Egalitarian and the choice of cell fates in Drosophila melanogaster oogenesis.

Oogenesis in Drosophila females begins with the formation of a cyst of 16 interconnected sister cells, one of which eventually becomes the definitive oocyte. The other 15 become nurse cells, performing nutritive roles for the oocyte. There are four alternative developmental pathways in each cyst: winning pro-oocyte, losing pro-oocyte and cells with three ring canals, all of which enter meiosis, and the twelve obligate pro-nurse cells, which do not. In flies homozygous for the female-sterile mutation egalitarian (egl) all 16 cells follow the same intermediate pathway. All nuclei enter meiosis (shown by their attainment of synaptonemal complex of at least mid-zygotene levels and by their attainment of pachytene numbers and locations of recombination nodules), then all exit it and become morphologically indistinguishable from nurse cells in wild-type flies. The wild-type allele of egl therefore performs two active but opposite roles in cell fate choice. Early on it is necessary for inhibition of meiotic entry in the 'obligate' pro-nurse cells; later it is necessary for meiotic maintenance in the 'winning' pro-oocyte. One can account for both roles from a single function by invoking gradients through the early cyst; the egl+ function appears to be required for the normal function of these gradients.