If you imagine a scenario where an individual had unlimited resources, anything that required some investment wouldn’t be a big deal. It would be able to send some of these resources to achieve the required task and go on about its business without worrying about cutting costs elsewhere. When resources are limited, this changes. An individual must allocate resources wisely to make the best of its lot. The immune system is, in my mind, a fairly nebulous concept. It includes all of the normal immune organs and cells that you might expect but, I would argue, also includes behaviours, metabolic pathways, and probably a great deal more than we know at the moment. But while nebulous, the immune system is clearly expensive. Producing immune compounds, or growing organs, or engaging in hygienic behaviours are all expensive and leave fewer resources for other things. In our recent paper Franziska Brunner and I explored how limiting a key component of nutritional resources altered immune responses as measured by gene expression. We found that if we deprived bumblebee workers from pollen, their protein source, for even a fairly short amount of time, they have much weaker immune expression. These bees are really robust, and this wasn’t a greatly stressful condition, but even so, they were unable to upregulate key components of their immune response. It seems that a number of effectors (components of the immune system that are produced to actively kill invading parasites) are protein limited.
We also found that when the bees were protein limited the variance in their immune response reduced. Bees became more similar in their immune responses when they didn’t have enough protein. This can have important consequences because we know that when populations of animals are under stressful conditions there can be outbreaks of diseases. This reduction in expressed immune variation could help explain why diseases are better able to spread in these stressed populations. For instance, imagine a population of individuals that have a distribution of phenotypes (the outcome of the genotype and environment). Parasites that are able to infect common phenotypes do very well since there are many hosts to infect and will thus be selected for.
Here a parasite has a range of host phenotypes that it can infect but there remain quite a few that it can’t. If, however the population becomes stressed and this shrinks the variation in phenotype, then the parasites are better able to infect a greater proportion of the population.
The variation in immune phenotype might be especially important in social insect colonies, like bumblebees, since workers are very highly related to one another. Because of the haplodiploid sex determination system, and because most bumblebee queens only mate with a single male, all the workers share 75% of their genome. This means that these populations are very homogenous and would be a prime target for invasion by parasites because if a parasite can infect one worker it is very likely able to infect other sister workers. Having workers with diverse immune phenotypes while sharing most of their genome might help buffer bumblebee colonies from parasite outbreaks within the colony. At least, for as long as life isn’t too stressful.
