Morphological aspects of cluster formation in the germarium of the sugarcane borer Diatraea saccharalis Fabricius (Lepidoptera: Pyralidae).

Diatraea saccharalis F. is one of the greatest pests of the sugar cane culture. This report aimed to characterize the germarium region of the sugarcane borer by light and transmission electron microscopy, emphasizing the morphological steps of the ovarian cluster formation. In the germarium of this insect, four zones could be morphologically identified during the cluster formation. In the most apical end of each ovariole--Zone I--the germ line stem cells undergo complete mitotic division, originating the cystoblasts. In the Zone II, each cystoblast produces a group of eight cells, the cystocytes, which are interconnected by the ring canals. Clusters containing all the cystocytes in the meiosis, characterizes the Zone III. Germ cells with ultrastructural features of apoptosis are also detected in this Zone. In the Zone IV the cystocytes differentiate, morphologically, into one oocyte and seven nurse cells. Interstitial somatic cells and pre-follicle cells exhibit, in their cytoplasm, heterogeneous vacuoles containing degenerated cellular fragments, characterized as apoptotic bodies. Our results pointed out to the morphological evidences related with important control mechanisms for new clusters/follicles production and for the cellular arrangement into the germarium, resulting from the programmed cell death. We believe that the morphological characterization of ovarian cluster formation in D. saccharalis provided valuable information for the understanding of the initial steps of oogenesis and contributed for the knowledge of the cellular mechanisms related with the oocyte production and with reproduction in insects.