RSBP15 interacts with and stabilizes dRSPH3 during sperm axoneme assembly in Drosophila

CG9920 is necessary for mitochondrial morphogenesis and individualization during spermatogenesis in Drosophila melanogaster

Drosophila melanogaster is an ideal model organism for investigating spermatogenesis due to its powerful genetics, conserved genes and visible morphology of germ cells during sperm production. Our previous work revealed that ocnus (ocn) knockdown resulted in male sterility, and CG9920 was identified as a significantly downregulated protein in fly abdomen after ocn knockdown, suggesting a role of CG9920 in male reproduction. In this study, we found that CG9920 was highly expressed in fly testes. CG9920 knockdown in fly testes caused male infertility with no mature sperms in seminal vesicles. Immunofluorescence staining showed that depletion of CG9920 resulted in scattered spermatid nuclear bundles, fewer elongation cones that did not migrate to the anterior region of the testis, and almost no individualization complexes. Transmission electron microscopy revealed that CG9920 knockdown severely disrupted mitochondrial morphogenesis during spermatogenesis. Notably, we found that CG9920 might not directly interact with Ocn, but rather was inhibited by STAT92E, which itself was indirectly affected by Ocn. We propose a possible novel pathway essential for spermatogenesis in D. melanogaster, whereby Ocn indirectly induces CG9920 expression, potentially counteracting its inhibition by the JAK-STAT signaling pathway.

Structure and molecular basis of spermatid elongation in the Drosophila testis

Spermatid elongation is a crucial event in the late stage of spermatogenesis in the Drosophila testis, eventually leading to the formation of mature sperm after meiosis. During spermatogenesis, significant structural and morphological changes take place in a cluster of post-meiotic germ cells, which are enclosed in a microenvironment surrounded by somatic cyst cells. Microtubule-based axoneme assembly, formation of individualization complexes and mitochondria maintenance are key processes involved in the differentiation of elongated spermatids. They provide important structural foundations for accessing male fertility. How these structures are constructed and maintained are basic questions in the Drosophila testis. Although the roles of several genes in different structures during the development of elongated spermatids have been elucidated, the relationships between them have not been widely studied. In addition, the genetic basis of spermatid elongation and the regulatory mechanisms involved have not been thoroughly investigated. In the present review, we focus on current knowledge with regard to spermatid axoneme assembly, individualization complex and mitochondria maintenance. We also touch upon promising directions for future research to unravel the underlying mechanisms of spermatid elongation in the Drosophila testis.

Single-cell RNA-sequencing reveals the transcriptional landscape of ND-42 mediated spermatid elongation via mitochondrial derivative maintenance in Drosophila testes

During spermatogenesis, mitochondria extend along the whole length of spermatid tail and offer a structural platform for microtubule reorganization and synchronized spermatid individualization, that eventually helps to generate mature sperm in Drosophila. However, the regulatory mechanism of spermatid mitochondria during elongation remains largely unknown. Herein, we demonstrated that NADH dehydrogenase (ubiquinone) 42 kDa subunit (ND-42) was essential for male fertility and spermatid elongation in Drosophila. Moreover, ND-42 depletion led to mitochondrial disorders in Drosophila testes. Based on single-cell RNA-sequencing (scRNA-seq), we identified 15 distinct cell clusters, including several unanticipated transitional subpopulations or differentiative stages for testicular germ cell complexity in Drosophila testes. Enrichments of the transcriptional regulatory network in the late-stage cell populations revealed key roles of ND-42 in mitochondria and its related biological processes during spermatid elongation. Notably, we demonstrated that ND-42 depletion led to maintenance defects of the major mitochondrial derivative and the minor mitochondrial derivative by affecting mitochondrial membrane potential and mitochondrial-encoded genes. Our study proposes a novel regulatory mechanism of ND-42 for spermatid mitochondrial derivative maintenance, contributing to a better understanding of spermatid elongation.

shrub is required for spermatogenesis of Drosophila melanogaster

Shrub (CG8055) encodes the vps32/snf7 protein, a filament-forming subunit of the ESCRT (endosomal sorting complexes required for transport)-III complex involved in inward membrane budding. It was reported that shrub was required for abscission in female germline stem cells. In this study, we showed that the expression level of shrub in the testis was significantly higher than that in the ovary of 1-day-old Drosophila melanogaster, suggesting a role in male reproduction. Then we used nosGal4 driver to knockdown shrub specifically in the fly testis and found that this resulted in a significantly lower paternal effect egg hatch rate relative to the control group. Immunofluorescence staining showed that shrub knockdown in fly testes caused an accumulation of early-stage germ cells and lack of spectrin caps. In the late stages (spermiogenesis), the control testis contained multiple compacted spermatid bundles and individualization complexes (ICs) consisting of actin cones, whereas there were scattered spermatid nuclei and only a few ICs with disorganized actin cones in the shrub knockdown testis. Finally, the control seminal vesicle was full of mature sperms with needle-like heads, but in shrub knockdown testis 75% of seminal vesicles had no mature sperms. We also found that knockdown of shrub in fly testes led to upregulated expression of several cytoskeleton-associated genes, and an accumulation of ubiquitylated proteins. These results suggest that knockdown of shrub in fly testes might damage spermatogenesis by affecting transportability.

mRNA localization mediates maturation of cytoplasmic cilia in Drosophila spermatogenesis

Cytoplasmic cilia, a specialized type of cilia in which the axoneme resides within the cytoplasm rather than within the ciliary compartment, are proposed to allow for the efficient assembly of very long cilia. Despite being found diversely in male gametes (e.g., Plasmodium falciparum microgametocytes and human and Drosophila melanogaster sperm), very little is known about cytoplasmic cilia assembly. Here, we show that a novel RNP granule containing the mRNAs for axonemal dynein motor proteins becomes highly polarized to the distal end of the cilia during cytoplasmic ciliogenesis in Drosophila sperm. This allows for the incorporation of these axonemal dyneins into the axoneme directly from the cytoplasm, possibly by localizing translation. We found that this RNP granule contains the proteins Reptin and Pontin, loss of which perturbs granule formation and prevents incorporation of the axonemal dyneins, leading to sterility. We propose that cytoplasmic cilia assembly requires the precise localization of mRNAs encoding key axonemal constituents, allowing these proteins to incorporate efficiently into the axoneme.

Combover interacts with the axonemal component Rsp3 and is required for Drosophila sperm individualization

Gamete formation is key to survival of higher organisms. In male animals, spermatogenesis gives rise to interconnected spermatids that differentiate and individualize into mature sperm, each tightly enclosed by a plasma membrane. In Drosophila melanogaster, individualization of sister spermatids requires the formation of specialized actin cones that synchronously move along the sperm tails, removing inter-spermatid bridges and most of the cytoplasm. Here, we show that Combover (Cmb), originally identified as an effector of planar cell polarity (PCP) under control of Rho kinase, is essential for sperm individualization. cmb mutants are male sterile, with actin cones that fail to move in a synchronized manner along the flagella, despite being correctly formed and polarized initially. These defects are germline autonomous, independent of PCP genes, and can be rescued by wild-type Cmb, but not by a version of Cmb in which known Rho kinase phosphorylation sites are mutated. Furthermore, Cmb binds to the axonemal component Radial spoke protein 3, knockdown of which causes similar individualization defects, suggesting that Cmb coordinates the individualization machinery with the microtubular axonemes.