A NOVEL GROUP OF PUMILIO MUTATIONS AFFECTS THE ASYMMETRIC DIVISION OFGERMLINE STEM-CELLS IN THE DROSOPHILA OVARY

Germline stem cells play a pivotal role in gametogenesis; yet little i s known about how they are formed, how they divide to self-renew and h ow these processes are genetically controlled. Here we describe the se lf-renewing asymmetric division of germline stem cells in the Drosophi la ovarian germline, as marked by the spectrosome, a cytoplasmic struc ture rich in membrane skeletal proteins. The ontogeny of the spectroso me marks the lineage of germline stem cells. We identified two new gro ups of mutations in which the divisional asymmetry is disrupted. The f irst, which me refer to as ovarette (ovt) mutations, was shown to corr espond to a novel class of mutations in the pumilio locus. Since pumil io is known to posttranscriptionally repress the expression of target genes at earlier stages of germ cell development, our results suggest that a similar activity is needed to maintain germ line stem cells. We have also identified a second and novel gene, piwi, whose mutations a bolish germline stem cell division.