Plant molecules stop worker bees from ascending throne

You are what you eat – especially if you’re a female honeybee. Whereas a larval diet of royal jelly will land you on the throne for a few years, one without is a ticket to a brief life of worker drudgery.

But a new study suggests that royal jelly – a glandular secretion rich in protein, sugars and fatty acids that’s produced by workers – isn’t the only essential ingredient to seal female bees’ fate.

Plant molecules that tinker with the developmental programs of bee larvae could also be playing a role, according to the study published in PLoS Genetics.

The molecules – tiny stretches of RNA, known as microRNAs – make their way into the diet of larvae destined to be workers as components of the pollen and honey combo ‘beebread’ they are fed. The team found that beebread was particularly rich in plant microRNAs.

microRNAs are known regulators of gene function, able to dial down the activity of a target gene. In this case, the authors suggest, the microRNAs could be regulating genes across the divide between the plant and animal kingdoms.

“Plant microRNAs in beebread inhibits ovary and bee development and they become worker bees,” says Nanjing University’s Chen-Yu Zhang, a senior author on the paper.

Female larvae fed an artificial diet containing the 16 most abundant beebread microRNAs grew slower, ended up around 15% lighter, had less developed ovaries, and grew into adults that were more worker-like in appearance compared to larvae fed a mock diet without the microRNAs. Fruit fly larvae also failed to develop as well when fed the microRNA-laced diet.

One microRNA in particular, miRNA162a, could target a key developmental gene called TOR. Previous studies have shown that dampening TOR activity in larvae fed a queen diet can turn them into workers. In this study, larvae fed miRNA162a alone were stunted, though not as much as those fed the full microRNA banquet.

“Dietary microRNA may be a novel class of nutrient,” says Zhang, for bees as well as for other animals including humans.

The team has previously identified plant microRNAs in human blood, though the idea of cross-kingdom microRNA effects “is still very controversial,” says honeybee geneticist Ryszard Maleszka from the Australian National University, who wasn’t involved in the study.

As for their role in determining bee caste, “it’s possibly part of the story,” he says.  “It’s a multifactorial, multilevel process,” he says, that is likely to involve a range of factors.

Analysing how genes or hormones in known bee developmental pathways are affected by the plant microRNAs could help to bolster the case for the molecules having bona fide biological effects, according to Maleszka. “I would like to see a more global network level explanation for how the process works,” he says. “It’s not a simple story.”