Background

The VSH behaviour has evolved in Apis mellifera as a response to being exposed to the Varroa destructor mites, which have co-evolved in an evolutionary arms race with Apis cerana for so long that they have become extremely virulent towards A. mellifera. New defensive behaviours in A.mellifera against the mites, like varroa sensitive hygiene (VSH), have appeared in some A. mellifera colonies, but it is also costly to fitness.

Moro et al. (2021) compared the rates of VSH behaviour in A. mellifera colonies as a response to infestation of V. destructor mites. They found that the rates of VSH expression were greatly reduced within their experimental colony over a three-year period, although the exact rate of change was not examined.

Research questions

In order to determine the rates of evolution of this behaviour, we aim to measure how the expression of VSH changes over time in a colony that is mite-infested compared to a colony where mites have been removed. We will also compare the rate of VSH behaviour change between colonies originating from a population that expresses VSH behaviour, such as the AWD population from Moro et al. (2021), and a “wild-type” population that does not express VSH.

Experiment design

This experiment will take place in a highly controlled laboratory environment to remove the impact of wild mite populations impacting the colonies. As shown in Figure 1, we would begin with sixteen colonies from two separate genetic populations. One population has been selected for the VSH behaviour, such as the AWD population from Moro et al. (2021), and the other wild-type population does not express VSH. Within each population we will have two control groups with four colonies each — one that is infected with a known quantity of mites, and one that has all of their mites removed. In both control groups, re-infection from wild mites will be prevented from occurring.

Using four colonies in each group allows for reproduction in the experiment and will minimize biases and random chance skewing our results, while also not making the experiment too large in scope. Each colony should be the same age and similar in every other way. Since the infected colonies will also be at risk of collapsing due to mite infestation, having multiple colonies will safeguard against us completely losing any control group.

After establishing the colonies, we will measure the rates of VSH expression within the colony after each winter using the same measurement methods as in the 2021 Moro et al. paper. By comparing the measured amounts of VSH expression between the populations and control groups, we can examine how this behaviour evolves in response to mite infestations.

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Figure 1. Flow chart showing the four control groups, methods and predicted outcomes of the experiment.

Expected outcomes

What outcomes do you expect from your treatment groups as supporting evidence of co-evolution?

We expect that the infected colonies from the AWD population would show a reduction over time in VSH expression, similar to that measured between the colonies in 2015 to 2018 in the 2021 study, due to the high cost to fitness of the VSH expression and the adoption of less costly defensive strategies. In the mite-free colonies from this same population, we would expect a larger reduction in VSH expression each season, since the VSH will not benefit them. In the wild-type population, which did not previously display VSH, we might expect this behaviour to evolve in response to a virulent mite infestation, although it is unclear how fast this might happen or if different strategies might be selected instead. In our control group of wild-type bees which are shielded from mite infestation, the we would expect no change in VSH.

Impacts on co-evolutionary processes

More generally, how does your aim/question shed light on our understanding of co-evolution processes (2-3 sentences)?

Based on the “Red Queen Hypothesis” from co-evolutionary theory, Apis mellifera will need to evolve new adaptations to resist the virulent varroa mites or risk extinction. By measuring the rate of evolution of this defensive behaviour in different conditions, we can determine if it is possible for the bees to adapt quick enough to overcome the virulence of the mites, and also measure what cost to fitness this strategy has in colonies without mite infestations.

References

Moro, A.; Blacquière, T.; Panziera, D.; Dietemann, V.; Neumann, P. Host-Parasite Co-Evolution in Real-Time: Changes in Honey Bee Resistance Mechanisms and Mite Reproductive Strategies. Insects 2021, 12, 120. https://doi.org/10.3390/insects12020120