Effect of aging on male reproduction in zebrafish (Danio rerio)

[Mechanisms of the Effect of Maternal Age-Related Oocyte Aging on Fertility: Transcriptomic Sequencing Analysis of a Zebrafish Model]

Objective

Female fertility gradually decreases with the increase in women's age. The underlying reasons include the decline in the quantity and quality of oocytes. Oocyte aging is an important manifestation of the decline in oocyte quality, including in vivo oocyte aging before ovulation and in vitro oocyte aging after ovulation. Currently, few studies have been done to examine oocyte aging, and the relevant molecular mechanisms are not fully understood. Therefore, we used zebrafish as a model to investigate oocyte aging. Three different age ranges of female zebrafish were selected to mate with male zebrafish of the best breeding age. In this way, we studied the effects of maternal age-related oocyte aging on fertility and investigated the potential molecular mechanisms behind maternal age-related fertility decline.

Methods

Eight female zebrafish aged between 158 and 195 d were randomly selected for the 6-month age group (180±12) d, 8 female zebrafish aged between 330 and 395 d were randomly selected for the 12-month age group (360±22) d, and 8 female zebrafish aged between 502 and 583 d were randomly selected for the 18-month age group (540±26) d. Male zebrafish of (180±29) d were randomly selected from zebrafish aged between 158 and 195 d and mated with female zebrafish in each group. Each mating experiment included 1 female zebrafish and 1 male zebrafish. Zebrafish embryos produced by the mating experiments were collected and counted. The embryos at 4 hours post-fertilization were observed under the microscope, the total number of embryos and the number of unfertilized embryos were counted, and the fertilization rate was calculated accordingly. The numbers of malformed embryos and dead embryos were counted 24 hours after fertilization, and the rates of embryo malformation and mortality were calculated accordingly. The primary outcome measure was the embryo fertilization rate, and the secondary outcome measures were the number of embryos per spawn (the total number of embryos laid within 1.5 hours after the beginning of mating and reproduction of the zebrafish), embryo mortality, and embryo malformation rate. The outcome measures of each group were compared. The blastocyst embryos of female zebrafish from each group born after mating with male zebrafish in their best breeding period were collected for transcriptomics analysis. Fresh oocytes of female zebrafish in each group were collected for transcriptomics analysis to explore the potential molecular mechanisms of maternal age-related fertility decline.

Results

Compared with that of the 6-month group (94.9%±3.6%), the embryo fertilization rate of the 12-month group (92.3%±4.2%) showed no significant difference, but that of the 18-month group (86.8%±5.5%) decreased significantly (P<0.01). In addition, the fertilization rate in the 18-month group was significantly lower than that in the 12-month group (P<0.05). Compared with that of the 6-month group, the embryo mortality of the female zebrafish in the 12-month group and that in the 18-month group were significantly higher than that in the 6-month group (P<0.000 1, P<0.001). There was no significant difference in the number of embryos per spawn or in the embryo malformation rate among the three groups. The results of the transcriptomics analysis of blastocyst embryos showed that some genes, including dusp5, bdnf, ppip5k2, dgkg, aldh3a2a, acsl1a, hal, mao, etc, were differentially expressed in the 12-month group or the 18-month group compared with their expression levels in the 6-month group. According to the KEGG enrichment analysis, these differentially expressed genes (DEGs) were significantly enriched in the MAPK signaling pathway, the phosphatidylinositol signaling system, and the fatty acid degradation and histidine metabolism pathway (P<0.05). The analysis of the expression trends of the genes expressed differentially among the three groups (the 6-month group, the 12-month group, and the 18-month group in turn) showed that the gene expression trends of fancc, fancg, fancb, and telo2, which were involved in Fanconi anemia pathway, were statistically significant (P<0.05). In the results of oocyte transcriptomics analysis, the genes that were differentially expressed in the 12-month group or the 18-month group compared with the 6-month group were mainly enriched in cell adhesion molecules and the protein digestion and absorption pathway (P<0.05). The results of the trends of gene expression in the zebrafish oocytes of the three groups (the 6-month group, the 12-month group, and the 18-month group in turn) showed that three kinds of gene expression trends of declining fertility with growing maternal age had significant differences (P<0.05). Further analysis of the three significantly differential expression trends showed 51 DEGs related to mitochondria and 5 DEGs related to telomere maintenance and DNA repair, including tomm40, mpc2, nbn, tti1, etc.

Conclusion

With the increase in the maternal age of the zebrafish, the embryo fertilization rate decreased significantly and the embryo mortality increased significantly. In addition, with the increase in the maternal age of the zebrafish, the expression of mitochondria and telomere-related genes, such as tomm40, mpc2, nbn, and tti1, in female zebrafish oocytes decreased gradually. Maternal age may be a factor contributing to the decrease in oocyte fertilization ability and the increase in early embryo mortality. Maternal age-related oocyte aging affects the fertility and embryo development of the offspring.

Timeless or tainted? The effects of male ageing on seminal fluid

Reproductive ageing can occur due to the deterioration of both the soma and germline. In males, it has mostly been studied with respect to age-related changes in sperm. However, the somatic component of the ejaculate, seminal fluid, is also essential for maintaining reproductive function. Whilst we know that seminal fluid proteins (SFPs) are required for male reproductive success across diverse taxa, age-related changes in SFP quantity and composition are little understood. Additionally, only few studies have explored the reproductive ageing of the tissues that produce SFPs, and the resulting reproductive outcomes. Here we provide a systematic review of studies addressing how advancing male age affects the production and properties of seminal fluid, in particular SFPs and oxidative stress, highlighting many open questions and generating new hypotheses for further research. We additionally discuss how declines in function of different components of seminal fluid, such as SFPs and antioxidants, could contribute to age-related loss of reproductive ability. Overall, we find evidence that ageing results in increased oxidative stress in seminal fluid and a decrease in the abundance of various SFPs. These results suggest that seminal fluid contributes towards important age-related changes influencing male reproduction. Thus, it is essential to study this mostly ignored component of the ejaculate to understand male reproductive ageing, and its consequences for sexual selection and paternal age effects on offspring.

Disentangling the effects of male age and mating history: Contrasting effects of mating history on precopulatory mating behavior and paternity success

Many studies ask whether young or older males are better at acquiring mates. Even so, how age affects reproductive success is still poorly understood because male age and mating history are confounded in most studies: older males usually have more mating experience. To what extent does mating history rather than age explain variation in male mating success? And how do mating history and male age determine paternity when there is also postcopulatory sexual selection? Here, we experimentally manipulated the mating history of old and young males in the eastern mosquitofish (Gambusia holbrooki). We then recorded male mating behavior and share of paternity (1259 offspring from 232 potential sires) when they competed for mates and fertilizations. Old males, and males with no mating experience, spent significantly more time approaching females, and attempting to mate, than did young males and those with greater mating experience. Male age and mating history interacted to affect paternity: old males benefited from having previous mating experience, but young males did not. Our results highlight that the age-related changes in male reproductive traits and in paternity that have been described in many taxa may be partly attributable to male mating history and not simply to age itself.

Male age alone predicts paternity success under sperm competition when effects of age and past mating effort are experimentally separated

Older males often perform poorly under post-copulatory sexual selection. It is unclear, however, whether reproductive senescence is because of male age itself or the accumulated costs of the higher lifetime mating effort that is usually associated with male age. To date, very few studies have accounted for mating history and sperm storage when testing the effect of male age on sperm traits, and none test how age and past mating history influence paternity success under sperm competition. Here, we experimentally manipulate male mating history to tease apart its effects from that of age on ejaculate traits and paternity in the mosquitofish, Gambusia holbrooki. We found that old, naive males had more sperm than old, experienced males, while the reverse was true for young males. By contrast, neither male age nor mating history affected sperm velocity. Finally, using artificial insemination to experimentally control the number of sperm per male, we found that old males sired significantly more offspring than young males independently of their mating history. Our results highlight that the general pattern of male reproductive senescence described in many taxa may often be affected by two naturally confounding factors, male mating history and sperm age, rather than male age itself.

Effect of age on the mercury sensitivity of zebrafish (Danio rerio) sperm

The effect of age on the sensitivity of zebrafish sperm against mercury exposure was investigated in the present study. Although results of the use of sperm from mature individuals for toxicity tests have been published, there is no information about the exact age of the fish in some cases, which can affect the results. During the experiments, pooled sperm was stripped from males of 7, 12, or 18 months of age, divided into 5 sub-groups, diluted with different concentrations of Hg (0, 0.5, 1, 2.5, and 5 mg/L Hg), and incubated for 240 min. The motility parameters of sperm (progressive motility (%), curvilinear velocity (VCL)) were measured by a computer-assisted sperm analysis system, at 30, 120, and 240 min of exposure. Regarding the age, significant differences were found in PMOT (p = 0.0267) as well as in VCL (p = 0.0004) among the three different age groups. The different concentrations of Hg also caused significant differences. The most significant differences in PMOT were between the 7- and 18-month-old groups; these differences were observed at 0.5, 1 and 2.5 mg/L Hg at 30 min, at 0.5 and 1 mg/L at 120 min, as well as at 0.5 mg/L at 240 min. In VCL the most significant differences were found between the 7- and 12-month-old groups; significant differences were found at each tested concentration at 30 min as well as at 0.5 and 2.5 mg/L at 240 min. According to the results, the age of zebrafish negatively influences the sensitivity of its sperm. This may concern not only toxicology tests but many techniques in fish breeding where the sperm is treated before use (cryopreservation, pressure shock, etc.).

Effects of age on sperm quality metrics in endangered Mississippi gopher frogs (Lithobates sevosus) from captive populations used for controlled propagation and reintroduction efforts

A decline in sperm quality with age is a common prediction of senescence-based hypotheses and empirical studies. While widely studied across taxa, there is little known on the effect of ageing on sperm quality in amphibians, especially in captive populations used for controlled propagation and reintroduction efforts. Here, we investigated variation in sperm quality metrics (i.e., motility, concentration, and morphology) in the endangered Mississippi gopher frog (Lithobates sevosus) among males of three age categories using individuals from captive breeding populations housed at three different zoological institutions. Different aged males across the species expectant lifespan (1-9, 1-2, 3-4, and 8-9-year-old subcategories) were chosen in an attempt to identify an optimal breeding age relevant for captive breeding programs. Moreover, we explored and statistically controlled for potential differences in sperm quality which may be attributed to the type of induction hormones and source populations that differed among institutions. Results indicated that males of different ages did not differ in sperm motility or concentration. However, we did find that older males (8-9 years old) had significantly longer sperm than other age categories and younger males (1-2 years old) had significantly more atypical sperm than other age categories. Furthermore, we found no significant differences in any sperm quality metrics between the different induction hormones or source populations used at the different institutions. Within a captive breeding program, this information is especially valuable as our results indicate that males that have only recently sexually matured may not be ready to breed, while older males maintain sperm quality metrics presumably related to fertilization success.

A comparison of behavioral and reproductive parameters between wild-type, transgenic and mutant zebrafish: Could they all be considered the same "zebrafish" for reglementary assays on endocrine disruption?

Transgenic zebrafish models are efficiently used to study the effects of endocrine disrupting chemicals (EDC); thereby informing on their mechanisms of action. However, given the reported differences between zebrafish strains at the genetical, physiological and behavioral levels; care should be taken before using these transgenic models for EDC testing. In the present study, we undertook a set of experiments in different transgenic and/or mutant zebrafish strains of interest for EDC testing: casper, cyp19a1a-eGFP, cyp19a1a-eGFP-casper, cyp11c1-eGFP, cyp11c1-eGFP-casper. Some behavioral traits, and some biochemical and reproductive physiological endpoints commonly used in EDC testing were assessed and compared to those obtained in WT AB zebrafish to ensure that transgene insertion and/or mutations do not negatively modify basal reproductive physiology or behavior of the fish. Behavioral traits considered as anxiety and sociality have been monitored. Sociality was evaluated by monitoring the time spent near congeners in a shuttle box while anxiety was evaluated using the Novel tank diving test. No critical difference was observed between strains for either sociality or anxiety level. Concerning reproduction, no significant difference in the number of eggs laid per female, in the viability of eggs or in the female circulating VTG concentrations was noted between the 5 transgenic/mutants and the WT AB zebrafish studied. In summary, the transgene insertion and the mutations had no influence on the endpoints measured in basal conditions. These results were a prerequisite to the use of these transgenic/mutant models for EDC testing. Next step will be to determine the sensitivity of these biological models to chemical exposure to accurately validate their use in existing fish assays for EDC testing.

Thymus development in the zebrafish (Danio rerio) from an ecoimmunology perspective

The thymus is present in all gnathostome vertebrates and is an essential organ for the adaptive immune system via the generation of functional mature T-cells. Over the life span of mammals, the thymus undergoes morphological and functional alterations, including an age-related involution, which in humans starts in early life. Life history tradeoffs have been suggested as possible reasons for thymus involution. While in teleost fish, only a few studies have investigated alterations of thymus structure and function over different life stages, resulting in a fragmented database. Here, we investigated the thymus growth of zebrafish (Danio rerio) from early life, throughout puberty and reproductive stage, up to 1-year-old. We assessed thymus growth by histological and morphometric analyses and thymocyte numbers. Thymus function was assessed by measuring the transcripts of the thymocyte marker genes, ikaros, tcrα, and tcrδ. Additionally, we analyzed gonad maturity and tail homogenate vitellogenin concentrations to align thymus status with the status of the reproductive system. Our results showed that the zebrafish thymus, in contrast to the human thymus, grew strongly during early life and puberty but started to undergo involution when the fish reached the reproductive age. The involution was characterized by reduced thymus area and thymocyte number, altered histoarchitecture, and decreasing thymocyte marker gene transcript levels. Our findings suggest that age-related changes of the zebrafish thymus do exist and could be partly explained in terms of resource tradeoffs, but also in terms of the ontogenetically late development of a functional adaptive immune system in teleosts.

An experimental test to separate the effects of male age and mating history on female mate choice

Should females prefer older males as mates? Male survival to old age might indicate the presence of fitness-enhancing genes that increase offspring fitness. However, many correlational studies show that mating with older males can lower female fecundity and even reduce offspring fitness due to epigenetic or germline mutation effects. One problem in quantifying female choice based on male age is that age is usually confounded with mating history. This begs a question: Do females choose males based on their age or their mating history? The answer requires an experimental approach, but few such studies exist. Here, we test if experimentally induced variation in the mating history of old and young males (12-week difference in postmaturity age) affects female choice in the eastern mosquitofish (Gambusia holbrooki). To vary mating history, adult males were either allowed to freely mate with females for 3 weeks or they only had visual contact with females. Immediately thereafter, we ran four-choice mating trials, using association time, to test the effects of male age and mating history (2 × 2 design) on male attractiveness. Females did not show a clear preference for males based on either characteristic. This was not due to a lack of female choice: females spent significantly more time with larger males. In addition, female choice was significantly repeatable across four trials: twice as a virgin and twice as a nonvirgin. Finally, female mating status (virgin or nonvirgin) did not affect her choice of mate, although virgin females spent significantly more time associating with test males.

Evidence that fertility trades off with early offspring fitness as males age

Models of ageing predict that sperm function and fertility should decline with age as sperm are exposed to free radical damage and mutation accumulation. However, theory also suggests that mating with older males should be beneficial for females because survival to old age is a demonstration of a male's high genetic and/or phenotypic quality. Consequently, declines in sperm fitness may be offset by indirect fitness benefits exhibited in offspring. While numerous studies have investigated age-based declines in male fertility, none has taken the integrated approach of studying age-based effects on both male fertility and offspring fitness. Here, using a cohort-based longitudinal study of zebrafish (Danio rerio), we report a decline in male mating success and fertility with male age but also compensating indirect benefits. Using in vitro fertilization, we show that offspring from older males exhibit superior early survival compared to those from their youngest counterparts. These findings suggest that the high offspring fitness observed for the subset of males that survive to an old age (approx. 51% in this study) may represent compensating benefits for declining fertility with age, thus challenging widely held views about the fitness costs of mating with older males.

Characterization of assortative mating in medaka: Mate discrimination cues and factors that bias sexual preference

Somatolactin alpha (SLα) is a peptide hormone that regulates skin color, and SLα-deficient and SLα-excess strains have been established in medaka (Oryzias latipes). Their skin colors differ conspicuously and males prefer to mate with females from the same strain. Pre-mating sexual isolation in this model vertebrate provides an ideal platform for investigating the molecular mechanisms of mate choice. Thus, we studied the sensory cues utilized by these fish to discriminate the same and different strains. When males were given a choice under monochromatic light, where the skin colors differed only in terms of brightness but not in hue, mating occurred but it was not assortative. This suggests that: (1) a visual cue is essential for mate discrimination rather than odor or acoustic cues; (2) the visual cue is color and not shape, size, or motion; and (3) the color cue needs to be perceived as the relative balance of brightness at multiple wavelengths rather than the brightness at a specific wavelength. In addition, we introduced another skin-color mutation into the SLα-excess strain and found that this new strain and the original SLα-excess strain, which also overexpressed SLα but exhibited distinct skin colors, preferred different colors. This demonstrates that SLα is not a primary determinant of sexual preference. The symmetrically biased sexual preferences of the SLα-deficient and SLα-excess strains may be acquired postnatally depending on their individual skin color or that of tank mates.

Rest mutant zebrafish swim erratically and display atypical spatial preferences

The Rest/Nrsf transcriptional repressor modulates expression of a large set of neural specific genes. Many of these target genes have well characterized roles in nervous system processes including development, plasticity and synaptogenesis. However, the impact of Rest-mediated transcriptional regulation on behavior has been understudied due in part to the embryonic lethality of the mouse knockout. To investigate the requirement for Rest in behavior, we employed the zebrafish rest mutant to explore a range of behaviors in adults and larva. Adult rest mutants of both sexes showed abnormal behaviors in a novel environment including increased vertical swimming, erratic swimming patterns and a proclivity for the tank walls. Adult males also had diminished reproductive success. At 6 days post fertilization (dpf), rest mutant larva were hypoactive, but displayed normal evoked responses to light and sound stimuli. Overall, these results provide evidence that rest dysfunction produces atypical swimming patterns and preferences in adults, and reduced locomotor activity in larvae. This study provides the first behavioral analysis of rest mutants and reveals specific behaviors that are modulated by Rest.

Spinal deformity in aged zebrafish is accompanied by degenerative changes to their vertebrae that resemble osteoarthritis

Age-related degenerative changes within the vertebral column are a significant cause of morbidity with considerable socio-economic impact worldwide. An improved understanding of these changes through the development of experimental models may lead to improvements in existing clinical treatment options. The zebrafish is a well-established model for the study of skeletogenesis with significant potential in gerontological research. With advancing age, zebrafish frequently develop gross deformities of their vertebral column, previously ascribed to reduced trunk muscle tone. In this study, we assess degenerative changes specifically within the bone and cartilage of the vertebral column of zebrafish at 1, 2 and 3-years of age. We show increased frequency and severity of spinal deformities/curvatures with age. Underlying the most severe phenotypes are partial or complete vertebral dislocations and focal thickening of the vertebral bone at the joint margins. MicroCT examination demonstrates small defects, fractures and morphological evidence suggestive of bone erosion and remodeling (i.e. osteophytes) within the vertebrae during aging, but no significant change in bone density. Light and electron microscopic examination reveal striking age-related changes in cell morphology, suggestive of chondroptosis, and tissue remodelling of the vertebral cartilage, particularly within the pericellular micro-environment. Glycosaminoglycan analysis of the vertebral column by HPLC demonstrates a consistent, age-related increase in the yield of total chondroitin sulfate disaccharide, but no change in sulfation pattern, supported by immunohistochemical analysis. Immunohistochemistry strongly identifies all three chondroitin/dermatan sulphate isoforms (C-0-S, C-4-S/DS and C-6-S) within the vertebral cartilage, particularly within the pericellular micro-environment. In contrast, keratan sulfate immunolocalises specifically with the notochordal tissue of the intervertebral disc, and its labelling diminishes with age. In summary, these observations raise the prospect that zebrafish, in addition to modelling skeletal development, may have utility in modelling age-related degenerative changes that affect the skeleton during senescence.

Male reproductive traits of full-sibs of different age classes in three-spined sticklebacks (Gasterosteus aculeatus)

The process of ageing is associated with negative effects of mutations acting late in life, which range from those affecting cells to those affecting the whole organism. In many animal taxa, the deterioration of the phenotype with age also affects traits such as males’ primary and secondary sexual characteristics. In three-spined sticklebacks (Gasterosteus aculeatus), males usually reproduce at one or two years of age. To see whether sexual selection has the potential to differ between young and old males, full-sib brothers of different age classes were compared, which were bred and raised under standardised laboratory conditions. During two simulated, successive breeding seasons males were allowed to build their nest in single tanks either in the first (“young males”) or in the second (“old males”) breeding season. A comparison of reproductively active brothers of the first and second breeding season showed that older males produce more but smaller sperm, which might be of lower quality. The fact that older males stored more sperm is size dependent as the results show that larger males possess a greater absolute testis mass, which is inextricably linked to sperm number. However, independent of body size, old males had a lower intensity of red/orange and UV breeding coloration as well as a reduced testis melanisation, which might have consequences in female mate choice and sperm competition.

GH overexpression decreases spermatic parameters and reproductive success in two-years-old transgenic zebrafish males

Growth hormone (GH) transgenesis has been postulated as a biotechnological tool for improving growth performance in fish aquaculture. However, GH is implied in several other physiological processes, and transgenesis-induced GH excess could lead to unpredictable collateral effects, especially on reproductive traits. Here, we have used two-years-old transgenic zebrafish males to evaluate the effects of GH-transgenesis on spermatic parameters and reproductive success. Transgenic spermatozoa were analyzed in terms of motility, motility period, membrane integrity, mitochondrial functionality, DNA integrity, fertility and hatching rate. We have also performed histological analyses in gonad, in order to verify the presence of characteristic cell types from mature testes. The results obtained have shown that, even in transgenic testes present in all cells in normal mature gonads, a significant general decrease was observed in all spermatic and reproductive parameters analyzed. These outcomes raise concerns about the viability of GH-transgenesis appliance to aquaculture and the environmental risks at the light of Trojan gene hypothesis.

Comparison of neurobehavioral effects of methylmercury exposure in older and younger adult zebrafish (Danio rerio)

It is widely recognized that the nature and severity of responses to toxic exposure are age-dependent. Using active avoidance conditioning as the behavioral paradigm, the present study examined the effect of short-term methylmercury (MeHg) exposure on two adult age classes, 1- and 2-year-olds to coincide with zebrafish in relatively peak vs. declining health conditions. In Experiment 1, 2-year-old zebrafish were randomly divided into groups and were exposed to no MeHg, 0.15% ethanol (EtOH), 0.01, 0.03, 0.1, or 0.3 μM of MeHg (in 0.15% ethanol) for 2 weeks. The groups were then trained and tested for avoidance responses. The results showed that older zebrafish exposed to no MeHg or EtOH learned and retained avoidance responses. However, 0.01 μM or higher concentrations of MeHg exposure impaired avoidance learning in a dose-dependent manner with 0.3 μM of MeHg exposure producing death during the exposure period or shortly after the exposure but before the avoidance training. In Experiment 2, 1-year-old zebrafish were randomly divided into groups and were exposed to the same concentrations of MeHg used in Experiment 1 for 2 weeks. The groups were then trained and tested for avoidance responses. The results showed that younger zebrafish exposed to no MeHg, EtOH, or 0.01 μM of MeHg learned and retained avoidance responses, while 0.1 or 0.3 μM of MeHg exposure impaired avoidance learning in a dose-dependent manner. The study suggested that MeHg exposure produced learning impairments at a much lower concentration of MeHg exposure and more severely in older adult compared against younger adult zebrafish even after short exposure times.

The genetic pleiotropy of musculoskeletal aging

Musculoskeletal aging is detrimental to multiple bodily functions and starts early, probably in the fourth decade of an individual's life. Sarcopenia is a health problem that is expected to only increase as a greater portion of the population lives longer; prevalence of the related musculoskeletal diseases is similarly expected to increase. Unraveling the biological and biomechanical associations and molecular mechanisms underlying these diseases represents a formidable challenge. There are two major problems making disentangling the biological complexity of musculoskeletal aging difficult: (a) it is a systemic, rather than "compartmental," problem, which should be approached accordingly, and (b) the aging per se is neither well defined nor reliably measurable. A unique challenge of studying any age-related condition is a need of distinguishing between the "norm" and "pathology," which are interwoven throughout the aging organism. We argue that detecting genes with pleiotropic functions in musculoskeletal aging is needed to provide insights into the potential biological mechanisms underlying inter-individual differences insusceptibility to the musculoskeletal diseases. However, exploring pleiotropic relationships among the system's components is challenging both methodologically and conceptually. We aimed to focus on genetic aspects of the cross-talk between muscle and its "neighboring" tissues and organs (tendon, bone, and cartilage), and to explore the role of genetics to find the new molecular links between skeletal muscle and other parts of the "musculoskeleton." Identification of significant genetic variants underlying the musculoskeletal system's aging is now possible more than ever due to the currently available advanced genomic technologies. In summary, a "holistic" genetic approach is needed to study the systems's normal functioning and the disease predisposition in order to improve musculoskeletal health.

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.