First posted on 04-26-2012
Age determines stereotypical roles worker honeybees assume. When they are only a few days old, they take on the tasks of feeding the brood and producing beeswax to seal cells in the hive where larvae develop. After a week, she moves on to other tasks, such as grooming other bees and packing pollen in the nest. Only as her life comes to an end does she become a forager, leaving the hive to collect nectar for the colony.
Honeybee-studying scientists considered the possibility that this hierarchy of specialized tasks was determined genetically.
Dr. Yehuda Ben-Shahar of Washington University in St. Louis led a team of researches from across the country, and determined that the presence of tiny snippets of ribonucleic acid, called micro-RNA or miRNA, actually suppresses gene expression in bees.
The group found that bees which had assumed forager tasks had higher levels of miRNAs in their brains than those performing nursery duties.
Ben-Shahar said that miRNAs are known to regulate both developmental processes and progression of certain diseases, such as cancer.
“We wondered if they weren’t playing a role in regulating social behaviors,” he says, “because recent studies have implicated them in complex nervous-system functions such as neurodevelopment, psychiatric disease, and circadian clocks.”
The team discovered that, while task allocation is tightly scripted in honeybees, it’s still fluid enough to allow behavioral changes during periods of tight staffing inside the hive. For example, if nurses are in short supply, those bees performing the task continue to do so well past the age limit, and if foragers are lacking, younger bees take on that role.
The scientists undertook the arduous task of sorting tiny RNA fragments after extracting them from bees. They could then sequence the RNA and compare what they found with a bee genome, which has already been sequenced in its entirety. This cataloguing effort resulted in a list of 97 miRNAs, including 17 that had never been identified before. Several had been found in flies and some mammals, but never in bees.
“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.
Ben-Shahar also wonder whether miRNAs could play a role beyond task regulation in honeybees, which are eusocial, meaning they behave more like a single organism than individuals. Deeper research revealed the presence of 19 miRNAs honeybees share with other eusocial animals, such as wasps and ants.
Ben-Shahar concedes that human behavior is more complicated than that of insects, but he observed that human genome sequencing shows nearly 2,000 miRNAs, including a few shared with honeybees. Scientists believe those 2,000 human miRNAs target about 60 percent of our genes.
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