Three recent sex chromosome-to-autosome fusions in a Drosophila virilis strain with high satellite content

Abstract

The karyotype, or number and arrangement of chromosomes, has varying levels of stability across both evolution and disease. Karyotype changes often originate from DNA breaks near the centromeres of chromosomes, which generally contain long arrays of tandem repeats or satellite DNA. Drosophila virilis possesses among the highest relative satellite abundances of studied species, with almost half its genome composed of three related 7bp satellites. We discovered a strain of D. virilis that we infer recently underwent three independent chromosome fusion events involving the X and Y chromosomes, in addition to one subsequent fission event. Here we isolate, characterize and propose a timeline for the chromosome fusions in this strain which we believe demonstrates a remarkable karyotype instability. We discovered that one of the substrains with an X-autosome fusion has a X-to-Y chromosome nondisjunction rate 20x higher than the D. virilis reference strain (21% vs. 1%). Finally, we found an overall higher rate of DNA breakage in the substrain with higher satellite DNA compared to a genetically similar substrain with less satellite DNA. This suggests satellite DNA abundance may play a role in the risk of genome instability. Overall, we introduce a novel system consisting of a single strain with four different karyotypes, which we believe will be useful for future studies of genome instability, centromere function, and sex chromosome evolution.