Meiotic Chromosome Dynamics Dependent Upon the rec8⁺, rec10⁺ and rec11⁺ Genes of the Fission Yeast Schizosaccharomyces pombe

Corresponding author: Wayne P. Wahls, Department of Biochemistry, Vanderbilt University School of Medicine, 621 Light Hall, Nashville, TN 37232-0146., wahlswp{at}ctrvax.vanderbilt.edu (E-mail)

Communicating editor: M. LICHTEN

	ABSTRACT

TOP ABSTRACT MATERIALS AND METHODS RESULTS DISCUSSION LITERATURE CITED

During meiosis homologous chromosomes replicate once, pair, experience recombination, and undergo two rounds of segregation to produce haploid meiotic products. The rec8⁺, rec10⁺, and rec11⁺ genes of the fission yeast Schizosaccharomyces pombe exhibit similar specificities for meiotic recombination and rec8⁺ is required for sister chromatid cohesion and homolog pairing. We applied cytological and genetic approaches to identify potential genetic interactions and to gauge the fidelity of meiotic chromosome segregation in the mutants. The rec8⁺ gene was epistatic to rec10⁺ and to rec11⁺, but there was no clear epistatic relationship between rec10⁺ and rec11⁺. Reciprocal (crossover) recombination in the central regions of all three chromosomes was compromised in the rec mutants, but recombination near the telomeres was nearly normal. Each of the mutants also exhibited a high rate of aberrant segregation for all three chromosomes. The rec8 mutations affected mainly meiosis I segregation. Remarkably, the rec10 and rec11 mutations, which compromised recombination during meiosis I, affected mainly meiosis II segregation. We propose that these genes encode regulators or components of a "meiotic chromatid cohesion" pathway involved in establishing, maintaining, and appropriately releasing meiotic interactions between chromosomes. A model of synergistic interactions between sister chromatid cohesion and crossover position suggests how crossovers and cohesion help ensure the proper segregation of chromosomes in each of the meiotic divisions.

MEIOSIS is a key component of sexual reproduction in eukaryotes. After premeiotic DNA replication, each chromosome is composed of two sister chromatids that remain associated with each other. Homologous chromosomes (two sets of sister chromatids) then pair to form a bivalent and undergo recombination. This is followed by the meiosis I (MI) reductional division, in which homologous chromosomes segregate from each other, and then the meiosis II (MII) equational division, in which sister chromatids segregate from each other to complete meiosis.

Recombination occurs at a high frequency during meiosis and serves to generate genetic diversity and, in most organisms, to help ensure the appropriate segregation of chromosomes at the first meiotic division (MOENS 1994 ; ROEDER 1997 ; MOORE and ORR-WEAVER 1998 ). Crossovers (reciprocal recombination events) generate chiasmata, which are cytologically visible nucleoprotein structures that help to stabilize connections between homologous chromosomes so they can orient properly on the MI spindle and thus segregate accurately.

The rec8⁺, rec10⁺, and rec11⁺ genes of fission yeast were identified in a screen for mutations that decrease meiotic recombination at the ade6 locus (PONTICELLI and SMITH 1989 ). Their expression is induced in meiosis (LIN et al. 1992 ; LIN and SMITH 1995 ; LI et al. 1997 ); they are reportedly specific for recombination on chromosome III (DEVEAUX and SMITH 1994 ), but their primary function may be in some other meiotic process. Upon chromosome synapsis, Schizosaccharomyces pombe does not form a tripartite synaptonemal complex (SC), but it does form discontinuous patches of filamentous structures, known as "linear elements," similar to axial elements of SC (BAHLER et al. 1993 ; SCHERTHAN et al. 1994 ). The rec8 mutants do not properly pair the internal regions of homologous chromosomes, they fail to form proper linear elements, and they suffer precocious separation of sister chromatids and nondisjunction of homologous chromosomes during MI (MOLNAR et al. 1995 ; KRAWCHUK and WAHLS 1999 ).

After correcting (PARISI et al. 1999 ) for multiple errors in the published coding sequence (LIN et al. 1992 ), rec8⁺ has homology to a family of mitotically expressed genes defined by the rad21⁺ gene of S. pombe. The rad21⁺ gene and other rad21⁺ "cohesin" gene family members are required for the proper mitotic transmission of chromosomes (BIRKENBIHL and SUBRAMANI 1992 , BIRKENBIHL and SUBRAMANI 1995 ; MCKAY et al. 1996 ; GUACCI et al. 1997 ; MICHAELIS et al. 1997 ; HEO et al. 1998 ; TATEBAYASHI et al. 1998 ). The rec10⁺ gene (LIN and SMITH 1995 ) encodes a pioneer protein with no significant primary sequence homology to proteins or translations of DNA sequences currently present in computer databases (unpublished observations). The predicted Rec11 polypeptide has sequence conservation with several proteins of unknown function present in the databases, including a hypothetical protein encoded in S. pombe genomic DNA (GenBank accession no. Z98597; our unpublished observations).

We have characterized the roles of the rec8⁺, rec10⁺, and rec11⁺ genes in meiosis. We report that they exhibit epistatic interactions and are variously required for aspects of chromosome dynamics ranging from the initial pairing of homologous chromosomes to chromosome segregation in the second meiotic division. Models in which a "meiotic chromatid cohesion" pathway might participate in these functions are presented.

	MATERIALS AND METHODS

TOP ABSTRACT MATERIALS AND METHODS RESULTS DISCUSSION LITERATURE CITED

S. pombe culture:
The S. pombe strain genotypes are listed in Table 1. Culture media and genetic methods were as previously described (GUTZ et al. 1974 ; KON et al. 1997 ). Yeast extract agar (YEA)-B was YEA containing 2.5 µg/ml of Phloxin-B (Sigma, St. Louis) and 100 µg/ml of adenine. Synthetic sporulation agar (SPA), nitrogen base agar (NBA), and nitrogen base liquid (NBL) were supplemented with required amino acids, adenine, and uracil at 100 µg/ml.

Recombinant frequency determination:
Mating, meiosis, and preparation of free spores were as previously described (GUTZ et al. 1974 ; KON et al. 1997 ). Intergenic (crossover) recombination was measured by plating spores on media with various combinations of growth factors to select for double prototrophic recombinants. The recombinant frequency was calculated relative to the viable spore titer. Because diploid spores could contain complementing markers and be mistaken for recombinants, we tested all spore colonies for diploidy by replica plating to media containing Phloxin-B (YEA-B). On Phloxin-B-containing plates, haploid cells produce light pink colonies whereas diploid cells produce dark pink colonies (GUTZ et al. 1974 ). Diploid meiotic products were excluded from recombinant frequency determinations (below; Table 4).

Diploid spore isolation and haploidization analysis:
Spores were plated on YEA-B and incubated for 3 days at 32° to distinguish the diploid spore colonies from the haploid spore colonies. The ploidy was confirmed by microscopic examination of the cells. Diploid cells are both longer and wider than haploid cells and can be readily distinguished under the microscope.

The parental haploid strains that were crossed had different alleles at loci on the three chromosomes. The genotypes of the resulting diploid spore colonies were determined by analysis of haploidized colonies on diagnostic plates. Diploid spore colonies were haploidized with m-fluorophenylalanine (m-FPA; KOHLI et al. 1977 ). Fifty haploidized colonies derived from each individual diploid spore colony were replica plated to various differentially supplemented minimal media to genotype the ade6 and lys1 loci. The tps13 alleles were scored by replica plating colonies onto rich (YEA) media and scoring for growth at the permissive (22°) and restrictive (35°) temperatures. The mat1 alleles were determined by test crossing to strains of known mating type and iodine staining of the resultant asci (GUTZ et al. 1974 ).

Microscopic analysis:
To monitor chromosome segregation, asci from meiotic cultures were fixed with 95% ethanol, washed with PBS, and stained with 4,6-diamidino-2-phenylindole (DAPI) at a final concentration of 1 µg/ml. Cells were spread on poly-L-lysine-coated slides and visualized by differential interference contrast (DIC) and fluorescence microscopy with a Zeiss axiophot (Carl Zeiss, Thornwood, NY). Images were captured using the MetaMorph software package (Universal Imaging, West Chester, PA).

	RESULTS

TOP ABSTRACT MATERIALS AND METHODS RESULTS DISCUSSION LITERATURE CITED

Aberrant meiotic development in the rec8, rec10, and rec11 mutants:
S. pombe is a particularly attractive model organism in which to study meiotic chromosome dynamics because it has only three chromosome pairs and all four products of each meiosis (the spores) are held together in an ascus. To gain insight into the functions of the rec8⁺, rec10⁺, and rec11⁺ genes, asci from meiotic cultures were stained with a DNA-specific fluorescent dye (DAPI) and visualized by DIC and fluorescence microscopy (Figure 1). The morphology of a typical wild-type zygotic ascus was slightly curved and contained four well-rounded spores, each with a single DAPI-staining body of equal intensity (Figure 1A). The rec8, rec10, and rec11 mutants were proficient at meiosis and underwent two meiotic divisions as revealed by ascus development and the distribution of chromosomes (Figure 1, B–J). Strikingly, each mutant produced a high frequency of chromosome segregation errors that were sometimes accompanied by defects in ascus development (Figure 1, B–J). The data from a large number of asci from each mutant are presented schematically in Table 2.

View larger version (96K): In this window In a new window Download PPT slide   Figure 1. Representative cytological phenotypes of wild-type and rec mutant meiotic products. (A–J) DIC (top) and DAPI fluorescence (bottom) images of asci from (A) wild-type; (B–D) rec8; (E–G) rec10; and (H–J) rec11 mutant meioses. The correspondence between each of the cytological phenotypes and their schematic representation is provided in Table 2.

The cytological phenotypes of the rec8 mutant were consistent with the reported (MOLNAR et al. 1995 ; KRAWCHUK and WAHLS 1999 ) precocious separation of sister chromatids and nondisjunction of homologous chromosomes during MI: ascus development was nearly normal, but the chromosomes partitioned abnormally (Figure 1 and Table 2). In contrast, the chromosome segregation defects of the rec10 and rec11 mutants were often accompanied by additional defects such as abnormal spore placement, reduced spore numbers, and asynchronous spore maturation (Figure 1 and Table 2). These ascus phenotypes are characteristic of an MII defect because spore formation in S. pombe is controlled by the spindle pole body of the MII spindle (TANAKA and HIRATA 1982 ; SHIMODA et al. 1985 ; HIRATA and SHIMODA 1992 , HIRATA and SHIMODA 1994 ). In some cases a defect in the segregation of sister chromatids during MII was directly inferred. For example, in Figure 1I it appears that the sister chromatids began to segregate from one another during MII and then became stuck. Thus, the rec10⁺ and rec11⁺ genes affect some function in MII in addition to their role in recombination during MI.

The rec8, rec10, and rec11 mutants produce aneuploid meiotic products:
In organisms with many chromosomes it is very difficult to study mutations affecting meiotic chromosome dynamics. Few products of such mutant meioses receive the appropriate number of chromosomes, and most of the meiotic products are aneuploid (of abnormal chromosome content) and inviable. Because S. pombe has only three pairs of chromosomes, aberrant segregation might produce relatively high frequencies of viable haploid and diploid missegregants if the chromosomes partition with some degree of randomness. This seems to be the case. The rec8, rec10, and rec11 mutants produced 16, 66, and 79% viable meiotic products relative to wild-type cells (Figure 2A).

View larger version (34K): In this window In a new window Download PPT slide   Figure 2. Formation of aneuploid meiotic products. (A) Spore viabilities. (B) Frequencies of diploid meiotic products. Between 962 and 1538 spore colonies were genotyped for each value. (C) Spore viabilities in cytologically normal tetrads. Seventy-six tetrads with normal morphology were dissected and classified by the number of spores that were viable on nonselective medium. In each mutant the viability pattern is significantly different (P 0.05) from that of wild type.

Random assortment of three chromosome pairs during one of the meiotic divisions should produce four classes of meiotic products: haploids, nullosomic aneuploids (missing one or more chromosomes), disomic aneuploids (having one or two additional chromosomes), and diploids. In S. pombe the haploids and diploids are viable, nullosomic aneuploids are inviable, and disomic aneuploids are unstable and rapidly lose the extra chromosome or chromosomes to become haploid (NIWA and YANAGIDA 1985 ). We therefore scored for the production of diploid spores with the understanding that this underestimates the frequency of missegregation events. Each of the rec mutants generated diploid spores at a frequency that was 10- to 50-fold higher than that from rec⁺ crosses (Figure 2B).

Because S. pombe contains only three pairs of chromosomes, aberrant assortment during one or both of the meiotic divisions might produce some asci with aneuploid spores that, by chance, each contain a similar amount of DNA. We therefore dissected tetrads with normal morphology and we gauged the viability of each spore within each tetrad. The rec8, rec10, and rec11 mutants each produced normal looking asci in which there was a high frequency of one or more inviable spores (Figure 2C). In each mutant the tetrad spore viability pattern was significantly different than that of wild-type cells. This indicates that the majority of the cytologically normal tetrads of the rec8, rec10, and rec11 mutants were genetically abnormal.

In summary, in each mutant the severity of the meiotic chromosome segregation defects scored cytologically (Figure 1 and Table 2) was roughly proportional to the frequency of inviable meiotic products scored genetically (Figure 2A), to the frequency of meiotic diploid spores (Figure 2B), and to the frequency of cytologically normal, genetically abnormal asci (Figure 2C). Together, these data indicate that the majority of the rec8, rec10, and rec11 mutant meioses were defective.

The rec8⁺ gene is required for MI chromosome segregation, but rec10⁺ and rec11⁺ are required for segregation of sister chromatids during MII:
Because the three mutants had aberrant chromosome segregation (Figure 1 and Table 2), produced a relatively high frequency of viable meiotic products (Figure 2A), and produced a relatively high frequency of meiotic diploids (Figure 2B), we were able to monitor directly the segregation patterns of individual chromosomes in the mutants. Meiotic diploids were genotyped for heteroallelic, centromere-linked genetic markers on each chromosome. Those data were compared to the various patterns of marker inheritance predicted to result from three different types of aberrant chromosome segregation (Figure 3, A–D).

View larger version (43K): In this window In a new window Download PPT slide   Figure 3. Segregation patterns of chromosomes. (A) Normal meiosis produces four haploid products. The pattern by which centromeric markers are inherited reveals (B) MI nondisjunction, (C) MII nondisjunction, and (D) precocious separation of sister chromatids. If sister chromatids segregate at random during each of the meiotic divisions, 67% of diploid spores will be heterozygous for the centromere (KRAWCHUK and WAHLS 1999 ). (E) Diploid spore colonies were genotyped for heteroallelic marker loci that were tightly linked to each of the three centromeres. The loci used were lys1 (chromosome I), tps13 (chromosome II), and ade6 (chromosome III) and each value is based upon genotyping between 63 and 130 diploid spore colonies. For the analysis of double mutants, the mat1 locus (chromosome II) was used and each value is based upon genotyping between 26 and 123 diploid spore colonies.

The rec8 mutants produced meiotic diploids that were predominantly heterozygous for each of the centromere-linked markers on chromosomes I, II, and III (Figure 3E). The average heterozygosity for loci on all three chromosomes was 75 ± 7%. We conclude that the rec8⁺ gene is required mainly for the proper segregation of chromosomes during MI.

In contrast to the rec8 mutants, the majority of the diploids produced by the rec10 mutants were homozygous for each of the centromere-linked markers on chromosomes I, II, and III (Figure 3E). The average homozygosity for loci on all three chromosomes was 69 ± 7%. We conclude that the rec10⁺ gene is required principally for the proper segregation of sister chromatids during MII.

The rec11 mutants produced diploids that were predominantly homozygous for each of the centromere-linked markers on chromosomes I, II, and III, with an average homozygosity of 88 ± 8% (Figure 3E). We conclude that the rec11⁺ gene is required mainly for the proper segregation of sister chromatids during MII.

The availability of centromere-linked markers allowed us to monitor simultaneously the segregation of each of the three chromosomes within individual cells (Table 3). The majority of the rec8 mutants missegregated all three chromosomes during MI, and the majority of the rec10 and rec11 mutants suffered MII segregation errors for all three chromosomes. We conclude that the MI defect in the rec8 mutants and the MII defects in the rec10 and rec11 mutants affect each of the three chromosomes within an individual cell with an approximately equal probability. Almost half of the individual diploids exhibited patterns characteristic of MI or MII missegregation for different chromosomes (Table 3). These segregation patterns provide additional evidence that two meiotic divisions had occurred and that the diploids resulted from meiotic chromosome segregation defects.

The rec8⁺, rec10⁺, and rec11⁺ genes are required for recombination toward the centers of chromosomes:
The rec8^-, rec10^-, and rec11^- mutations were identified based upon a recombination defect at the ade6 locus on chromosome III (PONTICELLI and SMITH 1989 ). Because these mutations affect the segregation of all three chromosomes (Figure 1 Figure 2 Figure 3; Table 2 and Table 3), it seemed implausible that the recombination defect was restricted mainly to chromosome III, as previously reported (DEVEAUX and SMITH 1994 ). We therefore extended that study here and elsewhere (KRAWCHUK and WAHLS 1999 ) to determine whether the rec8⁺, rec10⁺, and rec11⁺ genes were required for crossover recombination at specific regions of all three chromosomes. The new data are presented in Table 4 and are summarized together with the published data in Figure 4. Intergenic (crossover) recombination near the center of chromosomes I, II, and III was compromised in the rec8, rec10, and rec11 mutants. In contrast, recombination toward the ends of chromosomes I and II was nearly normal in the mutants. (It is not practical to measure intergenic recombination in the subtelomeric regions of chromosome III because those regions, which encompass about one-third of the chromosome, are occupied by the rDNA repeats.) In each mutant, and on each chromosome, the defects in recombination were similar and appeared to be a function of the physical distance from the approximate center of the chromosome. We conclude that the rec8⁺, rec10⁺, and rec11⁺ genes are each required for recombination toward the centers of all three chromosomes during MI. This contradicts the previous claim (DEVEAUX and SMITH 1994 ) that rec8⁺, rec10⁺, and rec11⁺ are chromosome III-specific activators of recombination.

View larger version (46K): In this window In a new window Download PPT slide   Figure 4. Intergenic recombinant frequencies. The histogram bars indicate the approximate locations of the intergenic intervals tested on each chromosome and the frequency of crossing over within each interval relative to that in wild-type cells. The shaded bars indicate the upper limit to the frequency determined specifically in those meioses with aberrant chromosome segregation producing diploid spores. A curve plotting crossing over as a function of marker location (dashed line) was made for each chromosome and superimposed upon the data sets of each mutant for the sake of comparison. The rDNA repeats on the ends of chromosome III (about one-third of the chromosome length) are not shown on the schematic. The data are from this study (Table 4), DEVEAUX and SMITH 1994 , and KRAWCHUK and WAHLS 1999 and wild-type genetic map coordinates are those of MUNZ 1994 .

The rec8⁺ gene is epistatic to rec10⁺ and to rec11⁺:
Because the rec8, rec10, and rec11 mutants displayed some common phenotypes and some phenotypes that were distinct, we tested for genetic interactions between the three mutated genes. The rec8 rec10 and rec8 rec11 double mutants each produced the same predominant cytological phenotypes as the rec8 single mutant (Table 2). Furthermore, in both cases the rec8^- mutation suppressed the predominant mutant phenotypes conferred by the rec10^- and rec11^- mutations. These data demonstrate that rec8⁺ is epistatic to both rec10⁺ and rec11⁺. Analyses of spore numbers in the asci (Table 2), spore viabilities (Figure 2A), frequencies of aneuploid meiotic products (Figure 2B), and patterns of chromosome missegregation for centromere-linked genetic markers (Figure 3E) in the single and double mutants each support this pathway assignment.

While rec8⁺ was epistatic to rec10⁺ and rec11⁺, there was no clear epistatic relationship between rec10⁺ and rec11⁺. The rec10 rec11 double mutant expressed cytological phenotypes that were distinct from those of the rec10 and rec11 single mutants, and the data were characteristic of an additive or intermediate phenotype (Table 2). However, other data suggest that a partial epistasis relationship may exist: There was no additivity for meiotic inviability (Figure 2A), or for the generation of meiotic diploidy (Figure 2B), or for the aberrant segregation pattern of a centromere-linked genetic marker (Figure 3E) in the rec10 rec11 double mutant relative to the rec10 and rec11 single mutants. We conclude that rec10⁺ and rec11⁺ each participate in some distinct functions, but there may be one or more additional functions in which the rec10⁺ and rec11⁺ genes participate together or share an epistasis relationship.

	DISCUSSION

TOP ABSTRACT MATERIALS AND METHODS RESULTS DISCUSSION LITERATURE CITED

There are four principal findings of this study. First, the three rec genes have a partial epistasis relationship that places rec8⁺ upstream of rec10⁺ and rec11⁺. Second, each of these three genes is required for a normal frequency and distribution of meiotic crossover recombination, in particular toward the centers of each of the three chromosomes. Third, the rec8⁺ gene is required for proper chromosome segregation during MI, as reported (MOLNAR et al. 1995 ). And fourth, the rec10⁺ and rec11⁺ genes are required for the proper segregation of sister chromatids during MII.

An epistasis pathway of genes that are required for multiple meiotic functions:
The rec8⁺ gene is a member of a family of cohesin genes (PARISI et al. 1999 ) and is required for meiotic sister chromatid cohesion and homologous chromosome pairing (MOLNAR et al. 1995 ). Because rec8⁺ is epistatic to both rec10⁺ and rec11⁺ (Table 2; Figure 2 and Figure 3), and the rec10⁺ and rec11⁺ genes are each required for the proper segregation of sister chromatids during MII (Figure 3), we propose that these genes define a pathway of "meiotic chromatid cohesion."

The lack of clear genetic interactions between rec10⁺ and rec11⁺ indicates that the inferred pathway is not simply linear. The presence of shared (Figure 2 Figure 3 Figure 4; Table 3 and Table 4) and distinct (Figure 1 and Figure 2; Table 2) phenotypes in the single and double mutants suggests that rec10⁺ and rec11⁺ might function together for some processes, but function independently for others.

rec10⁺ and rec11⁺ are required for recombination (Table 4; Figure 4) and must therefore have some function in meiotic prophase or MI. Intriguingly, the rec10 and rec11 mutants exhibit mainly MII segregation errors (Table 3; Figure 3). This is paradoxical because hyporecombination mutants should affect MI segregation (ROEDER 1997 ; MOORE and ORR-WEAVER 1998 ). However, the paradigm is from organisms in which MI defects activate a checkpoint or produce inviable meiotic products, precluding genetic identification of potential MII phenotypes.

S. pombe is different in two regards. First, the recombination defects in the rec8, rec10, and rec11 mutants do not confer a checkpoint arrest or barrier to development; meiosis proceeds and meiotic products are formed (Figure 1). Second, because S. pombe contains only three pairs of chromosomes, abnormal or random assortment can produce a relatively high frequency of meiotic products that receive at least one copy of each chromosome and are therefore viable (Figure 2). The high viability of meiotic products from the S. pombe rec10 and rec11 mutants allowed us to visualize the MII errors (Figure 3).

A model of chromosome pairing dependent upon Rec8, Rec10, and Rec11:
In fission yeast, as in many organisms, telomeres become clustered during meiotic prophase to form a "bouquet" structure prior to pairing of other regions of chromosomes (CHIKASHIGE et al. 1994 , CHIKASHIGE et al. 1997 ; SHIMANUKI et al. 1997 ; COOPER et al. 1998 ; NIMMO et al. 1998 ). The available data suggest a model in which the Rec8, Rec10, and Rec11 proteins are required for subsequent pairing of internal chromosome regions (Figure 5).

View larger version (23K): In this window In a new window Download PPT slide   Figure 5. A model of homologous chromosome pairing. Lines represent homologous chromosomes (a pair of chromatids) and circles represent the centromeres. (A) Telomeres cluster and associate with the spindle pole body (asterisk) to form the bouquet structure. (B) The telomeres are pulled back and forth by the spindle pole body. (C) This results in a rough alignment of individual chromosomes with multiple interstitial points of contact between homologous sequences (triangles). (D) Components of the meiotic chromatid cohesion pathway stabilize these interstitial contacts and promote further pairing between nearby regions of the homologous chromosomes (squares). (E) Discontinuous patches of linear elements (rectangles) form to complete synapsis. In the absence of full pairing and synapsis, recombination can still occur at the multiple interstitial interactions between homologous DNA sequences (C), which are much more abundant near the telomeres due to the telomere clustering.

After telomere clustering (Figure 5A), migration of the spindle pole body leads the clustered telomeres back and forth and the unpaired central regions of the chromosomes trail behind in a structure called a "horsetail" (Figure 5, B–C; KOHLI 1994 ; DE LANGE 1998 ). The net effect is that the internal regions of the chromosomes of similar length fall into rough alignment (Figure 5C). This may facilitate local searches for homology or there may simply be fortuitous, chance encounters where homologous DNA sequences come into close contact with one another. We suggest that the Rec10 and Rec11 proteins, like Rec8 (MOLNAR et al. 1995 ), are required to stabilize the interstitial points of contact between the homologous chromosomes (Figure 5D) and to promote the pairing of regions in the vicinity (Figure 5E). While pairing has not been directly studied in the rec10 and rec11 mutants, the similar regional recombination defects in each of the rec8, rec10, and rec11 mutants (Figure 4) may simply be a consequence of defects in pairing: Recombination could still occur at a high rate near the telomeres, where homologous chromosomes are in close proximity due to telomere clustering, but might be severely compromised at the more central regions, where homologs are not properly paired (Figure 5C).

An alternative (not mutually exclusive) possibility is that the Rec8, Rec10, and Rec11 proteins have a more direct role in distributing recombination events along chromosomes. For example, certain recombination enzymes may serve some active function in the pairing process, such as testing the homologous register of the interstitial points of contact between chromosomes (Figure 5D and Figure E). As another example, the Rec8, Rec10, or Rec11 proteins might nucleate the assembly of recombination enzymes to recombination nodules. It will be informative to determine whether these proteins and known enzymes of recombination colocalize with each other.

Functions for chromatid cohesion in both MI and MII segregation:
In most organisms chiasmata have a role in maintaining the paired state between homologous chromosomes prior to anaphase of MI. However, in the absence of some type of additional glue, chiasmata will migrate off the bivalent and the homologs will separate. DARLINGTON 1932 and others (e.g., MAGUIRE 1974 ) suggested that a cohesive interaction between sister chromatids distal to crossovers would stabilize the chiasmata to prevent their loss (reviewed by MOORE and ORR-WEAVER 1998 ).

A model for how crossovers and sister chromatid cohesion relate to meiotic chromosome segregation is depicted in Figure 6. Crossovers and cohesion distal to the point of exchange work synergistically to hold bivalents together. The distal cohesion must be dissolved during MI so that the homologs can segregate. However, cohesion at or near to the centromere (proximal to the point of exchange) must be maintained during MI to ensure that sister chromatids segregate to the same pole and to hold the sister chromatids together on the metaphase plate of MII while under spindle tension. During anaphase of MII, this centromere-proximal cohesion must be properly released to allow sister chromatids to segregate.

View larger version (18K): In this window In a new window Download PPT slide   Figure 6. Regulated dissolution of cohesive forces. The lines represent individual chromatids. (A) Mitotic cohesion (asterisks) holds sister chromatids together in opposition to spindle tension. A complete release of cohesion allows the chromatids to segregate during mitosis. (B) Meiotic cohesion (asterisks) provides cohesive forces between sister chromatids and contributes to pairing forces between homologs. A release of synapsis is insufficient to permit the homologous chromosomes to segregate, because they remain firmly attached to each other via the sister chromatid cohesion distal to the crossover (crossed chromatids). A release of chromatid cohesion distal to the crossover allows the homologous chromosomes to segregate from one another during MI. (C) Cohesion proximal to the crossover, at least at the centromere, must be maintained to keep sister chromatids associated and to oppose spindle forces as each chromosome subsequently aligns on the metaphase plate of MII. The second meiotic division then proceeds with a complete release of cohesion.

A hypothesis about Rec8 and the influence of crossover location on the stability of interhomolog cohesion during MI:
Fluorescence in situ hybridization of wild-type S. pombe meioses revealed that sister chromatids are normally in close proximity to one another, whereas in 20% of rec8 mutant meioses the sister chromatids are widely separated (MOLNAR et al. 1995 ). Also in that study, genetic analysis of chromosome III disomy patterns in tetrads demonstrated that 4.4% (3 of 68) of rec8 mutant meioses suffered precocious separation of one pair of chromosome III sister chromatids. However, as pointed out by the authors, the analysis of chromosome III disomics would have missed MI nondisjunction events involving more than just chromosome III, and the assay system precluded the identification of any MII nondisjunction events. Our approach to study segregation errors on all three chromosomes (Figure 3; Table 3) also has some limitations. In particular, we cannot distinguish between MI segregation errors due to nondisjunction and those due to precocious separation of sister chromatids. Because chromatid cohesion defects clearly occur but do not seem to affect all rec8 mutant meioses (MOLNAR et al. 1995 ), and chromatids of homologous chromosomes can recombine to (presumably) generate chiasmata (Table 4; Figure 4), nondisjunction may contribute to the MI segregation errors. This seems to be the case. Novel centromere mapping functions for aneuploid meiotic products revealed significant levels of MI nondisjunction in the rec8 mutants (KRAWCHUK and WAHLS 1999 ).

There are between 11 and 17 crossovers per chromosome in a wild-type S. pombe meiosis (MUNZ 1994 ) and reciprocal recombination in distal regions of chromosomes of rec8 mutants is nearly normal (Figure 4), so at least one crossover is statistically present on each chromosome in the majority of the mutant meioses. And yet, the majority of those meioses suffer chromosome segregation errors (Figure 1 Figure 2 Figure 3; Table 2 and Table 3). Thus, it appears that the residual crossovers are insufficient to ensure proper MI segregation in the rec8 mutants.

Some MI segregation errors in the rec8 mutants might be the result of either the decreased frequency or the abnormal distribution of crossovers. However, either of these seems unlikely to be the sole causative factor because the rec10 and rec11 mutants have a similar frequency and distribution of crossovers as the rec8 mutants (Figure 4) but do not suffer predominantly MI segregation errors (Figure 3). Intriguingly, chromosomes that have suffered spontaneous MI nondisjunction in Drosophila, budding yeast, and humans exhibit a preferential reduction in centromere-proximal crossovers (GETHMANN 1984 ; RASOOLY et al. 1991 ; MACDONALD et al. 1994 ; MOORE et al. 1994 ; SHERMAN et al. 1994 ; HASSOLD et al. 1995 ). This parallel to the recombination and segregation phenotypes of the rec8 mutants suggests that some combination of sister chromatid cohesion and crossover position is important for MI chromosome segregation.

The patches of discontinuous linear elements on synapsed S. pombe chromosomes, thought to be related to axial elements of the SC in other organisms, break down before the two meiotic divisions (BAHLER et al. 1993 ; KOHLI 1994 ; KOHLI and BAHLER 1994 ; SCHERTHAN et al. 1994 ). Thus, the structural scaffold supporting synaptic interhomolog interactions is probably disassembled prior to the two meiotic divisions. This leaves in place the sister chromatid cohesion interactions, which, when accompanied by crossovers, may be sufficient to hold together both pairs of sister chromatids and the pair of homologous chromosomes (Figure 6B). An attractive hypothesis is that the centromere-proximal crossovers are more proficient at ensuring proper MI chromosome segregation than distal crossovers because the proximal crossovers leave a greater extent of interhomolog sister chromatid cohesion than distal crossovers (Figure 6B). If the crossover is too close to the telomere or if the distal cohesion is defective, the interhomolog cohesive force would be inadequate to oppose the repeated, register-testing forces of the spindle apparatus. Once torn apart, the homologous chromosomes cannot form the proper bipolar spindle attachment and therefore segregate independently. By chance, half of the time they will segregate to different poles, and half of the time they will arrive at the same pole, thus giving rise to an MI nondisjunction phenotype.

A hypothesis about Rec10, Rec11, and the influence of crossover location on the stability of intersister cohesion during MII:
The rec10⁺ and rec11⁺ genes are required for the proper segregation of sister chromatids during MII (Figure 3; KRAWCHUK and WAHLS 1999 ). Mutations affecting MII chromatid cohesion and segregation are quite rare, but they have been described in a few organisms (CLAYBERG 1959 ; DAVIS 1971 ). As one well-characterized example, the mei-S332 mutants of Drosophila fail to maintain cohesion between sister chromatids during or just after anaphase of the MI division (KERREBROCK et al. 1992 ). Furthermore, the mei-S332 protein localizes to centromeric regions and dissociates at the onset of MII anaphase, when sister chromatids segregate from one another (KERREBROCK et al. 1995 ; MOORE et al. 1998 ). It will be of interest to determine whether the Rec10 and Rec11 proteins behave in a similar fashion.

An elevated frequency of centromere-proximal crossing over has been found on chromosomes that suffered spontaneous MII nondisjunction in humans and Drosophila, suggesting that crossover position can also affect the segregation of sister chromatids during MII (MACDONALD et al. 1994 ; FISHER et al. 1995 ; LAMB et al. 1996 ; KOEHLER et al. 1996A , KOEHLER et al. 1996B ). The rec10 and rec11 mutants exhibit MII segregation errors (Figure 3) but have a reduced frequency of centromere-proximal crossing over (Figure 4; KRAWCHUK and WAHLS 1999 ). We suggest that Rec10 and Rec11 are required for proper centromere-proximal sister chromatid cohesion during MII and that their absence recapitulates the effect of placing a crossover too close to the centromere. In each case, the intersister cohesive force would be inadequate to oppose the repeated, register-testing forces of the MII spindle apparatus (Figure 6C). Once torn apart, the sister chromatids would segregate independently. By chance, half of the time they will segregate to different poles, and half of the time they will arrive at the same pole, thus giving rise to an MII nondisjunction phenotype.

The machinery of the reductional division is sufficiently intact in the rec10 and rec11 mutant meioses to ensure proper segregation of homologs during MI most of the time. However, the rec10 and rec11 mutants do experience 31 and 12% of MI segregation errors, respectively (Figure 3). Whether these MI segregation errors are a consequence of homologous chromosome pairing defects, of defects in the overall crossover frequency, of defects in the distribution of crossovers, or of defects in sister chromatid cohesion is a matter for conjecture.

Summary:
Analysis of the rec8, rec10, and rec11 mutants revealed a "meiotic chromatid cohesion" pathway that links together sister chromatid cohesion, pairing of internal regions of homologous chromosomes, centromere proximal recombination, and the proper segregation of chromosomes during each of the meiotic divisions. Additional functions might include the preferential selection of homologs (rather than sister chromatids) as partners for recombination, stabilization of chiasmata (crossovers), and ensuring that the additional kinetochores are shielded during MI and presented during MII for proper spindle attachment.

	FOOTNOTES

¹ Present address: Department of Physics, Idaho State University, Campus Box 8106, Pocatello, ID 83209-8106.

	ACKNOWLEDGMENTS

We thank Charlie Albright, Mari Davidson, Aaron Graff, Jürg Kohli, Ning Kon, Michael Lichten, Gisela Mosig, Wallace Sharif, and Gerry Smith for helpful discussions; Calley Hardin and Steve Lindsey for laboratory assistance; Kathy Gould, Nancy Hollingsworth, Jürg Kohli, and Gerry Smith for yeast strains; and Jürg Kohli for sharing results prior to publication. This work was supported by a grant from the National Institutes of Health (GM54671) and pilot project funds from the Vanderbilt University Research Council. M.D.K. was supported in part by a training grant from the National Institutes of Health (CA09582), L.C.D. was supported by a grant from the National Institutes of Health (GM31693), and W.P.W. was a Leukemia Society of America Special Fellow (3021-94) for a portion of this research.

Manuscript received January 14, 1999; Accepted for publication May 27, 1999.

	LITERATURE CITED

TOP ABSTRACT MATERIALS AND METHODS RESULTS DISCUSSION LITERATURE CITED

BÄHLER, J., T. WYLER, J. LOIDL, and J. KOHLI, 1993  Unusual nuclear structures in meiotic prophase of fission yeast: a cytological analysis. J. Cell Biol. 121:241-256[Abstract/Free Full Text].

BIRKENBIHL, R. P. and S. SUBRAMANI, 1992  Cloning and characterization of rad21 an essential gene of Schizosaccharomyces pombe involved in DNA double-strand-break repair. Nucleic Acids Res. 20:6605-6611[Abstract/Free Full Text].

BIRKENBIHL, R. P. and S. SUBRAMANI, 1995  The rad21 gene product of Schizosaccharomyces pombe is a nuclear, cell cycle-regulated phosphoprotein. J. Biol. Chem. 270:7703-7711[Abstract/Free Full Text].

CHIKASHIGE, Y., D. Q. DING, H. FUNABIKI, T. HARAGUCHI, and S. MASHIKO et al., 1994  Telomere-led premeiotic chromosome movement in fission yeast. Science 264:270-273[Abstract/Free Full Text].

CHIKASHIGE, Y., D. Q. DING, Y. IMAI, M. YAMAMOTO, and T. HARAGUCHI et al., 1997  Meiotic nuclear reorganization: switching the position of centromeres and telomeres in the fission yeast Schizosaccharomyces pombe.. EMBO J. 16:193-202[Medline].

CLAYBERG, C., 1959  Cytogenetic studies of precocious meiotic centromere division in Lycopersicon esculentum.. Genetics 44:1335-1346[Free Full Text].

COOPER, J. P., Y. WATANABE, and P. NURSE, 1998  Fission yeast Taz1 protein is required for meiotic telomere clustering and recombination. Nature 392:828-831[Medline].

DARLINGTON, C. D., 1932 Recent Advances in Cytology. Churchill, London.

DAVIS, B., 1971  Genetic analysis of a meiotic mutant resulting in precocious sister-centromere separation in Drosophila melanogaster.. Mol. Gen. Genet. 113:251-271[Medline].

DE LANGE, T., 1998  Ending up with the right partner. Nature 392:753-754[Medline].

DEVEAUX, L. C. and G. R. SMITH, 1994  Region-specific activators of meiotic recombination in Schizosaccharomyces pombe.. Genes Dev. 8:203-210[Abstract/Free Full Text].

DEVEAUX, L. C., N. A. HOAGLAND, and G. R. SMITH, 1992  Seventeen complementation groups of mutations decreasing meiotic recombination in Schizosaccharomyces pombe.. Genetics 130:251-262[Abstract].

FISHER, J. M., J. F. HARVEY, N. E. MORTON, and P. A. JACOBS, 1995  Trisomy 18: studies of the parent and cell division of origin and the effect of aberrant recombination on nondisjunction. Am. J. Hum. Genet. 56:669-675[Medline].

GETHMANN, R. C., 1984  The genetic analysis of a chromosome-specific meiotic mutant that permits a premature separation of sister chromatids in Drosophila melanogaster.. Genetics 107:65-77[Abstract/Free Full Text].

GUACCI, V., D. KOSHLAND, and A. STRUNNIKOV, 1997  A direct link between sister chromatid cohesion and chromosome condensation revealed through the analysis of MCD1 in S. cerevisiae.. Cell 91:47-57[Medline].

GUTZ, H., H. HESLOT, U. LEUPOLD and N. LOPRIENO, 1974 Schizosaccharomyces pombe, pp. 395–446 in Handbook of Genetics, edited by R. C. KING. Plenum Press, New York.

HASSOLD, T., M. MERRILL, K. ADKINS, S. FREEMAN, and S. SHERMAN, 1995  Recombination and maternal age-dependent nondisjunction: molecular studies of trisomy 16. Am. J. Hum. Genet. 57:867-874[Medline].

HEO, S. J., K. TATEBAYASHI, J. KATO, and H. IKEDA, 1998  The RHC21 gene of budding yeast, a homologue of the fission yeast rad21⁺ gene, is essential for chromosome segregation. Mol. Gen. Genet. 257:149-156[Medline].

HIRATA, A. and C. SHIMODA, 1992  Electron microscopic examination of sporulation-deficient mutants of the fission yeast Schizosaccharomyces pombe.. Arch. Microbiol. 158:249-255[Medline].

HIRATA, A. and C. SHIMODA, 1994  Structural modification of spindle pole bodies during meiosis II is essential for normal formation of ascospores in Schizosaccharomyces pombe: ultrastructural analysis of spo mutants. Yeast 10:173-183[Medline].

KERREBROCK, A. W., W. Y. MIYAZAKI, D. BIRNBY, and T. L. ORR-WEAVER, 1992  The Drosophila mei-S332 gene promotes sister-chromatid cohesion in meiosis following kinetochore differentiation. Genetics 130:827-841[Abstract].

KERREBROCK, A. W., D. P. MOORE, J. S. WU, and T. L. ORR-WEAVER, 1995  Mei-S332, a Drosophila protein required for sister-chromatid cohesion, can localize to meiotic centromere regions. Cell 83:247-256[Medline].

KOEHLER, K. E., C. L. BOULTON, H. E. COLLINS, R. L. FRENCH, and K. C. HERMAN et al., 1996a  Spontaneous X chromosome MI and MII nondisjunction events in Drosophila melanogaster oocytes have different recombinational histories. Nat. Genet. 14:406-413[Medline].

KOEHLER, K. E., R. S. HAWLEY, S. SHERMAN, and T. HASSOLD, 1996b  Recombination and nondisjunction in humans and flies. Hum. Mol. Genet. 5:1495-1504[Abstract].

KOHLI, J., 1994  Meiosis. Telomeres lead chromosome movement. Curr. Biol. 4:724-727[Medline].

KOHLI, J. and J. BAHLER, 1994  Homologous recombination in fission yeast: absence of crossover interference and synaptonemal complex. Experientia 50:295-306[Medline].

KOHLI, J., H. HOTTINGER, P. MUNZ, A. STRAUSS, and P. THURIAUX, 1977  Genetic mapping in Schizosaccharomyces pombe by mitotic and meiotic analysis and induced haploidization. Genetics 87:471-489[Abstract/Free Full Text].

KON, N., M. D. KRAWCHUK, B. G. WARREN, G. R. SMITH, and W. P. WAHLS, 1997  Transcription factor Mts1/Mts2 (Atf1/Pcr1, Gad7/Pcr1) activates the M26 meiotic recombination hotspot in S. pombe.. Proc. Natl. Acad. Sci. USA 94:13765-13770[Abstract/Free Full Text].

KRAWCHUK, M. D. and W. P. WAHLS, 1999  Centromere mapping functions for aneuploid meiotic products: analysis of rec8, rec10, and rec11 mutants of the fission yeast Schizosaccharomyces pombe.. Genetics 153:49-55[Abstract/Free Full Text].

LAMB, N., S. B. FREEMAN, A. SAVAGE-AUSTIN, D. PETTAY, and L. TAFT et al., 1996  Susceptible chiasmate configurations of chromosome 21 predispose to non-disjunction in both maternal meiosis I and meiosis II. Nat. Genet. 14:400-405[Medline].

LI, Y. F., M. NUMATA, W. P. WAHLS, and G. R. SMITH, 1997  Region-specific meiotic recombination in Schizosaccharomyces pombe: the rec11 gene. Mol. Microbiol. 23:869-878[Medline].

LIN, Y. and G. R. SMITH, 1995  Molecular cloning of the meiosis-induced rec10 gene of Schizosaccharomyces pombe.. Curr. Genet. 27:440-446[Medline].

LIN, Y., K. L. LARSON, R. DORER, and G. R. SMITH, 1992  Meiotically induced rec7 and rec8 genes of Schizosaccharomyces pombe.. Genetics 132:75-85[Abstract].

MACDONALD, M., T. HASSOLD, J. HARVEY, L. H. WANG, and N. E. MORTON et al., 1994  The origin of 47,XXY and 47,XXX aneuploidy: heterogeneous mechanisms and role of aberrant recombination. Hum. Mol. Genet. 3:1365-1371[Abstract/Free Full Text].

MAGUIRE, M. P., 1974  The need for a chiasma binder. J. Theor. Biol. 48:485-487[Medline].

MCKAY, M. J., C. TROELSTRA, P. VAN DER SPEK, R. KANAAR, and B. SMIT et al., 1996  Sequence conservation of the rad21 Schizosaccharomyces pombe DNA double-strand break repair gene in human and mouse. Genomics 36:305-315[Medline].

MICHAELIS, C., R. CIOSK, and K. NASMYTH, 1997  Cohesins: chromosomal proteins that prevent premature separation of sister chromatids. Cell 91:35-45[Medline].

MOENS, P. B., 1994  Molecular perspectives of chromosome pairing at meiosis. Bioessays 16:101-106[Medline].

MOLNAR, M., J. BAHLER, M. SIPICZKI, and J. KOHLI, 1995  The rec8 gene of Schizosaccharomyces pombe is involved in linear element formation, chromosome pairing and sister-chromatid cohesion during meiosis. Genetics 141:61-73[Abstract].

MOORE, D. P. and T. L. ORR-WEAVER, 1998  Chromosome segregation during meiosis: building an unambivalent bivalent. Curr. Top. Dev. Biol. 37:263-299[Medline].

MOORE, D. P., W. Y. MIYAZAKI, J. E. TOMKIEL, and T. L. ORR-WEAVER, 1994  Double or nothing: a Drosophila mutation affecting meiotic chromosome segregation in both females and males. Genetics 136:953-964[Abstract].

MOORE, D. P., A. W. PAGE, T. T. TANG, A. W. KERREBROCK, and T. L. ORR-WEAVER, 1998  The cohesion protein MEI-S332 localizes to condensed meiotic and mitotic centromeres until sister chromatids separate. J. Cell Biol. 140:1003-1012[Abstract/Free Full Text].

MUNZ, P., 1994  An analysis of interference in the fission yeast Schizosaccharomyces pombe.. Genetics 137:701-707[Abstract].

NIMMO, E. R., A. L. PIDOUS, P. E. PERRY, and R. C. ALLSHIRE, 1998  Defective meiosis in telomere-silencing mutants of Schizosaccharomyces pombe.. Nature 392:825-828[Medline].

NIWA, O. and M. YANAGIDA, 1985  Triploid meiosis and aneuoploidy in Schizosaccharomyces pombe: an unstable aneuploid disomic for chromosome III.. Curr. Genet. 9:463-470.

PARISI, S., M. J. MCKAY, M. MOLNAR, M. A. THOMPSON, and P. J. VAN DER SPEK et al., 1999  Rec8p, a meiotic recombination and sister chromatid cohesion phosphoprotein of the Rad21p family, conserved from fission yeast to humans. Mol. Cell. Biol. 19:3515-3528[Abstract/Free Full Text].

PONTICELLI, A. S. and G. R. SMITH, 1989  Meiotic recombination-deficient mutants of Schizosaccharomyces pombe.. Genetics 123:45-54[Abstract/Free Full Text].

RASOOLY, R. S., C. M. NEW, P. ZHANG, R. S. HAWLEY, and B. S. BAKER, 1991  The lethal(1)TW-6cs mutation of Drosophila melanogaster is a dominant antimorphic allele of nod and is associated with a single base change in the putative ATP-binding domain. Genetics 129:409-422[Abstract].

ROEDER, G. S., 1997  Meiotic chromosomes: it takes two to tango. Genes Dev. 11:2600-2621[Free Full Text].

SCHERTHAN, H., J. BAHLER, and J. KOHLI, 1994  Dynamics of chromosome organization and pairing during meiotic prophase in fission yeast. J. Cell Biol. 127:273-285[Abstract/Free Full Text].

SHERMAN, S. L., M. B. PETERSEN, S. B. FREEMAN, J. HERSEY, and D. PETTAY et al., 1994  Non-disjunction of chromosome 21 in maternal meiosis I: evidence for a maternal age-dependent mechanism involving reduced recombination. Hum. Mol. Genet. 3:1529-1535[Abstract/Free Full Text].

SHIMANUKI, M., F. MIKI, D.-Q. DING, Y. CHIKASHIGE, and Y. HIRAOKA et al., 1997  A novel fission yeast gene, kms1⁺, is required for the formation of meiotic prophase-specific nuclear architecture. Mol. Gen. Genet. 254:238-249[Medline].

SHIMODA, C., A. HIRATA, M. KISHIDA, T. HASHIDA, and K. TANAKA, 1985  Characterization of meiosis-deficient mutants by electron microscopy and mapping of four essential genes in the fission yeast Schizosaccharomyces pombe.. Mol. Gen. Genet. 200:252-257[Medline].

TANAKA, K. and A. HIRATA, 1982  Ascospore development in the fission yeast Schizosaccharomyces pombe and S. japonicus.. J. Cell Sci. 56:263-279[Abstract/Free Full Text].

TATEBAYASHI, K., J. KATO, and H. IKEDA, 1998  Isolation of a Schizosaccharomyces pombe rad21ts mutant that is aberrant in chromosome segregation, microtubule function, DNA repair and sensitive to hydroxyurea: possible involvement of Rad21 in ubiquitin-mediated proteolysis. Genetics 148:49-57[Abstract/Free Full Text].

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