Drosophila male germline stem cells do not asymmetrically segregate chromosome strands.

Adult stem cells continuously supply differentiated cells throughout the life of organisms. This increases the risk of replicative senescence or neoplastic transformation due to mutations that accumulate over many rounds of DNA replication. The immortal strand hypothesis proposes that stem cells reduce the accumulation of replication-induced mutations by retaining the older template DNA strands. Other models have also been proposed in which stem cells asymmetrically segregate chromosome strands for other reasons, such as retention of epigenetic memories. Recently, the idea has emerged that the mother centrosome, which is stereotypically retained within some asymmetrically dividing stem cells, might be utilized as a means of asymmetrically segregating chromosome strands. We have tested this hypothesis in germline stem cells (GSCs) from Drosophila melanogaster testis, which undergo asymmetric divisions marked by the asymmetric segregation of centrosomes and the acquisition of distinct daughter cell fates (stem cell self-renewal versus differentiation). Using 5-bromo-2-deoxyuridine labeling combined with direct visualization of GSC-gonialblast (differentiating daughter) pairs, we directly scored the outcome of chromosome strand segregation. Our data show that, in male GSCs in the Drosophila testis, chromosome strands are not asymmetrically segregated, despite asymmetrically segregating centrosomes. Our data demonstrate that asymmetric centrosome segregation in stem cells does not necessarily lead to asymmetric chromosome strand segregation.