DNA methylation in cockroaches is essential in early embryo development and reduces gene expression noise

The influence of DNA methylation on gene behavior, and its consequent phenotypic effects appear to be very important, but the details are not well understood. Insects offer a diversity of DNA methylation modes, making them an excellent lineage for comparative analyses. However, functional studies have tended to focus on quite specialized holometabolan species, such as wasps, bees, beetles, and flies. Here we have studied DNA methylation in a hemimetabolan insect, the cockroach Blattella germanica, a model of early-branching insects. In this cockroach, one of the main genes responsible for DNA methylation, DNA methyltransferase 1 (DNMT1), is expressed in early embryogenesis. In our experiments, DNMT1 interference by RNAi reduces DNA methylation and impairs blastoderm formation. Using Reduced Representation Bisulfite Sequencing (RRBS) and transcriptomic analyses, we observed that hypermethylated genes are associated with metabolism and are highly expressed, whereas hypomethylated genes are related to signaling and have low expression levels. Moreover, the expression change in hypermethylated genes is greter than that in hypomethylated genes, whereas hypermethylated genes have less expression variability than hypomethylated genes. The latter observation has also been reported for humans and in Arabidopsis plants. A reduction in expression noise may therefore be one of the few universal effects of DNA methylation.