Neural cell fate in rca1 and cycA mutants: the roles of intrinsic and extrinsic factors in asymmetric division in the Drosophila central nervous system

In the central nervous system (CNS) of Drosophila embryos lacking regulator of cyclin A (rcal) or cyclin A, we observe that several ganglion mother ce lls (GMCs) fail to divide. Whereas GMCs normally produce two sibling neuron s that acquire different fates ('A/B'), nondividing GMCs differentiate excl usively in the manner of one of their progeny ('B'). In zygotic numb mutant s, sibling neuron fate alterations ('A/B' to 'A/A') occur infrequently or d o not occur in some sibling pairs; we have determined that depletion of bot h maternal and zygotic numb causes sibling neurons to acquire equalized fat es ('A/A') with near-complete expressivity. In real, numb mutant embryos, w e observe binary cell fate changes ('B' to 'A') in several GMCs as well. Fi nally, we have demonstrated that expression of Delta in the mesoderm is suf ficient to attain both sibling fates. Our results indicate that the intrins ic determinant Numb is absolutely required to attain differential sibling n euron fates. While the extrinsic factors Notch and Delta are also required to attain both fates, our results indicate that Delta signal can be receive d from outside the sibling pair. (C) 1999 Elsevier Science Ireland Ltd. All rights reserved.