Recombination is a fundamental process in all sexually reproducing organisms. Recombination describes the process where one's paternally and maternally derived chromosomes exchange genetic material before being passed to offspring. This process helps partition the proper number of chromosomes into egg and sperm and creates genetic diversity by shuffling different gene copies. Despite the importance of recombination, little is known about what causes recombination events to occur in particular locations of the genome. One possible mechanism for selecting the position of recombination events is epigenetics, describing modifications to one's inherited genetic material independent from what is coded in one's genes. Some research suggests that the epigenetic modification DNA methylation could affect the recombination process by inhibiting certain sites from experiencing a recombination event. Therefore, this research will utilize the model system Drosophila melanogaster to directly test for a role of DNA methylation in recombination. The experiments will measure recombination with several different techniques in flies missing the gene responsible for DNA methylation, flies in which the gene is over-expressed, and flies with the normal gene, to understand how these processes are interrelated.
The mechanisms of recombination and DNA methylation are fundamental to the development, evolution, and general health of organisms from flies to humans. Errors in recombination result in chromosomal defects that are lethal or developmentally debilitating. Methylation has been implicated in human diseases, particularly in the silencing of tumor suppressor genes in cancer. Understanding the interaction and regulation of these processes will have direct implications in human health and medicine.
Meiotic recombination is an essential process that occurs during egg and sperm production in nearly all sexually reproducing organisms. Recombination, or crossing over, is the reciprocal exchange of genetic material between one’s homologous chromosomes, which serves two purposes. First, crossing over creates a physical link between chromosome pairs, ensuring each gamete ultimately contains the proper number of chromosomes. The failure of this process results in miscarriage or developmentally debilitating syndromes like Down’s. Second, crossing over is vital for bringing together advantageous gene variants and allowing the purging of deleterious gene variants, thereby providing the raw material for evolution. Despite recombination’s central role in reproduction and evolution, studies across all species examined show that the rate and distribution of recombination varies across the genome, and between individuals and populations. Why and how this variation exists is an area of active research in the Noor lab, where we use fruit flies as a model system. In human and mouse, a sequence motif has been found to be important in determining sites of recombination, however, previous research in fruit flies has not found any compelling sequence features. It’s probable that epigenetic factors are important; epigenetics describes a set of modifications apart from DNA sequence that control the organization of genetic material and processes like when certain genes are turned on or off. DNA methylation is a common epigenetic mark involving the tagging of cytosine residues (the "C" bases of your DNA) with a methyl group (CH3); this mark often blocks access to DNA and turns off gene expression. How DNA methylation affects recombination rate is unclear. Fruit flies are helpful in studying the effects of DNA methylation on recombination rate because they only have one gene responsible for methylating DNA, known as Dnmt2, which we can "knock out," or make non-functional. To test whether DNA methylation changes the frequency or distribution of recombination in the fruit fly, we compared recombination in flies with one copy of the Dnmt2 gene (control) to flies having no functioning Dnmt2 gene (experimental). We studied two regions of the genome, one gene-rich with moderate recombination rates in wild type flies, and one gene-poor, repeat-rich region with low recombination rates in wild type flies. We saw no difference in recombination rates between experimental and control flies in either region, and therefore conclude that DNA methylation has no impact on recombination in the fruit fly. The results of this project point shed light on determining factors of recombination rate in fruit flies, suggesting that other epigenetic factors (yet unidentified) play an important role. This highlights key differences across the tree of life, for example, DNA methylation appears to be associated with recombination in humans, whereas in plants, recombination is reduced in methylated regions. These data have been presented at several academic seminars at three different institutions, and the manuscript "No detectable effect of the DNA methyltransferase DNMT2 on Drosophila meiotic recombination" is in press at G3: Genes, Genome, Genetics. Furthermore, two undergraduate students participated in data collection for this experiment before going on to conduct their own independent research projects in the lab, significantly contributing to their scientific training, and to my training as a mentor. Finally, I had the opportunity to continue running labs and talking with students in local middle schools and a high school over the course of this project.
