John B Free/NaturePL
A simple meal is all that’s needed to determine the fate of a honeybee larva. It turns out that fragments of genetic material from flowers in their food control the bees’ destinies.
When female larvae are fed royal jelly, which is secreted by other bees, they develop into large-bodied, fertile queens. But most larvae eat beebread, a mixture of pollen and nectar. These larvae develop into smaller, sterile worker bees.
Xi Chen at Nanjing University in China and colleagues have now found that beebread contains lots of small RNA molecules called microRNAs. These regulate the expression of genes, and in plants they help regulate essential processes like making leaves and flowers.
Advertisement
“Plants utilise certain miRNAs to influence the size, morphology, colour and development of flowers,” says Chen. “Such characteristics of flowers guide [honeybees] in pollen collection.” As a result, a lot of these miRNAs end up in beebread, where larvae eat them.
Controlled by plant RNA
The researchers collected pollen, honey, royal jelly and beebread from hives and measured their miRNA levels. They found that beebread and pollen had much higher concentrations of plant miRNAs than royal jelly.
The team then reared bee larvae in the laboratory, feeding them a beebread mimic — a lab diet enriched with the same miRNAs as in pollen, at the same amounts. Larvae grown with miRNAs ended up as worker bees, with reduced weight and size, and smaller ovaries.
A closer look at the miRNAs in beebread suggested that the 16 most abundant could bind to 96 bee genes, some of which are known to affect development. One was amTOR, a gene known to be more active in queens. Further experiments showed that an miRNA called miR162a inhibits the function of amTOR.
However, when bee larvae were given only miR162a it didn’t prolong development time. This indicates that miR162a isn’t the only miRNA involved.
None of these effects were seen with royal jelly. “Queen-destined larvae [are] exempted from [miRNA] regulation,” says Chen.
Power of plants
But why should miRNAs from plants affect development in an entirely different species?
The authors point out that honeybees and flowering plants benefit from a long-standing “coevolutionary relationship”. The plant miRNAs could be part of that.
“When honeybees gather pollen as food for the entire colony, they pollinate plants; meanwhile, plants donate miRNAs to stabilise the honeybee colony,” Chen says.
Not everyone agrees with this interpretation, however. “I don’t think the plant is benefited by being involved in mechanisms that control insect development,” says Diana Wheeler at the University of Arizona in Tucson. “The simplest interpretation for me is that the miRNA is involved in signalling diet quality. It affects amTOR, which is also involved in the insulin signalling pathway.”
Journal reference: PloS Genetics, DOI: 10.1371/journal.pgen.1006946
More on these topics: