The Sister Act

This blog post is brought to you by a special guest writer, Kristyn Ing, who’s currently studying Environmental Science at the University of Western Ontario. She recently became fascinated with bees while taking an Animal Behaviour course and marveled at their eusocial behaviour. 

While they may appear to be working together, these sisters could be doin’ it for themselves. 

In our last post, we discussed the epigenetics of bees and how they change roles in the hive, so we’re going to dive a little deeper this time into their genetics and how it affects their reproductive behaviour.

Honeybee societies are often used to explain a well-functioning operation or a network of co-operative components. Yet what most people do not realize is that the queen bee and her workers are in an ongoing evolutionary conflict that is unlikely to end. The conflict between the two parties stems from their reproductive interests being misaligned. Conflict is often seen as an undesired outcome, but if stability were realized, there would be no room for biological and behavioural change.

Just as human parents are equally related to both their offspring by 50%, the same goes for female queen bees, who are 50% related to both their daughters and sons. However, if one were to analyze the relatedness of a worker, who is biologically always female, to either one of their full sisters or brothers, you would notice that they are more related to a sister than a brother by 25%! Hence the name “super-sisters” that has been attributed to full sister honeybees.

How might this be? Queen bees are diploidy, only partially sharing their genome with their offspring. Female honeybees stem from fertilized eggs and require a male to be fertilized. Conversely, male honeybees come from unfertilized eggs and are thus solely related to their mother, the queen. While queen bees are diploidy, male bees are haploid and therefore share 100% of their genetic material with their offspring, creating a haplodiploid interaction for super-sisters. Super-sisters share half, and therefore have 50% of genes from their mutual father (100%/2 = 50%). They also have half of their genes in common from their mother, the queen (50%/2 = 25%), making super-sisters related to each other by a grand total of 75%, leaving their brothers to be only a mere 25% related. This shows that a female worker would be three times more related to her sister (75%) than to her brother (25%). Therefore, it is genetically more favourable to workers for the queen to rear more sisters than brothers.

This may sound absurd and only theoretical, yet it has been well-studied that sisters have manipulated the sex ratio in their favour as they are the workers who tend to the queen’s eggs. Due to the imbalance in relatedness of workers to their sisters versus their brothers, they would preferentially treat eggs destined to be female while destroying those who would be male. Though this would be advantageous to the workers, it would be very detrimental to the queen’s evolutionary fitness, as her future sons are being terminated even though she is equally related to both her sons and daughters. At that moment in evolutionary history, honeybee workers were “winning” this evolutionary battle.

As time passed, queen bees slowly evolved to (unintentionally) realize a solution to this conundrum by mating with multiple males and not remaining monogamous, which had no impact on her relatedness, nor to her personal indirect fitness. This in turn had a tremendous negative effect on her daughters/workers. As workers no longer shared a common father, their relatedness was slowly driven down to a point where they were equally related to their sisters and brothers (25%). Therefore, the worker’s incentive to preferentially rear sisters has been stripped away and has now placed the queen in a “winning” position in this evolutionary conflict she had been having with her daughters.

From eggs to pupae, the bee brood will soon emerge as future workers and drones.

To combat this issue, worker bees are sometimes inclined to try their luck and lay some of their own unfertilized eggs within the group to rear sons of their own, which would result in having their own offspring that are 50% related to themselves, compared to only being 25% related to their siblings. As a result, it is genetically more favourable for workers to have offspring of their own and not solely help raise their siblings. However, this is not beneficial to her sisters and the rest of the worker community as a whole because workers are less related to the offspring of their partial siblings rather than their other siblings. Lone reproduction by a worker is not commonly practiced because due to this imbalance in relatedness*, the individuals who perform this act often get their eggs destroyed by their siblings as it is not genetically favourable for workers to care for those eggs.

Honey bees are often compared to being a well-oiled machine, working together as one, yet in reality, their evolutionary interests remain unresolved and therefore they continue to be in conflict, much like fighting to win the Iron Throne. This goes to show that not only do humans have family feuds, but so do bees!

*One exception to the norm is seen in Cape honeybee workers, which possess a unique reproductive system where they can lay unfertilized eggs to become a queen or a worker bee, thus creating a clone that is genetically identical to them!