Beauveria bassiana infection in Drosophila melanogaster

Drosophila melanogaster is a model organism to study innate
immunity in invertebrates. Temperate and tropical D. melanogaster
populations, being exposed to different environments, potentially
face distinct parasites and parasite pressure. Indeed, there is
experimental evidence suggesting that tropical D. melanogaster
populations survive longer than temperate ones to infection by the
fungal parasite Beauveria bassiana. In the present work we test the
generality of this conclusion and investigate if host populations
differ in their molecular response to infection. We first exposed
to B. bassiana infection two tropical (from Africa and South-East
Asia) and two temperate (from Europe and North America) D.
melanogaster out-crossed populations. We consistently found a
significant effect of B. bassiana on Drosophila mortality, but we
were not able to identify a significant difference in survival to
infection among populations. These results indicate that tropical
populations may not always survive better than temperate ones, and
suggest that other environmental factors, such as humidity or local
species richness may be more accurate predictors of immune
competence. Subsequently, we recorded transcriptional response to
B. bassiana in all D. melanogaster populations, both by microarray
and RNA sequencing. To our knowledge this is the first time that
transcriptional response to fungal infection has been determined in
multiple D. melanogaster out-crossed populations. We found few or
no genes significantly induced 8 hours after infection. On the
other hand, we identified between 200 and 1,300 genes induced 24
hours after infection depending on the population. This means that
transcriptional response to B. bassiana begins between 8 and 24
hours after infection. We reveal here that host populations respond
differently at the molecular level, as shown by the large variation
in the number of induced genes. We report that gene ontology
categories related to translation, biosynthesis and reproduction
are enriched in genes down-regulated upon infection, suggesting a
metabolic cost of mounting the defence response. Next, we wanted to
assess the selective pressures acting on induced candidate genes.
We compared the genes induced in all populations to the ones
induced specifically in each population and computed population
genetic statistics for a subset of genes in each category. We
noticed higher conservation at non-synonymous sites for commonly
induced genes compared to population specific ones. This hints that
common genes are under stronger selective constraints. Another
topic we addressed in the present work is the effect of
endosymbionts and trans-generational immune priming on D.
melanogaster survival to B. bassiana. We tested for a protective
effect of the endosymbiont Wolbachia pipientis in two D.
melanogaster inbred lines. We did not find an effect of Wolbachia
on survival to infection in two independent experimental
replicates. In absence of infection, flies bearing Wolbachia had a
lower fitness than cured ones. Therefore W. pipientis appears to
have a negative effect on Drosophila general vigour, but no effect
on mortality upon infection. Finally, we tested if flies whose
parents were exposed to B. bassiana were less susceptible when
infected by the same parasite. This would imply a transfer of
immune memory from parents to offspring, which is called
trans-generational immune priming. However, no evidence of immune
transfer for two D. melanogaster out-crossed populations could be
found. Yet, as trans-generational immune priming depends on host
and parasite genotype, more experiments are needed to determine its
generality in the D. melanogaster – B. bassiana system.