What is in this article?:
- The division of labor among honey bees coincides with the presence in their brains of tiny snippets of noncoding RNA, called micro-RNAs, or miRNAs, that suppress the expression of genes.
Five prime suspects
To design a manageable behavioral experiment, the scientists then selected five of the 97 miRNAs for closer inspection. These five were either very abundant or had been implicated in neural function in other organisms.
The scientists then manipulated two colonies of bees to produce cohorts of nurse and forager bees that were the same age, either young for foragers or old for nurses.
They dissected out the brains of their precocious foragers and overage nurses and measured the level of expression of the miRNAs in the brains with a sensitive test called the Northern Blot.
“We found that the level of expression of four of these miRNAS correlated with the task the bee was performing. Four of them were expressed at higher levels in foragers than in nurses. Because miRNAs typically suppress gene expression, this means genes actively transcribed in nurses were silenced in foragers,” Ben-Shahar says.
“There is clearly a task-related difference, but we don’t yet know what the gene targets of the miRNAs are,” he says.
An ancient regulatory system
Could miRNAs be playing a much broader role in the behavior of bees, not just regulating the tasks workers performed but also their social behavior more generally?
Honey bees are eusocial insects, meaning that a colony behaves more like a superorganism than a gathering of individuals. The scientists knew that the genomes of several other eusocial insects had recently been sequenced.
Did the eusocial insects share miRNAs, they wondered?
The grand survey of miRNAs had identified 20 miRNAs that seemed to be honeybee-specific. To test their idea, they looked for these miRNAs in the genomes of four other eusocial insects within the hymenoptera (an order of insects that consists of ants, bees and wasps) and in that of a solitary wasp.
A total of 19 out of the 20 miRNAs that had initially appeared to be honeybee-specific were also identified in the genomes of the other eusocial insects. Moreover, five found in all the eusocial hymenoptera were found in no other species. And none of the 20 miRNAS found in the eusocial insects were found in the genome of the solitary wasp.
Once a miRNA assumes a functional role it is rarely lost from an animal’s genome, Ben-Shahar says, because it typically regulates multiple genes and is too thoroughly enmeshed in the cell’s regulation to be easily extracted. This makes miRNAs a valuable marker for evolutionary relationships among species.
The relationships among eusocial species could do with clarification. Ants and bees diverged a long time ago, and all ant species are eusocial, but bee species run the gamut from solitary to eusocial.
That pattern makes sense, Ben-Shahar says, only if the eusocial trait evolved more than once as new species evolved. Something in hymenoptera DNA may have made that group of animals more sensitive than others to whatever evolutionary pressures led to social behavior, he says.
Genetic control of human behavior is undoubtedly more complicated, Ben-Shahar says, but he points out that the human genome encodes close to 2,000 miRNAS, including two of the five he studied in bee brains, and these 2,000 miRNAs are thought to target roughly 60 percent of our genes.