STUDIES ON FES, A MUTATION AFFECTING CYSTOCYTE CYTOKINESIS, IN DROSOPHILIA MELANOGASTER

¹ Department of Biological Sciences, Northwestern University, Evanston, Illinois 60201

Females of Drosophila-mclanogaster homozygous for the autosomal, recessive gene fes are sterile, and their ovaries contain "tumorous" cysts that continue to grow mitotically and may eventually posses thousands of undifferentiated cells. To study the earliest steps in the formation of a fes "tumor" we determined the three dimensional interrelations of the cells in a single mutant germarium utilizing electron micrographs taken of serial ultrathin sections. This germarium contained a large number of unconnected cells and clusters made up of only a few interconnected cystocytes. The distributions of dividing cells in fes and wild type germaria, some of which were treated with colchicine, were also studied. All of the cystocyte divisions take place in the anterior third of the wild type germarium. Here a few isolated metaphases were seen in stem line oogonia and cystoblasts, and the rest of the metaphase figures were found in groups of 2, 4, and 8 and presumably represented dividing cystocytes. Metaphases were found throughout the fes germarium. The number of isolated metaphase figures observed in mutant germaria was 15-20 times higher than in wild type. Metaphases were also found in groups. Clusters of two were twice as abundant in fes as in wild type, and clusters of four were equally abundant. Clusters of eight were seen about six times more often in wild type than in fes, but clusters of 3, 5, 6, 7, 9, 10, and 11 metaphases (which were never observed in wild type germaria) were found in fes. We estimated that the average fes cystocyte undergoes one supernumerary division before leaving the germarium.

We concluded that while all cystocytes undergo incomplete division in wild type germaria, a significant fraction of fes cystocytes undergo complete cytokinesis. An algebraic model developed from this hypothesis predicts the relative frequencies of single cells and clusters containing between 2 and 8 cells and enables us to calculate q, the probability that cystocytes will undergo complete cytokinesis. The predicted frequencies were not significantly different from those observed, and the hypothesis was also consistent with the observed rate of division found in the colchicine-treated ovaries and the patterns of cytocyte interconnections found in the reconstructed fes germarium. Germaria from fes females reared at 18° and 25° C gave q values of 0.356 and 0.403, respectively. The mitotic rates were the same at both temperatures. We conclude that the product of the fes + gene is required for the formation of a stable canal system and suggest that the product of the mutant gene is defective in this regard and thermolabile. The fes + substance may function to prevent the constriction of the contractile ring during cystocyte cytokinesis.

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