The mother centrosome (labelled by embryonic expression of GFP–PACT) remains close to the hub-GSC interface, whereas the non-labelled daughter centrosome migrates toward the opposite side of the GSC. Green, GFP–PACT; Red, FasIII (hub) and γ-tubulin (centrosome); blue, Vasa (germ cells). The centrosomes are indicated by white arrows. The scale bar represents 10μm.

A defining characteristic of stem cells is their ability to divide asymmetrically into a self renewed mother cell and a differentiating daughter cell. Yamashita et al. (Science 315, 518–521; 2007) have recently shown that in the Drosophila male germ line differential centrosome inheritance regulates this process.

Drosophila male germline stem cells (GSCs) are induced to self renew by signals from the stem cell niche composed of proximal somatic hub cells in the testes. Asymmetric GSCs division is governed by spindle orientation perpendicular to the hub cells, to produce a stem cell attached to the niche and a differentiating gonialblast. The positioning of centrosomes during interphase orientates the spindle: in G1, the single centrosome is located adjacent to the hub, and in G2 the duplicated centrosomes separate before spindle formation, so that one remains next to the hub and the other migrates to the opposite side of the cell.

Differential labelling of mother and daughter centrosomes by transient expression of the fluorescently labelled PACT domain of pericentrin like protein (which is only incorporated into centrioles during centrosome duplication) established that GSCs preferentially retain the mother centrosomes, whereas the daughter centrosome is inherited by the gonialblast destined to differentiate outside the niche microenvironment.

GSC centrosomes proximal to the hub maintain microtubules throughout the cell cycle, which in some cases extended towards the adherens junctions between GSC and hub cells. Thus, mother centrosomes may remain anchored to the GSC–hub interface via astral microtubules. In contrast, daughter centrosomes only had a few associated microtubules in interphase, which may allow them to move away from the stem-cell niche. Mutants in centrosomin (cnn), which is required for the anchoring of astral microtubules to centrosomes, show random mother and daughter centrosome separation.

These results indicate that the two centrosomes in dividing GSCs have different fates: the mother centrosome remains anchored to the hub in the self-renewed stem cell, whereas the newly formed daughter centrosome is free to move to the differentiating gonialblast, possibly because it is too immature to be hooked by the niche microtubules.

The anchoring of the mother centrosome may govern asymmetric division of the GSCs. The strategy of Yamashita et al. can now be used to determine whether differential centrosome inheritance is characteristic of other stem-cell types.