Genetic researchers have sequenced and analyzed the complete genomes of two key bumblebee species – the European buff-tailed bumblebee (Bombus terrestris) and the North American common eastern bumblebee (Bombus impatiens). The results appear in two papers published in the journal Genome Biology.
The buff-tailed bumblebee (Bombus terrestris). Image credit: Simon Koopmann / CC BY-SA 2.0 DE.
“The sequencing and annotation of these bumblebee genomes constitute a great leap forward for the study of bee biology and understanding the organization of insect societies,” said Dr Ben Sadd of Illinois State University, a co-author of one of the papers.
Both studies involved large international collaborations and included genetic scientists from eight countries. Using individual bees from Thurgau, Switzerland, and Michigan, they focused on identifying similarities and differences between the transatlantic cousins to understand their basic biology and to shed light on possible clues about their susceptibility to threats like pesticides and diseases.
“We remain far from understanding the threats to bumblebees, but molecular insights about their genes are vital to help us understand how bumblebees and diseases interact and what is putting populations of bumblebees at risk,” Dr Sadd said.
Bumblebees have different colony lifestyles than honeybees. They live in colonies of hundreds rather than thousands of individuals and have less advanced social behaviors than honeybees.
On the spectrum of social organization, most bumblebees have what is described as an intermediate structure, lying between completely solitary bees and the extreme social adaptations seen in highly social honeybees.
The limited immune gene repertoire of honeybees was believed to be a consequence of sociality. Honeybees tend to live in densely populated colonies with thousands of closely related individual bees, which are ideal conditions for the spread of infectious diseases.
Despite the greater risk of disease, the highly social honeybee was thought to have fewer immune genes than non-bee solitary insects because of the extensive social defenses available in colonies like grooming or other sanitary behaviors. Surprisingly though, a reduced number of immune genes is found also in the bumblebees and is consistent with the highly social honeybee and a solitary bee.
“The catalogue of genes involved in immune defense responses is well conserved among different bee species regardless of their level of social organization, but it is much smaller than in solitary insects such as flies and mosquitoes that often live in more pathogen-rich environments,” said co-author Dr Robert Waterhouse of the University of Geneva and the SIB Swiss Institute of Bioinformatics.
“Nevertheless, variations in evolutionary signatures of selection amongst immune genes from bumblebees and honeybees may point to different pressures exerted by the distinct pathogens that threaten these bees.”
“Despite bumblebees having simpler colony organization and shorter colony lifespan, they have a remarkably similar repertoire of immune genes to the highly social honeybee (Apis mellifera), and even more surprising, are similar to the solitary leaf-cutting bee (Megachile rotundata),” said co-author Dr Seth Barribeau of East Carolina University.
The common eastern bumblebee (Bombus impatiens). Image credit: Bob Peterson / CC BY-SA 2.0.
The similarity between social and solitary bees suggests that the immune system of bees has remained largely the same since before the origin of bee sociality.
The scientists speculate that this reduced immunity may even have provided a route to advanced social living.
“However, despite the similarity across bees in terms of the broad complement of genes present we do find distinct signatures of selection in the different lineages suggesting that these bees have faced selection from different parasites in their past,” Dr Barribeau said.
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