Non-random Sister Chromatid Segregation Mediated by Ribosomal DNA in Drosophila Male Germline Stem Cells

Germline stem cells (GSCs) in the testis are responsible for producing sperm. In Drosophila, GSCs almost always divide asymmetrically, resulting in one self-renewing GSC and one differentiating cell called the gonialblast (GB), which then develops into sperm. This continuous asymmetric division ensures sperm production throughout the animal's life. During division, some asymmetries are observed. For example, sister chromatids-products of DNA replication that are generally considered identical-are segregated nonrandomly: they are equally segregated into two daughter cells, but one particular chromatid of the sister chromatids is preferentially segregated into the GSC in a strongly biased manner (~85%). Interestingly, only X and Y chromosomes, not autosomes, show such nonrandom sister chromatid segregation (NRSS). Recent studies indicate that NRSS is mediated by an asymmetry in sister chromatids of ribosomal DNA (rDNA), which is unique to the X and Y chromosomes in this species. rDNA is both necessary and sufficient to trigger NRSS. Moreover, NRSS appears to be essential for maintaining germline continuity across generations, mainly by preserving the unstable rDNA copy number. In this review, I will discuss the mechanisms and reasons behind the rDNA-mediated NRSS in Drosophila male GSCs.