Actualización sobre COVID-19 e histofisiología del sistema reproductor masculino / Update on COVID-19 and histophysiology of the male reproductive system

RESUMEN: La reciente pandemia de la COVID-19 ha sacudido a la sociedad teniendo una importante repercusión en el campo de la salud y de la investigación. Dada su relevancia, se han llevado a cabo estudios sobre los efectos del SARS-CoV-2 en la fisiología humana. En concreto, sobre la posible presencia y transmisión del virus a través del sistema reproductor masculino y su posible efecto en el éxito reproductivo. Conocer si la presencia del virus altera los órganos responsables del desarrollo y maduración de las células de la serie espermatogénica podría revelarnos su implicación en la calidad seminal. Por ello, nos planteamos esta revisión, con el fin de analizar las principales evidencias científicas sobre los efectos del SARS-CoV-2 en la histofisiología del sistema reproductor masculino y sobre la capacidad fecundante de los espermatozoides.

SUMMARY: The recent COVID-19 pandemic has shaken up society, having a significant impact on the field of health and research. Given its relevance, studies have been performed on the effects of SARS-CoV-2 on human physiology. In particular, the possible presence and transmission of the virus through the male reproductive system could affect reproductive success. Knowing if the presence of the virus disrupts the organs responsible for the development and maturation of the cell lines involved in spermatogenesis could reveal its implications in sperm quality. For that reason, we proposed this review, in order to analyze the main scientific evidence on the effects of SARS-CoV-2 on the histophysiology of the male reproductive system and sperm fertilizing capacity.

Evaluation of SARS-CoV-2 in semen, seminal plasma, and spermatozoa pellet of COVID-19 patients in the acute stage of infection.

To date, there is limited information about the presence of SARS-CoV-2 in semen especially in the acute phase of the infection. While available data from cohort studies including a total of 342 patients in the acute or recovery phase of the infection are reassuring, one study mentioned detecting virus in the semen of 6/38 COVID-19 patients. Here we assessed SARS-CoV-2 presence in the semen of COVID-19 positive patients in the acute stage of infection, within 24 hours of the positive nasopharyngeal swabs. Semen, seminal plasma and spermatozoa pellet were screened for SARS-CoV-2 and manual or airborne contamination during semen sampling. Among the 32 COVID-19 volunteers, the median interval from the onset of symptoms to semen collection was 4 days [IQR: 0-8]. Only one presented positive SARS-CoV-2 PCR in semen and seminal plasma fractions, although the spermatozoa pellet was negative. Viral cultures were all negative. We observed slightly higher concentrations of bacterial DNA in the SARS-CoV-2 positive specimen than in all negative samples. The bacteria identified neither confirm nor rule out contamination by oropharyngeal secretions during collection. SARS-CoV-2 was rarely present in semen during the acute phase of the disease. This very rare situation could be connected to oral or manual contamination during semen collection. The possible presence of SARS-CoV-2 in semen calls for nasopharyngeal viral testing and strict hygiene protocols during semen collection before assisted reproductive attempts.

Does COVID-19 Worsen the Semen Parameters? Early Results of a Tertiary Healthcare Center.

INTRODUCTION: The coronavirus disease 2019 (COVID-19) is a global pandemic which may affect multiple organs and systems including testes and disrupt the gonadal functions. The current study aimed to evaluate the effect of COVID-19 on the semen parameters and sex-related hormone levels in infertile men. METHODS: The study included 21 patients who were evaluated in Ankara City Hospital, Andrology Clinic, for male infertility and have had the diagnosis of COVID-19. All the patients were evaluated in terms of semen parameters. The follicle-stimulating hormone, luteinizing hormone, and testosterone (T) levels were also evaluated in 8 of the patients. The results were presented through 2 dependent group analyses, based on the data of the patients collected before and after the diagnosis of COVID-19. RESULTS: None of the patients needed to be hospitalized at any time through the course of COVID-19. There was a significant decrease in semen volume, percentage of total motility, percentage of progressive motility, and normal sperm morphology after COVID-19 (3 [1-8] vs. 2.5 [1.5-5], p = 0.005; 48.6 ± 22.1 vs. 34.7 ± 20.7, p = 0.001; 35.1 ± 21.7 vs. 21.8 ± 15.9, p < 0.001; 6 [3-24] vs. 5 [3-18], p = 0.015; respectively). There was also a significant decline in T level of the patients after the diagnosis of COVID-19 (350.1 ± 115.5 vs. 289.8 ± 103.3, p = 0.009). CONCLUSION: COVID-19 may have unfavorable effects on the gonadal functions and may lead to further deterioration of the semen parameters in infertile men, which should be considered through the evaluation for infertility.

SARS-CoV-2 vs. human gametes, embryos and cryopreservation.

The COVID-19 pandemic, caused by the SARS-CoV-2 virus, is an unprecedented global situation, and all countries have adopted their own measurements to mitigate the spread of the virus in the first as well as in the subsequent waves of infection. All measures, especially in the first wave of the pandemic, were in combination with recommendations provided by professional and scientific organizations. Similar measures were applied to specific procedures, such as the management of infertility, including in vitro fertilization-embryo transfer (IVF-ET) treatments. Although there is no clear scientific evidence yet that the SARS-CoV-2 may exert negative effects on IVF outcome, especially at the early stages, several clinical reports indicate that the virus may impact male fertility through specific receptors presented at the somatic cells of the testis and used by the virus in order to gain entry to the respective cells. Nevertheless, it is not unreasonable to suspect that the virus may affect sperm function as well as oocyte performance directly through specific receptors or indirectly through other signaling pathways. Despite the good practice of IVF laboratory techniques, culture media may also be contaminated during equilibration when airborne virus's particles can contaminate culture media from an already infected embryology area or staff. Furthermore, although there is no clinical evidence, liquid nitrogen could be a route of infection for gametes and embryos when it has been contaminated during production or transportation. Therefore, cryopreservation of gametes and embryos must be virus-free. This communication aims to provide some aspects of the possible impact of the virus on gametes and embryos and how it may affect the cryopreservation procedures.Abbreviations: ACE2: angiotensin- converting enzyme 2; ART: assisted reproductive technology; ASRM: American Society for Reproductive Medicine; CDC: Centers for Disease Control and Prevention; COVID-19: coronavirus disease 2019; ESHRE: European Society of Human Reproduction and Embryology; ET: embryo transfer; FSH: follicle stimulating hormone; IFFS: International Federation of Fertility Societies; IVF: in vitro fertilization; LH: luteinizing hormone; LN: liquid nitrogen; SARS-CoV-2: severe acute respiratory syndrome coronavirus 2; T: testosterone; WHO: World Health Organization.

A cohort study of men infected with COVID-19 for presence of SARS-CoV-2 virus in their semen.

INTRODUCTION: Whether severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) can be detected in semen and transmitted sexually is a vital question that has, thus far, been inconclusive. Prior studies, with limited numbers, have included men in various stages of infection with most in the recovery phase of the illness. The timing of test results and severity of illness has made recruiting study participants a significant challenge. Our pilot study will examine semen from men with a recent diagnosis of COVID-19 as well as those in the convalescent phase to determine if SARS-CoV-2 can be detected and its relationship, if any, with the severity of the disease. METHODS: Eighteen men with a median age of 32 (range, 24-57) who tested positive for COVID-19 by rt-PCR analysis were enrolled and provided a semen sample. The study group demonstrated symptoms of COVID-19 ranging from asymptomatic to moderate and none required hospitalization. Samples were subjected to viral RNA extraction and then processed by real-time RT-PCR using the US Centers for Disease Control and Prevention (CDC, USA) panel of 2019-Novel Coronavirus (2019-nCoV) primers and probes to detect the presence of SARS-CoV-2 RNA. RESULTS: Length of time from diagnosis to providing a specimen ranged from 1 to 28 days (median, 6 days). Fifteen participants were symptomatic and three were asymptomatic, including recovering men, at the time of semen collection. No SARS-CoV-2 was detected in any of the semen samples. CONCLUSION: Based on these preliminary results and consistent with prior findings, we suggest SARS-CoV-2 is not present in semen during the acute or convalescent phase of COVID-19.

COVID-19 and male reproductive function: a prospective, longitudinal cohort study.

The existing evidence suggests that the human reproductive system may be potentially vulnerable to COVID-19 infection. However, little is known about the virus-host interaction of COVID-19 in sperm cells. We are the first to address the connection between changes in multiple seminal biomarkers and reproductive function in male patients recovering from COVID-19. In a prospective longitudinal cohort study, seminal ACE2 activity, markers of inflammation and oxidative stress, apoptotic variables, and semen quality parameters were evaluated at 10-day intervals for a maximum follow-up time of 60 days among male patients with laboratory-confirmed COVID-19 (n = 84) and healthy controls (CON; n = 105). At the baseline and the subsequent follow-ups, the COVID-19 group revealed significantly higher levels of seminal plasma ACE2 enzymatic activity, IL-1ß, IL-6, IL-8, IL-10, TGF-ß, TNF-α, IFN-α, IFN-γ, ROS, caspase-8, caspase-9, and caspase-3 activity as well as lower levels of SOD activity than those in the CON group (P < 0.05). These perturbations tended to persist over time and were correlated with significant impairments in semen volume, progressive motility, sperm morphology, sperm concentration, and the number of spermatozoa. We provide the direct experimental evidence that the male reproductive system could be targeted and damaged by the COVID-19 infection. These findings go beyond our current understanding of the disease, suggesting that the reproductive function of the patients recovering from the disease should be precisely followed and evaluated to detect and avoid more serious reproductive problems in the future, as they may develop a transient state of male subfertility like those with oligoasthenoteratozoospermia.

Evaluating the impact of COVID-19 on male reproduction.

Invasion or damage of the male reproductive system is one of the reported outcomes of viral infection. Current studies have documented that SARS-CoV-2, which causes COVID-19, can damage the male reproductive system in large part by inflammatory damage caused by a cytokine storm. However, whether SARS-CoV-2 can infect the human testis directly and enter semen is controversial. Other adverse effects of SARS-CoV-2 on male reproduction are also of concern and require comprehensive evaluation. Here, we analyze the invasiveness of SARS-CoV-2 in the testis and examine reported mechanisms by which SARS-CoV-2 interferes with male reproduction. Long-term implications of SARS-CoV-2 infection on male reproduction are also discussed. It should be emphasized that although COVID-19 may induce testicular damage, a substantial decrease in male reproductive capacity awaits clinical evidence. We propose that there is an urgent need to track male COVID-19 patients during their recovery. The development of suitable experimental models, including human reproductive organoids, will be valuable to further investigate the viral impact on reproduction for current and future pandemics.

Zika RNA and Flavivirus-Like Antigens in the Sperm Cells of Symptomatic and Asymptomatic Subjects.

Zika virus (ZIKV) RNA has been found to remain in human semen for up to one year after infection, but the presence of Flavivirus antigens in the different compartments of semen has been largely unexplored. Following the introduction of ZIKV in Nicaragua (2016), a prospective study of patients with clinical symptoms consistent with ZIKV was conducted in León to investigate virus shedding in different fluids. ZIKV infection was confirmed in 16 male subjects (≥18 years of age) by RT-qPCR in either blood, saliva or urine. Of these, three provided semen samples at 7, 14, 21, 28, 60 and 180 days postsymptom onset (DPSO) for Flavivirus antigens and RNA studies. These cases were compared with 19 asymptomatic controls. Flavivirus antigens were examined by immunofluorescence (IF) using the 4G2 Mabs, and confocal microscopy was used to explore fluorescence patterns. The three (100%) symptomatic subjects and 3 (16%) of the 19 asymptomatic subjects had Flavivirus antigens and viral RNA in the spermatozoa fraction. The percentage of IF Flavivirus-positive spermatozoa cells ranged from 1.9% to 25% in specimens from symptomatic subjects, as compared with 0.8% to 3.8% in specimens from asymptomatic controls. A marked IF-pattern in the cytoplasmic droplets and tail of the spermatozoa was observed. The sperm concentrations (45 × 106/mL vs. 63.5 × 106/mL, p = 0.041) and the total motility percentage (54% vs. 75%, p = 0.009) were significantly lower in specimens from ZIKV-positive than in those of ZIKV-negative. In conclusion, this study demonstrated the presence of Flavivirus antigens and RNA within a time frame of 28 DPSO in sperm cells of symptomatic and asymptomatic subjects during the ZIKV epidemic. These findings have implications for public health, in terms of nonarthropod-born, silent transmission facilitated by sperm cells and potential transmission from asymptomatic males to pregnant women, with consequences to the fetus.

Viral pathogenesis of SARS-CoV-2 infection and male reproductive health.

Coronavirus disease 2019 (COVID-19) has emerged as a new public health crisis, threatening almost all aspects of human life. Originating in bats, severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is transmitted to humans through unknown intermediate hosts, where it is primarily known to cause pneumonia-like complications in the respiratory system. Organ-to-organ transmission has not been ruled out, thereby raising the possibility of the impact of SARS-CoV-2 infection on multiple organ systems. The male reproductive system has been hypothesized to be a potential target of SARS-CoV-2 infection, which is supported by some preliminary evidence. This may pose a global threat to male fertility potential, as men are more prone to SARS-CoV-2 infection than women, especially those of reproductive age. Preliminary reports have also indicated the possibility of sexual transmission of SARS-CoV-2. It may cause severe complications in infected couples. This review focuses on the pathophysiology of potential SARS-CoV-2 infection in the reproductive organs of males along with their invasion mechanisms. The risks of COVID-19 on male fertility as well as the differences in vulnerability to SARS-CoV-2 infection compared with females have also been highlighted.

The Role of Male Reproductive Organs in the Transmission of African Swine Fever-Implications for Transmission.

African swine fever (ASF) has evolved from an exotic animal disease to a threat to global pig production. An important avenue for the wide-spread transmission of animal diseases is their dissemination through boar semen used for artificial insemination. In this context, we investigated the role of male reproductive organs in the transmission of ASF. Mature domestic boars and adolescent wild boars, inoculated with different ASF virus strains, were investigated by means of virological and pathological methods. Additionally, electron microscopy was employed to investigate in vitro inoculated sperm. The viral genome, antigens and the infectious virus could be found in all gonadal tissues and accessory sex glands. The viral antigen and viral mRNAs were mainly found in mononuclear cells of the respective tissues. However, some other cell types, including Leydig, endothelial and stromal cells, were also found positive. Using RNAScope, p72 mRNA could be found in scattered halo cells of the epididymal duct epithelium, which could point to the disruption of the barrier. No direct infection of spermatozoa was observed by immunohistochemistry, or electron microscopy. Taken together, our results strengthen the assumption that ASFV can be transmitted via boar semen. Future studies are needed to explore the excretion dynamics and transmission efficiency.

COVID-19 and human spermatozoa-Potential risks for infertility and sexual transmission?

As COVID-19 infections wreak havoc across the globe, attention has rightly been focused on the vital organ systems (lung, kidney and heart) that are vulnerable to viral attack and contribute to the acute pathology associated with this disease. However, we should not lose sight of the fact that COVID-19 will attack any cell type in the body expressing ACE2 - including human spermatozoa. These cells possess the entire repertoire of receptors (AT1R, AT2R, MAS) and ligand processing enzymes (ACE1 and ACE2) needed to support the angiotensin signalling cascade. The latter not only provides COVID-19 with a foothold on the sperm surface but may also promote integration, given the additional presence of a range of proteases (TMPRSS2, TMPRSS11B, TMPRSS12, furin) capable of promoting viral fusion. This article reviews the roles played by these various cellular constituents in maintaining the vitality of human spermatozoa and their competence for fertilization. The reproductive consequences of a viral attack on these systems, in terms of fertility and the risk of sexual transmission, are currently unknown. However, we should be alive to the possibility that there may be reproductive consequences of COVID-19 infection in young males that go beyond their capacity to survive a viral attack.

Prevention of HIV transmission with sperm washing within fertile serodiscordant couples undergoing non-stimulated intrauterine insemination.

The purpose of this prospective non-randomized study was to study the effectiveness of semen washing followed by intrauterine insemination (IUI) in Human Immune Deficiency Virus (HIV)-discordant couples in which the male partner was infected, in preventing HIV transmission to uninfected partner and offspring. The study was performed in a private assisted reproductive center specialized in couples with infectious diseases and enrolled sixty-nine fertile couples in which male partner tested positive for HIV, seeking for reproductive treatment. Triple sperm washing followed by viral RNA purification and real-time polymerase chain reaction was performed prior to IUI intervention. HIV transmission to female partner and newborns, and clinical pregnancy rate were the main outcome measures. A total of 180 IUI treatment cycles were performed in 69 couples. There were 16 clinical pregnancies (clinical pregnancy rate/cycle 9.0%, clinical pregnancy rate/patient 23.2%), one of which resulted in miscarriage (6.3%). No seroconversion was detected in the 69 women treated with sperm washing followed by IUI or in any of the newborns (tested at birth and at 3 months of age). Sperm washing followed by IUI is a safe and effective treatment option for serodiscordant couples wishing to conceive and to prevent HIV virus transmission to the mothers and newborns.

It is currently unknown whether SARS-CoV-2 is viable in semen or whether COVID-19 damages spermatozoa.

Research is needed to understand the presence of the SARS-CoV-2 virus in semen, sexual transmissibility, and impact on sperm quality. Several studies have examined men recovering from COVID-19, but large-scale community-based testing is needed to ascertain the effects on the male reproductive tract, and the potential for prolonged transmission.

SARS-COV-2 (Covid-19) and male fertility: Where are we?

Severe acute respiratory syndrome coronavirus 2 (SARS-COV-2), a single-stranded RNA virus, was found to be the causal agent of the disease called coronavirus disease. During December 2019, China informed the World Health Organization (WHO) of an outbreak of cases of pneumonia of unknown etiology, which caused severe-acute respiratory distress. The disease was termed coronavirus disease 2019 (Covid-19). Due to alarming levels of spread and severity, on the 11th of March 2020, the WHO declared the outbreak as a global pandemic. As of September 14, 2020, more than 29 million cases have been reported, with over 900,000 deaths globally. Since the outbreak, although not conclusive, discoveries have been made regarding the understanding of the epidemiology, etiology, clinical features, clinical treatment, and prevention of the disease. SARS-COV-2 has been detected in saliva, respiratory fluids, blood, urine, and faeces. Findings are however controversial regarding its presence in the semen or the testis. Hence, this review aimed to further analyse the literature concerning (i) the effects of previously identified human coronaviruses on male fertility (ii) the impact of Covid-19 on male fertility and (iii) the implication for general health in terms of infection and transmission.

Host microRNAs regulate expression of hepatitis B virus genes during transmission from patients' sperm to embryo.

Human sperm nucleus contains diverse RNA populations. This study aimed to screen and identify host microRNAs (miRs) that regulate gene expression of hepatitis B virus (HBV) during transmission from patients' sperm to sperm-derived embryos. Using microarrays, 336 miRs were found to be differentially expressed. After validation using real-time quantitative RT-PCR (RT-qPCR), four miRs were selected as targets. Using RT-qPCR and enzyme-linked immunosorbent assays, when patients' sperm were treated with mimics (or inhibitors) specific for hsa-miR-19a-3p and hsa-miR-29c-3p, the S gene transcription in sperm and translation in sperm-derived embryos was downregulated (or upregulated). There were significant differences in transcriptional and translational levels of the S gene between the test and control groups. These findings suggest that hsa-miR-19a-3p and hsa-miR-29c-3p significantly suppressed expression of the S gene, offering potential therapeutic targets for treating patients with HBV infection, and further reducing the negative impact of HBV infection on sperm fertilizing capacity.

Hyaluronidase-based swim-up for semen selection in patients with human papillomavirus semen infection.

Infection by human papillomavirus (HPV) represents one of the most common sexually transmitted diseases in both men and women worldwide. Recently, the detection of HPV virions in the semen of a large percentage of sexually active men has been associated with detrimental effects on both sperm parameters and on assisted reproductive technologies (ART) treatment outcomes. Conventional semen washing procedure used in ART have proved to be ineffective in removing HPV bound to sperm, requiring the identification of more effective and specific methods. In the present study, we assessed the possible use of hyaluronidase for the detachment of HPV from sperm cell surface. Semen samples from five normozoospermic control subjects (CTRL) were incubated with HPV virus-like particles (HPV-VLP) and treated with hyaluronidase by both a modified swim-up procedure (M-SU) and single-cell approach (SCA). The treatment with hyaluronidase was associated with the complete loss of HPV-VLP signal on sperms by both M-SU and SCA. In addition, semen samples from 12 HPV-positive infertile patients were treated with hyaluronidase 80 IU/mL by M-SU, resulting in the complete loss of HPV-DNA signal from sperm surface. Finally, the possible impact of hyaluronidase treatment on sperm parameters was assessed on both sperms from the five CTRL subjects and on further five oligo-astheno-terato-zoospermic (OAT) patients, both HPV negative. The treatment with hyaluronidase was equally associated with a slight reduction of sperm viability and progressive motility in both CTRL and OAT. In conclusion, the treatment with hyaluronidase removed efficiently and safely HPV virions bound to spermatozoa.

Progress in research on the detection of the novel coronavirus in human samples of different groups.

OBJECTIVE: Among the illnesses that may develop from COVID-19, the disease caused by the novel coronavirus (SARS-CoV-2), is pneumonia, a severe acute respiratory infectious disease. SARS-CoV-2 continues to spread worldwide and has caused hundreds of thousands of deaths thus far and has disrupted the world economy. PATIENTS AND METHODS: This review summarized the reported distributions of SARS-CoV-2 in 13 biological samples of the human body, including nose, feces, sperm, tears, breast milk, cerebrospinal fluid, urine, organs, sputum, cell lines, bronchial brush, blood, throat, and bronchoalveolar lavage fluid. Moreover, this review briefly describes the detection of SARS-CoV-2 in human body samples of five other coronaviruses. CONCLUSIONS: This review offers several recommendations for controlling the spread of SARS-CoV-2 control, specifically, sample collection from suspected cases from foreign countries and risk assessment of imported special goods (biological materials).

Hepatitis B virus surface protein induces oxidative stress by increasing peroxides and inhibiting antioxidant defences in human spermatozoa.

Hepatitis B virus (HBV) infection may affect sperm motility in patients with HBV. HBV surface protein (HBs) decreases mitochondrial membrane potential, impairs motility and induces apoptotic-like changes in human spermatozoa. However, little is known about how human spermatozoa respond to reactive oxygen species (ROS; mainly peroxides) induced by HBs. In this study, HBs induced supraphysiological ROS levels in human spermatozoa and reduced the formation of 2-cell embryos (obtained from hamster oocytes and human spermatozoa). HBs induced a pre-apoptotic status in human spermatozoa, as well as antioxidant defences by increasing glutathione peroxidase 4 (GPX4) and peroxiredoxin 5 (PRDX5) levels. These results highlight the molecular mechanism responsible for the oxidative stress in human spermatozoa exposed to HBV and the antioxidant defence response involving GPX4 and PRDX5.

Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) and its effect on gametogenesis and early pregnancy.

SARS-CoV-2 infection and pregnancy has been the topic of hundreds of publications over the last several months; however, few studies have focused on the implications of infection in early pregnancy and reproductive tissues. Here, we analyzed available evidence pertaining to SARS-CoV-2 infection, in early pregnancy, and in reproductive tissues. We searched PubMed and Embase databases in accordance with guidelines of Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) for publications from inception to June 4, 2020. Four reviewers screened titles and abstracts and obtained full-text articles for analysis. Sixty-two studies were included in the review. Biological plausibility for infection with SARS-CoV-2 exists in testis, ovaries, and placenta as they express ACE2 receptor activity. In males, SARS-CoV-2 infection could lead to functional abnormalities leading to spermatogenic failure and male infertility. In females, an alteration of the ACE2 cascade via SARS-CoV-2 infection could lead to impairment in important follicular and luteal processes. There is also evidence of significant placental pathology in SARS-CoV-2 infection, but it is unclear what effects there may be for early pregnancy, though available data suggest less severe effects compared to other respiratory virus outbreaks. Further investigation is needed regarding SARS-CoV-2 in reproductive function and early pregnancy.

Endogenous retroviruses drive species-specific germline transcriptomes in mammals.

Gene regulation in the germline ensures the production of high-quality gametes, long-term maintenance of the species and speciation. Male germline transcriptomes undergo dynamic changes after the mitosis-to-meiosis transition and have been subject to evolutionary divergence among mammals. However, the mechanisms underlying germline regulatory divergence remain undetermined. Here, we show that endogenous retroviruses (ERVs) influence species-specific germline transcriptomes. After the mitosis-to-meiosis transition in male mice, specific ERVs function as active enhancers to drive germline genes, including a mouse-specific gene set, and bear binding motifs for critical regulators of spermatogenesis, such as A-MYB. This raises the possibility that a genome-wide transposition of ERVs rewired germline gene expression in a species-specific manner. Of note, independently evolved ERVs are associated with the expression of human-specific germline genes, demonstrating the prevalence of ERV-driven mechanisms in mammals. Together, we propose that ERVs fine-tune species-specific transcriptomes in the mammalian germline.