13, Rusan and Peifer reveal that the separation of a dominant and a secondary centrosome helps define an asymmetric division axis in fly neuroblasts.
Asymmetric divisions in stem cells give rise to one daughter that will differentiate and one that will replace the stem cell. This asymmetry can be set by the orientation of the mitotic spindle through microtubule interactions with the cortex. But Nasser and Peifer find that fly neural stem cells begin to align their spindles before they are even built.
Spindle orientation in male germline stem cells was recently shown to be defined by separation of the mother centrosome, which remains near the stem cell niche, and the daughter centrosome, which travels to the opposite side. In neuroblasts, the authors now find, centrioles similarly separate in interphase to define the division axis.
Whether the traveling centriole is consistently the daughter is not known. But only the stationary centriole retained its microtubule-organizing capacity; the traveler shed its pericentriolar material and microtubules until the following mitosis. The stationary centriole also kept its Polo kinase, which might help provide its unique abilities.
Without centrioles, fly neuroblasts still create spindles, whose interactions with the cortex often resulted in normal orientations. But if the spindle formed too far off-center, cortical cues could not correct the problem, thwarting the asymmetric division.