Symposium on reproduction of arthropods of medical and veterinary importance. I. Insect gametogenesis.

Abstract

In higher insects both the male and female gametocytes undergo cycles of mitoses accompanied by incomplete cytokinesis. This results in clusters of sister cells joined to one another by systems of canals. The final number of joined “cystocytes” is constant for a given species and sex. Subsequently all male cystocytes, but only 1 female cystocyte in a cluster complete meiosis. The remaining female cystocytes grow and transfer cytoplasmic components through the canal system to the oocyte. Endomitosis in these “nurse cells” serves to multiply the cistrons that transcribe the components of the ribosomes which are assembled and then stored in the ooplasm in vast numbers. In the nuclei of oocytes and spermatocytes only 1 chromosomal replication cycle takes place, and this is generally followed by the assembly of synaptonemal complexes. Crossingover occurs, and the chiasmata which result keep the bivalents intact until the first meiotic division. The meiotic divisions of the oocyte are unique in that they occur in the absence of centrioles. The fertilizing sperm brings in a centriole, and this generates the centrioles required for subsequent cleavages. In Drosophila melanogaster studies of aberrant forms of gametogenesis in insects possessing mutant genes that markedly influence their fertility provide insights into the genetic control of these phenomena.