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Pairing and Transvection Position Effects in Drosophila Homologous Chromosomes Thomas King, Class of 2017 In my research this summer, I aided the Bateman lab in its exploration of the genetic phenomenon of transvection. Transvection occurs when regulatory DNA sequences called enhancers on one chromosome interact with promoters (a different type of regulatory sequence) on a neighboring chromosome. These interactions can lead to gene expression that would not be accounted for under standard models of molecular genetics, in which it is often assumed that the regulatory elements on each chromosome operate in isolation, independent of effects from nearby chromosomes. The Bateman lab studies this phenomenon in fruit flies (Drosophila melongaster) in part due to the fact that their two copies of each chromosome are tightly bound to each other (“paired”) along their entire length, increasing the likelihood that the crosschromosomal interactions necessary for transvection will occur. The Bateman lab had previously found that the same genetic construct, when integrated into different locations in the Drosophila genome, activated transvection at vastly different levels. Multiple hypotheses exist regarding the reason for this phenomenon (broadly described as “position effects”), but one potential explanation is that the sister chromosomes in Drosophila could be more likely to be tightly paired at some locations in the genome than others, causing differences in the amount of transvection throughout the genome. In my project, I investigated the relationship between pairing and transvection to determine whether there was any correlation between the two. My work involved using DNA fluorescent in situ hybridization, a technique that allows the precise targeting of a location on a chromosome with glowing dye molecules, to visualize the state of the chromosomes at particular regions of interest. I used specially designed DNA probes called oligopaints to highlight the specific regions of the genome where the Bateman lab had earlier quantified the amount of transvection. Using fluorescent microscopy, I was then able to score multiple nuclei based on the whether or not the chromosomes were closely paired. I used five different probes targeting different genomic regions, and I applied the same technique to three different cell types: embryonically derived S2 cell cultures, neuronal BG3 cultures, and eye discs (the tissues that develop into eyes in adult flies) taken from larvae. Thus far, I have only been able to obtain a complete and satisfactory set of results from the BG3 cells. At least in this cell type, I found significant differences in the degree of pairing among each genomic position I tested. This is exciting in that it implies that the homologous chromosomes in Drosophila are not necessarily uniformly paired, but instead display consistent differences in pairing throughout the genome. However, preliminary analysis indicates that there is no correlation between the level of pairing and the degree of transvection observed—more tightly paired locations on the genome seem no more likely to exhibit transvection. I will be continuing my research during the upcoming academic year, and hope to replicate these results across all three cell types. I also hope to alter the expression of genes that influence pairing and determining whether this impacts transvection. I am grateful for the opportunity to have contributed to this field of inquiry, and look forward to continuing this research experience during the rest of my time at Bowdoin. Faculty Mentor: Dr. Jack Bateman Funded by the National Science Foundation