Stem cells can produce differentiated progeny as well as renew the stem cell pool. During the process of self-renewal, they must avoid the accumulation of deleterious DNA mutations, but how they do this is poorly understood. The ‘immortal-strand’ hypothesis suggests a mechanism: during cell division, daughter stem cells retain the original chromosome strand, whereas the new strand – together with any replication-induced mutations – is passed on to the differentiating daughter cell. However, the validity of this hypothesis in vivo remains controversial. Therefore, on page , Yukiko M. Yamashita and colleagues set out to test the hypothesis by using Drosophila melanogaster male germline stem cells (GSCs). These cells have the advantage of always dividing asymmetrically, with one daughter maintaining GSC identity and the other becoming a differentiating gonialblast. Their other advantage is that the mitotic spindle is always oriented in the same way in relation to a cell cluster anchoring the GSCs, so that the segregation of strands into each daughter cell can be precisely determined. By feeding flies BrdU-containing food, the authors show that all daughter cells inherit the labelled chromosome strands equally. So, in this system at least, daughter stem cells do not retain the initial template DNA – an observation that could disprove the immortal-strand hypothesis.
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