NEW YORK (GenomeWeb) – In Genome Biology this week, an international team led by investigators at Illinois State University, Baylor College of Medicine, and Switzerland's Institute of Integrative Biology reported on findings from a genome sequencing study of two bumblebee species.
After putting together high-quality draft genome sequences for the European buff-tailed bumblebee, Bombus terrestris, and the North American common eastern bumblebee, B. impatiens, the researchers compared the genomes to another and to sequences from other bee species, including the eusocial honeybee species Apis mellifera, which was first sequenced more than a decade ago.
The analysis revealed immune and other gene similarities in the bumblebees and their more socially structured cousin, the honeybee, hinting that such functions evolved in a shared common ancestor of the bee lineages rather than in conjunction with eusociality. The genomes also contained clues to the genetic basis of bumblebee-specific traits and behaviors.
"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," the study's corresponding author Ben Sadd, a biological sciences and experimental ecology researcher affiliated with Illinois State University and the Institute of Integrative Biology, said in a statement.
"We remain far from understanding the threats to bumblebees," Sadd added, "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."
Honeybees and a select number of other Hymenoptera order insect species have evolved to participate in eusocial systems that involve large, highly organized colonies comprised of specialized worker females that do not reproduce, a queen that does reproduce, and, in some cases, distinct physical features that mark members of different castes.
While bumblebees are similar in some respects, the researchers explained, they live in far smaller colonies that form annually rather than perennially and are less strictly structured. Because their social structures fall between advanced eusocial bees such as the honeybee and solitary bees, for instance, the solitary leaf-cutting bee, bumblebees are considered primitively eusocial.
"There is a clear value to investigating bumblebees as they hold a key, intermediate position on the eusocial spectrum," Sadd and his co-authors noted.
The researchers used Illumina instruments to sequence genomic DNA from European buff-tailed bumblebees collected in Switzerland and North American common eastern bumblebees from Michigan, which they used to put together high-quality draft genome assemblies for the bumblebees.
Transcriptome sequences generated from queen head, queen ovary, and male head tissues bolstered genome annotation and gene expression analyses, while restriction-site-associated DNA marker sequencing on nearly two dozen more B. impatiens worker bees provided a look at B. impatiens SNP patterns.
Within these B. terrestris and B. impatiens assemblies — which spanned 249 and 247 million bases, respectively — the team saw very similar gene and repeat sequence repertoires. Likewise, the bumblebee sequences were largely syntenic and displayed few rearrangements despite an estimated 18 million years of divergence between the species.
Both bee species had similar immune and detoxification gene sets when compared with honeybees and the solitary leaf-cutting bee, Megachile rotundata, in an accompanying Genome Biology study by members of the same team, arguing against the notion that pared down bee immunity is a feature of eusociality.
The team found conserved features when it came to genes believed to govern some facets of social interaction in the honeybee and bumblebee species, too, though the bumblebee genomes contained a robust repertoire of 25 gustatory receptor genes compared with the honeybee's 12 such genes.
The bumblebee genomes also contained different suites of microRNAs than the honeybee genome, despite sharing a core miRNA set, hinting that these molecules may play regulatory roles that affect the bees' distinct social behaviors.