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Comparative Genomics Provides Clues to Evolutionary Processes Behind Bee Sociality

NEW YORK (GenomeWeb) – A study appearing online today in Science used comparative genomics to probe the regulatory roots of sociality in bees, uncovering parallel, but independent, genomic routes used to attain more complex and socially interdependent colonies.

Using genome sequences for five newly sequenced bee species and five bee species sequenced previously, an international team led by investigators in the US and China looked for genetic differences that might explain why some of these organisms live in highly complex and interdependent colonies while others remain solitary or more loosely affiliated with one other.

The researchers noted that gene networks appeared to gain complexity alongside increasing social complexity, for example, while processes driving natural selection tended to ease up. Nevertheless, their results pointed to distinct genetic underpinnings behind social evolution between different bee lineages.

"[T]here is no single road map to eusociality — the complex, cooperative social system in which animals behave more like superorganisms than individuals fending for themselves," co-corresponding author Gene Robinson, an entomology researcher and head of the University of Illinois' Carl R. Woese Institute for Genomic Biology, said in a statement.

Eusociality in bees ranges from small colonies with sterile worker bees and a reproductively active queen to far more complicated super-organism groups made up of thousands of individuals with highly specialized roles.

In a study published in Genome Biology last month, researchers from the US and Switzerland took a look at the evolution of bee sociality using newly generated bumblebee genome sequences — an analysis that revealed similar sets of immune and detoxification genes in solitary and more socially complicated bees.

For their own look at the evolution of bee sociality, Robinson and his colleagues amalgamated available genome sequences for five bee species and did de novo genome sequencing on five more.

The 10 species included solitary bees as well as bees with simple eusociality, and complex eusociality. At least some of the eusocial bees believed to have evolved this social structure independently, they noted, providing an opportunity to retrace distinct eusocial evolution routes.

When the researchers scrutinized promoter regions from nearly 5,900 single-copy genes that were orthologous across the bee species tested, they found that enhanced social complexity in bees tends to coincide with enhanced gene regulation — including a rise in transcription factor capacity — and a dip in the diversity and abundance of the transposable elements peppering the bee genome.

Genes involved in processes such as transcription, RNA splicing, and translational regulation were among those that seemed to be rapidly evolving, the team reported, in part due to stronger directional selection for many of these genes in species with more complex social structures such as eusociality.

On the other hand, roughly one third of the genes showing rapid rates of evolution in the eusocial bees were under relaxed purifying selection — a pattern the study's authors suspected may stem from reduced effective population size in bee colonies.

Despite some broad similarities in the process, they concluded that a variety of protein changes, gene family expansions, and gene regulatory tweaks could lead to eusociality.

"It is now clear that there are lineage-specific genetic changes associated with independent origins of eusociality in bees, and independent elaborations of eusociality in both bees and ants," they wrote. "This suggests that … eusociality may arise through different mechanisms each time, but would likely always involve an increase in the complexity of gene networks."