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Genomes of Two Key Bumblebee Species Sequenced; Surprising Paucity of Immune Genes and Detoxification Genes Found

A research collaboration spearheaded by ETH Zurich has shed light on the genome of two commercially important species of bumblebees. The findings provide unexpected insights into the ecology and evolution of bumblebees and honeybees. Bumblebees are considered peaceful and industrious creatures, and their commercial value has increased in the wake of the decline of honeybees around the world. The bumblebees are therefore now bred on a large scale and used as pollinators for economically valuable crops. Yet, these cute little, buzzing creatures, of which there are approximately 250 different species worldwide, are doing poorly in some places. The large shadow cast by the honeybee collapse has distracted from the fact that in recent years in the U.S., as well as in other areas, some previously common bumblebee species have also become rare or endangered, or disappeared altogether. For this reason, two former ETH researchers, Dr. Seth Barribeau and Dr. Ben Sadd, together with Professor Paul Schmid-Hempel from the group for experimental ecology, started a bumblebee genome project eight years ago. The objective of the project was to analyze the genomes of two commercially important species: the European Buff-Tailed Bumblebee (Bombus terrestris) and its American counterpart, the Common Eastern Bumblebee (Bombus impatiens). The researchers were hoping that the genomic data would shed light on the biology, ecology, and evolution of the bumblebee more generally. Dr. Barribeau, Dr. Sadd, and 80 other researchers from around the world paid particular attention to the genes of the bumblebee immune system. Evolutionary biologists, ecologists, bioinformaticians, and geneticists were involved in the project. The researchers also compared the genomes of the two bumblebee species with genomes of other insects that had already been mapped, such as the honeybee, the parasitoid wasp Nasonia, and the fruit fly Drosophila melanogaster. The scientists published their findings online on April 24, 2015 in two open-access articles in Genome Biology. One article was titled “The Genomes of Two Key Bumblebee Species with Primitive Eusocial Organization.” The other was titled “A Depauperate Immune Repertoire Precedes Evolution of Sociality in Bees.”

The genomes of the two bumblebee species strongly resemble each other and contain about 20,000 different genes on 18 chromosomes. The scientists discovered that only a small fraction of these are genes involved in the immune response. In fact, the genetic repertoire of the immune system in the two bumblebee species has only approximately 150 genes, a relatively small portion compared with flies or mosquitoes – Drosophila has twice as many, for example. The honeybee and parasitoid wasp (Nasonia), on the other hand, also have only a small immunogenetic repertoire.

Dr. Schmid-Hempel is puzzled that the bumblebee, which exhibits relatively weak social organization, has just as few immune genes as the honeybee, with its complex social organization. He says that, until now, researchers had assumed that insects with a high degree of social organization could afford to invest less in their immune system. In turn, a simple social system would promote a stronger individual defense. This previous theory suggested that highly social insects had other ways to ward off pathogens than to rely solely on their individual immune defenses – for example, defense cooperation, such as mutual body care.

Dr. Schmid-Hempel thinks that the weak immune systems of the honeybee and bumblebee could be related to their diet. Flies and fruit flies such as Drosophila melanogaster eat from surfaces such as rotten fruit, which are contaminated with bacteria and fungi, whereas bees eat from relatively clean food sources, such as plant pollen and nectar. This should reduce the risk of infection and thus also reduce the selective pressure for a well-developed immune system.

But it is more than just weak immune systems that have been making the lives of bumblebees (and honeybees) difficult lately. The researchers were able to identify only very few genes that regulate the body’s mechanism for detoxification. According to Dr. Schmid-Hempel, this would at least be in line with the worry that these insects are sensitive to environmental toxins, such as agricultural pesticides.

The genetic analysis has, however, also revealed significant differences between the honeybee and bumblebee. For example, the bumblebee has more genes that contribute to the sense of taste, whereas the honeybee has more that contribute to the sense of smell.

This makes sense as the bumblebee relies on its sense of taste to find food, testing with its tongue virtually every flower. In contrast, the honeybee relies more on odors to find the right food, as it plays a role in the waggle dance, which honeybees use to communicate the presence of a good food source to each other. “This fundamental difference in the way these two bees live is clearly reflected in their genes,” explains Dr. Schmid-Hempel.

The researchers were surprised to find that they were able to associate relatively few specific genes to social organization and behavior. “The genes for this don’t differ very much between bumblebees and honeybees,” says Dr. Schmid-Hempel.

But the scientists did discover different sets of microRNAs, tiny snippets of ribonucleic acids, in these insects. These miRNAs regulate genes by blocking gene copies, which serve as blueprints for proteins in the cells. It is specifically this type of gene regulation that seemingly takes a ‘normal,’ primitively social insect, such as the bumblebee, and transforms it into a highly social organism like the honeybee.

[Press release] [Genome Biology article "The Genomes of Two Key Bumblebee Species with Primitive Eusocial Organization"] [Genome Biology article "A Depauperate Immune Repertoire Precedes Evolution of Sociality in Bees]