Dnmt1 has an essential function despite the absence of CpG DNA methylation in the red flour beetle Tribolium castaneum

Abstract

Epigenetic mechanisms, such as CpG DNA methylation enable phenotypic plasticity and rapid adaptation to changing environments. CpG DNA methylation is established by DNA methyltransferases (DNMTs), which are well conserved across vertebrates and invertebrates. There are insects with functional DNA methylation despite lacking a complete set of Dnmts. But at least one of the enzymes, DNMT1, appears to be required to maintain an active DNA methylation system. The red flour beetle, Tribolium castaneum, lacks Dnmt3 but possesses Dnmt1 and it has been controversial whether it has a functional DNA methylation system. Using whole genome bisulfite sequencing, we did not find any defined patterns of CpG DNA methylation in embryos. Nevertheless, we found Dnmt1 expressed throughout the entire life cycle of the beetle, with mRNA transcripts significantly more abundant in eggs and ovaries. A maternal knockdown of Dnmt1 caused a developmental arrest in offspring embryos. We show that Dnmt1 plays an essential role in T. castaneum embryos and that its downregulation leads to an early developmental arrest. This function appears to be unrelated to DNA methylation, since we did not find any evidence for this modification. This strongly suggests an alternative role of this protein.

Silencing of the DNA methyltransferase 1 associated protein 1 (DMAP1) gene in the invasive ladybird Harmonia axyridis implies a role of the DNA methyltransferase 1‐DMAP1 complex in female fecundity

“…ovaries. Further, maternal silencing of this gene caused a developmental arrest in offspring embryos (Schulz et al, 2018). Taking together these studies and our findings, we postulate an evolutionarily conserved role of the DNMT1-DMAP1 complex beyond embryonic development including a role in ovary maturation and female fecundity.…”

More Than DNA Methylation: Does Pleiotropy Drive the Complex Pattern of Evolution of Dnmt1?

“…While Dnmt1 is expressed at all life stages, it is upregulated in ovaries, suggesting a role in reproduction. Finally, parental RNAi of Dnmt1 demonstrated that T. castaneum requires maternal expression of Dnmt1 for early embryogenesis (Schulz et al, 2018). Together, these studies suggest a germ cell specific function for Dnmt1 in insects.…”

“…Expression of Dnmt1 is high in eggs and embryos relative to other tissues in several insect species (Yan et al, 2015;Schulz et al, 2018, this study). Knocking down Dnmt1 expression in eggs and embryos results in embryo lethality (Zwier et al, 2012;Schulz et al, 2018;Bewick et al, 2019). Third, there is a suggestion that DNA methylation may be important in stability of germ cells (Takashima et al, 2009;Bewick et al, 2019).…”

“…This is consistent with similar results in other insects. Tribolium castaneum is an insect with no detectable DNA methylation (Zemach et al, 2010;Cunningham et al, 2015;Bewick et al, 2016;Schulz et al, 2018), but which has maintained the Dnmt1 gene in its genome (Bewick et al, 2016). When Dnmt1 is knocked down during pupal development, it results in embryo lethality (Schulz et al, 2018).…”

“…The phylogenetic distribution of DNMT1 in insects is more widespread than DNMT3 and it is duplicated in some groups. Curiously, DNMT1 is found in insect taxa with low or no DNA methylation, including the red flour beetle Tribolium castaneum (Schulz et al, 2018). This variation raises questions both about the function of DNA methylation in insects and the selection leading to differences among taxa.…”