ABERRANT FUSOMES IN THE OVARIAN CYSTOCYTES OF THE FS (1) 231 MUTANT OF DROSOPHILA-MELANOGASTER MEIGEN (DIPTERA, DROSOPHILIDAE)

The three-dimensional arrangements of tumor cells were analyzed in reconstructions made from electron micrographs of serially sectioned ovaries of Drosophila melanogaster homozygous for the female sterile (1) 231 mutation. Special attention was paid to the pattern of fusomes extending through ring canals between interconnected tumor cells. Cystocytes within mutant ovaries undergo 3-4 supernumerary cell divisions; the probability of complete cytokinesis being 0.53. The occurrence of long, unbranched chains of interconnected cells suggests that the spindles are sometimes oriented abnormally at mitosis, parallel to the long axis of the cluster. The rims of canals between cystocytes do not appear to undergo the normal morphological changes during development. Tumor cells with 4 or 5 canals were observed, but these did not differentiate into oocytes. Fusomes were found in all sectioned canals, but fusomes often were not joined within cells having 2 or more canals. A loss of mitotic synchrony was observed within certain clusters. The evidence suggests that aberrant fusome formation is the underlying cause of the atypical patterns of cytokinesis characterizing the fs231 mutation. © 1979.