Structural Organization and Dynamics of the Endoplasmic Reticulum during Spermatogenesis of Drosophila melanogaster: Studies Using Pdi-GFP Chimera Protein

Endoplasmic reticulum (ER) is a complex, multifunctional structure of the eukaryotic cell undergoing essential morphological transformation in the cell cycle. The spermatogenesis is a complicated developmental process, including mitosis, meiosis, cell shape changes, and remodeling of subcellular organelles from the nucleus to mitochondria. We performed our study on this model system and used Pdi-GFP (Protein disulfide isomerase) hybrid protein as an ER cytological marker. In interphase ER is represented by tubular material and cistern net filling intracellular space between nucleus and plasma membrane. During prometaphase, ER elements appear as concentric rings edging the nucleus membrane. In the late meiotic prometaphase this ER sphere dissociates at presumptive spindle poles, and this structure can be found in metaphase, anaphase, and telophase - up to cytokinesis. During cytokinesis, GFP-labeled membrane vesicles can be found near presumptive contractile ring and form a cloud-like area. After the contraction of the ring, ER fiber-like structures bend and close down around the newly formed nucleus membrane. In the onion stage spermatids the reticular ER structure restores. At this stage, both nucleus and nebenkern are covered by the ER net. At the elongation stage, the ER is represented by a system of tubular-like membrane material. This system dissociates into a small fragments at the beginning of individualization stage. The GFP-labeled membrane granules concentrate at the region immediately ahead of the moving individualization complex. Our results on the ER morphological transformation in spermatogenesis agree well with previous ultrastructural data. In addition, we recorded the ER alterations at the individualization stage, which had not been described before. The process of the ER transformation in the cell cycle, when reticular ER transforms into fiber-like structures, attracts most attention. Two different models were suggested for this transition. One of them assumes that this transition occurs through a vesicular intermediate stage. According to the other model, vesicular intermediates do not exist. Our data concerning the ER morphology in the cell cycle agree with the second model, while the ER transformation at the individualization stage can be better explained by the first model.