Influence of B chromosomes on gene expression in the D. melanogaster germline
Authors: Paulo Belato; Kaylah Samuelson; Stacey Hanlon
Affiliation: Department of Molecular and Cell Biology, University of Connecticut, Storrs, CT
Keywords: b. meiosis; c. meiosis
B chromosomes are supernumerary genetic elements found in hundreds of species across a wide range of taxa. Even though many B chromosomes are present in low copy numbers (1-2), the B chromosomes recently discovered in a stock of Drosophila melanogaster are carried at a relatively high copy number of 10-12 B chromosomes. Though these B chromosomes do not appear to carry active genes and are mostly heterochromatin, wild-type females with B chromosomes exhibit a significantly increased frequency of Chromosome 4 nondisjunction, leading us to speculate that the presence of B chromosomes may have an influence on other important meiotic processes. We set out to investigate the impact of these B chromosomes on transcription of protein coding genes during gametogenesis in both males and females by performing RNAseq on mRNA extracted from whole testes and the tips of ovaries (germarium through stage 6). Unfortunately, the original B chromosome stock carries a null mutation of matrimony (mtrm126), a critical regulator of Polo kinase during female meiosis, held over the balancer chromosome TM3, Sb Ser, making it difficult to assign significant transcriptional changes due solely to the B chromosomes since a genotypically comparable stock does not exist. To overcome this obstacle, we outcrossed the original B chromosome stock to a Pr Dr/TM3, Sb Ser stock that does not carry B chromosomes to create two new stocks, both with B chromosomes and either Pr Dr or mtrm126 held over the TM3, Sb Ser balancer. Together with the Pr Dr/TM3, Sb Ser stock without B chromosomes, these three genotypes allow us to triangulate genes that are differentially expressed in the germline due to the presence of these B chromosomes and not because of other confounding genetic factors. Our transcriptome data is currently being analyzed as part of a collaboration with the Cabral De Mello Lab from São Paulo University, and we hypothesize that the gene expression of mechanical and regulatory proteins involved in chromosome segregation, such as microtubules, kinetochore proteins, and Bub kinases, will be elevated in response to the presence of supernumerary B chromosomes. We anticipate that our RNAseq analysis will serve as the foundation for new investigations in the genetic pathways that are disrupted due to the presence of B chromosomes during Drosophila gametogenesis.