A novel pattern of germ cell divisions in the production of Hymenopteran insect eggs

Egg development is a defining process of reproduction in higher eukaryotes. In the fruit fly, Drosophila melanogaster, this process begins with four mitotic divisions starting from a single germ cell, producing a cyst of 16 cystocytes; one of these cells will become the oocyte and the others supporting nurse cells. These mitotic divisions are exceptional because cytokinesis is incomplete, resulting in the formation of cytoplasmic bridges known as ring canals that interconnect the cystocytes. This organization allows all cystocytes to divide synchronously during each mitotic round, resulting in a final, power-of-2 number of germ cells. Given that numerous insects obey this power-of-2 rule, we investigated if strict cell doubling is a universal, underlying cause. Using confocal microscopy, we found striking departures from this paradigm in three different power-of-2 insects belonging to the Apocrita suborder (ants, bees and wasps). In these insects, the earliest-formed cystocytes cease to divide during the latter mitotic cycles while their descendants undergo further division, thereby producing an outward, or "radial," direction of division activity. Broadly, our results suggest that the unique cystocyte division patterns that depart from strict cell doubling in some insects may be "fine-tuned" in order to maintain a final, power-of-2 germ cell number.