Franziska Brünner was my first student here at the ETH and a paper from her semesterarbeit has been published in PLoS One. Franzi’s paper explores the expression of immune genes in the bumblebee Bombus terrestris when they are infected with the trypanosome gut parasite Crithidia bombi. Unlike most other papers looking at gene expression in bumblebees we used colonies that were collected from the wild. We found that while there was a great deal of variation among workers within a colony, we were still able to detect changes in the immune system after infection. One of the nice things about this result is that it suggests that these kinds of gene expression methods are applicable to generally noisy biological systems.
We also found something surprising. Bees that came from different locations in northern Switzerland, had very different responses to this same parasite. We tend to collect from two main sites around Basel and an hour or so North on Zürich. This suggests that while the European population of our bee is panmictic (mixed across the region with no obvious differences between regions) there are actually quite distinct patterns of immune responses between bees that were collected within a few hours drive from one another. This might reflect the parasite prevalence in these two locations. In one location, bees are more likely to be infected with our common trypanosome parasite than the other location (12.5% vs 5.1%). The bees that were from the more parasitised location, had lower expression of immune genes than those in the less parasitised location. The differences in immune expression could have a few causes. Bees from the high risk location may differ genetically from the bees in the less risky location leading to them producing a weaker response. Otherwise historical exposure to other parasites might have tweaked their expression to respond to different parasites at the expense of this parasite. Bumblebees have a form of immune memory, sometimes called immune priming, where they are better able to respond to repeated exposure to parasites of the same kind. This immune memory can even be passed from mothers (queens) to their daughter workers.
While we found variation across locations and with infection, we also found that colonies differed from one another in their immune profile. This is important, because while bees from different colonies have different susceptibilities we don’t know why. Differences in immune gene expression was strong enough that we were pretty reliably able to assign an individual to their colony just based on their immune expression profile. These differences in immune expression among colonies may explain why colonies differ in resistance to parasites.
