Abstract
While DNA methylation is an important chromatin modification in many groups of organisms, the function of DNA methylation within the insects is unclear. The taxonomic distribution of DNA methyltransferase genes in insects is highly variable, as is the presence of methylated genomes. In the large milkweed bug, Oncopeltus fasciatus, we have shown the maintenance methyltransferase Dnmt1 is required for oocyte production but this appears to be unrelated to methylation given that demethylating somatic cells causes no loss of somatic cell function. One hypothesis is that Dnmt1 is affecting meiosis. Here we used RNAi to downregulate Dnmt1 in males at two stages where meiosis is occurring; during testis development and in adults replenishing sperm stores following sperm depletion. We found that downregulation of Dnmt1 in stages where meiosis is required resulted in the greatest disruption to spermatogenesis. Our results support the hypothesis that Dnmt1 is required for the transition of germ cells to gametes in O. fasciatus and that this function is conserved in male and female gametogenesis. In addition, the role of Dnmt1 was specific to the germ cells. Downregulation of Dnmt1 across all tissues resulted in a germline-specific phenotype. These results suggest that the reduction of methylation has a phenotype restricted to the germ cells. Our results raise the question of how a gene so critical in fitness across multiple insect species can have diverged widely across the insect tree of life.
Significance Statement Given the importance of DNA methylation in protection of the genome against transposable elements and transcriptional regulation in other taxonomic groups, the diversity in both levels and patterns of DNA methylation in the insects raises questions about its function and evolution. We show that the maintenance DNA methyltransferase, DNMT1, affects meiosis and is essential to fertility in milkweed bugs, Oncopeltus fasciatus, while DNA methylation is not required in somatic cells. Our results suggest that DNMT1 has a function independent of DNA methylation in germ cells. The evolutionary lability of a gene with such a fundamental fitness activity suggests that the function Dnmt1 in germ cell development is easily lost or replaced.
Competing Interest Statement
The authors have declared no competing interest.
Footnotes
↵* Patricia J. Moore, 413 Biological Sciences Building, Department of Entomology, University of Georgia, Athens, GA, 30602; (706) 542-0169, Email: pjmoore{at}uga.edu
This version of the manuscript has been revised to reflect minor changes in the significance statement and acknowledgements.
