Oogenesis in Dacus tryoni (Frogg.) (Diptera: Trypetidae)

Abstract

Oogenesis in D. tryoni is typical of cyclorrhaphous Diptera. The ovariolar germarium produces a linear succession of 16-cell cysts enclosed by follicle cells. The cells of a cyst are interconnected by cytoplasmic canals and differentiate as 15 nurse cells and a posterior oocyte. Previtellogenesis occupies 3 days, vitellogenesis 1 day. The oocyte grows slowly during previtellogenesis, with little differentiation, rapidly during vitellogenesis, when protein and fatty yolk deposition, axial differentiation, and nuclear breakdown to first maturation metaphase, take place. The nurse cells grow rapidly during previtellogenesis and early vitellogenesis, developing large polytene nuclei and RNA-rich cytoplasm, and pour an RNA-rich nutrient stream into the oocyte during early vitellogenesis. The stream also contains P.A.S.-positive material, lipid droplets, possibly protein precursors, and nucleotides. Later, the nurse cells degenerate. Both growth and degeneration of the nurse cells are polarized, the posterior cells leading the more anterior cells. The follicular epithelium, cuboidal during previtellogenesis, differentiates as columnar around the oocyte, squamous outside the nurse cells, and anteriorly as border cells which migrate between the nurse cells to the anterior end of the oocyte. Late in vitellogenesis, the follicular epithelium secretes the chorion and vitelline membrane. It is not yet possible to discern in oogenesis the establishment in the oocyte of the prepattern essential for normal epigenesis.