Age, tissue, genotype and virus infection regulate Wolbachia levels in Drosophila

The bacterial symbiont Wolbachia can protect insects against viral
pathogens, and the varying levels of antiviral protection are correlated
with the endosymbiont load within the insects. To understand why Wolbachia
strains differ in their antiviral effects, we investigated the factors
controlling Wolbachia density in five closely related strains in their
natural Drosophila hosts. We found that Wolbachia density varied greatly
across different tissues and between flies of different ages, and these
effects depended on the host-symbiont association. Some endosymbionts
maintained largely stable densities as flies aged while others increased,
and these effects in turn depended on the tissue being examined. Measuring
Wolbachia rRNA levels in response to viral infection, we found that viral
infection itself also altered Wolbachia levels, with FHV causing
substantial reductions in symbiont loads late in the infection. This
effect, however, was virus-specific as DCV had little impact on Wolbachia
in all of the five host systems. Since viruses have strong tissue tropisms
and antiviral protection is thought to be cell autonomous, these effects
are likely to affect the virus-blocking phenomenon. However, we were
unable to find any evidence of a correlation between Wolbachia and viral
titers within the same tissues. We conclude that Wolbachia levels within
flies are regulated in a complex host-symbiont-virus dependent manner and
this trinity is likely to influence the antiviral effects of Wolbachia.