Diversification of the SprT-like Gene Family and Functional Consequences for Chromosome Transmission
This past summer, I had the opportunity to work in the Levine Lab on research centered around the evolutionary diversification of the SprT-like gene family (consisting of four genes in Drosophila melanogaster) and the functional consequences on chromosome transmission. The so-called SprT-like gene family is integral to maintaining genome integrity in many chromatin-mediated processes including replication, transcription, and chromatin remodeling. Previous research had indicated rapid sequence evolution and gene birth in a Sprt-like gene called MH. I sought to build on that research to investigate whether other genes in the Sprt-like family were also diversifying between related Drosophila species. I found evidence of gene losses in four Drosophila species and possible gene births in three others. Previous research in the Levine lab had indicated that a coevolutionary relationship between a gene in the SprT-like family (MH) and a deleterious DNA satellite (called “359”) could lead to this diversification. Specifically, the coevolutionary model asserts that deleterious mutations in a DNA satellite select for mutations in the chromatin proteins that package the satellite to restore genome integrity and chromosome transmission. For my project, I was curious as to how a deleterious satellite (in this case 359) could have originally evolved. I tested the hypothesis that this rapid evolution of 359 was due to “selfish” transmission distortion. Specifically, I investigated whether, in heterozygotes, the chromosome with 359 would be transmitted to progeny more than the allele for a 359 deletion. While I did not find statistically significant evidence that transmission was being distorted, there was a trend of 359 transmission bias in D. melanogaster flies with a coevolved MH and 359. If this trend holds with higher numbers, my findings would suggest that 359 rose to high frequency by gaining a transmission advantage. Alternatively, 359 may evolve neutrally in the presence of a coevolved MH to package the satellite.
Through these two projects, I gained proficiency with many laboratory techniques integral to fly genetics, evolution, and molecular biology. For instance, I spent the first few weeks of the summer optimizing a PCR (polymerase chain reaction) protocol that allowed me to distinguish flies with and without 359, which was important in order for me to determine if transmission was being distorted. I also had the opportunity to assist with a number of other projects in the lab where I quantified cell fluorescence, set up fly crosses, dissected fly ovaries and counted eggs. In parallel, I learned to search computationally for genes from different species’ genomes using BLAST and BLAT algorithms. I also learned to align DNA sequences in CLUSTAL and infer gene loss events from poor genome assembly. Beyond the technical skills, I developed a patience for the often long and repetitive tasks required for scientific research and an appreciation for the daily work that leads to new scientific findings. I am grateful to PURM for the opportunity to take my interest in biology beyond the classroom and for the start of my research career. I am also grateful to Dr. Mia Levine and all the researchers in the Levine Lab for their invaluable guidance this summer, and I look forward to continuing research with them in the future.