In a honey bee colony, the queen bee is responsible for laying most of the eggs but she does not otherwise forage for nectar, make honey, defend against intruders, or even provision her own young. Instead, this latter set of behaviours are performed by her worker daughters, who selflessly live as sterile servants of the queen. But how does this type of reproductive self-sacrifice evolve?
As first described by renowned English biologist and evolution theorist William D. Hamilton in the 1960s, natural selection could actually favour genes for selfless qualities, like those expressed by sterile workers, if their effect was to increase the fitness of reproducing relatives. For honey bees, if workers direct their altruism towards their queen mother, then she can pass on copies of the worker’s own genes, including those genes for sterility and other selfless qualities. Finding real life examples of genes underlying behavioural altruism is a major focus of insect sociobiology.
Working with honey bees, scientists at Western University have now discovered a sweet – albeit unexpected – sample.
In a study published by Scientific Reports, Western PhD graduate Alison Camiletti, Western biology professor Graham Thompson and their collaborators describe how they unexpectedly found that application of honey bee queen pheromone – the chemical that normally renders workers sterile – is, oddly, effective on female fruit flies, an insect that is neither social nor closely related to honey bees.
The researchers found that by administering the ‘queen substance’ to fruit flies they will shut off their ovaries and stop laying eggs in a manner that is comparable to the pheromone’s normal effect on the workers themselves. The scientific team then used this accidental discovery, together with some sophisticated molecular techniques, to find the genes involved.
Using gene ‘knock-downs,’ in which strains of flies can be bred with particular genes switched off, the researchers were able to systematically identify which genes are important to perceiving the ovary-inhibiting pheromone.
Camiletti and Thompson found that three specific genes normally associated with the fly’s ability to smell – called olfactory receptors – are necessary for the pheromone to have its effect.
“Are these ‘genes for sterility’ of the type predicted from theory? Arguably, yes – or at least they represent candidate copies of these genes present in flies that, when expressed by worker bees, render workers sterile in the presence of their queen. The fly is not social and thus has no altruistically sterile caste, but flies nonetheless share many genes by common descent with honey bees and indeed all other insects,” explains Thompson.
The findings in Scientific Reports suggest that the basic building blocks that have come to regulate female reproductive behaviour in honey bee societies has probably been in place for hundreds of millions of years, or at least since the honey bee and fruit fly last shared a common ancestor.
The study also notes that it is not yet feasible to knock down the olfactory receptor genes in the bee itself, as was achieved in the fly, but the study now makes clear that this is a worthwhile goal.
“If Alison (Camiletti) has accidentally discovered the famous genes for honey bee sterility – in the fruit fly – it would provide an important validation of a 50-year-old theory, and would further suggest that even complex societies like that of the honey bee is best understood as an evolutionary elaboration of a basic reproductive and behavioural groundplan that first evolved in non-social insects,” says Thompson.
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