Ribosomal DNA Instability as a Potential Cause of Karyotype Evolution

Karyotype refers to the configuration of the genome into a set of chromosomes. The karyotype difference between species is expected to impede various biological processes, such as chromosome segregation and meiotic chromosome pairing, potentially contributing to incompatibility. Karyotypes can rapidly change between closely related species and even among populations of the same species. However, the forces driving karyotype evolution are poorly understood. Here we describe a unique karyotype of a Drosophila melanogaster strain isolated from the Seychelles archipelago. This strain has lost the ribosomal DNA (rDNA) locus on the X chromosome. Because the Y chromosome is the only other rDNA-bearing chromosome, all females carry at least one Y chromosome as the source of rDNA. Interestingly, we found that the strain also carries a truncated Y chromosome (Y-S) that is stably maintained in the population despite its inability to support male fertility. Our modeling and cytological analysis suggest that the Y chromosome has a larger negative impact on female fitness than the Y-S chromosome. Moreover, we generated an independent strain that lacks X rDNA and has a karyotype of XXY females and XY males. This strain quickly evolved multiple karyotypes: two new truncated Y chromosomes (similar to Y-S), as well as two independent X chromosome fusions that contain the Y-derived rDNA fragment, eliminating females' dependence on the Y chromosome. Considering that Robertsonian fusions frequently occur at rDNA loci in humans, we propose that rDNA loci instability may be one of driving forces of karyotype evolution.

Automated summary

Karyotype refers to the configuration of the genomes in a set of chromosomes. Differences in karyotype between species can hinder biological processes and contribute to incompatibility. This study describes a unique karyotype of a Drosophila melanogaster strain isolated from the Seychelles archipelago, which has lost the ribosomal DNA locus on the X chromosome. The strain also carries a truncated Y chromosome that is stably maintained in the population, despite its inability to support male fertility. The study suggests that the Y chromosome has a larger negative impact on female fitness than the truncated Y chromosome. Furthermore, an independent strain lacking X rDNA quickly evolved multiple karyotypes, including new truncated Y chromosomes and X chromosome fusions containing the Y-derived rDNA fragment, eliminating females' dependence on the Y chromosome. The study proposes that rDNA loci instability may be one of the driving forces of karyotype evolution, a phenomenon also observed in humans.