EVOLUTION: Aging and Sexual Conflict

Polystyrene nanoplastics exposure triggers spermatogenic cell senescence via the Sirt1/ROS axis

Polystyrene nanoplastics (PS-NPs) have been reported to accumulate in the testes and constitute a new threat to reproductive health. However, the exact effects of PS-NPs exposure on testicular cells and the underlying mechanisms remain largely unknown. The C57BL/6 male mice were orally administered with PS-NPs (80 nm) at different dosages (0, 10, and 40 mg/kg/day) for 60 days, and GC-1 cells were treated with PS-NPs in this study. Enlarged seminiferous tubule lumens and a loose and vacuolated layer of spermatogenic cells were observed in PS-NPs-exposed mice. Spermatogenic cells which may be one of the target cells for this reproductive damage, were decreased in the mice from PS-NPs group. PS-NPs caused spermatogenic cells to undergo senescence, manifested as elevated SA-β-galactosidase activity and activated senescence-related signaling p53-p21/Rb-p16 pathways, and induced cell cycle arrest. Mechanistically, Gene Ontology (GO) enrichment suggested the key role of reactive oxygen species (ROS) in PS-NPs-induced spermatogenic cell senescence, and this result was confirmed by measuring ROS levels. Moreover, ROS inhibition partially attenuated the senescence phenotype of spermatogenic cells and DNA damage. Using the male health atlas (MHA) database, Sirt1 was filtrated as the critical molecule in the regulation of testicular senescence. PS-NPs induced overexpression of the main ROS generator Nox2, downregulated Sirt1, increased p53 and acetylated p53 in vivo and in vitro, whereas these disturbances were partially restored by pterostilbene. In addition, pterostilbene intervention significantly alleviated the PS-NPs-induced spermatogenic cell senescence and attenuated ROS burst. Collectively, our study reveals that PS-NPs exposure can trigger spermatogenic cell senescence mediated by p53-p21/Rb-p16 signaling by regulating the Sirt1/ROS axis. Importantly, pterostilbene intervention may be a promising strategy to alleviate this damage.

Effects of Methionine Restriction from Different Sources on Sperm Quality in Aging Mice

Decreased sperm quality causing poor pregnancy outcomes in aging males is a common problem. The aim of this study was to investigate the ameliorative effect of methionine restriction on sperm quality in aging mice, using methionine or 2-hydroxy-4-(methylthio)butanoate (HMTBA) as the methionine source, with a view to providing nutritional strategies to mitigate the decline in sperm quality in aging livestock. Fifty-one 6-week-old male mice were randomly divided into four groups: the non-aging group (NA, 0.86% methionine), the control diet group (CD, 0.86% methionine), the methionine-restricted group (MR, 0.17% methionine) and the HMTBA-restricted group (HR, 0.17% methionine). The mice in the CD, MR and HR groups were injected with a daily dose of 0.25 mL/20 g body weight of 10% D-galactose to establish an aging model. The test period was 42 days. The results showed that aging mice in the CD group had impaired testicular morphology and significantly decreased sperm quality compared to those in the NA group. Aging mice in the MR and HR groups showed attenuated impaired testicular morphology and improved sperm quality, especially sperm acrosomal integrity and membrane integrity, compared to mice in the CD group. In addition, mice in the MR and HR groups had reduced testicular inflammation and oxidative stress, increased spermidine levels, and reduced sperm RNA N6-methyladenosine (m6A) and DNA 5-methylcytosine (5mC) levels. Spermidine levels were positively correlated, whereas sperm RNA m6A and DNA 5mC levels were negatively correlated with sperm quality parameters. Our study suggests that methionine restriction alleviates the decline in sperm quality in aging mice, which may be related to changes in methionine metabolism and inhibition of sperm DNA and RNA methylation.

Longitudinal covariation of testosterone and sperm quality across reproductive stages in the zebra finch

Birds that breed opportunistically maintain partial activation of reproductive systems to rapidly exploit environmental conditions when they become suitable for breeding. Maintaining reproductive systems outside of a breeding context is costly. For males, these costs are thought to include continual exposure to testosterone. Males of seasonally breeding birds minimise these costs by downregulating testosterone production outside of a breeding context. Opportunistically breeding birds trade off the need to rapidly initiate reproduction with the costs of elevated testosterone production. One way opportunistically breeding males could minimise these costs is through fine scale changes in testosterone production across discrete reproductive stages which have a greater or lesser requirement for active sperm production. Although spermatogenesis broadly depends on testosterone production, whether changes in testosterone levels across the reproductive stages affect sperm quality and production is unknown. Here, we measured testosterone, sperm quality, and body condition in male zebra finches at discrete stages within reproductive bouts (egg laying, incubation, nestling provisioning, and fledging) and across two consecutive reproductive events in captive male zebra finches (Taeniopygia castanotis). We also examined associations between male testosterone, sperm quality/production, body condition, and nestling body condition. We found that testosterone levels varied across discrete reproductive stages with the lowest levels during incubation and the highest following chick fledging. Testosterone levels were positively associated with sperm velocity and the proportion of motile sperm but were not associated with male body condition. We found no associations between paternal body condition, testosterone levels, or sperm traits with nestling body condition (a proxy for the reproductive quality of a male and his partner). This study is the first to show that opportunistically breeding males vary testosterone synthesis and sperm traits at discrete stages within a reproductive event.

Condition-dependent mortality exacerbates male (but not female) reproductive senescence and the potential for sexual conflict

Disentangling the relationship between age and reproduction is central to understand life-history evolution, and recent evidence shows that considering condition-dependent mortality is a crucial piece of this puzzle. For example, nonrandom mortality of 'low-condition' individuals can lead to an increase in average lifespan. However, selective disappearance of such low-condition individuals may also affect reproductive senescence at the population level due to trade-offs between physiological functions related to survival/lifespan and the maintenance of reproductive functions. Here, we address the idea that condition-dependent extrinsic mortality (i.e. simulated predation) may increase the age-related decline in male reproductive success and with it the potential for sexual conflict, by comparing reproductive ageing in Drosophila melanogaster male/female cohorts exposed (or not) to condition-dependent simulated predation across time. Although female reproductive senescence was not affected by predation, male reproductive senescence was considerably higher under predation, due mainly to an accelerated decline in offspring viability of 'surviving' males with age. This sex-specific effect suggests that condition-dependent extrinsic mortality can exacerbate survival-reproduction trade-offs in males, which are typically under stronger condition-dependent selection than females. Interestingly, condition-dependent extrinsic mortality did not affect mating success, hinting that accelerated reproductive senescence is due to a decrease in male post-copulatory fitness components. Our results support the recent proposal that male ageing can be an important source of sexual conflict, further suggesting this effect could be exacerbated under more natural conditions.

Reproductive senescence: new perspectives in the wild

According to recent empirical studies, reproductive senescence, the decline in reproductive success with increasing age, seems to be nearly ubiquitous in the wild. However, a clear understanding of the evolutionary causes and consequences of reproductive senescence is still lacking and requires new and integrative approaches. After identifying the sequential and complex nature of female reproductive senescence, we show that the relative contributions of physiological decline and alterations in the efficiency of parental care to reproductive senescence remain unknown and need to be assessed in the light of current evolutionary theories of ageing. We demonstrate that, although reproductive senescence is generally studied only from the female viewpoint, age-specific female reproductive success strongly depends on male-female interactions. Thus, a reduction in male fertilization efficiency with increasing age has detrimental consequences for female fitness. Lastly, we call for investigations of the role of environmental conditions on reproductive senescence, which could provide salient insights into the underlying sex-specific mechanisms of reproductive success. We suggest that embracing such directions should allow building new bridges between reproductive senescence and the study of sperm competition, parental care, mate choice and environmental conditions.

Are old males still good males and can females tell the difference? Do hidden advantages of mating with old males off-set costs related to fertility, or are we missing something else?

Sperm function generally declines with male age. Paradoxically, females of many species still choose to mate with old males rather than young males. Females choosing old mates may suffer reduced fertilization rates and an increased incidence of birth defects in offspring, lowering fitness which may in turn lead to conflict between the sexes. This apparent paradox has generated much interest from theorists, but whether this paradox presents in nature remains equivocal. Empirical studies have found mixed support for both a decline in fertility with male age and age-based female mate preference. Here, we examine recent evidence for this paradox, identify confounding variables, highlight areas that deserve further investigation, and suggest avenues for future research.

Multiple deleterious effects of experimentally aged sperm in a monogamous bird

Sperm aging is known to be detrimental to reproductive performance. However, this apparently general phenomenon has seldom been studied in an evolutionary context. The negative impact of sperm aging on parental fitness should constitute a strong selective pressure for adaptations to avoid its effects. We studied the impact of sperm aging on black-legged kittiwakes (Rissa tridactyla), a monogamous seabird. Kittiwakes comprise a model system because (i) of evidence that females eject their mates' sperm to prevent fertilization by sperm that would be old and degraded by the time of fertilization and result in reduced reproductive performance and (ii) the lack of extra-pair fertilization in this species makes cryptic female choice an unlikely explanation of postcopulatory sperm ejection by females. We experimentally manipulated the age of the sperm fertilizing kittiwake eggs by fitting males with anti-insemination rings for variable periods of time preceding egg-laying. We found evidence that sperm aging negatively affected four sequential stages of reproduction: fertilization potential, rate of embryonic development, embryonic mortality, and chick condition at hatching. These results may be produced by a continuum of a single process of sperm aging that differentially affects various aspects of development, depending on the degree of damage incurred to the spermatozoa. The marked impact of sperm age on female fitness may thus drive postcopulatory sperm ejection by females. These results provide experimental evidence of deleterious effects of sperm aging on a nondomestic vertebrate, underlining its taxonomic generality and its potential to select for a wide array of adaptations.

Evolution of female preference for younger males

Previous theoretical work has suggested that females should prefer to mate with older males, as older males should have higher fitness than the average fitness of the cohort into which they were born. However, studies in humans and model organisms have shown that as males age, they accumulate deleterious mutations in their germ-line at an ever-increasing rate, thereby reducing the quality of genes passed on to the next generation. Thus, older males may produce relatively poor-quality offspring. To better understand how male age influences female mate preference and offspring quality, we used a genetic algorithm model to study the effect of age-related increases in male genetic load on female mate preference. When we incorporate age-related increases in mutation load in males into our model, we find that females evolve a preference for younger males. Females in this model could determine a male's age, but not his inherited genotype nor his mutation load. Nevertheless, females evolved age-preferences that led them to mate with males that had low mutation loads, but showed no preference for males with respect to their somatic quality. These results suggest that germ-line quality, rather than somatic quality, should be the focus of female preference in good genes models.