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Meiosis in mammals: recombination,
non-disjunction and the environment
P.A. Hunt1
School of Molecular Biosciences and Center for Reproduction, Fulmer Hall 539, Washington State University, P.O. Box 644660, Pullman,
WA 99164-4660, U.S.A.
Abstract
By comparison with other species, the meiotic process in the human female is extraordinarily error-prone.
In addition to the well-known effect of advancing maternal age, recent studies have demonstrated that the
number and location of meiotic recombination events influences the likelihood of meiotic non-disjunction
in our species. Although this association extends to many other organisms, the factors that influence the
number and placement of exchanges within a cell remain poorly understood. Like other aspects of meiosis,
the control of recombination is likely to be subject to variation among species. In this review we summarize
data from recent studies in mammals; the combined data suggest that both genetic and environmental
factors influence recombination in mammals and, importantly, that control mechanisms probably differ
between males and females.
Errors in chromosome segregation during meiosis result in
the production of eggs or sperm with an inappropriate number of chromosomes and these, upon fertilization, give rise
to aneuploid conceptions. In humans, most aneuploidy is incompatible with survival and causes loss of the conceptus
during the first trimester. More than 20% of human conceptions are lost as a result of meiotic errors and, although errors
can occur either during spermatogenesis or oogenesis, the
vast majority of human aneuploidy is caused by errors during
female meiosis (reviewed in [1]). Indeed, by comparison with
other species, the incidence of errors in the human female is
extraordinarily high and, for reasons that remain unknown,
is strongly influenced by age. Current estimates suggest that,
among 20–24-year-old women, approx. 2% of all clinically
recognized pregnancies involve a chromosomally abnormal
fetus [1]. In contrast, among women aged 40 years and older,
the value increases to almost one-third of all recognized pregnancies [1]. However, because these values are based on
pregnancies that survive long enough to be clinically recognized, the actual incidence of chromosomally abnormal
pregnancies for both age groups is substantially higher.
During the past several years, human studies have demonstrated that, in addition to maternal age, the location of recombination events influences the ability of homologues to
segregate during the first meiotic division (reviewed in [1]).
As chromosomes condense in preparation for division, the
sites of recombination (known as chiasmata) physically tether
homologues together, facilitating their orientation on the first
meiotic spindle. The resolution of these physical associations
at anaphase allows homologues to segregate. In humans,
sites of exchange that are placed close to the centromere or
to the telomere are associated with non-disjunction errors.
Key words: bisphenol A, human aneuploidy, meiosis, non-disjunction, oocyte, recombination.
Abbreviations used: BPA, bisphenol A; ER, oestrogen receptor.
1
email pathunt@wsu.edu
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Based on these findings, it has been hypothesized that human
non-disjunction involves events that occur during two distinct developmental time points [2]: (i) during fetal development, when sites of recombination are established and some
chromosomes become tethered by poorly placed recombination events, and (ii) in the adult where increasing age of the
woman acts (by a mechanism that remains unclear) to increase
the likelihood that these ‘vulnerable’ homologues will missegregate. Thus an understanding of human aneuploidy
requires knowledge of the factors that control the number
and placement of the sites of recombination as well as the
mechanism(s) by which age influences the meiotic process. In
the present review, we focus on recombination, summarizing
recent results that provide new insight into the factors that
influence recombination in mammals.
Mammalian meiosis: the basics
Although the basics are the same, male and female mammalian
meioses differ in many important respects, including the time
of initiation, duration, outcome and response to disturbances
in the normal sequence of events. In females, meiotic cell
division is initiated during fetal development. The paradigm
that ‘all’ oocytes initiate meiosis during prenatal development
has been challenged recently by the publication of two papers
by Tilly and co-workers [3,4] that postulate the existence of
a stem cell population that gives rise to new oocytes in the
adult ovary. Although these papers raise tantalizing questions
and have sparked considerable debate, currently there is no
evidence to support the suggestion that any oocytes – other
than the cohort that enters meiosis prenatally – contribute
to the population of eggs ovulated by the sexually mature
female. During fetal development, oocytes progress in a semisynchronous fashion through the leptotene, zygotene, pachytene and diplotene stages of prophase (i.e. the period of
synapsis and recombination between homologues). Prior to
Meiosis and the Causes and Consequences of Recombination
or just after birth, depending on the species, oocytes enter
a sustained period of meiotic arrest, termed dictyate, and
shortly thereafter, each oocyte becomes enclosed by somatic
cells, forming a primordial follicle. Extensive growth of the
follicle and of the oocyte it contains is required before the oocyte becomes capable of resuming meiosis. The first meiotic
division (MI) is completed in the sexually mature female just
prior to ovulation, and the second division (MII) is completed only if the egg is fertilized. Thus, in the human female,
the process takes years to complete. In contrast, male meiosis
is much less complicated. Cells enter meiosis in the sexually
immature testis, but the production of mature sperm coincides with the onset of puberty. Unlike in the female, the
process is not interrupted by arrest phases (i.e. spermatocytes
progress from prophase through MI and MII in a matter
of days), and mature gametes are produced continuously
throughout adult life.
In both sexes, the complex events of prophase are prone
to error and dramatic germ cell loss is sustained, presumably
as a result of the actions of a control mechanism analogous
to the DNA damage checkpoint that operates during mitotic
cell division [5]. In addition to monitoring recombination
intermediates, there is compelling evidence that, at least
in mammals, defects in chromosome synapsis also activate
this checkpoint mechanism, leading to cell-cycle arrest and
apoptosis. In females, this is evident as a massive wave of germ
cell atresia in the perinatal ovary. There is, however, growing
evidence that this checkpoint mechanism is more stringent in
males. Specifically, the introduction of mutations in meiotic
genes that disrupt the normal processes of synapsis and recombination frequently results in complete spermatogenic arrest during prophase (and thus sterility) in the male, but only
shortens the reproductive lifespan and increases the likelihood of genetically abnormal gametes in the female (reviewed
in [6]). Thus, although prophase is clearly a vulnerable stage in
both sexes, agents that induce damage at this stage are
likely to have quite different effects on spermatogenesis and
oogenesis.
How is the number and placement of
exchanges controlled in mammals?
Given the effect of the placement of meiotic exchange events
on chromosome segregation in the human, an important
question is: what controls the number and placement of
exchanges? Variation in exchange frequency among different
strains or species has been observed in a number of different organisms, including flies [7–11], maize [12] and mice
[13]. More recently, direct analysis of pachytene spermatocytes and oocytes in humans has demonstrated significant
variation in recombination levels both among and within
individuals [14,15]. This raises a host of important questions.
For example, if these differences are under genetic control,
certain individuals may be predisposed to meiotic aneuploidy
by virtue of the allele(s) they carry for genes that mediate
recombination differences. If so, identifying and understanding these genetic differences would have a tremendous impact
on genetic counselling and prenatal diagnosis. Dissecting
the genetic control of recombination to identify alleles or
mutations that introduce variation in the human, however, is
a daunting task – especially since the phenotype must be based
on the analysis of recombination patterns in a large number of
spermatocytes or oocytes. Fortunately, as for much of human
genetics, the mouse provides a logical starting point in the
identification of genes that affect the rate of recombination.
To identify possible genetic effects on recombination in the
mouse, we took advantage of recently developed immunostaining techniques that allow direct examination of meiotic
exchanges. Specifically, by analysing the number and distribution of MLH1 (mutL homologue 1) foci in pachytene
spermatocytes from males of different inbred strains, we
were able to assess the effect of genetic background [16]. Our
results suggest that genetic background is a major determinant
of recombination rate as we observed significant variation
among inbred strains. Because we identified strains with low
(Castaneus), medium (A/J) and high (C57BL/6 and Spretus)
levels of recombination, we have been able to use intercrosses
to determine the mode of inheritance. F1 males resulting from
a cross between low and high recombining strains exhibit
an intermediate phenotype, suggesting a co-dominant mode
of inheritance. Further, analysis of F2 animals suggests that
the trait is controlled by a very small number of genes
(T.J. Hassold, personal communication). Intriguingly, although females exhibit similar strain-specific differences, the
differences are not completely correlated with the results of
male studies (T.J. Hassold, personal communication). For
example, a strain that exhibits one of the lowest levels of
recombination in the male has one of the highest levels in
the female. Thus, while intercrosses and genome scans of F2
progeny provide a means of identifying the genes that mediate these differences, it is clear that the control of recombination in mammals, like an increasing number of meiotic
phenotypes, may differ in males and females.
Sex-specific differences in recombination:
sex, not genotype, matters
Recombination is a sexually dimorphic trait. When it occurs
in both sexes of an organism, the overall rate of recombination
is usually higher in one sex. All mammals are not consistent
in this regard; recombination rates are higher in the female
than in the male in the human, dog, pig and in most mouse
strains [17–19], but the opposite is seen in the sheep and
wallaby [20,21]. The placement of exchanges also exhibits
sex-specific differences. For example, in both the human
and mouse, exchanges in telomeric regions are a feature of
spermatogenesis. This is particularly striking in the human,
where the overall rate of recombination in the female is nearly
twice that of the male, but males exhibit significantly higher
rates of recombination in telomeric regions [17,22–24].
These differences could be under genetic control, that is,
mediated by sex chromosome constitution. Alternatively, the
primary determinant of recombination could be sex-specific
factors, that is, an environmental rather than a genetic effect.
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Several years ago, we realized that the available sex chromosome abnormalities in the mouse provided a means of addressing this question experimentally. To determine if sex
chromosome constitution influences recombination, we used
a naturally occurring XY sex reversal condition (the placement of a Mus domesticus Y chromosome on to the C57BL/6
inbred strain background) to compare genetically identical
cells undergoing oogenesis and spermatogenesis [25]. In addition, to control for a possible effect of X chromosome dosage,
we compared XX and XO females on the same genetic background. The question was simple: is the number and placement of recombination events influenced by sex chromosome composition or is it a reflection of whether the cell
undergoes oogenesis or spermatogenesis? Based on the results
of our analysis, the answer was straightforward: sex chromosome constitution by itself has no bearing on recombination
levels. We found that XY sex-reversed females and XO
females displayed a pattern of recombination – both the frequency of exchanges and their placement along the length
of the chromosome – that was indistinguishable from their
normal XX siblings. Further, the pattern was markedly different in XY cells undergoing oogenesis by comparison with
genetically identical cells undergoing spermatogenesis [25].
These results suggest that sex-specific differences in the recombination patterns of mammals are a reflection of differences in the somatic environment of the testis and the
ovary.
Environmental influences on
recombination in mammals
The idea that environmental influences can affect recombination is not new. Recombination levels have been reported to
be altered by nutritional status in Saccharomyces cerevisiae
[26], high temperature in the grasshopper [27] and X-ray
irradiation or chemicals that induce DNA damage in a variety
of organisms (e.g. [27,28]). The results in mammals are limited
to the effects of the chemotherapeutic agent, cisplatin. However, because the results from studies in different laboratories
are not in agreement [29–31], there are as yet no welldocumented examples of environmental effects on recombination in mammals.
We previously reported that exposure to low doses of BPA
(bisphenol A) during the final stages of oocyte growth disrupts meiotic chromosome behaviour, resulting in the production of chromosomally abnormal eggs [32]. Humans are
exposed to BPA on a daily basis; it is a component of polycarbonate plastics, resins lining food/beverage containers, and
additives in a variety of consumer products. Importantly, it
is an endocrine-disrupting chemical, that is, a chemical that
mimics the actions of endogenous hormones. Because mammalian oogenesis is a complex process that is initiated during
fetal development but not completed until after fertilization,
defining critical exposure periods requires assessment of the
effects of fetal, neonatal and adult exposures. Accordingly, we
initiated studies to assess the effect of low, environmentally
relevant doses of BPA during the prophase events of female
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meiosis. To achieve this, we implanted time-release BPA
pellets in pregnant C57BL/6 females at gestation day 11.5.
Because the first cells initiate meiosis at 13.5 days of gestation,
this ensured low-level, continuous exposure of all oocytes
during meiotic prophase. Ovaries were isolated from female
fetuses 7 days later (18.5 days of gestation) for analysis.
Pachytene stage cells from exposed female fetuses exhibited
a striking array of abnormalities, including an increase in
synaptic abnormalities, increased levels of recombination
and slight alterations in the placement of exchange events
(M. Susiarjo and P.A. Hunt, unpublished work). Because
the placement of exchange events is critical for the proper
segregation of homologues at metaphase I, we were interested
in determining whether the aberrations in recombination had
meiotic consequences. Analysis of oocytes at metaphase I
confirmed the elevation in recombination levels observed
among pachytene oocytes, and cytogenetic studies of metaphase II-arrested oocytes demonstrated a striking increase
in aneuploid eggs. BPA is thought to act by mimicking
the effects of endogenous oestrogen and, since targeted disruptions of the two known ERs (oestrogen receptors)
have been generated in the mouse, we were interested in
determining whether one of these mutations would ameliorate the effects of BPA. Indeed, we found that the ERβnull mutation did abolish the sensitivity to BPA, but with
an unexpected twist: the untreated null female exhibited all
of the features of a BPA-treated female, and treatment with
BPA did not further exacerbate the phenotype (M. Susiarjo
and P.A. Hunt, unpublished work).
Our results are both exciting and a cause for alarm. They
demonstrate a heretofore-unrecognized effect of oestrogen
during the earliest stages of oocyte development. Understanding the influence of oestrogen on synapsis and recombination between homologous chromosomes may prove key
to understanding the factors that influence recombination
levels in female mammals. At the same time, however, the fact
that this aspect of oogenesis is influenced by hormones raises
serious concerns: BPA is only one of many chemicals with
hormone-like actions, and our results suggest that lowdose exposure during fetal development has the potential to
impact the entire cohort of oocytes produced by a female.
Further, because this is a ‘grandmaternal’ effect (i.e. exposure
of a pregnant female influences the genetic quality of the
offspring of her female fetuses), documenting an effect of this
type in humans would require studies spanning several generations.
Summary
Recent studies of human trisomies provide compelling evidence that the prenatal events of female meiosis are a key component of human aneuploidy [1,2]. Specifically, abnormalities
in the number or location of meiotic exchanges contribute to
all trisomies studied. Accordingly, we have been interested
in defining and understanding the factors that influence the
number and placement of exchange events in mammals. Using
the mouse as a model system, we have investigated the effects
Meiosis and the Causes and Consequences of Recombination
of sex, mutations, genetic background and environmental
exposures. Our results suggest that both genetic and environmental factors influence exchange distribution and, intriguingly, that the genetic control mechanisms are probably
different in males and females. Thus a true understanding
of the control of recombination in mammals will necessitate
sex-specific studies to identify the genes that mediate these
effects and to understand how environmental factors exert an
influence.
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Received 16 March 2006
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