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 ). 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 . In contrast, among women aged 40 years and older, the value increases to almost one-third of all recognized pregnancies . 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 ). 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. Based on these findings, it has been hypothesized that human non-disjunction involves events that occur during two distinct developmental time points : (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 mis-segregate. 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 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 . 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 ). 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  and mice . 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 (mutLhomologue 1) foci in pachytene spermatocytes from males of different inbred strains, we were able to assess the effect of genetic background . 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. 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 . 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 . 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 , high temperature in the grasshopper  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 well-documented 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 . 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 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 ERβ-null 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 low-dose 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.
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 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.
Meiosis and the Causes and Consequences of Recombination: Biochemical Society Focused Meeting held at University of Warwick, U.K., 29–31 March 2006. Organized by R. Borts (Leicester, U.K.), D. Charlesworth (Edinburgh, U.K.), A. Eyre-Walker (Sussex, U.K.), A. Goldman (Sheffield, U.K.), G. McVean (Oxford, U.K.), D. Monckton (Glasgow, U.K.), G. Moore (John Innes Centre, U.K.), J. Richards (Roslin Biocentre, U.K.) and M. Stark (Glasgow, U.K.). Edited by D. Monckton.