Cell fusions during formation of the oocyte-nurse chamber complex in the ovary of the dipteran insectMycophila speyeri

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Abstract
Formation of the oocyte-nurse chamber complex in the cecidomyid insectMycophila speyeri was studied in situ and in vitro by electron microscopy and time-lapse cinemicrography. At the end of the oogonial divisions each oogonium passes through a mitotic division with incomplete cytokinesis. This division gives rise to two sister cells, a prospective nurse cell and the oocyte, which remain connected by an intercellular bridge. In two phases of nurse chamber formation, first four and then (usually) one or two ovarian cells of mesodermal origin fuse with the prospective nurse cell. This results in a syncytial nurse chamber containing one germ-cell-derived nucleus and a varying number of mesoderm-cell-derived nuclei. In two subsequent fusion steps, two mesodermal cells fuse with the oocyte, giving rise to an oocyte containing one large and two small nuclei. Thus, four fusion steps lead to the formation of the complete oocyte-nurse chamber complex. Characteristics of the cell fusions are: (1) in each case one or more somatic cell(s) fuse with a germ-line cell and (2) cell contact between the fusing cells is established by the somatic cell, which approaches the germ-line cells.