
however, the bacteria might work differently, Starr cautions.
"We can't tell if what we find in fruit flies will be related
to how Wolbachia operate in wasps or other insects, but
we know it will be interesting," said Starr, who continues
to track down the details of how the bacterium interacts
with fruit fly genes or proteins. "We'll discover something
about how this bacterium works, but it will tell us a lot
about sex determination in general."
The research was supported by the National Institutes of