The Cell Cycle and Cell Differentiation in the Drosophila Ovary

Abstract

Females belonging to certain evolutionary advanced groups of insects (such as the Diptera, Lepidoptera, and the Hymenoptera) are characterized by ovarioles that contain egg chambers in which the oocyte is one member of a cluster of interconnected cells (Brown and King, 1964; King and Aggarwal, 1965; Cassidy and King, 1972). The other cells of the cluster, the “nurse cells”, grow and simultaneously transfer their cytoplasm to the oocyte by a system of intercellular canals. In Drosophila melanogaster, as in most Diptera belonging to the superfamily Muscoida, each egg chamber consists of an oocyte, its fifteen “nurse” cells, and a surrounding monolayer of follicle cells (see Fig. 1B). The egg and its fifteen interconnected nurse cells are fourth generation descendants of a single cell, the germinal “cystoblast”. The interconnected cells formed by the division of a cystoblast are called “cystocytes”. It is within the apical portion of the ovariole (the germarium) that the consecutive divisions occur which produce the sixteen cell cluster, and it is here also that the cluster becomes enveloped by profollicle cells. However, it is within the more posterior part of the ovariole (the vitellarium) that the major growth of the egg chamber occurs (see Fig. 1A). Here, under optimal conditions it takes only three days for the cytoplasmic volume of the oocyte to increase ninety thousand times (King, 1970, his Fig. 11–19).