The effects of ageing on the meiotic chromosomes of male and female mice

Maternal obesity enhances oocyte chromosome abnormalities associated with aging

Obesity is increasing globally, and maternal obesity has adverse effects on pregnancy outcomes and the long-term health of offspring. Maternal obesity has been associated with pregnancy failure through impaired oogenesis and embryogenesis. However, whether maternal obesity causes chromosome abnormalities in oocytes has remained unclear. Here we show that chromosome abnormalities are increased in the oocytes of obese mice fed a high-fat diet and identify weakened sister-chromatid cohesion as the likely cause. Numbers of full-grown follicles retrieved from obese mice were the same as controls and the efficiency of in vitro oocyte maturation remained high. However, chromosome abnormalities presenting in both metaphase-I and metaphase-II were elevated, most prominently the premature separation of sister chromatids. Weakened sister-chromatid cohesion in oocytes from obese mice was manifested both as the terminalization of chiasmata in metaphase-I and as increased separation of sister centromeres in metaphase II. Obesity-associated abnormalities were elevated in older mice implying that maternal obesity exacerbates the deterioration of cohesion seen with advancing age.

Shugoshin protects centromere pairing and promotes segregation of nonexchange partner chromosomes in meiosis

Faithful chromosome segregation during meiosis I depends upon the formation of connections between homologous chromosomes. Crossovers between homologs connect the partners, allowing them to attach to the meiotic spindle as a unit, such that they migrate away from one another at anaphase I. Homologous partners also become connected by pairing of their centromeres in meiotic prophase. This centromere pairing can promote proper segregation at anaphase I of partners that have failed to become joined by a crossover. Centromere pairing is mediated by synaptonemal complex (SC) proteins that persist at the centromere when the SC disassembles. Here, using mouse spermatocyte and yeast model systems, we tested the role of shugoshin in promoting meiotic centromere pairing by protecting centromeric synaptonemal components from disassembly. The results show that shugoshin protects the centromeric SC in meiotic prophase and, in anaphase, promotes the proper segregation of partner chromosomes that are not linked by a crossover.

Variation in recombination frequency and distribution across eukaryotes: patterns and processes

Recombination, the exchange of DNA between maternal and paternal chromosomes during meiosis, is an essential feature of sexual reproduction in nearly all multicellular organisms. While the role of recombination in the evolution of sex has received theoretical and empirical attention, less is known about how recombination rate itself evolves and what influence this has on evolutionary processes within sexually reproducing organisms. Here, we explore the patterns of, and processes governing recombination in eukaryotes. We summarize patterns of variation, integrating current knowledge with an analysis of linkage map data in 353 organisms. We then discuss proximate and ultimate processes governing recombination rate variation and consider how these influence evolutionary processes. Genome-wide recombination rates (cM/Mb) can vary more than tenfold across eukaryotes, and there is large variation in the distribution of recombination events across closely related taxa, populations and individuals. We discuss how variation in rate and distribution relates to genome architecture, genetic and epigenetic mechanisms, sex, environmental perturbations and variable selective pressures. There has been great progress in determining the molecular mechanisms governing recombination, and with the continued development of new modelling and empirical approaches, there is now also great opportunity to further our understanding of how and why recombination rate varies.This article is part of the themed issue 'Evolutionary causes and consequences of recombination rate variation in sexual organisms'.

Meiotic recombination gets stressed out: CO frequency is plastic under pressure

Meiotic recombination ensures the fertility of gametes and creates novel genetic combinations. Although meiotic crossover (CO) frequency is under homeostatic control, CO frequency is also plastic in nature and can respond to environmental conditions. Most investigations have focused on temperature and recombination, but other external and internal stimuli also have important roles in modulating CO frequency. Even less is understood about the molecular mechanisms that underly these phenomenon, but recent work has begun to advance our knowledge in this field. In this review, we identify and explore potential mechanisms including changes in: the synaptonemal complex, chromatin state, DNA methylation, and RNA splicing.

Bivalent separation into univalents precedes age-related meiosis I errors in oocytes

The frequency of chromosome segregation errors during meiosis I (MI) in oocytes increases with age. The two-hit model suggests that errors are caused by the combination of a first hit that creates susceptible crossover configurations and a second hit comprising an age-related reduction in chromosome cohesion. This model predicts an age-related increase in univalents, but direct evidence of this phenomenon as a major cause of segregation errors has been lacking. Here, we provide the first live analysis of single chromosomes undergoing segregation errors during MI in the oocytes of naturally aged mice. Chromosome tracking reveals that 80% of the errors are preceded by bivalent separation into univalents. The set of the univalents is biased towards balanced and unbalanced predivision of sister chromatids during MI. Moreover, we find univalents predisposed to predivision in human oocytes. This study defines premature bivalent separation into univalents as the primary defect responsible for age-related aneuploidy.

Evidence for paternal age-related alterations in meiotic chromosome dynamics in the mouse

Increasing age in a woman is a well-documented risk factor for meiotic errors, but the effect of paternal age is less clear. Although it is generally agreed that spermatogenesis declines with age, the mechanisms that account for this remain unclear. Because meiosis involves a complex and tightly regulated series of processes that include DNA replication, DNA repair, and cell cycle regulation, we postulated that the effects of age might be evident as an increase in the frequency of meiotic errors. Accordingly, we analyzed spermatogenesis in male mice of different ages, examining meiotic chromosome dynamics in spermatocytes at prophase, at metaphase I, and at metaphase II. Our analyses demonstrate that recombination levels are reduced in the first wave of spermatogenesis in juvenile mice but increase in older males. We also observed age-dependent increases in XY chromosome pairing failure at pachytene and in the frequency of prematurely separated autosomal homologs at metaphase I. However, we found no evidence of an age-related increase in aneuploidy at metaphase II, indicating that cells harboring meiotic errors are eliminated by cycle checkpoint mechanisms, regardless of paternal age. Taken together, our data suggest that advancing paternal age affects pairing, synapsis, and recombination between homologous chromosomes--and likely results in reduced sperm counts due to germ cell loss--but is not an important contributor to aneuploidy.

Couples, pairs, and clusters: mechanisms and implications of centromere associations in meiosis

Observations of a wide range of organisms show that the centromeres form associations of pairs or small groups at different stages of meiotic prophase. Little is known about the functions or mechanisms of these associations, but in many cases, synaptonemal complex elements seem to play a fundamental role. Two main associations are observed: homology-independent associations very early in the meiotic program-sometimes referred to as centromere coupling-and a later association of homologous centromeres, referred to as centromere pairing or tethering. The later centromere pairing initiates during synaptonemal complex assembly, then persists after the dissolution of the synaptonemal complex. While the function of the homology-independent centromere coupling remains a mystery, centromere pairing appears to have a direct impact on the chromosome segregation fidelity of achiasmatic chromosomes. Recent work in yeast, Drosophila, and mice suggest that centromere pairing is a previously unappreciated, general meiotic feature that may promote meiotic segregation fidelity of the exchange and non-exchange chromosomes.

Synaptonemal complex components persist at centromeres and are required for homologous centromere pairing in mouse spermatocytes

Recent studies in simple model organisms have shown that centromere pairing is important for ensuring high-fidelity meiotic chromosome segregation. However, this process and the mechanisms regulating it in higher eukaryotes are unknown. Here we present the first detailed study of meiotic centromere pairing in mouse spermatogenesis and link it with key events of the G2/metaphase I transition. In mouse we observed no evidence of the persistent coupling of centromeres that has been observed in several model organisms. We do however find that telomeres associate in non-homologous pairs or small groups in B type spermatogonia and pre-leptotene spermatocytes, and this association is disrupted by deletion of the synaptonemal complex component SYCP3. Intriguingly, we found that, in mid prophase, chromosome synapsis is not initiated at centromeres, and centromeric regions are the last to pair in the zygotene-pachytene transition. In late prophase, we first identified the proteins that reside at paired centromeres. We found that components of the central and lateral element and transverse filaments of the synaptonemal complex are retained at paired centromeres after disassembly of the synaptonemal complex along diplotene chromosome arms. The absence of SYCP1 prevents centromere pairing in knockout mouse spermatocytes. The localization dynamics of SYCP1 and SYCP3 suggest that they play different roles in promoting homologous centromere pairing. SYCP1 remains only at paired centromeres coincident with the time at which some kinetochore proteins begin loading at centromeres, consistent with a role in assembly of meiosis-specific kinetochores. After removal of SYCP1 from centromeres, SYCP3 then accumulates at paired centromeres where it may promote bi-orientation of homologous centromeres. We propose that, in addition to their roles as synaptonemal complex components, SYCP1 and SYCP3 act at the centromeres to promote the establishment and/or maintenance of centromere pairing and, by doing so, improve the segregation fidelity of mammalian meiotic chromosomes.

[Correlation analysis between meiotic recombination frequencies and age in human spermatocyte]

Faithful meiotic recombination is essential for the segregation of homologous chromosomes and the formation of normal haploid gametes. Little is known about the mechanism of meiotic recombination in human germ cells. MLHl (a DNA mismatch repair protein) foci on synaptonemal complexes (SCs) at prophase I of meiosis can be used to examine recombination frequency. In 10 fertile men, the mean number of MLH1 foci per cell in all donors was 49.4 with a range from 33 to 63. There was significant variation in the recombination frequency found among 10 normal individuals: the mean frequencies of chromosomal recombination foci ranged from 47 to 52.7. The bivalents without recombination focus were rare, with a frequency of only 0.4%. Thus, achiasmate chromosomes appeared to be rare in human male meiosis. Spearman correlation analysis between age and the frequencies of recombination foci failed to get any significantly statistical correlation, suggesting that aging contributes nothing to the variation among individuals.

Gamma-irradiation increased meiotic crossovers in mouse spermatocytes

In mice, the occurrence of immunofluorescent foci for mismatch repair protein MLH1 correlates closely with the occurrence of crossovers, as detected genetically, and MLH1 foci represent virtually all prospective crossover positions. To examine the effects of γ-irradiation on meiotic crossovers in mouse spermatocytes, male mice were subjected to whole-body γ-irradiation at different sub-stages of meiotic prophase and crossovers on synaptonemal complexes (SCs) were analysed by visualising and quantifying the immunofluorescent MLH1 foci. At both 24 and 48 h after exposure, significant dose-dependent increases in the number of total MLH1 foci per spermatocyte were observed at late zygotene-early pachytene with the gradient increase of radiation dose from 0, 1.5, 3-6 Gy. Furthermore, irradiation at preleptotene-leptotene still led to significant dose-dependent increased meiotic crossovers in the spermatocytes analysed 120 h after exposure. In further analysis, these dose-dependent increases in the number of total MLH1 foci per cell were attributed to significant dose-dependent decreases in autosomal SCs with 0 MLH1 focus, and the dose-dependent increases in autosomal SCs with 2 MLH1 foci and the percentage of cells with MLH1 focus on XY bivalent. The increased number of cells with an MLH1 focus on the pseudoautosomal regions (PARs) may indicate that there is a delay in meiotic progression in the irradiated cells. Although significant dose-dependent increases in the number of total MLH1 foci per cell were examined 24, 48 or 120 h after exposure with the gradient increase of radiation doses, these increases were mild compared to the control groups. This suggests that there is tight control of crossover formation (at least with respect to MLH1 foci number). The mechanisms underlying irradiation-induced DNA lesion repair, cellular responses independent of DNA damage and meiotic crossover homeostasis in mammals will be the subjects of future study.

An investigation into the origins of meiotic aneuploidy using ascus analysis

Aneuploidy can result from a variety of defects at meiosis. Results are presented of crosses ofSordaria brevicollisin which aneuploid spores are detected through complementation of spore colour mutants at thebufforgrey-6 loci on linkage groups II and IV respectively. By using ascus analysis, the underlying cause of the aneuploidy can be deduced in many cases. Thus non-conjuction (pairing failure) and non-disjunction at the first meiotic division, premature centromere division, non-disjunction at the second division, and pre-meiotic errors such as extra replication of the chromosomes can be distinguished. Both linkage groups were found to give a similar proportion of the different errors. Non-conjunction and first-division non-disjunction formed 60–70% of detectable cases, whilst premature centromere division and second-division non-disjunction comprised 10% and 5% of aneuploids respectively. However, only a small proportion of second-division errors are detected.

It is proposed that the systems described in this paper can form the basis of a valuable screening method for detecting agents which increase aneuploid frequency. The advantages and disadvantages of using lower eukaryotes in this way are discussed.

Bisphenol A exposure in utero disrupts early oogenesis in the mouse

Estrogen plays an essential role in the growth and maturation of the mammalian oocyte, and recent studies suggest that it also influences follicle formation in the neonatal ovary. In the course of studies designed to assess the effect of the estrogenic chemical bisphenol A (BPA) on mammalian oogenesis, we uncovered an estrogenic effect at an even earlier stage of oocyte development—at the onset of meiosis in the fetal ovary. Pregnant mice were treated with low, environmentally relevant doses of BPA during mid-gestation to assess the effect of BPA on the developing ovary. Oocytes from exposed female fetuses displayed gross aberrations in meiotic prophase, including synaptic defects and increased levels of recombination. In the mature female, these aberrations were translated into an increase in aneuploid eggs and embryos. Surprisingly, we observed the same constellation of meiotic defects in fetal ovaries of mice homozygous for a targeted disruption of ERβ, one of the two known estrogen receptors. This, coupled with the finding that BPA exposure elicited no additional effects in ERβ null females, suggests that BPA exerts its effect on the early oocyte by interfering with the actions of ERβ. Together, our results show that BPA can influence early meiotic events and, importantly, indicate that the oocyte itself may be directly responsive to estrogen during early oogenesis. This raises concern that brief exposures during fetal development to substances that mimic or antagonize the effects of estrogen may adversely influence oocyte development in the exposed female fetus.

The potential effects on reproduction of chemicals with hormone-like activity is a growing concern. One estrogenic chemical, bisphenol A (BPA), has received considerable attention because low-dose exposures have been reported to induce a variety of reproductive effects in rodents. In the course of studies to assess the effects of BPA on the mouse oocyte, we have uncovered a novel “grandmaternal” effect: exposure to BPA during pregnancy disturbs oocyte development in unborn female fetuses. When these fetuses reach adulthood, the perturbations are translated into an increase in chromosomally abnormal eggs and embryos. Thus, low-dose BPA exposure during pregnancy has multigenerational consequences; it increases the likelihood of chromosomally abnormal grandchildren. Our studies also provide mechanistic insight, and, surprisingly, suggest that BPA acts in the fetal ovary not by mimicking the actions of estrogen but by interfering with the function of one of the known estrogen receptors. Thus, our data suggest that estrogen plays a far earlier role in oocyte development than previously suspected and, importantly, raise the possibility that a variety of substances—both synthetic and naturally occurring—that mimic the actions of estrogen or act as estrogen antagonists may affect early oocyte development.

Influence of follicular fluid meiosis-activating sterol on aneuploidy rate and precocious chromatid segregation in aged mouse oocytes

BACKGROUND

Follicular fluid meiosis-activating sterol (FF-MAS) protects young oocytes from precocious chromatid separation (predivision). Reduced expression of cohesion and checkpoint proteins and predivision has been hypothesized to occur in age-related aneuploidy in oocytes.

METHODS

To know whether FF-MAS also protects aged oocytes from predivision and from age-related non-disjunction, we analysed chromosome constitution in mouse oocytes matured spontaneously with or without 10 microM FF-MAS and in hypoxanthine (HX)-arrested young and aged oocytes induced to resume maturation by FF-MAS. Messenger RNA for checkpoint protein MAD2 and cohesion protein SMC1beta was compared between oocytes matured with or without FF-MAS.

RESULTS

Aged oocytes possessed many bivalents with single distal chiasma at meiosis I. Predivision was especially high in aged oocytes cultured sub-optimally to metaphase II in alpha-minimum essential medium (alpha-MEM). FF-MAS reduced predivision significantly (P < 0.001) but neither reduced non-disjunction nor induced aneuploidy in aged oocytes. Polyploidy was high in FF-MAS-stimulated maturation, in particular in the aged oocytes (P > 0.001). Relative levels of Smc1beta mRNA appeared increased by maturation in FF-MAS, and mitochondrial clustering was restored.

CONCLUSIONS

Sister chromatids of aged oocytes appear to be highly susceptible to precocious chromatid separation, especially when maturation is under sub-optimal conditions, e.g. in the absence of cumulus and FF-MAS. This may relate to some loss of chromatid cohesion during ageing. FF-MAS protects aged oocytes from predivision during maturation, possibly by supporting Smc1beta expression, thus reducing risks of meiotic errors, but it cannot prevent age-related non-disjunction. Aged oocytes appear prone to loss of co-ordination between nuclear maturation and cytokinesis suggesting age-related relaxed cell cycle control.

Variation in meiotic recombination frequencies among human males

Meiotic recombination is essential for the segregation of homologous chromosomes and the formation of normal haploid gametes. Little is known about patterns of meiotic recombination in human germ cells or the mechanisms that control these patterns. Here, newly developed immunofluorescence techniques, based on the detection of MLH1 (a DNA mismatch repair protein) foci on synaptonemal complexes (SCs) at prophase I of meiosis, were used to examine recombination in human spermatocytes. The mean number of MLH1 foci per cell in all donors was 48.0 with range from 21 to 65. Remarkable variation in the recombination frequency was noted among 11 normal individuals: the mean frequencies of chromosomal recombination foci ranged from a low of 42.5 to a high of 55.0 exchanges. Donor age did not contribute to this variation. There was no correlation between this variation and the frequency of gaps (discontinuities) or splits (unpaired chromosome regions) in the SCs. The mean percentage of cells with gaps was 35% (range: 20% to 58%) and with splits was 7% (range: 0% to 37%). Bivalents without a recombination focus were rare, with a frequency of only 0.3%. Thus, achiasmate chromosomes appear to be rare in human male meiosis.

Meiotic exchange and segregation in female mice heterozygous for paracentric inversions

Inversion heterozygosity has long been noted for its ability to suppress the transmission of recombinant chromosomes, as well as for altering the frequency and location of recombination events. In our search for meiotic situations with enrichment for nonexchange and/or single distal-exchange chromosome pairs, exchange configurations that are at higher risk for nondisjunction in humans and other organisms, we examined both exchange and segregation patterns in 2728 oocytes from mice heterozygous for paracentric inversions, as well as controls. We found dramatic alterations in exchange position in the heterozygotes, including an increased frequency of distal exchanges for two of the inversions studied. However, nondisjunction was not significantly increased in oocytes heterozygous for any inversion. When data from all inversion heterozygotes were pooled, meiotic nondisjunction was slightly but significantly higher in inversion heterozygotes (1.2%) than in controls (0%), although the frequency was still too low to justify the use of inversion heterozygotes as a model of human nondisjunction.

Chromosomal rearrangements and evolution of recombination: comparison of chiasma distribution patterns in standard and robertsonian populations of the house mouse

The effects of chromosomal rearrangements on recombination rates were tested by the analysis of chiasma distribution patterns in wild house mice. Males and females of two chromosomal races from Tunisia differing by nine pairs of Robertsonian (Rb) fusions (standard all-acrocentric, 2N = 40 and 2N = 22) were studied. A significant decrease in chiasma number (CN) was observed in Rb mice compared to standard ones for both sexes. The difference in CN was due to a reduction in the number of proximal chiasmata and was associated with an overall more distal redistribution. These features were related to distance of chiasmata to the centromere, suggesting that the centromere effect was more pronounced in Rb fusions than in acrocentric chromosomes. These modifications were interpreted in terms of structural meiotic constraints, although genic factors were likely involved in patterning the observed differences between sexes within races. Thus, the change in chromosomal structure in Rb mice was associated with a generalized decrease in recombination due to a reduction in diploid number, a lower CN, and a decrease in the efficiency of recombination. The effects of such modifications on patterns of genic diversity are discussed in the light of models of evolution of recombination.

Genetic Control of Mammalian Meiotic Recombination. I. Variation in Exchange Frequencies Among Males From Inbred Mouse Strains

Genetic background effects on the frequency of meiotic recombination have long been suspected in mice but never demonstrated in a systematic manner, especially in inbred strains. We used a recently described immunostaining technique to assess meiotic exchange patterns in male mice. We found that among four different inbred strains—CAST/Ei, A/J, C57BL/6, and SPRET/Ei—the mean number of meiotic exchanges per cell and, thus, the recombination rates in these genetic backgrounds were significantly different. These frequencies ranged from a low of 21.5 exchanges in CAST/Ei to a high of 24.9 in SPRET/Ei. We also found that, as expected, these crossover events were nonrandomly distributed and displayed positive interference. However, we found no evidence for significant differences in the patterns of crossover positioning between strains with different exchange frequencies. From our observations of &gt;10,000 autosomal synaptonemal complexes, we conclude that achiasmate bivalents arise in the male mouse at a frequency of 0.1%. Thus, special mechanisms that segregate achiasmate chromosomes are unlikely to be an important component of mammalian male meiosis.

X chromosome effect on maternal recombination and meiotic drive in the mouse

We observed that maternal meiotic drive favoring the inheritance of DDK alleles at the Om locus on mouse chromosome 11 was correlated with the X chromosome inactivation phenotype of (C57BL/6-Pgk1(a) x DDK)F(1) mothers. The basis for this unexpected observation appears to lie in the well-documented effect of recombination on meiotic drive that results from nonrandom segregation of chromosomes. Our analysis of genome-wide levels of meiotic recombination in females that vary in their X-inactivation phenotype indicates that an allelic difference at an X-linked locus is responsible for modulating levels of recombination in oocytes.

The effects of Robertsonian fusions on chiasma frequency and distribution in the house mouse (Mus musculus domesticus) from a hybrid zone in northern Scotland

Chiasma frequency and distribution were studied in male Mus musculus domesticus from the John O'Groats-standard chromosomal hybrid zone in northern Scotland. Individuals of the John O'Groats race (2n=32; homozygous for the Robertsonian fusions 4.10, 6.13, 9.12 and 11.14) and the standard race (2n=40, all telocentric), and hybrids with various karyotypes, were examined. Chiasma frequency was significantly negatively correlated with the number of Robertsonian configurations in the meiotic cell. The decrease of chiasma frequency can be attributed to intrachromosomal effects that reduce the number of chiasmata in Robertsonian bivalents (formed in homozygotes for Robertsonian fusions) and trivalents (formed in heterozygotes). However, the reduction is more pronounced in Robertsonian bivalents and is related to a shift of chiasmata to the distal ends of the chromosome arms. A different type of repatterning occurs in trivalents where there is a significant increase in proximal and interstitial chiasmata.

Distribution of crossing over on mouse synaptonemal complexes using immunofluorescent localization of MLH1 protein

We have used immunofluorescent localization to examine the distribution of MLH1 (MutL homolog) foci on synaptonemal complexes (SCs) from juvenile male mice. MLH1 is a mismatch repair protein necessary for meiotic recombination in mice, and MLH1 foci have been proposed to mark crossover sites. We present evidence that the number and distribution of MLH1 foci on SCs closely correspond to the number and distribution of chiasmata on diplotene-metaphase I chromosomes. MLH1 foci were typically excluded from SC in centromeric heterochromatin. For SCs with one MLH1 focus, most foci were located near the middle of long SCs, but near the distal end of short SCs. For SCs with two MLH1 foci, the distribution of foci was bimodal regardless of SC length, with most foci located near the proximal and distal ends. The distribution of MLH1 foci indicated interference between foci. We observed a consistent relative distance (percent of SC length in euchromatin) between two foci on SCs of different lengths, suggesting that positive interference between MLH1 foci is a function of relative SC length. The extended length of pachytene SCs, as compared to more condensed diplotene-metaphase I bivalents, makes mapping crossover events and interference distances using MLH1 foci more accurate than using chiasmata.

Genetics of oocyte ageing

OBJECTIVES

Correlations between parental age, aneuploidy in germ cells and recent findings on aetiological factors in mammalian trisomy formation are reviewed.

METHODS

Data from observations in human oocytes, molecular studies on the origin of extra chromosomes in trisomies, experiments in a mouse model system, and transgenic approaches are shown.

RESULTS

Errors in chromosome segregation are most frequent in meiosis I of oogenesis in mammals and predominantly predispose specific chromosomes and susceptible chiasmate configurations to maternal age-related nondisjunction. Studies on spindle structure, cell cycle and chromosome behaviour in oocytes of the CBA/Ca mouse used as a model for the maternal age-effect suggest that hormonal homeostasis and size of the follicle pool influence the quality, maturation competence and spindle size of the mammalian oocyte. Predisposition to errors in chromosome segregation are critically dependent on altered cell cycles. Compromised protein synthesis and mitochondrial function affect maturation kinetics and spindle formation, and cause untimely segregation of chromosomes (predivision), mimicking an aged phenotype.

CONCLUSIONS

Altered cell cycles and untimely resolution of chiasmata but also nondisjunction of late segregating homologues caused by asynchrony in cytoplasmic and nuclear maturation appear to be causal to errors in chromosome segregation with advanced maternal age. Oocytes appear to lack checkpoints guarding against untimely chromosome segregation. Genes and exposures affecting pool size, hormonal homeostasis and interactions between oocytes and their somatic compartment and thus quality of follicles and oocytes have the potential to critically influence chromosome distribution in female meiosis and affect fertility in humans and other mammals.

First-meiotic-division nondisjunction in human oocytes

Reject oocytes from in vitro-fertilization patients are currently the only practical source of human oocyte material available for meiotic studies in women. Two hundred clearly analyzable second meiotic (MII) metaphase oocytes from 116 patients were examined for evidence of first meiotic (MI) division errors. The chromosome results, in which 67% of oocytes had a normal 23,X chromosome complement but none had an extra whole chromosome, cast doubt on the relevance, to human oocytes, of those theories of nondisjunction that propose that both chromosomes of the bivalent fail to disjoin at MI so that both move to one pole and result in an additional whole chromosome at MII metaphase. The only class of abnormality found in the MII oocytes had single chromatids (half-chromosomes) replacing whole chromosomes. Analysis of the chromosomally abnormal oocytes revealed an extremely close correlation with data on trisomies in spontaneous abortions, with respect to chromosome distribution, frequency, and maternal age, and indicated the likelihood of the chromatid abnormalities being the MI-division nondisjunction products that lead to trisomy formation after fertilization. The most likely derivation of the abnormalities is through a from of misdivision process usually associated with univalents, in which the centromeres divide precociously at MI, instead of MII, division. In the light of recent data that show that altered recombination patterns of the affected chromosomes are a key feature of most MI-division trisomies, the oocyte data imply that the vulnerable meiotic configurations arising from altered recombination patterns are processed as functional univalents in older women. Preliminary evidence from MI-metaphase oocytes supports this view.

Parental age-related aneuploidy in human germ cells and offspring: a story of past and present

Parental age is the most important aetiological factor in trisomy formation in humans. Cytogenetic studies on germ cells reviewed here imply that (i) 2-4% sperm are aneuploid and 8.6% oocytes from IVF are hyperploid (ii) a paternal age effect may exist, and (iii) oocytes of aged women contain precociously separated chromatids in metaphase II. Trisomy data suggest that most aneuploidy is generated during meiosis I of oogenesis and is maternal age-dependent. Trisomy 18 is unique, originating mostly from maternal meiosis II errors. The extra gonosome in 47, XXY derives mostly from a paternal meiosis I error. Trisomy of individual chromosomes may remain low, linearly rise, or exponentially increase with advanced maternal age. Maternal age related trisomies involve achiasmatic and normochiasmate chromosomes, and chromosomes with disturbed recombination and distally located chiasmata. Hypotheses on the origin of the maternal age effect are critically reviewed. One model is presented that relates to altered cell cycle and protein phosphorylation in oocytes of aged mammals and accounts for most of the observed data in humans and in experimental studies. Aneuploidy may thus involve a predetermined component but is possibly also influenced by extrinsic factors reducing oocyte quality or depleting the oocyte pool precociously. Areas of future research are proposed to elucidate (i) the significance of early disturbances in the prenatal ovary, (ii) parameters diminishing the quality of oocytes in dictyate stage, and (iii) mechanisms enabling oocytes to process all chromosomal configurations successfully during later stages of oogenesis. Studies with newly developed and existing animal models appear indispensable to identify exposures affecting chromosome disjunction during meiosis, especially in the aging female.

Chiasma frequency, distribution and interference maps of mouse autosomes

Chiasma frequencies were analysed and chiasma positions measured in diakinesis/metaphase I autosomal bivalents from oocytes and spermatocytes of F1 hybrid C3H/HeHx101/H mice. Twenty chromosome size ranks, including the presumptive X bivalent, could be distinguished in oocytes, and nineteen autosomal ranks plus the XY pair spermatocytes. Overall, mean cell chiasma frequencies of the two sexes did not differ significantly once the contribution of the presumptive X bivalent and the XY pair were taken into account. Sex related differences in chiasma distribution patterns were evident, however. In monochiasmate bivalents, the chiasma was most commonly located interstitially in oocytes while in spermatocytes it could be either interstitial or distal. In dichiasmate bivalents, the chiasmata tended to be more centrally located in oocytes than in spermatocytes. Minimum inter-chiasma distances did not appear to show any great variation in chromosome pairs of different sizes, however, mean inter-chiasma distances did increase with the bivalent length. The minimum-inter chiasma distance data suggest that chiasma interference is complete over a chromosomal segment equating to approximately 60Mb. Measurement of the positions of chiasmata along chromosome arms open up the possibility of producing chiasma-based genetic maps for all the autosomes of the mouse.

Etiology of nondisjunction in humans

Aneuploidy is the most common class of chromosome abnormality in humans, occurring in at least 0.3% of newborns and approximately 50% of spontaneous abortions. Considered as a class, it is the most common known cause of mental retardation and the leading cause of pregnancy loss. Despite the high frequency of aneuploidy, its obvious clinical importance, its severe impact on human reproduction, and the 35 years of research since the first human chromosome abnormality was described, we still know very little about its causes, let alone the contribution of environmental exposures. Recently, however, with the advent of molecular and molecular cytogenetic techniques and advances in reproductive biology, a body of evidence has been generated that is beginning to shed light on the incidence, origin, and etiology of human aneuploid conditions. The bulk of this evidence comes from two sources: 1) studies of the incidence of aneuploidy in the cells of origin, namely oocytes and sperm; and 2) examinations of meiotic stage, parent of origin, and meiotic recombination in trisomic conceptuses, both liveborn and abortuses. Using a multicolor fluorescence in situ hybridization (FISH) approach, it is now possible to screen on extremely large number of human sperm to determine chromosome-specific rates of disomy. Likewise, because of the introduction in the past decade of in vitro fertilization technology, a population of human oocytes suitable for aneuploidy screening became available. The examination of the cells of origin of aneuploidy, the sperm and oocytes, has provided data on the incidence of chromosome aberrations and valuable insight into possible mechanisms of nondisjunction. Additionally, the recent identification of multiple, highly informative DNA polymorphisms on all human chromosomes has made the determination of parental origin and the analysis of recombination a straightforward matter. We now know that the vast majority of trisomic conceptuses are maternal in origin, that increased maternal age is associated with nondisjunction, and that the amount and position of recombination on nondisjoined chromosomes is altered. In this review we will restrict discussions to these recent developments and to new models of the mechanism(s) of human nondisjunction based on the molecular cytogenetic analyses. Additionally, we will discuss the direction of future epidemiological research made possible through the development of molecular and molecular cytogenetic techniques. These technological advances have now allowed for a systematic search for genetic and environmental components to human nondisjunction.

Sex differences in chiasma distribution along two marked mouse chromosomes: differences in chiasma distribution as a reason for sex differences in recombination frequency

Chiasma distributions along bivalents 1 and 14 in female and male mice were studied. It was shown that the average chiasma number in both chromosomes show no sex difference. There are however, significant sex differences in chiasma distribution along 1 and 14 chromosomes. In males there are two terminal chiasma peaks in chromosome 1 and one subtelomeric peak of chiasmata in chromosome 14. In females chiasma distributions are more even. According to genetic data, females produce more recombinants between loci of chromosome 1 than males do. By means of a computer simulation it was demonstrated that the differences in the average recombination frequency result from differences in chiasma distribution.

A test of the production line hypothesis of mammalian oogenesis

Germ cells in female mammals become committed to meiosis and enter its prophase sequentially in fetal life and, according to the Production Line Hypothesis, the oocytes thus generated are released after puberty as mature ova in the same sequence as that of meiotic entry in fetu. This hypothesis in its original and complete form has a subordinate proposition that concerns chiasma (Xma) frequency; it postulates that Xma number would decrease with fetal age. Consequently, univalents would increase, leading to errors of chromosome disjunction at the first meiotic division (MI), and thus to maternal age-dependent numerical chromosome anomalies. By using an in vitro/in vivo approach, we radioactively labelled the DNA of germ cells at premeiotic synthesis as they sequentially entered meiosis, while the fetal ovaries were in culture. At the end of this in vitro phase, pachytene/diplotene (P/D) stages were studied to determine their labelled fraction. The ovaries were then transplanted to spayed females and, after the in vivo phase, mature ova were harvested and the proportion of labelled first and second meiotic metaphases (MI/MII) determined. By marking the germ cells with label while in vitro during periods equivalent to early and late gestation, and by comparing the observed proportions of labelled MI/MII with those of oocytes labelled at P/D, we concluded that, in the mouse, ova do not mature at random for release, but are formed according to a production line system in which the time of release after puberty is related to the time of entry into meiosis in fetu.

Further examination of the production-line hypothesis in mouse foetal oocytes. II. T(14; 15)6Ca heterozygotes

Pachytene oocytes from foetal mice heterozygous for the translocation T(14; 15)6Ca were screened for evidence of a "production-line" effect on chromosome pairing. Metaphase I oocytes from adult heterozygotes were also examined to determine whether any such effect on pahytene chromosome pairing is subsequently repeated during adult reproductive life as anticipated by the production-line hypothesis. It was found that as gestation proceeded the proportion of pachytene oocytes with a translocation quadrivalent declined and that with a trivalent and univalent correspondingly increased. That is, there was evidence of variation in pairing behaviour of the translocation at different times of gestation. In contrast, the proportions of metaphase I cells with either a quadrivalent or a trivalent plus univalent did not vary between adult females of different ages. Thus if the variation observed at pachytene was the result of a production-line effect, clearly this was not reflected in the behaviour of the translocation at metaphase I. Our observations therefore do not support the production-line hypothesis for the maternal age effect on nondisjunction.

The CBA mouse as a model for age-related aneuploidy in man: studies of oocyte maturation, spindle formation and chromosome alignment during meiosis

To elucidate the possible mechanism of disturbances in chromosome segregation leading to the increase in aneuploidy in oocytes of aged females we examined the meiotic spindles of CBA/Ca mice. Employing immunofluorescence with an anti-tubulin antibody, and human scleroderma serum, as well as 4'-6-diamidino-2-phenylindole (DAPI) staining of chromosomes the microtubular cytoskeleton could be visualized, and the behaviour of chromosomes and centromeres of oocytes spontaneously maturing in vitro could be studied. The morphology of spindles during the first meiotic division was not conspicuously different in oocytes from young and aged mice as far as the cytoskeletal elements were concerned. Neither multipolar spindles nor pronounced cytoplasmic asters appeared in oocytes of mice approaching the end of their reproductive life (9 months and older). Oocytes of aged females also did not exhibit any sign of premature separation of parental chromosomes at prophase, obvious malorientations of bivalents, or significant lagging of chromosomes during ana- and telophase. Metaphase I with all bivalents aligned at the spindle equator appeared to be a relatively brief stage in oocyte development compared with pro- and prometaphase. Therefore, already slight disturbances occurring in the timing of the developmental programme which leads to a premature anaphase transition may be responsible for the high incidence of chromosomally unbalanced gametes in aged females, rather than non-separation and lagging of chromosomes during late ana- and telophase. In a second set of experiments we compared the metaphase II spindles of spontaneously ovulated oocytes obtained from animals at different ages. Previous studies have shown that spindle length and chromosome alignment may be altered in cells predisposed to aneuploidy.(ABSTRACT TRUNCATED AT 250 WORDS)

Relationship between frequencies of univalents and meiotic segregation in different mouse strains and their hybrids

Although univalents in diakinesis are often used as an estimator for chromosome mis-segregation during meiosis, no clear-cut relationship was demonstrated between both phenomena. In this study, the frequencies of autosomal and gonosomal univalents in diakinesis were related to the frequencies of aneuploid metaphase-II gonocytes during spermatogenesis and oogenesis of different mouse strains and their hybrids (inbred strains: DBA/2J, C57Bl; outbred strain; Swiss, inbred x outbred hybrids: Swiss X C57Bl, C57Bl X Swiss, inbred X inbred hybrids: DBA/2J X C57Bl, C57Bl X DBA/2J). As far as the frequencies of univalents are concerned, they were shown to be strain-dependent and similar in both sexes. Moreover, there is a high non-disjunction rate of DBA males and PMSG-HCG-primed DBA females. Aneuploidy in metaphase II is also strain-dependent but different in both sexes; in the male, a clear decrease of aneuploidy frequencies is observed as compared to the frequency of univalents. This decrease does not occur in females.

Influence of age on chiasma number in male Locusta migratoria

The influence of age on chiasma formation in Locusta migratoria has been studied both in wild-type and irradiated males. Samples were obtained on the one hand from different individuals and on the other hand from the same individuals at different ages through repeated biopsies. It was observed that mean chiasmata per cell decreased in ageing animals. Chi-squared tests and correlation coefficients showed that the decrease of chiasmata with age is progressive but not continuous. Intermediate values coincide with the first mating. It seems that older individuals tend to transmit their genetic combinations unaltered, increasing the probability of transmitting gene combinations of adaptive value.

Meiotic nondisjunction in oocytes from aged Djungarian hamsters correlates with an alteration in meiosis rate but not in univalent formation

The effect of maternal ageing on the meiotic rate, on chiasma and univalent frequency as well as on heteroploidy in secondary oocytes from Djungarian hamsters was examined. The frequency of hyperhaploid oocytes increased from 0.6% in young (8-14 weeks) to 2.8% in middle-aged (26-46 weeks) and reached 3.6% in the oldest females (49-75 weeks). On the basis of malsegregated bivalents per oocyte, nondisjunction occurred most often in the middle-aged group (5.42 X 10(-2) bivalents per oocyte). Hereby, the large meta- and submetacentric A-D chromosomes were preferentially involved. Furthermore, the pattern of nondisjunction was not different from that expected on the basis of chromosome length or induced by colchicine. The large A-D chromosomes did not show any alteration in chiasma or univalent frequency. Terminalized chiasmata were only detected in the E group and univalents increased slightly, but not significantly in the small chromosomes (G group). At higher ages, both chromosome groups were not preferentially involved in nondisjunction. Presegregation slightly increased with age and affected more or less all bivalents, whereas the incidence of diploidy significantly decreased. With respect to the rate of meiosis in oocytes from aged females, the resumption was delayed at metaphase I. Our data suggest that failures in the control of oocyte proliferation are involved in nondisjunction rather than the "production-line." Furthermore, a model is proposed to explain nondisjunction of specific bivalents at certain maternal ages.

Chromosome segregation at meiosis I in female T(2;4)1Gö/+ mice: no evidence for a decreased crossover frequency with maternal age

The influence of age and hormones on chromosome segregation at meiosis I was studied in female mice heterozygous for the T(2;4)1Gö translocation. Females of two age groups (18-22 and 40-56 weeks old) were stimulated for ovulation with different doses of gonadotropins (1.5 IU PMS/1.0 IU HCG or 10 IU PMS/10 IU HCG). Analysis of metaphase II oocytes revealed the highest level of hyperhaploidy (1.8%) and presegregation (4.4%) in the young females receiving the low dose. Presegregation preferentially affected the small 4(2) marker chromosome. There was no significant interference of the tetravalent with disjunction of the nontranslocated normal bivalents. Moreover, no remarkable difference in the mode of segregation (adjacent I, II or alternate) was observed. Recombination within the interstitial pairing segments of the chromosomes involved in the translocation allowed us to calculate cross-over frequencies in ovulated oocytes. For both the large 2(4) and the small 4(2) marker chromosomes, this frequency was higher in old than in young T(2;4)1Gö/+ females. Our data do not support the production line hypothesis of Henderson and Edwards (1968) which claims that chiasma frequency in oocytes decreases with maternal age.

Lack of correlation between mutagen-induced chromosomal univalency and aneuploidy in mouse spermatocytes

Aneuploidy represents the predominant type of chromosomal abnormality found in human newborns with birth defects. The concern that environmental agents may cause aneuploidy in germ cells has prompted development of assay systems for detection of potentially aneuploidy-producing agents. One of the most frequently used methods involves cytogenetic analysis of murine spermatogenic cells at the stages of meiotic metaphases. However, criteria for aneuploidy induction have not been standardized in this test system. Many investigators consider the ability of an agent to induce univalents an appropriate measure of its potential to induce aneuploidy. In the present study, the relationship between univalency and aneuploidy was examined in mouse spermatocytes after various mutagen treatments. 45 Swiss mice were treated with 4 different agents; viz., adriamycin, vinblastine sulfate, cytosine arabinoside, and radiation (cobalt 60) and 10 untreated animals served as controls. From each animal, 50-200 MIs were examined for both sex-chromosomal and autosomal univalency (X-Y U and AU), and equal numbers of MIIs were examined for aneuploidy (hyperhaploidy). No significant correlations between univalency (either X-Y U or AU) and aneuploidy were found in the mutagen-treated mice; nor were they found in the untreated animals. These results indicate that induction of univalents by a mutagen is not necessarily predictive of the aneuploidy-inducing ability of this agent.

Maternal ageing and nondisjunction: a comparative study of two chromosomal techniques on the formation of univalents in first meiotic metaphase oocytes of the mouse

The incidence of univalents was compared between slides prepared according to two clearly different chromosomal methods, i.e. Tarkowski's method and ours, in order to examine whether a univalent pair could be formed artifactually at the first meiotic metaphase (MI). The oocytes used were obtained from young (2-3 months) and old (12-15 months) age groups of both C57BL/6 and dd mice. In Tarkowski's method only a single fixative was used, while in our method three different fixatives were used successively in order to fix oocytes without their being ruptured. Despiralized, fuzzy and loosely associated chromatids were seen frequently in the slides prepared by Tarkowski's method, while such features were seen less frequently in the slides prepared by our method. The incidence of oocytes with univalents in the slides made by Tarkowski's method was much higher than in those made by ours in both age and strain groups (P less than 0.05-0.001). Thus, it was confirmed that the so-called univalents could be produced artifactually. The results did not support the production line hypothesis of Henderson and Edwards (1968) which was based on their observation of an increased incidence of univalents in MI oocytes from aged female mice.

Review of literature on chemical-induced aneuploidy in mammalian male germ cells

80 papers published between 1970 and 1984 were evaluated for results pertaining to chemical-induced aneuploidy in mammalian male germ cells. Diverse assays and end points were represented. The assays considered to involve direct measures of aneuploidy were based upon chromosome counts in premeiotic, meiotic, and embryonic cells, and the male pronucleus, or upon phenotypic expression of X-linked genetic markers. Assays in which indirect measures were interpreted as evidence for aneuploidy included those primarily assessing chiasma frequencies, univalent frequencies, and spermatid/sperm sex chromosome body counts. An initial screening to reject studies with insufficient data and those which did not involve a single chemical test agent led to the elimination of 39 papers from further review. The remaining 41 papers reported effects from 46 different chemicals. These papers were rigorously assessed for adequacy of experimental protocols, relevance of end points as direct measures of aneuploidy, and completeness of data presentation and statistical analysis. Criteria specific to each assay were also considered. 4 chemical tests were considered to provide reliable positive or negative aneuploidy data. Cyclophosphamide and chloral hydrate each caused metaphase II hyperploidy when injected into mice. Very limited analyses of trenimon and isoniazid provided negative results. Test findings for 44 chemicals were viewed as inconclusive. It was concluded that standardization of tests to evaluate chemical-induced aneuploidy in male germ cells and the application of these tests towards increasing the data base are badly needed.

An hypothesis regarding the origin of aneuploidy in man: indirect evidence from an experimental model

Recent studies have clearly demonstrated that aneuploidy may be induced in about 10 to 20% of oocytes and recently ovulated eggs when female mice are given an intragastric injection of a dilute solution of ethanol. Similar rates of aneuploidy have also been observed when recently ovulated eggs are briefly exposed in vitro to a dilute solution of ethanol in tissue culture medium. These findings are briefly reviewed, and observations made on the possible underlying mechanism of induction of chromosome malsegregation in the ethanol exposed groups. Attention is drawn to evidence from a wide range of studies on the effect of ethanol, acetaldehyde (its primary metabolite), and anaesthetics on cell division and chromosome segregation in an attempt to substantiate an hypothesis regarding the mode of action of these agents. In the light of this information, it is hypothesised that exposure to ethanol probably interferes with the normal functioning of the cytoskeletal elements of the spindle apparatus, or its precursor elements, during the first or second meiotic divisions. An attempt is also made to account for the very high incidence of aneuploid conceptuses in man, a high proportion of which are spontaneously aborted. It is hypothesised that exposure to ethanol and other spindle active agents during appropriate stages of oogenesis (in particular during the first meiotic division), and possibly also during spermatogenesis, may be important aetiological factors in a proportion of those cases of spontaneous abortion with a numerical chromosome anomaly for which no other obvious cause is recognised. If it is valid to extrapolate from these experimental findings to the clinical situation in man, it is suggested that attention should also be drawn to the potentially greater hazard to the conceptus which could result from maternal alcohol consumption at and shortly before conception.

Experimental approaches for the detection of chromosomal malsegregation occurring in the germline of mammals

Existing and newly proposed methods to detect the induction of heritable aneuploidy are summarized, and their favorable and unfavorable features discussed. Among the tests involving direct chromosomal examination, those involving study of pronuclear chromosomes at first cleavage are judged to be the most universally informative and reliable, provided tertiary trisomy can be ruled out. Measurement of postmidterm (fetal) death is proposed as a trisomy prescreen that can be readily combined with a dominant-lethal test. Among the genetic procedures for identifying the results of malsegregation events, direct detection of aneuploids has a number of advantages over complementation methods, in which only a fraction of the products of aneuploid gametes is detectable. Direct detection of aneuploids must, however, be restricted to sex chromosomes if postnatal animals are examined. A genetic marker system to detect autosomal trisomies in fetuses is proposed. An examination of experimental parameters that might maximize induction of malsegregation leads to the recommendation to include preleptotene among exposed germ-cell stages.

Meiosis in the foetal mouse ovary. II. Oocyte development and age-related aneuploidy. Does a production line exist?

A systematic search for chromosome pairing defects in foetal mouse oocytes has been carried out in two different strains (Swiss and CBA/Ca) over days 15-19 of gestation and on day 1 post-partum. The aim was to seek direct cytological evidence for a "production line" of oocyte development, or the occurrence of pairing anomalies at meiotic prophase that might lead, in the adult female, to nondisjunction at anaphase I. No evidence for either was found. The data argue against the "production line" hypothesis as the basis for maternal age-related increases in aneuploidy in the mouse. Attempts to analyse chiasmata in oocytes at diplotene were unsuccessful.

Similar dose-related chromosome non-disjunction in young and old female mice after X-irradiation

Radiation-induced chromosome non-disjunction has been investigated in young and old female mice. Chromosome numbers were screened in maternal and paternal pronuclei of 1-cell embryos. To avoid bias due to preparatory artefacts, only the numbers of hyperploid pronuclei were used as measures of chromosome non-disjunction. Spontaneous non-disjunction occurred more frequently in oocytes of old females (1.54%) compared to young females (0.19%). After X-irradiation, both young and old females displayed significant linear increases in non-disjunction with absorbed dose. The slopes of the regressions did not differ significantly. Maternal pronuclei of 1-cell embryos recovered from irradiated females occasionally contained chromosome structural aberrations. The numbers of aberrations showed significant linear relationships to absorbed dose in young and old females. No significant difference was present between the numbers of aberrations in young and old females. Analyses of numerical and structural chromosome anomalies have demonstrated, therefore, no increase in sensitivity to radiation to occur with maternal age.