Gonadal atrophy is the most typical and dramatic manifestation of intraspecific hybrid dysgenesis syndrome leading to sterility in Drosophila melanogaster dysgenic progeny. The P-M system of hybrid dysgenesis is primarily associated with germ cell degeneration during the early stages of Drosophila embryonic development at elevated temperatures. In the present study, we have defined the phase of germ cell death as beginning at the end of embryogenesis immediately following gonad formation. However, the temperature-dependent screening of germ cell developmental patterns in the dysgenic background showed that early germ cells are susceptible to the hybrid dysgenesis at any Drosophila life-cycle stage, including in the imago. Electron microscopy of germ cells after dysgenesis induction revealed significant changes in subcellular structure, especially mitochondria, prior to cellular breakdown. The mitochondrial pathology can promote the activation of cell death pathways in dysgenic germ cells, which leads to gonadal atrophy.

The Drosophila Trithorax-like (Trl) gene encodes a GAGA factor which regulates a number of developmentally important genes. In this study, we identify a new function for Drosophila GAGA factor in male germ cell development. Trl mutants carrying strong hypomorphic alleles display loss of primordial germ cells during their migration in embryogenesis and severe disruption in mitochondria structure during early spermatogenesis. The mutation resulted in small testes formation, a deficit of germ cells, abnormal mitochondrial morphogenesis, spermatocyte death through autophagy, and partial or complete male sterility. Pleiotropic mutation effects can be explained by the misexpression of GAGA factor target genes, the products of which are required for germ cell progression into mature sperm.

In Drosophila, the ubiquitin ligase Hyd (hyperplastic disc) is required for regulation of cell proliferation during development [Martin et al. (1977) Dev Biol 55, 213-232; Mansfield et al. (1994) Dev Biol 165, 507-526]. Earlier, we demonstrated that the Drosophila tumour suppressor Merlin participates not only in imaginal discs proliferation control, but also performs a separate Nebenkern structural function in Drosophila spermatogenesis [Dorogova et al. (2008) BMC Cell Biol 9, 1. Here, we show that the hyd mutants also have spermatogenesis defects: chromosome condensation and attachment to the spindle, centrosome behaviour and cytokinesis in meiosis. The process of spermatid elongation was also greatly affected: nuclei were scattered along the cyst and had an abnormal shape, Nebenkern-axoneme angular relation and attachment was distorted, axonemes themselves lost correct structure. Since Hyd and pAbp protein families share a common PABC [poly(A)-binding protein C-terminal] protein domain, we also studied spermatogenesis in pAbp homozygotes and found defects in cytokinesis and spermatid elongation. However, our study of hyd and pAbp genetic interaction revealed only the phenotype of defective nuclei shape at the final stage of spermatid differentiation. So, the PABC domain is unlikely to be responsible for meiotic defects. Thus, our data document that, in addition to the tumour suppressor Merlin, another tumour suppressor, Hyd, also has a function in spermatogenesis.

Summary Investigation of Drosophila oogenesis provides the opportunity to understand conservative genetic mechanisms underlying fertile female gamete development. In this study, we showed that the Drosophila DNA‐binding protein GAGA factor (GAF) had a multifunctional role in oogenesis and it is involved in the regulation of this process genetic program. We studied the influence on Drosophila oogenesis of a number of mutations in the 5′ region of the Trl gene that encodes GAF. We found that our originally generated Trl mutations lead to a decrease in transcriptional gene activity and levels of GAF expression in both germline and follicular cells. Cytological (fluorescence and electron microscopy) analysis showed that GAF loss resulted in multiple oogenesis defects. Mutations affected the actin cytoskeleton, leading to decrease of cytoplasmic filaments in nurse cells and basal actin in follicular cells. GAF depletion also leads to abnormal follicular cells migration, both border and centripetal. In addition, mutant ovaries demonstrated abnormalities in germ cells, including mitochondria, endoplasmic reticulum, karyosome organization, yolk granule formation and selective transport. Loss of GAF also promoted excessive cell death and egg chamber degradation. In sum, these defects caused very high or full female sterility. Since one of the main GAF activities is regulation of transcription, the complex phenotypes of the Trl mutants might be the consequence of its multiple target genes misexpression.

Drosophila protein GAGA (GAF) is a factor of epigenetic transcription regulation of a large group of genes with a wide variety of cellular functions. GAF is encoded by the Trithorax-like (Trl) gene, which is important for the formation of various organs and tissues at all stages of ontogenesis. In our previous works, we showed that this protein is necessary for the development of the reproductive system, both in males and females of Drosophila. Decreased expression of the Trl gene led to multiple disorders of spermatogenesis and oogenesis. One of the significant disorders was associated with massive degradation and loss of cells in the germline. In this work, we carried out a more detailed cytological study to determine what type of germ cell death is characteristic of Trl mutants, and whether there are disturbances or changes in this process compared to the norm. The results obtained showed that the lack of GAF protein causes massive germ cell death in both females and males of Drosophila, but this death manifests itself in different ways, depending on the sex. In Trl females, this process does not differ phenotypically from the norm. In the dying egg chambers, signs of apoptosis and autophagy were revealed, as well as morphological features that are characteristic of the wild type. In males, Trl mutations induce mass germ cell death through autophagy, which is not typical of Drosophila spermatogenesis, and has not been previously described, neither in the norm nor in other genes’ mutations. Thus, GAF lack in Trl mutants leads to increased germ cell death through apoptosis and autophagy. Ectopic cell death and germ line atrophy are probably associated with impaired expression of the GAGA factor target genes, among which there are genes that regulate both apoptosis and autophagy.