By their very nature, parasites have intimate contact with their hosts. While some may be relatively benign, others can have grave consequences for the organism they have infected – depleting food resources, or carrying debilitating disease. For the honey bee, such threats come in multiple forms that can be exacerbated by their high-density, hive-dwelling lifestyle. Once established, infection and disease can spread rapidly through a colony, passed between individuals as they go about their daily tasks in close contact with their kin. Despite the honey bee’s huge economic importance in agriculture, precisely how they combat the spread of infection in the hive is not well understood.
Now, a large interdisciplinary study published in BMC Ecology from researchers at INRA (French National Institute for Agricultural Research) has combined chemical, neurogenomic, and behavioral experiments to investigate just what happens when infection enters the hive – and how smell, brain activity, and behavior is affected as a result.
In situations of infection, the hive may mount a challenge through a mechanism known as ‘social immunity’. This phenomenon involves behavioral adaptations by the hosts to limit the spread of infection, either by forcibly ejecting diseased bees from the hive, or through selfless removal of infected individuals. Yves Le Conte and colleagues probed how parasitic infection could result in these modified behaviors.
Two of the most harmful honey bee parasites were chosen –the mite Varroa destructor, and the microsporidian fungi Nosema ceranae – in order to investigate whether the underlying mechanisms behind this ‘social immune’ response are parasite-specific or part of a larger, more conserved immune response. Crucially, these parasites have very different modes of infection: the Varroa mite feeds externally on the ‘blood’ of developing larvae, whereas Nosema enters the gut cells of fully developed adults, feeding off of their cellular energy. Both can severely weaken the host.
By creating experimentally infected hives harboring colonies of either mite-infected or fungi-infected bees, the research team compared how each parasite alters the physiological and behavioral traits of infected bees compared to healthy controls. Each bee emits a characteristic scent or chemical signature from its outer cuticle, used as a means of recognition by nest-mates in social interactions, but which can also induce aggressive encounters. Analysis of this scent from bees infected by both parasites revealed distinct changes to the emitted chemical signatures, but no differences among levels of an antiseptic chemical derived from royal jelly, which is thought to play a role in social immunity.
Interestingly, although parasitized bees were clearly exhibiting altered chemical scent profiles there was no evidence that healthy nest-mates became more aggressive toward them, or that forcible ejection of parasitized individuals was occurring as a response to infection. Le Conte and colleagues looked to the brain for answers.
Despite the respective pathologies of neither parasite directly targeting brain or neuronal pathways, distinct changes were apparent via Digital Gene Expression analysis. Unlike the individual signatures found in the chemical scent analysis, here the differences pointed toward a more conserved, generalized response to infection: patterns of gene expression in the brain were more similar among these two infection-types than they were even among different castes of bees.
The authors argue that the weight of evidence in the field so far favors a mechanism of altruistic self-removal of infected bees, to spare the rest of the colony. However, in a related Commentary published concurrently in BMC Ecology, Olav Rueppell and colleagues from the University of North Carolina at Greensboro, USA caution that further experimental data will be required to verify this. Acknowledging that this multi-faceted approach is an important step forward in our understanding of honey bee health, they also highlight how alternative hypotheses may still account for the patterns observed. For example, changes in brain gene expression could feasibly be the result of behavioral manipulation to encourage parasitized individuals to leave the nest in search of new hives to infect. Advances in insect-scale GPS technologies could help track down the answer.
Written by Simon Harold (@sid_or_simon), Senior Executive Editor for the BMC Series.
Ecto- and endoparasite induce similar chemical and brain neurogenomic responses in the honey bee (
BMC Ecology 2013, 13:25
Go to article >>
BMC Ecology 2013, 13:26
Go to article >>