Courtesy of Mark Petronczki and Juergen Knoblich

Contrary to the Marxist belief, not being equal can actually be a good thing. Asymmetric cell divisions generate diversity, and an avalanche of papers now shed some light on this process.

During asymmetric cell division, the precursor cell is polarized to segregate cell fate determinants predominantly into just one daughter cell, and the mitotic spindle is orientated along the appropriate axis before cytokinesis to ensure this. In the Drosophila central nervous system, neuroblasts' divide asymmetrically along the apical?basal axis. Petronczki and Knoblich, and Wodarz et al. show that this process is similar to the first cell division in Caenorhabditis elegans. In neuroblasts the apical PDZ domain protein Bazooka is in a complex with DmPAR-6 (shown in red in the picture) and an atypical protein kinase C, and this complex controls apical?basal polarity, necessary for the correct basal localization of cell fate determinants (such as the Notch antagonist Numb and the transcription factor Prospero) and asymmetric cell division. Ohshiro et al. and Peng et al. show that the tumour-suppressor genes lgl (lethal giant larvae) and dlg (discs large) are also essential to position cell fate determinants at the basal cortex of neuroblasts, independently of the Bazooka complex. As for the position of the spindle, this is known to be controlled by the Bazooka complex and the apical protein Inscuteable with its partner Pins.

In the Drosophila peripheral nervous system, a series of asymmetric divisions generates the external sensory organ from a single precursor cell. Roegiers et al. and Bellaïche et al. find that, as the precursor pI cell divides, Numb and its partner PON localize to the anterior pole of the cell before the spindle rotates to position itself along the anterior?posterior axis. Numb localization and correct spindle orientation depend on frizzled and flamingo , two genes involved in planar polarity.

Roegiers et al. also show that division of the pIIb cell is similar to neuroblast division. They propose that this difference within the same lineage could arise because pIIb cells express Inscuteable, whereas pI cells do not. The Bazooka complex polarity cue (acting through Inscuteable) could be dominant over the Frizzled cue, leading to an apical?basal polarization. But in the absence of Inscuteable the spindle orientates along an anterior?posterior axis, which could be specified by Frizzled.