Tracking the infection of flies
Although it is known that Wolbachia
— a bacterial genus present in 20 to 80 percent of all insects — is
transmitted from female hosts to their offspring through their germ line, it has
only been suspected that infectious transmission must occur between carriers and
noncarriers without passing through any germ line. Now, using confocal microscopy,
researchers at Princeton University in New Jersey have shown that Wolbachia
introduced to a host eventually crosses several tissues to ultimately infect the
germ line of flies.
The investigators, led by Horacio M. Frydman of
the department of molecular biology, injected the abdominal cavities of several
Drosophila species of flies with Wolbachia and imaged the insects’
ovaries using a Zeiss confocal microscope. They report in the May 25 issue of Nature
that, over the course of several days, the bacteria crossed the peritoneal sheath
membrane surrounding the ovary, the muscle epithelium that encloses each ovariole
and the somatic tissue that envelops the germ-line cells. The pattern of infection
suggests that Wolbachia reaches the germ-line cells through the forward section
of the ovariole, or germarium.
Interestingly, the germarium is the
location of several somatic stem cells, leading the researchers to believe that
these cells provide a niche that presents a preferential infection site for Wolbachia.
In experiments with long-term, maternally
transmitted infections, they found that female Drosophila had higher bacterial
levels in their germ-line cells than in, for example, follicle cells. The only exception
was that the Wolbachia also strongly accumulated along the border of the
germarium, similar to the behavior of transient infections.
Frydman’s group notes that the
mechanism by which Wolbachia enters and accumulates within the stem cell
niche remains unclear, but that future research should reveal the answer to that
question as well as to the cellular basis for horizontal transfer.
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