Social insects, such as honeybees (Apis mellifera), provide fascinating examples of natural social structures. These insects have a well-defined caste system, whereby tasks are divided depending on the “social class”. Queen bees are characterised by their remarkable reproductive capacity, large body size, and a marked longevity of life. Given their reproductive capacity, the main role of the queen bees is to sustain the beehive colony. In contrast, worker bees are mostly sterile and have short life spans. Their main role is to ensure a sustained food supply, act as nurse bees to the larvae produced by the queen bees, and to produce wax cells, which constitute the physical scaffold of the beehive. However, the factors determining differentiation into either queen or worker beers are not fully understood yet. At a molecular level, a protein called Target Of Rapamycin (TOR) plays a central role in stimulating and inducing queen-like physical traits in honey bees. Other factors, such as food type, are also necessary for determining the worker phenotype; for example, royal jelly (a glandular secretion produced by nurse bees) induces the development of queen bees, whereas “beebread”, a mixture of pollen and honey, stimulates the development of worker bees.
In a recent study published in PLoS Genetics, Kegan Zhu et al. demonstrated that plant-derived microRNAs are involved in the development of worker bees. microRNAs are short ribonucleic acids (RNAs), synthesised from the DNA, that play important roles in switching on and off genes within the cell, otherwise known as gene expression. Given their function as master regulators of gene expression, they have a tremendous impact on many physiological processes in plants and animals, including their development. In this study, the authors first demonstrated that beebread was particularly enriched in a total of 16 plant-derived microRNAs when compared to royal jelly, which was preferentially enriched in animal-derived microRNAs. The authors then asked whether these 16 plant-derived microRNAs were involved in honey bee larvae development. They found that when honey bee larvae were fed with beebread enriched in a particular subset of plant-derived microRNAs, they displayed a significant impairment in growth, and emerged as adults with worker-like features, such as reduced weight, size, and decreased ovary size. Consistent with the hypothesis that plant microRNAs can control and suppress the expression of genes in honey bees, using a combination of computational and experimental approaches, the authors showed that a specific plant microRNA, miR-162a, can bind to the gene responsible for the synthesis of TOR, thereby suppressing the levels of TOR protein and favouring the acquisition of a worker-like trait in bees.
Interestingly, the regulation of TOR protein levels mediated by plant microRNAs is not limited to social insects. The authors also provided evidence of a similar effect of plant miR-162a in reducing weight, length, and ovary and brood size in the non-social insect Drosophila. This provides strong evidence to support an evolutionarily conserved interaction between plants and insects potentially mediated by microRNAs. However, it is important to stress that plant microRNAs are not expected to be the sole factors mediating caste development in social insects, as other factors such as protein, sugars, p-coumaric acid and fatty acids, have all been also proposed to play a key role in this process. Nevertheless, taken together, this study uncovers a new layer of complexity in the development of a caste system in social insects, and provides strong evidence to support the hypothesis that the existence of cross-kingdom social interactions and co-evolution is partially mediated by microRNAs.
This article was written by Juan Quintana and edited by Bonnie Nicholson.