The eusocial bee colony is characterised by division of labour: While the queen is responsible for laying the eggs, the usually sterile workers take on tasks in nest building, brood care and foraging. It is known from species with highly developed social behaviour such as the honey bee that queens control the behaviour and fertility of their workers with specific scents ('pheromones'). Professor Manfred Ayasse and Iris Steitz from Ulm University have now been able to show for the first time that queen pheromones also influence the social life of primitively eusocial sweat bees. The chemical composition of this queen pheromone, however, differs from that of highly eusocial insects, allowing conclusions about the complex evolution of the queen pheromone. The study was recently published in the journal Current Biology.
The reproductive division of labour and the caste system are regarded as the basis of the evolutionary success of eusocial insects such as bees, wasps or ants. It is known from previous studies that species with more highly developed social behaviour organise their coexistence through chemical communication. Queen pheromones play a key role in this process, most notably by inhibiting the reproductive ability of female workers. The queens of primitively eusocial species with a low number of nestmates, on the other hand, were thought to prevent their workers from reproducing by means of physical aggression. However, studies on the social behaviour of these insects, which live together in small, annual colonies and whose queens' external appearance is hard to distinguish from the workers, are lacking. Biologists from the Institute of Evolutionary Ecology and Conservation Genomics in Ulm hope to close this research gap.
The scientists investigated possible queen pheromones in the primitively eusocial furrow bee (Lasioglossum malachurum). 'This bee species is particularly well suited as a model organism, as the chemical substances on its body surface have already been sufficiently examined and identified,' explains first author Iris Steitz. A comparative chemical analysis of the odour profile of furrow bee queens and workers revealed differences in the amounts of so-called macrocyclic lactones. Differences were also found in the amounts of cuticular hydrocarbons that cover the skin of the insects and primarily protect them from dehydration, and that serve as queen pheromones in many more highly developed species.
Experiments in the tube and in the artificial nest
The researchers experimented to identify the mixture that makes up the characteristic scent profile of a furrow bee queen. For this purpose, they simulated the underground nest situation: Two workers from the same nest were placed in a transparent tube to observe their behaviour when encountering. In each round, one of the bees was treated with the original scent of a queen or with synthetic mixtures of cuticular hydrocarbons or macrocyclic lactones respectively. 'This odour manipulation allowed us to observe the behavioural repertoire of workers who encounter their queen in the nest. The typical reaction involves submissive, backing movements,' Professor Manfred Ayasse describes. This pattern was particularly evident when the other worker had been treated with the original scent of a queen or with a synthetic mixture of macrocyclic lactones.
Cuticular hydrocarbons alone, however, did not trigger typical reactions in the untreated workers. They seem to function merely as the chemical background of the furrow bee queen pheromone.
In a second experiment, the researchers investigated the extent to which the queen pheromone actually influences the reproductive ability of the workers. For this purpose, workers were placed in artificial nests without queens but into which different scent mixtures found in queens were introduced. When the biologists examined the ovarian activity of the workers after seven days, they found that it was particularly reduced when macrocyclic lactones were offered in the nest. 'Overall, it can be concluded that the identified queen pheromones from macrocyclic lactones induce submissive behaviour in female sweat bee workers and significantly reduce their ovarian activity,' summarises Iris Steitz. The researchers are thus the first to succeed in identifying a queen pheromone in less complex eusocial species of bees. In contrast to most of the previously known olfactory profiles found in queens of species with more highly developed social behaviour, this one does not belong to the substance class of cuticular hydrocarbons. 'Most studies published to date have suggested that cuticular hydrocarbons have evolved into queen pheromones in eusocial insects. Our study challenges this assumption and presents a much more complex picture of evolution,' says Professor Ayasse. Further comparable studies on less highly developed eusocial insect species would thus be desirable.
Media contact: Annika Bingmann
Iris Steitz and Manfred Ayasse: Macrocyclic Lactones act as a Queen Pheromone in a Primitively Eusocial Sweat Bee. Current Biology (2020). DOI 10.1016/j.cub.2020.01.026