DNMT1 and MBD2/3 Modulate Population Density-Dependent Plasticity in Terminal Oocyte Development in Bean Beetle, Callosobruchus maculatus

Background/Objectives: The bean beetle (Callosobruchus maculatus) exhibits population density-dependent plasticity in the terminal oocyte maturation rate. DNA methyltransferase 1 (DNMT1) plays a conserved function in reproduction that is independent of DNA methylation. However, whether DNMT1 is involved in the population density-dependent reproductive plasticity of bean beetles remains unclear. Methods: Two and twenty pairs of beetles were reared with approximately 100 seeds per bottle to establish a low-density population and a high-density population, respectively. Quantitative real-time PCR was used to unveil the mRNA levels of DNMT1, MBD2/3, and insulin-like peptides (ILPs). RNA interference was used to determine the function of DNMT1 and MBD2/3 in terminal oocyte development. The length of terminal oocytes was measured under a microscope. Results: Individuals reared under high-population-density conditions showed a faster terminal oocyte maturation rate compared to those under low-density conditions. The bean beetle genome encodes DNMT1 but lacks DNMT3, and only a single methyl-DNA-binding domain protein (MBD2/3) was identified. Population density could modulate the expression levels of both DNMT1 and MBD2/3. RNA interference (RNAi)-mediated knockdown demonstrated that suppressing either DNMT1 or MBD2/3 significantly reduced the terminal oocyte maturation rate. Moreover, silencing DNMT1 and MBD2/3 resulted in decreased expression of ILP3 and all ILPs in the fat body, respectively. ILPs are known to be involved in regulating terminal oocyte development. Conclusions: Taken together, these findings suggest that DNMT1 and MBD2/3 modulate the population density-dependent terminal oocyte maturation rate in the bean beetle by influencing the expression of ILPs.