A comparison of spermatogenesis between flies and men-conserved processes of male gamete production

Abstract

Background: Spermatogenesis is a dynamic process that involves the co-ordinated development of millions of cells, from stem cells to highly polarized sperm capable of motility and fertility. It is, therefore, not surprising that many thousand genes are required for male fertility. Mutant mouse models are routinely employed to test the function of these genes as well as to validate genetic variants that may be causing human male infertility. The use of mice and other animal models has led to significant knowledge gain regarding the genetic regulation of mammalian male fertility. However, due to the sheer number of genes and genetic variants to be tested these approaches are expensive and time-consuming. We and others have investigated the use of alternate model organisms to expedite validation approaches, including the utility of the fruit fly Drosophila melanogaster.

Objective and rationale: This review explores the conserved mechanisms of sperm production between mammals and flies, with a focus on the human setting where possible.

Search methods: Studies were identified via PubMed using searches including keywords related to the focus of this review, including human, mammalian, and fly or Drosophila spermatogenesis and male fertility. Follow-up searches including using search terms for specific structures and processes for comparison between species included, but were not limited to, male reproductive tract, spermatogenesis, spermatogonia and stem cell niche, meiosis, spermiogenesis and its sub-processes, and sperm/spermatozoa. No time frame or species restrictions were placed on searches.

Outcomes: We identify key phases of spermatogenesis that are highly conserved between humans and flies, including the early germ cell divisions and the ratio of haploid germ cells generated for each spermatogonial stem cell, allowing their use as a model organism to explore such processes. Some processes are moderately well conserved between mammals and flies, including meiosis with the notable absence of 'crossing over' in flies. We also identify some processes that are poorly conserved, such as a divergence in sperm tail accessory structures, for which flies are not likely a suitable model organism to decipher human biology or for mammals broadly. Examples of where the fly has been or could be useful to study mammalian gene function in male fertility have also been described.

Wider implications: Drosophila melanogaster is undoubtedly a useful model organism for studying a wide range of human diseases with genetic origins, including male infertility. Both humans and flies possess a pair of testes with the primary role of generating sperm. The formation of cysts in Drosophila testes allows germ cells to constantly proliferate and stay synchronized at the respective maturation phase, as is the case for humans. While both organisms use a method of sperm storage, mammalian sperm undergo post-testicular modifications and are stored in the epididymis. In Drosophila, sperm are stored in the seminal vesicle, and do not appear to undergo any overt post-testicular modifications in this epididymis-like structure. The seminal vesicle is a separate organ in mammals that is responsible for generation of the seminal fluid. It is important to note that male fertility and thus spermatogenesis are subject to significant evolutionary pressure, and there is a degree of variation in its processes between all species. As such, the absence of a phenotype in mutants would not determine that the gene is dispensable for fertility in humans. While flies are useful for genetic studies to confirm human disease causality, we propose they should be used primarily to pre-screen and select strong candidates for further interrogation in mammalian species for translational pathways in the context of human fertility.

Registration number: N/A.

Keywords: animal model; genetic disorders; infertility; male infertility; sperm morphology.

Drosophila spermatogenesis shows significant similarities in spermatogenesis to humans, with identical germ cell ratios and function but some differences in meiosis and sperm structure.

Figure 1.

Male reproductive organs and tracts in humans and flies. Major male reproductive organs are shown for (A) humans and (B) flies. Organs of the same colour are analogous between species. The seminal vesicles and prostate are presumed to be collectively analogous to the accessory glands and ejaculatory bulb. Not to scale.

Figure 2.

A comparison of human and fly spermatogenesis. (A) In humans, multiple seminiferous tubules exist within the testis, each harbouring millions of germ cells along their length through the depth of their epithelium. A longitudinal section of a testis is shown, highlighting the seminiferous tubules that connect to the rete testis, as well as a cross-section of a single seminiferous tubule to highlight germ cell development. Sertoli cells and SSCs line the basement membrane of seminiferous tubules, which are surrounded by peritubular myoid cells. The Sertoli cell cytoplasm extends deep into the centre of tubules to provide the nutritional and structural support of male germ cells that remain in contact with this cytoplasm throughout their entire development from spermatogonia to spermatozoa. Spermatogonia divide and commit to spermatogenesis, then as spermatocytes traverse the blood–testis barrier, a collection of specialized junctions between neighbouring Sertoli cells to sequester immunologically foreign germ cells from the immune system. As germ cells divide, clones remain in contact with each other by intercellular germ cell bridges that allow the sharing of protein and RNA. Spermatocytes undergo meiosis to form round spermatids, which undergo a dramatic transformation as they are remodelled into highly specialized spermatozoa and then individualized for release via the processes of spermiation. PM, peritubular myoid cell; BM, basement membrane; SSC, spermatogonial stem cell; Sg, spermatogonia; BTB, blood–testis barrier; Sc, spermatocyte; SC, Sertoli cell; RS, round spermatid; ES, elongating spermatid; Sp, spermatozoa. (B) In flies, each testis comprised a single blunt-ended tubule in which continuous germ cell proliferation occurs in isolated cysts. Each cyst is originally populated by two cyst cells and one gonialblast (akin to spermatogonia), each arising from their own stem cell population located at the hub. The hub is a small mass of somatic cells that co-ordinates stem cell maintenance and division. As gonialblasts (and all male germ cells) mature, they remain encased with the same two cyst cells, which allows the germ cell cyst to migrate as a discrete unit along the length of the testis. At the 16-cell stage, spermatogonia mature into spermatocytes and undergo meiosis to produce round spermatids. Spermiogenesis permits the remodelling of round into elongating spermatids, which then undergo individualization and a process of coiling due to the immense length of fly sperm tails. CySC, cyst stem cell; GSC, germline stem cell; CC, cyst cell; Gb, gonialblast; HC, head cyst cell; TC, tail cyst cell; iSp, individualized spermatid. Panel (A) is adapted from Houston et al. (2021a).

Figure 3.

Male germ cell divisions in humans and flies. Cell types are shown above the respective clusters, while total numbers of sister germ cells at each stage are shown underneath. In both humans and flies, a single spermatogonial/germline stem cell generates 64 spermatids from each division. Red insets indicate the cell divisions of each of the eight committing spermatogonia prior to meiosis—i.e. once for each of the eight cells. (A) Human spermatogenesis begins with spermatogonial stem cells (A_dark and/or A_pale), which undergo asymmetric cell division to replenish the stem cell population (self-renewal) as well as to generate spermatogonia that undergo further mitotic divisions to commit to spermatogenesis. Type B spermatogonia mature to primary spermatocytes that undergo meiosis to form spermatids. Spg, spermatogonia; 1° spc, primary spermatocyte; 2°, secondary spermatocyte; sptd, spermatid. (B) In flies, spermatogenesis similarly begins with germline stem cells that undergo asymmetric division to self-renew and generate a gonialblast that commits to spermatogenesis. Subsequent cell divisions are highly similar to the human setting and result in identical germ cell numbers at each stage. * at the 16-cell stage, primary spermatocytes are formed after the cyst of spermatogonia undergoes S-phase prior to meiosis. GSC, germline stem cell; GB, gonialblast. Panel (A) is inspired by Fayomi and Orwig (2018).

Figure 4.

Sperm tail development and axoneme structure in humans and flies. (A) Sperm tail development in humans begins in round spermatids (i), with the docking of a basal body to the nuclear and plasma membranes, in order to construct the axoneme in a restricted ciliary compartment (ii), and the formation of a transition zone to regulate entry into this compartment. Following formation of the core microtubule axoneme (iii), components of the fibrous sheath and, at an overlapping time, outer dense fibres, are loaded into the sperm tail (iii–iv). Following, the annulus migrates and forms a diffusion barrier between the mid and principal pieces (iv). At the same time, the annulus drags the plasma membrane down, exposing a short region of the axoneme to the spermatid cytoplasm, which becomes the midpiece compartment (iv). Mitochondria are rapidly assembled into this compartment (iv) and remaining cytoplasm is stripped during the process of sperm individualization (v). as, acrosome; n, nucleus; bb, basal body; tz, transition zone; mc, mitochondria; an, annulus; ax, axoneme; fs, fibrous sheath; ht, head–tail coupling apparatus. (B) In flies, sperm tail development begins in earnest in spermatocytes (i) with the docking of basal bodies to the plasma membrane and the development of a cellular gate, the transition zone in a small ciliary pocket. In round spermatids (ii) the basal body docks to the nuclear membrane and the nebenkern forms as a precursor of the mitochondrial derivatives. As the axoneme is constructed, the transition zone migrates and maintains a fixed distance from the tip of the tail (ciliary compartment). At the same time, the mitochondrial derivatives unfold and aid in driving axoneme elongation within the cytoplasmic compartment. cyto, cytoplasm; ab, acroblast; nk, nebenkern; md, mitochondrial derivative. (C) While the core sperm axoneme structure is highly conserved between humans and flies, a set of accessory microtubules surrounds the ‘9 + 2’ axoneme and bear some resemblance to the central pair doublet structure. In human (and mammalian) but not fly sperm, outer dense fibres and a fibrous sheath encase the axoneme. Human and fly sperm both contain a mitochondrial sheath structure—in humans, this comprised a set of intercalated mitochondria in a left-handed helix and is restricted to the midpiece; in flies, this is represented by a major and minor mitochondrial derivative that sit adjacent to, and span the entire length of, the sperm tail. MiM, minor mitochondrial derivative. Parts of this figure are adapted from Dunleavy et al. (2019). Some structures are based on electron microscopy from Tokuyasu (1974a, b), Li et al. (1998), and Gottardo et al. (2018).