DNA methyltransferases

And the honey bee can help out. As a member of the honey bee genome consortium, he and his colleagues, especially Hugh Robertson from the department of entomology at the University of Illinois at Urbana-Champaign, noticed during the annotation process that the set of DNA methyltransferase genes found in mammals was also there in bees. In 2006, Maleszka, along with Robertson and Gene Robinson from Illinois and a team from Barcelona, published a paper in Science on a functional CpG methylation system in bees.

(Last year, Matthias Schaefer and Frank Lyko from the division of epigenetics at the German Cancer Research Centre in Heidelberg published a commentary in Bioessays on the fully active DNA methylation system in bees, highlighting the importance of this finding to human cancer research. As shown by several studies, DNA hypermethylation plays a role in certain types of cancer. An example is Sydney’s Robyn Ward and Megan Hitchens’ work on germline inheritance of hypermethylation of the MLH1 DNA repair gene in colorectal cancer.)

Of course, most suspected that many of the phenotypic alterations in social insects, such as those found in the queen bee versus the worker bee, were due to environmental factors under epigenetic control, but at the time there was no hard evidence.

One problem was that the model invertebrate genomes – Drosophila and C. elegans – seem to have lost many of the genes for DNA methylation. “So everyone thought insects don’t have DNA methylation but it turns out the Drosophila is an exception,” Malescka says.

“The Drosophila genome is in some aspects very similar to the bee genome, but it is a fast-evolving species that lost some of those genes because it didn’t need them anymore. It didn’t have this phenotypic plasticity noticeable in bees and other social insects. Drosophila is somewhat of a misleading organism in some aspect for broad insect studies.”

To the researchers, it came as a nice surprise that bees have a full complement of DNA cytosine-5-methylatransferases (Dnmts), as do some other arthropod genomes. In fact, it seems that many of the non-dipteran insects sequenced so far have the full complement, and that cytosine methylation occurs at cytosine-phosphate-guanine (CpG) sites, just like in mammals.

It is slightly different in insects – CpG methylation is found in transcription units but not at the three- and five-prime ends, and methylation seem to be less common in insects. Nevertheless, the discovery that honey bees use DNA methylation offers the opportunity to study epigenetics in a model insect, particularly as it affects development.