Cytophotometric evidence for the transformation of oocytes into nurse cells inDrosophila melanogaster

Summary A small proportion of ovarian chambers from females homozygous for theotu⁷ (forovariantumor) mutation contain an oocyte that in its nuclear morphology resembles a nurse cell. Such transformed oocytes also appear in colchicine-poisoned wild type ovaries. Cytophotometric estimates demonstrate that these oocytes have undergone 3–4 additional DNA replications, but that they lag behind the adjacent nurse cells by an average of 1.3 replication cycles. It follows that, under certain circumstances, the definitive oocyte can switch to the nurse cell developmental pathway and therefore that a mechanism normally exists for preventing the further replication of its DNA. In the case ofotu⁷, oocytes sometimes restart their endocyclic DNA replications and produce paired, polytene, homologous chromosomes.