Dynamics of Cellular Components in Meiotic and Premeiotic Divisions in Drosophila Males

Abstract

Meiosis in higher organisms is programmed as a part of gametogenesis. It should be discussed separately from a yeast meiosis that initiates in response to extracellular nutrient conditions. In this chapter, we would like to introduce current knowledge including our new findings about dynamics of meiosis in Drosophila males. At first, we will introduce general views of cell divisions and cell growth during Drosophila spermatogenesis as we illustrated in Fig. 1A. At the tip of the testis in adult Drosophila males, several germline stem cells (GSCs) are surrounding next to a cluster of smaller hub cells. The GSCs receive a signal to maintain their multi-potential stem cell characteristics secreted from the adjoining hub cells. A secreted protein encoded by the unpaired gene acts as a ligand of the maintenance signal (Kiger, et al., 2001, Fuller & Spradling, 2007). The insulin-like peptides in hemolymph first induce the cell cycle progression of the male GSCs from G2 phase to M phase (Ueishi et al., 2009). Between two daughter cells derived from asymmetric division of the GSC, a proximal daughter cell exclusively receives the Unpaird signal and becomes a self-renewed GSC. The other distal daughter cell leaves the niche and begins to differentiate as a spermatogonium. The spermatogonium then undergoes four cell cycles and generates a 16 cell unit known as a cyst (as a review Fuller, 1993). In ever mitosis, all spermatogonia within a cyst undergo cell divisions synchronously. Cytokinesis in spermatogonia mitoses as well as in following meiotic divisions terminates incompletely. The cleavage furrow ingression is arrested at the middle of cytokinesis and then the contractile rings transform into cytoplasmic bridges called ring canals (Hime et al., 1996). These 16 spermatocytes synchronously enter a growth phase during which the cells remarkably increase in volume by up to 25 times. Following completion of the enormous cell growth, primary spermatocytes carry out two consecutive meiotic divisions. A cyst of 16 spermatocytes gives rise to 64 spermatids simultaneously as a consequence of meiotic divisions. Every spermatid in a cyst at onion stage contains equally sized nucleus and Nebenkern that is a single large aggregate derived from mitochondria. This should be achieved as a consequence of proper chromosome segregation and cytokinesis in germ line cells as well as equal partition of mitochondria (Castrillon et al., 1993, Ichihara et al., 2007).