Meiotic recombination and segregation of human-derived artificial chromosomes in Saccharomyces cerevisiae

Centromere pairing enables correct segregation of meiotic chromosomes

Proper chromosome segregation in meiosis I relies on the formation of connections between homologous chromosomes. Crossovers between homologs provide a connection that allows them to attach correctly to the meiosis I spindle. Tension is transmitted across the crossover when the partners attach to microtubules from opposing poles of the spindle. Tension stabilizes microtubule attachments that will pull the partners toward opposite poles at anaphase. Paradoxically, in many organisms, non-crossover partners segregate correctly. The mechanism by which non-crossover partners become bioriented on the meiotic spindle is unknown. Both crossover and non-crossover partners pair their centromeres early in meiosis (prophase). In budding yeast, centromere pairing is correlated with subsequent correct segregation of the partners. The mechanism by which centromere pairing, in prophase, promotes later correct attachment of the partners to the metaphase spindle is unknown. We used live cell imaging to track the biorientation process of non-crossover chromosomes. We find that centromere pairing allows the establishment of connections between the partners that allows their later interdependent attachment to the meiotic spindle using tension-sensing biorientation machinery. Because all chromosome pairs experience centromere pairing, our findings suggest that crossover chromosomes also utilize this mechanism to achieve maximal segregation fidelity.

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.

A ZIP1 separation-of-function allele reveals that centromere pairing drives meiotic segregation of achiasmate chromosomes in budding yeast

In meiosis I, homologous chromosomes segregate away from each other-the first of two rounds of chromosome segregation that allow the formation of haploid gametes. In prophase I, homologous partners become joined along their length by the synaptonemal complex (SC) and crossovers form between the homologs to generate links called chiasmata. The chiasmata allow the homologs to act as a single unit, called a bivalent, as the chromosomes attach to the microtubules that will ultimately pull them away from each other at anaphase I. Recent studies, in several organisms, have shown that when the SC disassembles at the end of prophase, residual SC proteins remain at the homologous centromeres providing an additional link between the homologs. In budding yeast, this centromere pairing is correlated with improved segregation of the paired partners in anaphase. However, the causal relationship of prophase centromere pairing and subsequent disjunction in anaphase has been difficult to demonstrate as has been the relationship between SC assembly and the assembly of the centromere pairing apparatus. Here, a series of in-frame deletion mutants of the SC component Zip1 were used to address these questions. The identification of a separation-of-function allele that disrupts centromere pairing, but not SC assembly, has made it possible to demonstrate that centromere pairing and SC assembly have mechanistically distinct features and that the centromere pairing function of Zip1 drives disjunction of the paired partners in anaphase I.

Short- and long-term effects of chromosome mis-segregation and aneuploidy

Dividing cells that experience chromosome mis-segregation generate aneuploid daughter cells, which contain an incorrect number of chromosomes. Although aneuploidy interferes with the proliferation of untransformed cells, it is also, paradoxically, a hallmark of cancer, a disease defined by increased proliferative potential. These contradictory effects are also observed in mouse models of chromosome instability (CIN). CIN can inhibit and promote tumorigenesis. Recent work has provided insights into the cellular consequences of CIN and aneuploidy. Chromosome mis-segregation per se can alter the genome in many more ways than just causing the gain or loss of chromosomes. The short- and long-term effects of aneuploidy are caused by gene-specific effects and a stereotypic aneuploidy stress response. Importantly, these recent findings provide insights into the role of aneuploidy in tumorigenesis.

The frequency of precocious segregation of sister chromatids in mouse female meiosis I is affected by genetic background

Mammalian female gametes frequently suffer from numerical chromosomal aberrations, the main cause of miscarriages and severe developmental defects. The underlying mechanisms responsible for the development of aneuploidy in oocytes are still not completely understood and remain a subject of extensive research. From studies focused on prevalence of aneuploidy in mouse oocytes, it has become obvious that reported rates of aneuploidy are strongly dependent on the method used for chromosome counting. In addition, it seems likely that differences between mouse strains could influence the frequency of aneuploidy as well; however, up till now, such a comparison has not been available. Therefore, in our study, we measured the levels of aneuploidy which has resulted from missegregation in meiosis I, in oocytes of three commonly used mouse strains-CD-1, C3H/HeJ, and C57BL/6. Our results revealed that, although the overall chromosomal numerical aberration rates were similar in all three strains, a different number of oocytes in each strain contained prematurely segregated sister chromatids (PSSC). This indicates that a predisposition for this type of chromosome segregation error in oocyte meiosis I is dependent on genetic background.

High-resolution sex-specific linkage maps of the mouse reveal polarized distribution of crossovers in male germline

Since the publication of the first comprehensive linkage map for the laboratory mouse, the architecture of recombination as a basic biological process has become amenable to investigation in mammalian model organisms. Here we take advantage of high-density genotyping and the unique pedigree structure of the incipient Collaborative Cross to investigate the roles of sex and genetic background in mammalian recombination. Our results confirm the observation that map length is longer when measured through female meiosis than through male meiosis, but we find that this difference is modified by genotype at loci on both the X chromosome and the autosomes. In addition, we report a striking concentration of crossovers in the distal ends of autosomes in male meiosis that is absent in female meiosis. The presence of this pattern in both single- and double-recombinant chromosomes, combined with the absence of a corresponding asymmetry in the distribution of double-strand breaks, indicates a regulated sequence of events specific to male meiosis that is anchored by chromosome ends. This pattern is consistent with the timing of chromosome pairing and evolutionary constraints on male recombination. Finally, we identify large regions of reduced crossover frequency that together encompass 5% of the genome. Many of these "cold regions" are enriched for segmental duplications, suggesting an inverse local correlation between recombination rate and mutation rate for large copy number variants.

Building larger YACs by recombination

Despite the relatively large cloning capacity of YACs, many genomic regions or individual genes are not cloned intact, but are represented as a collection of overlapping clones or contigs. Fortunately, the relatively high frequency and fidelity of homologous recombination in Saccharomyces cerevisiae can be used to reconstruct intact genes within a single clone by splicing together overlapping DNA segments. This unit describes two protocols for carrying out such homologous recombination; one relies on the meiotic phase of the yeast cycle, while the other utilizes the mitotic phase of the yeast life cycle. Despite the relatively large cloning capacity of YACs, many genomic regions or individual genes are not cloned intact.

Effect of paternal age on the frequency of cytogenetic abnormalities in human spermatozoa

Many surveys have been performed to find etiological relationships between pregnancy outcome and specific risk factors, such as exposure to chemicals and radiation or parental age. Advanced maternal age is a strong risk factor for trisomic pregnancies, albeit there are considerable variations among the different chromosomes. The definite incidence of the various structural and numerical chromosome aberrations in spontaneous abortions and liveborns is well known, as well as the rate of maternally and paternally derived rearrangements. Nevertheless studies have failed to assert an age-dependent risk for men fathering chromosomally abnormal children. New techniques using fluorescence in situ hybridization render it possible to analyze spermatozoa directly for numerical and, to some extent, for structural aberrations. This article compiles the findings of studies on human spermatozoa over the last few years.

Competing crossover pathways act during meiosis in Saccharomyces cerevisiae

In Saccharomyces cerevisiae the MSH4-MSH5, MLH1-MLH3, and MUS81-MMS4 complexes act to promote crossing over during meiosis. MSH4-MSH5, but not MUS81-MMS4, promotes crossovers that display interference. A role for MLH1-MLH3 in crossover control is less clear partly because mlh1Delta mutants retain crossover interference yet display a decrease in crossing over that is only slightly less severe than that seen in msh4Delta and msh5Delta mutants. We analyzed the effects of msh5Delta, mlh1Delta, and mms4Delta single, double, and triple mutants on meiotic crossing over at four consecutive genetic intervals on chromosome XV using newly developed computer software. mlh1Delta mms4Delta double mutants displayed the largest decrease in crossing over (13- to 15-fold) of all mutant combinations, yet these strains displayed relatively high spore viability (42%). In contrast, msh5Delta mms4Delta and msh5Delta mms4Delta mlh1Delta mutants displayed smaller decreases in crossing over (4- to 6-fold); however, spore viability (18-19%) was lower in these strains than in mlh1Delta mms4Delta strains. These data suggest that meiotic crossing over can occur in yeast through three distinct crossover pathways. In one pathway, MUS81-MMS4 promotes interference-independent crossing over; in a second pathway, both MSH4-MSH5 and MLH1-MLH3 promote interference-dependent crossovers. A third pathway, which appears to be repressed by MSH4-MSH5, yields deleterious crossovers.

Involvement of Sir2/4 in silencing of DNA breakage and recombination on mouse YACs during yeast meiosis

Yeast artificial chromosomes (YACs) that contain human DNA backbone undergo DNA double-strand breaks (DSBs) and recombination during yeast meiosis at rates similar to the yeast native chromosomes. Surprisingly, YACs containing DNA covering a recombination hot spot in the mouse major histocompatibility complex class III region do not show meiotic DSBs and undergo meiotic recombination at reduced levels. Moreover, segregation of these YACs during meiosis is seriously compromised. In meiotic yeast cells carrying the mutations sir2 or sir4, but not sir3, these YACs show DSBs, suggesting that a unique chromatin structure of the YACs, involving Sir2 and Sir4, protects the YACs from the meiotic recombination machinery. We speculate that the paucity of DSBs and recombination events on these YACs during yeast meiosis may reflect the refractory nature of the corresponding region in the mouse genome.

Etoposide exposure during male mouse pachytene has complex effects on crossing-over and causes nondisjunction

In experiments involving different germ-cell stages, we had previously found meiotic prophase of the male mouse to be vulnerable to the induction of several types of genetic damage by the topoisomerase-II inhibitor etoposide. The present study of etoposide effects involved two end points of meiotic events known to occur in primary spermatocytes--chromosomal crossing-over and segregation. By following assortment of 13 microsatellite markers in two chromosomes (Ch 7 and Ch 15) it was shown that etoposide significantly affected crossing-over, but did not do so in a uniform fashion. Treatment generally changed the pattern for each chromosome, leading to local decreases in recombination, a distal shift in locations of crossing-over, and an overall decrease in double crossovers; at least some of these results might be interpreted as evidence for increased interference. Two methods were used to explore etoposide effects on chromosome segregation: a genetic experiment capable of detecting sex-chromosome nondisjunction in living progeny; and the use of FISH (fluorescence in situ hybridization) technology to score numbers of Chromosomes X, Y, and 8 in spermatozoa. Taken together these two approaches indicated that etoposide exposure of pachytene spermatocytes induces malsegregation, and that the findings of the genetic experiment probably yielded a marked underestimate of nondisjunction. As indicated by certain segregants, at least part of the etoposide effect could be due to disrupted pairing of achiasmatic homologs, followed by precocious sister-centromere separation. It has been shown for several organisms that absent or reduced levels of recombination, as well as suboptimally positioned recombination events, may be associated with abnormal segregation. Etoposide is the only chemical tested to date for which living progeny indicates an effect on both male meiotic crossing-over and chromosome segregation. Whether, however, etoposide-induced changes in recombination patterns are direct causes of the observed malsegregation requires additional investigation.

Effects of male age on the frequencies of germinal and heritable chromosomal abnormalities in humans and rodents

OBJECTIVE

To review evidence regarding the effects of male age on germinal and heritable chromosomal abnormalities using available human and rodent studies and to evaluate possible underlying mechanisms.

DESIGN

Review of English language-published research using MEDLINE database, excluding case reports and anecdotal data.

RESULT(S)

There was little evidence from offspring or germ cell studies for a generalized male age effect on autosomal aneuploidy, except in rodents. Sex chromosomal nondisjunction increased with age in both human and rodent male germ cells. Both human and rodent data showed age-related increases in the number of sperm with chromosomal breaks and fragments and suggest that postmeiotic cells are particularly vulnerable to the effects of aging. Translocation frequencies increased with age in murine spermatocytes, at rates comparable to mouse and human somatic cells. Age-related mechanisms of induction may include accumulation of environmental damage, reduced efficiency of DNA repair, increased genomic instability, genetic factors, hormonal influences, suppressed apoptosis, or decreased effectiveness of antioxidants and micronutrients.

CONCLUSION(S)

The weight of evidence suggests that the increasing trend toward fathering at older ages may have significant effects on the viability and genetic health of human pregnancies and offspring, primarily as a result of structural chromosomal aberrations in sperm.

Meiotic segregation of a homeologous chromosome pair

During meiosis, the alignment of homologous chromosomes facilitates their subsequent migration away from one another to opposite spindle poles at anaphase I. Recombination is part of the mechanism by which chromosomes identify their homologous partners, and serves to link the homologs in a way that, in some organisms, has been shown to promote proper attachment to the meiotic spindle. We have built a diploid strain that contains a pair of homeologous chromosomes V': one is derived from Saccharomyces cerevisiae and one originates from S. carlsbergensis. Sequence analysis reveals that these chromosomes share 71% sequence identity. The homeologs experience high levels of meiotic double-stranded breaks. Despite their relatedness and their competence to initiate recombination, the meiotic segregation behavior of the homeologous chromosomes suggests that, in most meioses, they are partitioned by a meiotic segregation system that has been shown previously to partition non-exchange chromosomes and pairs with no homology. Though the homeologous chromosomes show a degree of meiotic segregation fidelity similar to that of other non-exchange pairs, our data provide evidence that their limited sequence homology may provide some bias in meiotic partner choice.

The yeast kinetochore protein Slk19 is required to prevent aberrant chromosome segregation in meiosis and mitosis

BACKGROUND

Slk19 is a coiled-coil protein, which locates to the kinetochores of S. cerevisiae. Most cells lacking Slk19 undergo incomplete meiosis and form dyads during sporulation. Endogenous chromosomes appeared to be predominantly divided in an equational manner during single-division meiosis of slk19 null mutants.

RESULTS

We have monitored the segregation of artificial chromosomes (YACs) in slk19 null mutants during both single-division meiosis and complete meiosis. In contrast to the results obtained with endogenous chromosomes, YACs only rarely undergo equational segregation during single division meiosis, although high rates of aberrant segregation were detected. This accounts for the high frequency of lethal spores among dyads of slk19 delta null mutants. The fraction of slk19 delta cells that were able to form tetrads solely exhibited YAC segregation defects in meiosis II, whereas the segregation of YACs in meiosis I was normal in these cells. This result might indicate that correct chromosome division in meiosis I is a prerequisite for tetrad formation. slk19 null mutants also showed YAC instability in mitosis and reduced survival after the induction of mitotic spindle damage.

CONCLUSION

Slk19 is required to avoid aberrant segregation of chromosomes in meiosis I and II and in mitosis. We suggest that the absence of Slk19 leads to uncoupling of chromosome movement from completion of microtubule attachment and resolution of chromosome cohesion.

To err (meiotically) is human: the genesis of human aneuploidy

Aneuploidy (trisomy or monosomy) is the most commonly identified chromosome abnormality in humans, occurring in at least 5% of all clinically recognized pregnancies. Most aneuploid conceptuses perish in utero, which makes this the leading genetic cause of pregnancy loss. However, some aneuploid fetuses survive to term and, as a class, aneuploidy is the most common known cause of mental retardation. Despite the devastating clinical consequences of aneuploidy, relatively little is known of how trisomy and monosomy originate in humans. However, recent molecular and cytogenetic approaches are now beginning to shed light on the non-disjunctional processes that lead to aneuploidy.

Meiotic chromosomes: integrating structure and function

Meiotic chromosomes have been studied for many years, in part because of the fundamental life processes they represent, but also because meiosis involves the formation of homolog pairs, a feature which greatly facilitates the study of chromosome behavior. The complex events involved in homolog juxtaposition necessitate prolongation of prophase, thus permitting resolution of events that are temporally compressed in the mitotic cycle. Furthermore, once homologs are paired, the chromosomes are connected by a specific structure: the synaptonemal complex. Finally, interaction of homologs includes recombination at the DNA level, which is intimately linked to structural features of the chromosomes. In consequence, recombination-related events report on diverse aspects of chromosome morphogenesis, notably relationships between sisters, development of axial structure, and variations in chromatin status. The current article reviews recent information on these topics in an historical context. This juxtaposition has suggested new relationships between structure and function. Additional issues were addressed in a previous chapter (551).

Integrity of human YACs during propagation in recombination-deficient yeast strains

Several isogenic strains with defects in recombination/repair genes (RAD1, RAD50, RAD51, RAD52, RAD54, and RAD55) were examined for their ability to propagate accurately a variety of linear and circular yeast artificial chromosomes (YACs) containing human DNA inserts. To assess YAC stability, the human DNA inserts were internally marked by an ADE2-pBR-URA3 cassette. Following selection for Ura- clones on 5-fluoroorotic acid containing medium, the following types of YAC deletions were identified: (i) those caused by homologous recombination with a telomeric pBR sequence; (ii) internal deletions, presumed to occur by recombination between commonly occurring DNA repeats such as Alu and LINE sequences; and (iii) deletions leading to loss of part of a YAC arm. rad52 host strains, but not other recombination-deficient strains, decreased the rate of all types of YAC deletions 25- to 400-fold. We have also developed and tested kar1 strains with a conditional RAD52 gene that allow transfer of a YAC from any host into a recombination-deficient background. These strains provide an efficient tool for stabilization of YACs and are useful for allowing additional recombinational modification of YACs.

Chiasmata, crossovers, and meiotic chromosome segregation

Meiotic recombination events are probably critical for the completion of several meiotic processes. In addition, recombination is likely to be involved in the events that lead up to synapsis of homologues in meiotic prophase. Recombination events that ultimately become resolved as exchanges are needed for the formation of chiasmata. Chiasmata maintain the association of paired homologues following loss of the synaptonemal complex and participate in the mechanism that signals that the bivalent has attached to the spindle in a bipolar orientation that will result in meiosis I disjunction.

Tam1, a telomere-associated meiotic protein, functions in chromosome synapsis and crossover interference

The TAM1 gene of Saccharomyces cerevisiae is expressed specifically during meiosis and encodes a protein that localizes to the ends of meiotic chromosomes. In a tam1 null mutant, there is an increase in the frequency of chromosomes that fail to recombine and an associated increase in homolog nondisjunction at meiosis I. The tam1 mutant also displays an increased frequency of precocious separation of sister chromatids and a reduced efficiency of distributive disjunction. The defect in distributive disjunction may be attributable to overloading of the distributive system by the increased number of nonrecombinant chromosomes. Recombination is not impaired in the tam1 mutant, but crossover interference is reduced substantially. In addition, chromosome synapsis is delayed in tam1 strains. The combination of a defect in synapsis and a reduction in interference is consistent with previous studies suggesting a role for the synaptonemal complex in regulating crossover distribution. tam1 is the only known yeast mutant in which the control of crossover distribution is impaired, but the frequency of crossing over is unaffected. We discuss here possibilities for how a telomere-associated protein might function in chromosome synapsis and crossover interference.

Ndj1p, a meiotic telomere protein required for normal chromosome synapsis and segregation in yeast

The Saccharomyces cerevisiae gene NDJ1 (nondisjunction) encodes a protein that accumulates at telomeres during meiotic prophase. Deletion of NDJ1 (ndj1Delta) caused nondisjunction, impaired distributive segregation of linear chromosomes, and disordered the distribution of telomeric Rap1p, but it did not affect distributive segregation of circular plasmids. Induction of meiotic recombination and the extent of crossing-over were largely normal in ndj1Delta cells, but formation of axial elements and synapsis were delayed. Thus, Ndj1p may stabilize homologous DNA interactions at telomeres, and possibly at other sites, and it is required for a telomere activity in distributive segregation.

Meiotic crossing over between nonhomologous chromosomes affects chromosome segregation in yeast

Meiotic recombination between artificial repeats positioned on nonhomologous chromosomes occurs efficiently in the yeast Saccharomyces cerevisiae. Both gene conversion and crossover events have been observed, with crossovers yielding reciprocal translocations. In the current study, 5.5-kb ura3 repeats positioned on chromosomes V and XV were used to examine the effect of ectopic recombination on meiotic chromosome segregation. Ura3 random spores were selected and gene conversion vs. crossover events were distinguished by Southern blot analysis. Approximately 15% of the crossover events between chromosomes V and XV were associated with missegregation of one of these chromosomes. The missegregation was manifest as hyperploid spores containing either both translocations plus a normal chromosome, or both normal chromosomes plus one of the translocations. In those cases where it could be analyzed, missegregation occurred at the first meiotic division. These data are discussed in terms of a model in which ectopic crossovers compete efficiently with normal allelic crossovers in directing meiotic chromosome segregation.

Stabilization of yeast artificial chromosome clones in a rad54-3 recombination-deficient host strain

The cloning and propagation of large fragments of DNA on yeast artificial chromosomes (YACs) has become a routine and valuable technique in genome analysis. Unfortunately, many YAC clones have been found to undergo rearrangements or deletions during the cloning process. The frequency of transformation-associated alterations and mitotic instability can be reduced in a homologous recombination-deficient yeast host strain such as a rad52 mutant. RAD52 is one member of an epistatic group of genes required for the recombinational repair of double-strand breaks in DNA. rad52 mutants grow more slowly and transform less efficiently than RAD + strains and are therefore not ideal hosts for YAC library construction. We have investigated the ability of both null and temperature-sensitive alleles of RAD54 , another member of the RAD52 epistasis group, to prevent rearrangements of human YAC clones containing tandemly repeated DNA sequences. Our results show that the temperature-sensitive rad54-3 allele blocks mitotic recombination between tandemly repeated DYZ3 satellite sequences and significantly stabilizes a human DYZ5 satellite-containing YAC clone. Yeast carrying the rad54-3 mutation can undergo meiosis, have growth and transformation rates comparable with RAD + strains, and therefore represent improved YAC cloning hosts.

Patterns of meiotic double-strand breakage on native and artificial yeast chromosomes

The preferred positions for meiotic double-strand breakage were mapped on Saccharomyces cerevisiae chromosomes I and VI, and on a number of yeast artificial chromosomes carrying human DNA inserts. Each chromosome had strong and weak double-strand break (DSB) sites. On average one DSB-prone region was detected by pulsed-field gel electrophoresis per 25 kb of DNA, but each chromosome had a unique distribution of DSB sites. There were no preferred meiotic DSB sites near the telomeres. DSB-prone regions were associated with all of the known "hot spots" for meiotic recombination on chromosomes I, III and VI.

Double-strand breaks on YACs during yeast meiosis may reflect meiotic recombination in the human genome

Meiotic recombination in the yeast Saccharomyces cerevisiae is initiated at double-strand breaks (DSBs), which occur preferentially at specific locations. Genetically mapped regions of elevated meiotic recombination ('hotspots') coincide with meiotic DSB sites, which can be identified on chromosome blots of meiotic DNA (refs 4,5; S.K. et al., manuscript submitted). The morphology of yeast artificial chromosomes (YACs) containing human DNA during the pachytene stage of meiosis resembles that of native yeast chromosomes. Homologous YAC pairs segregate faithfully and recombine at the high rates characteristic of S. cerevisiae (vs. approximately 0.4 cM/kb in S. cerevisiae versus approximately 10-3 cM/kb in humans). We have examined a variety of YACs carrying human DNA inserts for double-strand breakage during yeast meiosis. Each YAC has a characteristic set of meiotic DSB sites, as do yeast chromosomes (S.K. et al., manuscript submitted). We show that the positions of the DSB sites in the YACs depend on the human-derived DNA in the clones. The degree of double-strand breakage in yeast meiosis of the YACs in our study appears to reflect the degree of meiotic recombination in humans.

Exchanges are not equally able to enhance meiotic chromosome segregation in yeast

Homologous chromosomes pair, and then migrate to opposite poles of the spindle at meiosis I. In most eukaryotic organisms, reciprocal recombinations (crossovers) between the homologs are critical to the success of this process. Individuals with defects in meiotic recombination typically produce high levels of aneuploid gametes and exhibit low fertility or are sterile. The experiments described here were designed to test whether different crossovers are equally able to contribute to the fidelity of meiotic chromosome segregation in yeast. These experiments were performed with model chromosomes with which it was possible to control and measure the distributions of meiotic crossovers in wild-type cells. Physical and genetic approaches were used to map crossover positions on model chromosomes and to correlate crossover position with meiotic segregation behavior. The results show that crossovers at different chromosomal positions have different abilities to enhance the fidelity of meiotic segregation.

Meiotic recombination and germ cell aneuploidy

Data on human trisomic conceptuses suggest that the extra chromosome commonly has a maternal origin, and the amount and position of crossing-over on nondisjoined chromosomes is commonly altered. These observations may provide important clues to the etiology of human germ cell aneuploidy, especially in regard to evaluating whether environmental factors play a role. There is concordance of effects of environmental agents on fungi, plants, and animals, which suggests that the overall process of meiosis is well conserved and that chemical and physical agents can affect meiotic recombination, leading to aneuploidy. It seems likely that meiosis in humans will fit the general pattern of meiosis in terms of sensitivity to radiation and chemicals. Thus studies on other organisms provide some insight into the procedures necessary for obtaining useful human data. For example, frequencies of spontaneous meiotic recombination are not uniform per physical length in Drosophila, and different regions of a chromosome respond differently to treatment. Treatments that relieve constraints on the distribution of meiotic exchange, without changing greatly the overall frequency of exchange, may increase the number of univalents and give the impression that there are chromosome-specific responses. Recombination studies that monitor one or a few relatively short genetic regions may also give a false impression of the effects of a treatment on recombination. In addition, meiotic mutants in Saccharomyces and Drosophila highlight a number of processes that are important for production of an exchange event and the utility of that event in the proper segregation of both homologues and sisters. They also suggest that tests for pairing at pachytene, chiasmata at diplotene, and genetic crossing-over may give different results.

Effects of homology, size and exchange of the meiotic segregation of model chromosomes in Saccharomyces cerevisiae

In most eukaryotic organisms, chiasmata, the connections formed between homologous chromosomes as a consequence of crossing over, are important for ensuring that the homologues move away from each other at meiosis I. Some organisms have the capacity to partition the rare homologues that have failed to experience reciprocal recombination. The yeast Saccharomyces cerevisiae is able to correctly partition achiasmate homologues with low fidelity by a mechanism that is largely unknown. It is possible to test which parameters affect the ability of achiasmate chromosomes to segregate by constructing strains that will have three achiasmate chromosomes at the time of meiosis. The meiotic partitioning of these chromosomes can be monitored to determine which ones segregate away from each other at meiosis I. This approach was used to test the influence of homologous yeast DNA sequences, recombination intiation sites, chromosome size and crossing over on the meiotic segregation of the model chromosomes. Chromosome size had no effect on achiasmate segregation. The influence of homologous yeast sequences on the segregation of noncrossover model chromosomes was negligible. In meioses in which two of the three model chromosomes experienced a crossover, they nearly always disjoined at meiosis I.

Sex and the single cell: meiosis in yeast

Recent studies of Saccharomyces cerevisiae have significantly advanced our understanding of the molecular mechanisms of meiotic chromosome behavior. Structural components of the synaptonemal complex have been identified and studies of mutants defective in synapsis have provided insight into the role of the synaptonemal complex in homolog pairing, genetic recombination, crossover interference, and meiotic chromosome segregation. There is compelling evidence that most or all meiotic recombination events initiate with double-strand breaks. Several intermediates in the double-strand break repair pathway have been characterized and mutants blocked at different steps in the pathway have been identified. With the application of genetic, molecular, cytological, and biochemical methods in a single organism, we can expect an increasingly comprehensive and unified view of the meiotic process.

Morphology of a human-derived YAC in yeast meiosis

In meiosis of human males DNA is packaged along pachytene chromosomes about 20 times more compactly than in meiosis of yeast. Nevertheless, a human-derived yeast artificial chromosome (YAC) shows the same degree of compaction of DNA as endogenous chromosomes in meiotic prophase nuclei of yeast. This suggests that in yeast meiosis, human and yeast DNA adopt a similar organization of chromatin along the pachytene chromosome cores. Therefore meiotic chromatin organization does not seem to be an inherent chromosomal property but is governed by the host-specific cellular environment. We suggest that there is a correlation between the less dense DNA packaging and the increased rate of recombination that has been reported for human-derived YACs as compared with human DNA in its natural environment.

Cis-acting determinants affecting centromere function, sister-chromatid cohesion and reciprocal recombination during meiosis in Saccharomyces cerevisiae

We have employed a system that utilizes homologous pairs of human DNA-derived yeast artificial chromosomes (YACs) as marker chromosomes to assess the specific role(s) of conserved centromere DNA elements (CDEI, CDEII and CDEIII) in meiotic chromosome disjunction fidelity. Thirteen different centromere (CEN) mutations were tested for their effects on meiotic centromere function. YACs containing a wild-type CEN DNA sequence segregate with high fidelity in meiosis I (99% normal segregation) and in meiosis II (96% normal segregation). YACs containing a 31-bp deletion mutation in centromere DNA element II (CDEII delta 31) in either a heterocentric (mutant/wild type), homocentric (mutant/mutant) or monosomic (mutant/--) YAC pair configuration exhibited high levels (16-28%) of precocious sister-chromatid segregation (PSS) and increased levels (1-6%) of nondisjunction meiosis I (NDI). YACs containing this mutation also exhibit high levels (21%) of meiosis II nondisjunction. Interestingly, significant alterations in homolog recombination frequency were observed in the exceptional PSS class of tetrads, suggesting unusual interactions between prematurely separated sister chromatids and their homologous nonsister chromatids. We also have assessed the meiotic segregation effects of rare gene conversion events occurring at sites located immediately adjacent to or distantly from the centromere region. Proximal gene conversion events were associated with extremely high levels (60%) of meiosis I segregation errors (including both PSS and NDI), whereas distal events had no apparent effect. Taken together, our results indicate a critical role for CDEII in meiosis and underscore the importance of maintaining sister-chromatid cohesion for proper recombination in meiotic prophase and for proper disjunction in meiosis I.

Introduction of YACs into intact yeast cells by a procedure which shows low levels of recombinagenicity and co-transformation

Yeast artificial chromosomes (YACs) enable the cloning and analysis of large segments of genomic DNA and permit the isolation of sequences which are impossible to maintain in Escherichia coli. However, the construction of genome libraries in YAC vectors is beset by a number of technical problems, not least of which is the creation of cloned fragments which are not true representatives of the donor genome. These artefactual clones arise mainly due to intra-fragment rearrangements or inter-fragment chimaera formation, both phenomena resulting from the activity of the host yeast's mitotic recombination system. We demonstrate that this system is significantly stimulated by the spheroplasting step of the standard YAC transformation system. In contrast, the transformation of intact yeast cells by either the lithium method or a new lithium-free protocol is much less recombinagenic. It is not possible to introduce high molecular weight YACs into yeast using the lithium protocol, but we find that such molecules may be introduced into pde2-mutants using the lithium-free approach. Since intact cells are transformed by this method, automation of post-transformation steps in the construction of YAC libraries is facilitated. Moreover, the frequency of cotransformation (and, therefore, chimera formation) is significantly reduced. However, these advantages do incur a penalty. Yields of YAC transformants by this simplified intact cell approach are reduced some 25- to 30-fold compared to those obtained by the spheroplast transformation route. Nevertheless, the considerable advantages of the new system recommend it for a number of applications.

Recombination during transformation as a source of chimeric mammalian artificial chromosomes in yeast (YACs)

Mammalian DNAs cloned as artificial chromosomes in yeast (YACs) frequently are chimeras formed between noncontiguous DNAs. Using pairs of human and mouse YACs we examined the contribution of recombination during transformation or subsequent mitotic growth to chimeric YAC formation. The DNA from pairs of yeast strains containing homologous or heterologous YACs was transformed into a third strain under conditions typical for the development of YAC libraries. One YAC was selected and the presence of the second was then determined. Co-penetration of large molecules, as deduced from co-transformation of markers identifying the different YACs, was > 50%. In approximately half the cells receiving two homologous YACs, the YACs had undergone recombination. Co-transformation depends on recombination since it was reduced nearly 10-fold when the YACs were heterologous. While mitotic recombination between homologous YACs is nearly 100-fold higher than for yeast chromosomes, the level is still much lower than observed during transformation. To investigate the role of commonly occurring Alu repeats in chimera formation, spheroplasts were transformed with various human YACs and an unselected DNA fragment containing an Alu at one end and a telomere at the other. When unbroken YACs were used, between 1 and 6% of the selected YACs could incorporate the fragment as compared to 49% when the YACs were broken. We propose that Alu's or other commonly occurring repeats could be an important source of chimeric YACs. Since the frequency of chimeras formed between YACs or a YAC and an Alu-containing fragment was reduced when a rad52 mutant was the recipient and since intra-YAC deletions are reduced, rad52 and possibly other recombination-deficient mutants are expected to be useful for YAC library development.

SPO13 negatively regulates the progression of mitotic and meiotic nuclear division in Saccharomyces cerevisiae

The meiosis-specific yeast gene SPO13 has been previously shown to be required to obtain two successive divisions in meiosis. We report here that vegetative expression of this gene causes a CDC28-dependent cell-cycle arrest at mitosis. Overexpression of SPO13 during meiosis causes a transient block to completion of the meiosis I division and suppresses the inability of cdc28ts strains to execute meiosis II. The spo13 defect can be partially suppressed by conditions that slow progression of the first meiotic division. Based on the results presented below, we propose that SPO13 acts as a meiotic timing function by transiently blocking progression through the meiosis I division, thereby allowing (1) coordination of the first division with assembly of the reductional segregation apparatus, and (2) subsequent entry into a second round of segregation to separate replicated sister chromatids without an intervening S-phase.

Genetic selection of meiotic and mitotic recombinant yeast artificial chromosomes

We have developed a genetic screen for the isolation of larger or smaller recombinant yeast artificial chromosomes derived from overlapping YACs. Integration plasmids were used to modify the TRP1 and URA3 auxotrophic markers present respectively on the left and right vector arms of one of the parental YACs. Diploids containing the two parental YACs were studied through meiosis and mitosis. Tetrad analysis revealed the presence of meiotic recombinant YACs at a frequency comparable with what is expected for yeast DNA (about 3 kb/cM). More direct genetic selection of diploids on -TRP-LYS synthetic media in the presence of 5-fluoro-orotic acid (5-FOA), led to the isolation of mitotic recombinant YACs at a high frequency. Analysis of these yeast cells by pulsed-field gel electrophoresis, confirmed the loss of both parental artificial chromosomes, and the specific retention of a larger or smaller recombinant YAC.

Yeast artificial chromosome cloning

Yeast artificial chromosome (YAC) cloning systems enable the cloning of DNA stretches of 50 to well over 2000 kb. This makes it possible to study large intact regions of DNA in detail, by restriction mapping the YAC to produce a physical map and by examining the YAC for coding sequences or genes. YACs are important for their ability to clone the complete sequences of large genes or gene complexes that exceed the size limit for cloning in conventional bacterial cloning vectors like plasmids (up to 10 kb), bacteriophage (15 kb), and cosmids (50 kb). A major advantage of cloning in yeast, a eukaryote, is that many sequences that are unstable, underrepresented, or absent when cloned into prokaryotic systems, remain stable and intact in YAC clones. It is possible to reintroduce YACs intact into mammalian cells where the introduced mammalian genes are expressed and used to study the functions of genes in the context of flanking sequences. The correct protein processing mechanisms are present in the mammalian cells to ensure that a viable protein product is produced.

Double or nothing: a Drosophila mutation affecting meiotic chromosome segregation in both females and males

We describe a Drosophila mutation, Double or nothing (Dub), that causes meiotic nondisjunction in a conditional, dominant manner. Previously isolated mutations in Drosophila specifically affect meiosis either in females or males, with the exception of the mei-S332 and ord genes which are required for proper sister-chromatid cohesion. Dub is unusual in that it causes aberrant chromosome segregation almost exclusively in meiosis I in both sexes. In Dub mutant females both nonexchange and exchange chromosomes undergo nondisjunction, but the effect of Dub on nonexchange chromosomes is more pronounced. Dub reduces recombination levels slightly. Multiple nondisjoined chromosomes frequently cosegregate to the same pole. Dub results in nondisjunction of all chromosomes in meiosis I of males, although the levels are lower than in females. When homozygous, Dub is a conditional lethal allele and exhibits phenotypes consistent with cell death.

The centromere of budding yeast

Stable maintenance of genetic information during meiosis and mitosis is dependent on accurate chromosome transmission. The centromere is a key component of the segregational machinery that couples chromosomes with the spindle apparatus. Most of what is known about the structure and function of the centromeres has been derived from studies on yeast cells. In Saccharomyces cerevisiae, the centromere DNA requirements for mitotic centromere function have been defined and some of the proteins required for an active complex have been identified. Centromere DNA and the centromere proteins form a complex that has been studied extensively at the chromatin level. Finally, recent findings suggest that assembly and activation of the centromere are integrated in the cell cycle.

What determines whether chromosomes segregate reductionally or equationally in meiosis?

Normal meiosis consists of a single round of DNA replication followed by two nuclear divisions. In the 1st division the chromosomes segregate reductionally whereas in the 2nd division they segregate equationally (as they do in mitosis). In certain yeast mutants, a single-division meiosis takes place, in which some chromosomes segregate reductionally while others divide equationally. This autonomous segregation behaviour of individual chromosomes on a common spindle is determined by the centromeres they carry. The relationship between reductional segregation of a pair of chromosomes and their earlier recombinational history is also discussed.

Centromere DNA mutations induce a mitotic delay in Saccharomyces cerevisiae

Cytological observations of animal cell mitoses have shown that the onset of anaphase is delayed when chromosome attachment to the spindle is spontaneously retarded or experimentally interrupted. This report demonstrates that a centromere DNA (CEN) mutation carried on a single chromosome can induce a cell cycle delay observed as retarded mitosis in the yeast Saccharomyces cerevisiae. A 31-base-pair deletion within centromere DNA element II (CDEII delta 31) that causes chromosome missegregation in only 1% of cell division elicited a dramatic mitotic delay phenotype. Other CEN DNA mutations, including mutations in centromere DNA elements I and III, similarly delayed mitosis. Single division pedigree analysis of strains containing the CDEII delta 31 CEN mutation indicated that most (and possibly all) cells experienced delay in each cell cycle and that the delay was not due to increased chromosome copy number. Furthermore, a synchronous population of cells containing the CDEII delta 31 mutation underwent DNA synthesis on schedule with wild-type kinetics, but subsequently exhibited late chromosomal separation and concomitant late cell separation. We speculate that this delay in cell cycle progression before the onset of anaphase provides a mechanism for the stabilization of chromosomes with defective kinetochore structure. Further, we suggest that the delay may be mediated by surveillance at a cell cycle checkpoint that monitors the completion of chromosomal attachment to the spindle.

There are two mechanisms of achiasmate segregation in Drosophila females, one of which requires heterochromatic homology

There are numerous examples of the regular segregation of achiasmate chromosomes at meiosis I in Drosophila melanogaster females. Classically, the choice of achiasmate segregational partners has been thought to be independent of homology, but rather made on the basis of availability or similarities in size and shape. To the contrary, we show here that heterochromatic homology plays a primary role in ensuring the proper segregation of achiasmate homologs. We observe that the heterochromatin of chromosome 4 functions as, or contains, a meiotic pairing site. We show that free duplications carrying the 4th chromosome pericentric heterochromatin induce high frequencies of 4th chromosome nondisjunction regardless of their size. Moreover, a duplication from which some of the 4th chromosome heterochromatin has been removed is unable to induce 4th chromosome nondisjunction. Similarly, in the absence of either euchromatic homology or a size similarity, duplications bearing the X chromosome heterochromatin also disrupt the segregation of two achiasmate X chromosome centromeres. Although heterochromatic regions are sufficient to conjoin nonexchange homologues, we confirm that the segregation of heterologous chromosomes is determined by size, shape, and availability. The meiotic mutation Axs differentiates between these two processes of achiasmate centromere coorientation by disrupting only the homology-dependent mechanism. Thus there are two different mechanisms by which achiasmate segregational partners are chosen. We propose that the absence of diplotene-diakinesis during female meiosis allows heterochromatic pairings to persist until prometaphase and thus to co-orient homologous centromeres. We also propose that heterologous disjunctions result from a separate and homology-independent process that likely occurs during prometaphase. The latter process, which may not require the physical association of segregational partners, is similar to those observed in many insects, in Saccharomyces cerevisiae and in C. elegans males. We also suggest that the physical basis of this process may reflect known properties of the Drosophila meiotic spindle.