Download Pairing and Transvection Position Effects in Drosophila Homologous

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