All honey bee larvae are equal when they start out. But then, in a twist of fate, some develop into fertile queen bees, while others are relegated to a life of worker drudgery: and that twist is simply their diet. Larvae fed a nutritious gel – royal jelly – develop into queens, while larvae provided with a plainer diet develop into workers. However, the mechanism for this drastic diet-induced switch of fate wasn't clear. Adam Dolezal from Arizona State University, USA, explains that two metabolic pathways – the insulin-like signalling pathway and TOR (target of rapamycin) pathway – are known to sense the nutritional status of most creatures, adjusting their metabolism and development to make the most of an individual's diet. He adds that a key hormone – juvenile hormone – is also known to throw the switch from worker to queen development. So, Dolezal, his supervisor Gro Amdam and a team of collaborators from Arizona State University and Washington State University decided to find out whether these signalling pathways affect juvenile hormone levels in response to diet to turn workers into queens (p. 3977).
Knowing that both pathways are triggered by specific signalling proteins [insulin-like receptor substrate (IRS) and TOR], the team decided to inactivate IRS and TOR individually, and simultaneously, while feeding larvae on a diet of royal jelly. If the signalling pathways were carrying the message that the royal jelly-fed bees should develop into queens, then short circuiting the pathways should force the bees to develop into workers, despite the regal diet.
Dolezal and Navdeep Mutti became worker bee nurses to thousands of vulnerable honey bee larvae as they fed the insects a royal jelly diet laced with specially tailored double stranded mRNA molecules, to prevent the larvae from producing the IRS and/or TOR molecules and so inactivate the signalling pathways' responses to the diet. Then the team monitored how the larvae developed without the signalling pathways.
Teaming up with Florian Wolschin, Jasdeep Mutti and Kulvinder Gill, Dolezal and Nardeep Mutti found that the larvae developed into workers, despite their royal jelly diet. And when Dolezal monitored the larvae's juvenile hormone levels, instead of being high – which is what you would expect for larvae fed on royal jelly – the larvae had low levels of the hormone, which had forced them to develop into workers despite their diet.
Switching off the pathways prevented larvae on a royal jelly diet from producing juvenile hormone and developing as queens, but could the insects develop into queens if they received a juvenile hormone supplement? Sure enough, when Dolezal and Mutti gently applied juvenile hormone to the skins of royal jelly fed larvae that had lost insulin and/or TOR signalling, they developed into queens. So, the IRS and TOR signalling molecules are key links between the larvae's diet and their developmental fate.
However, Dolezal points out that another researcher, Masaki Kamakura, tested the link between diet and development at a different point in the insulin signalling pathway. Publishing his work in Nature, Kamakura found the opposite result: his royal jelly-fed larvae successfully developed into queens even though the signalling pathway had been short circuited at the signal's receptor. But Dolezal explains that the IRS trigger is also known to activate the epidermal growth factor (EGF) signalling pathway, in addition to the insulin pathway, by interacting with other receptors. The larvae could still develop into queens if the insulin receptor was inactivated because the IRS signal could trigger the alternative pathway, and the team is now keen to test the connection between IRS and EGF signalling in larval development.