Summary 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 the hemimetabolan insect Blattella germanica . In this cockroach, a gene involved in DNA methylation, DNA methyltransferase 1 ( DNMT1 ) , is expressed in early embryogenesis. In our experiments, RNAi of DNMT1 reduces DNA methylation and impairs blastoderm formation. Using reduced representation bisulfite sequencing and transcriptome analyses, we observed that methylated genes are associated with metabolism and are highly expressed, whereas unmethylated genes are related to signaling and show low expression. Moreover, methylated genes show greater expression change and less expression variability than unmethylated genes.

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Significance Mayflies are the only extant insects that molt after having formed wings, in a stage called subimago. Numerous authors have wondered whether this stage is a nymph, an adult, or a kind of intermediate. Another question is why mayflies have a subimago stage, when molting a wing is risky. Working with Cloeon dipterum , we found that metamorphosis is regulated as in neopteran insects and that it is determined prior to the formation of the subimago. Thus, it should be considered an instar of the adult stage. We also found that the forelegs grow dramatically between the last nymphal instar, the subimago, and the adult. That necessary growth may help to explain the functional sense of the subimago.