Germ cells undergo a series of mitotic and meiotic divisions, followed by a differentiation program to produce highly specialized haploid gametes that, at fertilization, fuse and give rise to a totipotent zygote, capable of generating all somatic lineages/tissues and the next-generation of gametes (Larose et al., 2019; Farini and De Felici, 2022). The proper completion of meiosis is essential for fertility and for ensuring the normal development of the offspring (Farini and De Felici, 2022). Therefore, revealing the molecular mechanisms and pathways underlying this unique process is pivotal for understanding human reproduction and fertility, as well as helping to identify therapeutic and diagnostic targets (Larose et al., 2019; Farini and De Felici, 2022).

Infertility affects 8%–12% of reproductive-age couples worldwide (Ombelet et al., 2008), reaching 30% in some regions of the world (Mascarenhas et al., 2012). In 33% of infertility cases, the inability to have children is due to female factors alone, in 20% to male factors, in 39% to both male and female factors, and in 8% the causes remain unknown, termed as unexplained infertility (Thonneau et al., 1991). Understanding meiosis may reveal possible mechanisms underlying the cases of unexplained infertility, since proper gametogenesis is important to the formation of mature gametes and to guarantee the first events of embryo development (Larose et al., 2019).

Both female and male germ cells undergo meiosis; however, the timing of meiosis and the quantity of gametes that are produced differs in males and females (Farini and De Felici, 2022). In mammals, meiosis begins during fetal life in females and continues until menopause, whilst in males it begins at puberty and is maintained throughout life (Farini and De Felici, 2022). Understanding the molecular mechanisms behind the differences in meiosis onset and progression in males and females may contribute to the elucidation of genes involved in conditions affecting human reproduction.

The current study aimed to investigate if the gene expression profile of meiosis-related genes in human male and female gonads and gametes could be associated with human reproductive disorders. To achieve that, the expression of 678 meiosis-related genes was evaluated in normal and pathological conditions affecting human reproduction, through publicly available datasets. The interesting pattern of expression demonstrated in this study sheds light on the importance of gene expression regulation programs in human reproductive disorders and elucidates potential biomarkers in the reproductive medicine field.

Both female and male gametogenesis begin with the mitotic proliferation of oogonia and spermatogonial stem cells, respectively, followed by meiotic divisions and modifications until the formation of mature germ cells (Larose et al., 2019). However, the timing and quantity of gametes developed are very different in the two gonads, and gene expression patterns may govern, at least in part, these differences (Handel and Schimenti, 2010). In males, the spermatogonial stem cells remain quiescent during the prenatal period, starting to proliferate and entering meiosis after puberty, resulting in continuous sperm production throughout the reproductive lifespan (Handel and Schimenti, 2010). On the other hand, female meiosis initiates in fetal ovaries and is arrested in the prophase I stage before birth, being reestablished only after puberty in periodical intervals in response to hormonal stimuli (Handel and Schimenti, 2010). To accomplish these processes, intracellular signals, such as the cAMP levels in the oocyte, are necessaire for the maintenance of meiotic arrest or for the meiotic resumption, which is characterized by the germinal vesicle (GV) breakdown, followed by completion of meiosis I, and arrest at the metaphase of meiosis II until fertilization (Pan and Li, 2019). Hence, differences in gene expression in the GV (prophase I-arrested oocytes) and MII (mature oocytes), as demonstrated in the present study, may be related to molecular mechanisms involved in the regulation of meiotic arrest and resumption; these molecular mechanisms might also be related to the acquisition of oocyte competence, pivotal to support the early embryo development (Conti and Franciosi, 2018).

Complex biological processes, such as those involved in human reproduction, require different genes working in an orchestrated way to ensure regulatory networks (de la Fuente, 2010). Recent studies of global gene expression in oocytes and sperm have demonstrated important target genes involved in conditions affecting human reproduction (Hermann et al., 2018; Li et al., 2021; Ntostis et al., 2021; Santiago et al., 2021; Sayutti et al., 2022). In the present study, prior filtering of meiosis-related genes was performed, and these genes had their differential gene expression evaluated in female and male gonads and gametes under physiological and pathological conditions.

It was demonstrated that during adulthood, 17 meiosis-related genes were upregulated in the testicle in comparison to the ovary (ADAD1, BRDT, DAZL, DDX4, GTSF1, HORMAD1, MAEL, MAJIN, OOEP, SERPINA5, SOX9, SPATA22, SYCE1, SYCP3, TDRD1, TDRD12, and TEX11), whilst in all the prenatal periods analyzed, downregulation of these genes was observed in the testicle in comparison to the ovary, except for SERPINA5 and SOX9, that maintain the upregulation in the testicle. The orchestrated expression of these genes during meiosis, according to time and place, may be crucial for meiosis entry and normal gamete formation (Figure 4), considering their involvement in important processes such as: meiotic regulation, homologous chromosomes pairing, recombination and segregation, DNA double-strand breaks (DSBs) repair, and formation of the synaptonemal complex (GeneCards, 2022). In addition, considering the 678 meiosis-related genes, it was observed that more genes were upregulated in the ovary during the prenatal period compared to the testicle, whilst in the adult gonads more genes related to meiosis were upregulated in the testicle.

During the meiosis prophase I, the synaptonemal complex mediates the homologous chromosomes pairing and synapsis, a fundamental step for the recombination, that is initiated by DNA double strand breaks, and the segregation events (Handel and Schimenti, 2010). In human fetal ovaries, meiotic entry is initiated asynchronously around 8-9 wpc, confirmed by the increased expression of meiotic proteins by this time, such as DAZL, DDX4, and SYCP3 (Farini and De Felici, 2022); this increased expression was corroborated in the present study. The conditions for meiotic beginning depend on intrinsic and extrinsic factors, being proposed that germ cells of both sexes receive at the same time stimuli for meiotic beginning, however, somatic cells in the developing gonads induce or prevent meiosis entry and progression (Farini and De Felici, 2022). Therefore, the distinct pattern of meiosis-related gene expression observed in the testicle and ovary according to the life period may reflect the differences in female and male meiosis onset, through differential gene expression of germ and somatic cells in fetal and adult gonads. The upregulation of the 17 meiosis-related genes in the fetal ovary and adult testicle, coinciding with the meiosis onset in both sexes, is in agreement with the roles of these genes in processes mainly related to prophase I, the first stage of meiosis.

SERPINA5 and SOX9 were upregulated in the testicle in both prenatal and adult periods. SOX9 is a transcription factor expressed in the Sertoli cells of the male gonad throughout life (Jiménez et al., 2021). At the time of sex determination, the expression of the Y-linked, sex determining-region - SRY - in the Sertoli cells leads to the upregulation of SOX9, which activates male-promoting genes necessary for the differentiation of Sertoli cells into the testicle cords (Jiménez et al., 2021). SERPINA5 encodes a glycoprotein with an inhibitory effect in several serine proteases; in seminal plasma, inactivates several serine proteases implicated in the reproductive system, and has a control on the sperm motility and fertilization (GeneCards, 2022). Although the roles of SERPINA5 are poorly understood (Yang and Geiger, 2017), studies have pointed to a role in male reproduction (Yang and Geiger, 2017; Selvam et al., 2019). Therefore, the constant upregulation of SOX9 and SERPINA5 in the testicle samples in comparison to the ovary is in agreement with their roles in processes related to male reproduction.

Chromosome segregation errors during meiosis, leading to aneuploidies, are one of the most common causes of infertility, pregnancy failure, and genetic disorders in the offspring (Mikwar et al., 2020). Although meiotic errors may occur in both male and female germ cells, the majority are of maternal origin, being the advanced maternal age (≥35 years) a risk factor for chromosome segregation errors (Mikwar et al., 2020). The present study demonstrated 48 meiosis-related genes differentially expressed in mature oocytes of advanced maternal age in comparison to young maternal age patients. Among them are TUBB8, which encodes the primary beta-tubulin subunit of oocytes and early embryos, and BUB3, which encodes a protein involved in spindle checkpoint function. TUBB8 and BUB3 were already associated with infertility due to oocyte meiotic arrest and mosaic variegated aneuploidy syndrome, respectively (GeneCards, 2022). Genes related to meiotic recombination, chromosome cohesion, spindle assembly checkpoint, post-translational modification of histones and tubulin, and mitochondrial functions, are great targets for research, since impairment in these mechanisms has been proposed to explain the higher incidence of oocyte aneuploidies in older females (Mikwar et al., 2020).

Aside from advanced maternal age, it was expected that alterations in meiosis-related gene expression could be associated with human reproductive disorders, considering their pattern of expression in male and female gonads in critical periods of gametogenesis. Nevertheless, no differences were observed in the oocytes’ gene expression of PCOS patients in comparison to the control group. PCOS is a multifactorial disorder, influenced by intrinsic and extrinsic factors, such as insulin resistance, hyperandrogenism, environmental factors, genetics, and epigenetics (Sadeghi et al., 2022). Therefore, as many factors are involved in the etiology of PCOS, the meiosis-related genes considered in the present study probably do not play a crucial role in the mechanisms behind PCOS etiology. Furthermore, a recent study on global gene expression of oocytes and cumulus cells from PCOS patients have not demonstrated the genes evaluated here as possible candidates (Li et al., 2021).

Contrastingly, when evaluating male reproductive disorders, 238 and 290 meiosis-related genes were differentially expressed in teratozoospermic and NOA patients in comparison to the control group, respectively. Most of these genes were downregulated in the conditions affecting human male reproduction, demonstrating that expression of meiosis-related genes may be necessaire in processes related to male fertility. Additionally, 145 genes were differentially expressed in both teratozoospermic and NOA patients compared to the control group, many of them already associated with spermatogenic failure and sperm maturation defect, according to our search for phenotype descriptions in the Ensembl database. Previous studies have demonstrated a downregulation profile in meiosis-related genes in both NOA and teratozoospermic patients (Fu et al., 2016; He et al., 2021). He et al. demonstrated 430 downregulated genes in NOA in comparison to the control group, many of them associated with spermatogenesis, multicellular organism development, cell differentiation, spermatid development, and sperm motility (He et al., 2021); Fu et al. demonstrated 2158 downregulated genes in teratozoospermic in comparison to normospermic patients, mostly related to reproduction, mitotic cell cycle, and spermatogenesis (Fu et al., 2016).

Considering the 17 meiosis-related genes differentially expressed in all the developmental periods analyzed, it was observed downregulation of ADAD1, BRDT, GTSF1, HORMAD1, OOEP, SPATA22, SYCP3, and TEX11 in both teratozoospermic sperm and NOA testicle samples, evidencing a role in male reproduction. Except for OOEP, all the other downregulated genes have been linked to male infertility disorders, including NOA and teratozoospermia (Aarabi et al., 2006; Miyamoto et al., 2012; Chen et al., 2020; Greither et al., 2020; Snyder et al., 2020; Her et al., 2021; Ji et al., 2021; Wu et al., 2021; Omolaoye et al., 2022). OOEP is a member of the SCMC, a maternal complex essential to the progression beyond the first embryonic cell divisions (Li et al., 2008; Zhu et al., 2014). Although mutations in the SCMC genes are associated with female reproductive disorders (Demond et al., 2019; Zhang et al., 2019), a relationship between SCMC components and male reproduction was only recently suggested by (Feng et al., 2020), evidencing Tle6 playing a role in the proliferation and cell cycle of the mouse spermatogonia (Feng et al., 2020).

Although no obvious association of OOEP with male reproduction the co-expression network performed for OOEP demonstrated the co-expression with genes located in the Y chromosome, related to human reproduction, spermatogenesis, and male infertility. Since co-expressed genes are simultaneously active (van Dam et al., 2018), downregulation of OOEP in patients with NOA and teratozoospermia could affect the entire gene expression regulation, even though the structure or even the protein expression might not be affected. Furthermore, Ooep has been related to processes involved in DNA repair (He et al., 2018; Zhao et al., 2018), and impairment in these mechanisms is associated with spermatogenesis arrest (Gunes et al., 2015), and teratozoospermia (Ammar et al., 2020).

This study has some limitations, such as not having the validation of gene expression results by qPCR and not demonstrating protein expression. However, all data used for analysis are from experimental studies of different datasets that pointed in the same direction. In this way, the results presented here contribute to the understanding of gene expression patterns involved in the differences between male and female meiosis, by (I) demonstrating that meiosis-related genes are differentially expressed in human male and female gonads throughout gametogenesis and (II) presenting altered gene expression in human male reproductive disorders and oocytes of advanced maternal age patients. Hence, concerning basic sciences and assisted reproduction application, future studies aiming to elucidate the molecular mechanisms of meiosis should consider the genes presented in this study. These results pinpoint to possible new candidate genes that could have clinical implications, therefore should be considered biomarkers in panels for patients with NOA or teratozoospermia.