User:Tessi87/DNA methylation in insects

Epigenetic mechanisms in insects DNA methylation is an epigenetic mechanism. It is a chemical modification of the DNA where a methyl group is attached to cytosine. This modification is set by DNA methyltransferases (Dnmts). There are three known typs of DNA methyltransferases in mammals. Those DNA methyltransferases are present in insects as well, although it varies between different species which specific Dnmt types are present. DNA methylation is known to play a role in a number of processes, such as development, behavioural plasticity, diseases and cell differentiation. It still is a matter of discussion what the specific role of DNA methylation in insects is, as some insects such as Drosophila melanogaster just have traces of DNA methylation in their genome and in general insect genomes are much less methylated compared to mammalian genomes. In a comparison of different insect species and their respective methylation levels, there was a clear relationship between cell turn over and DNA methylation, but not between genome size or the number of repetitive sequences and DNA methylation.

In Honeybees
Honeybees (Apis mellifera) posses homologs for all three DNA methyltransferases known in mammals. But in contrary to mammals they posses two DNA methyltransferases 1 and just one DNA methyltransferase 3. DNA methylation predominantly occurs in coding regions in honeybees.

Development
DNA methylation plays a major role in honeybee cast and subcast development. In honeybees there are two different casts which are workers and queens. They are genetically the same, but show morphological, physiological and behavioural differences. Among the worker cast there are two subcasts, which are nurses and foragers. Which subcast a worker bee belongs to depends on its age. The DNA methylation pattern in queens and workes, and between nurses and foragers is different. DNA methylation also increases in worker larvae with age, especially in coding regions and CpG island s. If DNA methyltransferase 3 is silenced in honeybee larvae they evolve into queens, whereas they otherwise would evolve into workers.

Associative learning
Using the DNA methyltransferase inhibitor zebularine, the role of DNA methyltransferases during learning and memory formation has been studied. If DNA methyltransferases are inhibited during an associative learning paradigm, in which the bee is trained to associate an odour with a food reward, the odour specific associative long-term memory of bees is impaired, as well as their extinction memory. Short-term memory formation and acquisition are not affected by DNA methyltransferase inhibition.

In Fruitflies
The fruitfly Drosophila melanogaster posses just one DNA methyltransferase, which is Dnmt 2-like. Dnmt 2 is not known to methylate DNA in mammals. In Drosophila however a knock down of Dnmt 2-like protein is sufficient to deplet DNA methylation completly and an overexpression of Dnmt 2 causes hypermethylation of the DNA.