The Epigenetics Revolution

Because honeybee maturation has so many hallmarks of an epigenetic phenomenon, researchers speculated that there would also be an involvement of the epigenetic machinery. The first indications that this is indeed the case came in 2006. This was the year when researchers sequenced the genome of this species, to identify the fundamental genetic blueprint[264]. Their research showed that the honeybee genome contained genes that looked very similar to the DNA methyltransferase genes of higher organisms such as vertebrates. The honeybee genome was also shown to contain a lot of CpG motifs. This is the two-nucleotide sequence that is usually the target for DNA methyltransferases.

In the same year, a group led by Gene Robinson in Illinois showed that the predicted DNA methyltransferase proteins encoded in the honeybee genome were active. The proteins were able to add methyl groups to the cytosine residue in a CpG motif in DNA[265]. Honeybees also expressed proteins that were able to bind to methylated DNA. Together, these data showed that honeybee cells could both ‘write’ and ‘read’ an epigenetic code.

Until these data were published, nobody had really wanted to take a guess as to whether or not honeybees would possess a DNA methylation system. This was because the most widely used experimental system in insects, the fruit fly Drosophila melanogaster, whom we met earlier in this book, doesn’t methylate its DNA.

It’s interesting to discover that honeybees have an intact DNA methylation system. But this doesn’t prove that DNA methylation is involved in the responses to royal jelly, or the persistent effects of this foodstuff on the physical form and behaviour of mature bees. This issue was addressed by some elegant work from the laboratory of Dr Ryszard Maleszka at the Australian National University in Canberra.

Dr Maleszka and his colleagues knocked down the expression of one of the DNA methyltransferases in honeybee larvae, by switching off the Dnmt3 gene. Dnmt3 is responsible for adding methyl groups to regions of DNA that haven’t been methylated before. The results of this experiment are shown in Figure 14.1.

^{Figure 14.1 When royal jelly is fed to honeybee larvae for extended periods, the larvae develop into queens. The same effect is seen in the absence of prolonged feeding with royal jelly if the expression of the Dnmt3 gene is decreased experimentally in the larvae. Dnmt3 protein adds methyl groups to DNA.}

When the scientists decreased the expression of Dnmt3 in the honeybee larvae, the results were the same as if they had fed them royal jelly. Most of the larvae matured as queens, rather than as workers. Because knocking down Dnmt3 had the same effects as feeding royal jelly, this suggested that one of the major effects of royal jelly is connected with alterations of the DNA methylation patterns on important genes[266].

To back up this hypothesis, the researchers also examined the actual DNA methylation and gene expression patterns in the different experimental groups of bees. They showed that the brains of queens and worker bees have a different DNA methylation pattern. The DNA methylation patterns in the bees where Dnmt3 had been knocked down were like those of the normal royal jelly-induced queens. This is what we would expect given the similar phenotypes in the two groups. The gene expression patterns in the normal queens and the Dnmt3-knockdown queens were also very similar. The authors concluded that the nutritional effects of continual feeding on royal jelly occurred via DNA methylation.

There are still a lot of gaps in our understanding of how nutrition in the honeybee larva results in altered patterns of DNA methylation. One hypothesis, based on the experiments above, is that royal jelly inhibits the DNA methyltransferase enzyme. But so far nobody has been able to demonstrate this experimentally. It’s therefore possible that the effect of royal jelly on DNA methylation is indirect.

What we do know is that royal jelly influences hormonal signalling in honeybees, and that this changes gene expression patterns. Changes in the levels of expression of a gene often influence the epigenetic modifications at that gene. The more highly a gene is switched on, the more its histones become modified in ways which promote gene expression. Something similar may happen in honeybees.