The Lethal(1)tw-6(cs) 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

The l(1)TW-6cs mutation is a cold-sensitive recessive lethal mutation in Drosophila melanogaster, that affects both meiotic and mitotic chromosome segregation. We report the isolation of three revertants of this mutation. All three revert both the meiotic and mitotic effects as well as the cold sensitivity, demonstrating that all three phenotypes are due to a single lesion. We further show that these revertants fail to complement an amorphic allele of the nod (no distributive disjunction) locus, which encodes a kinesin-like protein. These experiments demonstrate that l(1)TW-6cs is an antimorphic allele of nod, and we rename it nodDTW. Sequencing of the nod locus on a nodDTW-bearing chromosome reveals a single base change in the putative ATP-binding region of the motor domain of nod. Recessive, loss-of-function mutations at the nod locus specifically disrupt the segregation of nonexchange chromosomes in female meiosis. We demonstrate that, at 23.5 degrees, the meiotic defects in nodDTW/+ females are similar to those observed in nod/nod females; that is, the segregation of nonexchange chromosomes is abnormal. However, in nodDTW/nodDTW females, or in nodDTW/+ females at 18 degrees, we observe a more severe meiotic defect that apparently affects the segregation of both exchange and nonexchange chromosomes. In addition, nodDTW homozygotes and hemizygous males have previously been shown to exhibit mitotic defects including somatic chromosome breakage and loss. We propose that the defective protein encoded by the nodDTW allele interferes with proper chromosome movement during both meiosis and mitosis, perhaps by binding irreversibly to microtubules.     

The Genetic Analysis of Distributive Segregation in Drosophila Melanogaster. I. Isolation and Characterization of Aberrant X Segregation (Axs), a Mutation Defective in Chromosome Partner Choice

We describe the isolation and characterization of Aberrant X segregation (Axs), a dominant female-specific meiotic mutation. Although Axs has little or no effect on the frequency or distribution of exchange, or on the disjunction of exchange bivalents, nonexchange X chromosomes undergo nondisjunction at high frequencies in Axs/+ and Axs/Axs females. This increased X chromosome nondisjunction is shown to be a consequence of an Axs-induced defect in distributive segregation. In Axs-bearing females, fourth chromosome nondisjunction is observed only in the presence of nonexchange X chromosomes and is argued to be the result of improper X and fourth chromosome associations within the distributive system. In XX females bearing a compound fourth chromosome, the frequency of nonhomologous disjunction of the X chromosomes from the compound fourth chromosome is shown to account for at least 80% of the total X nondisjunction observed. In addition, Axs diminishes or ablates the capacity of nonexchange X chromosomes to form trivalents in females bearing either a Y chromosome or a small free duplication for the X. Axs also impairs compound X from Y segregation. The effect of Axs on these segregations parallels the defects observed for homologous nonexchange X chromosome disjunction in Axs females. In addition to its dramatic effects on the X chromosome, Axs exerts a similar effect on the segregation of a major autosome. We conclude that Axs defines a locus required for proper homolog disjunction within the distributive system.     

A Meiotic Mutant Defective in Distributive Disjunction in DROSOPHILA MELANOGASTER

The female meiotic mutant no distributive disjunction (symbol: nod) reduces the probability that a nonexchange chromosome will disjoin from either a nonexchange homolog or a nonhomolog; the mutant does not affect exchange or the disjunction of bivalents that have undergone exchange. Disjunction of nonexchange homologs was examined for all chromosome pairs; nonhomologous disjunction of the X chromosomes from the Y chromosome in XXY females, of compound chromosomes in females bearing attached-third chromosomes with and without a Y chromosome, and of the second chromosomes from the third chromosomes were also examined. The results suggest that the defect in nod is in the distributive pairing process. The frequencies and patterns of disjunction from a trivalent in nod females suggest that the distributive pairing process involves three separate events-pairing, orientation, and disjunction. The mutant nod appears to affect disjunction only.     

Meiotic chromosome distribution in Drosophila oocytes: Roles of two kinesin-related proteins

Summary The recent finding that two proteins required for proper chromosome distribution in Drosophila oocytes are related to the microtubule motor protein, kinesin, provides new insights into the forces involved in meiotic chromosome movement. ncd is a spindle motor in meiosis but may perform a different role in the early mitotic divisions of the embryo. nod, until recently, has been thought to be a component of the distributive process of chromosome segregation. The finding that nod is a kinesin protein provides an alternative explanation of the effect of mutants on nonexchange chromosomes and suggests that nonexchange chromosomes segregate with exchange chromosomes in a single process, rather than via a two-step distributive system.     

Meiotic spindle assembly in Drosophila females: behavior of nonexchange chromosomes and the effects of mutations in the nod kinesin-like protein

Mature Drosophila oocytes are arrested in metaphase of the first meiotic division. We have examined microtubule and chromatin reorganization as the meiosis I spindle assembles on maturation using indirect immunofluorescence and laser scanning confocal microscopy. The results suggest that chromatin captures or nucleates microtubules, and that these subsequently form a highly tapered spindle in which the majority of microtubules do not terminate at the poles. Nonexchange homologs separate from each other and move toward opposite poles during spindle assembly. By the time of metaphase arrest, these chromosomes are positioned on opposite half spindles, between the metaphase plate and the spindle poles, with the large nonexchange X chromosomes always closer to the metaphase plate than the smaller nonexchange fourth chromosomes. Nonexchange homologs are therefore oriented on the spindle in the absence of a direct physical linkage, and the spindle position of these chromosomes appears to be determined by size. Loss-of-function mutations at the nod locus, which encodes a kinesin-like protein, cause meiotic loss and nondisjunction of nonexchange chromosomes, but have little or no effect on exchange chromosome segregation. In oocytes lacking functional nod protein, most of the nonexchange chromosomes are ejected from the main chromosomal mass shortly after the nuclear envelope breaks down and microtubules interact with the chromatin. In addition, the nonexchange chromosomes that are associated with spindles in nod/nod oocytes show excessive poleward migration. Based on these observations, and the structural similarity of the nod protein and kinesin, we propose that nonexchange chromosomes are maintained on the half spindle by opposing poleward and anti-poleward forces, and that the nod protein provides the anti-poleward force.     

Morewright (Mwr), a New Meiotic Mutant of Drosophila Melanogaster Affecting Nonexchange Chromosome Segregation

A new meiotic mutation, morewright (mwr) was identified by screening for new mutations that act as dominant enhancers of the dosage-sensitive Drosophila melanogaster female meiotic mutant, nod(DTW). mwr is a recessive meiotic mutant, specifically impairing the segregation of nonexchange chromosomes. Cytological evidence suggests that the meitoic defect in mwr/mwr females is in homologue recognition because the chromosomes appear to be misaligned on an intact spindle. The mwr mutation was recovered during a screen of random P-element insertions on a chromosome with a single insertion located at 50C. The P-element insertion is a recessive female-sterile mutation. While excision of the P element from the mwr-bearing chromosome partially relieves the female sterility, the excisions retain the dominant nod(DTW)-enhancing activity. The mwr meiotic phenotype maps very close to the female-sterile P insertion. Thus the mwr locus appears to encode a function required for partner recognition in meiosis, although its relationship to the neighboring female-sterile mutation remains to be elucidated.     

Distributive Disjunction of Authentic Chromosomes in Saccharomyces Cerevisiae

Distributive disjunction is defined as the first division meiotic segregation of either nonhomologous chromosomes that lack homologs or homologous chromosomes that have not recombined. To determine if chromosomes from the yeast Saccharomyces cerevisiae were capable of distributive disjunction, we constructed a strain that was monosomic for both chromosome I and chromosome III and analyzed the meiotic segregation of the two monosomic chromosomes. In addition, we bisected chromosome I into two functional chromosome fragments, constructed strains that were monosomic for both chromosome fragments and examined meiotic segregation of the chromosome fragments in the monosomic strains. The two nonhomologous chromosomes or chromosome fragments appeared to segregate from each other in approximately 90% of the asci analyzed, indicating that yeast chromosomes were capable of distributive disjunction. We also examined the ability of a small nonhomologous centromere containing plasmid to participate in distributive disjunction with the two nonhomologous monosomic chromosomes. The plasmid appeared to efficiently participate with the two full length chromosomes suggesting that distributive disjunction in yeast is not dependent on chromosome size. Thus, distributive disjunction in S. cerevisiae appears to be different from Drosophila melanogaster where a different sized chromosome is excluded from distributive disjunction when two similar size nonhomologous chromosomes are present.     

The Genetic Analysis of Achiasmate Segregation in Drosophila Melanogaster. III. the Wild-Type Product of the Axs Gene Is Required for the Meiotic Segregation of Achiasmate Homologs

The regular segregation of achiasmate chromosomes in Drosophila melanogaster females is ensured by two distinct segregational systems. The segregation of achiasmate homologs is assured by the maintenance of heterochromatic pairing; while the segregation of heterologous chromosomes is ensured by a separate mechanism that may not require physical association. Axs(D) (Aberrant X segregation) is a dominant mutation that specifically impairs the segregation of achiasmate homologs; heterologous achiasmate segregations are not affected. As a result, achiasmate homologs frequently participate in heterologous segregations at meiosis I. We report the isolation of two intragenic revertants of the Axs(D) mutation (Axs(r2) and Axs(r3)) that exhibit a recessive meiotic phenotype identical to that observed in Axs(D)/Axs(D) females. A third revertant (Axs(r1)) exhibits no meiotic phenotype as a homozygote, but a meiotic defect is observed in Axs(r1)/Axs(r2) females. Therefore mutations at the Axs(D) locus define a gene necessary and specific for homologous achiasmate segregation during meiosis. We also characterize the interactions of mutations at the Axs locus with two other meiotic mutations (ald and ncd). Finally, we propose a model in which Axs(+) is required for the normal separation of paired achiasmate homologs. In the absence of Axs(+) function, the homologs are often unable to separate from each other and behave as a single segregational unit that is free to segregate from heterologous chromosomes.     

Genetic Analysis of Sex Chromosomal Meiotic Mutants in DROSOPHILA MELANOGASTER

A total of 209 ethyl methanesulfonate-treated X chromosomes were screened for meiotic mutants that either (1) increased sex or fourth chromosome nondisjunction at either meiotic division in males; (2) allowed recombination in such males; (3) increased nondisjunction of the X chromosome at either meiotic division in females; or (4) caused such females, when mated to males heterozygous for Segregation-Distorter (SD) and a sensitive homolog to alter the strength of meiotic drive in males.-Twenty male-specific meiotic mutants were found. Though the rates of nondisjunction differed, all twenty mutants were qualitatively similar in that (1) they alter the disjunction of the X chromosome from the Y chromosome; (2) among the recovered sex-chromosome exceptional progeny, there is a large excess of those derived from nullo-XY as compared to XY gametes; (3) there is a negative correlation between the frequency of sex-chromosome exceptional progeny and the frequency of males among the regular progeny. In their effects on meiosis these mutants are similar to In(1)sc(4L)sc(8R), which is deleted for the basal heterochromatin. These mutants, however, have normal phenotypes and viabilities when examined as X/0 males, and furthermore, a mapping of two of the mutants places them in the euchromatin of the X chromosome. It is suggested that these mutants are in genes whose products are involved in insuring the proper functioning of the basal pairing sites which are deleted in In(1)sc(4L)sc(8R), and in addition that there is a close connection, perhaps causal, between the disruption of normal X-Y pairing (and, therefore, disjunction) and the occurrence of meiotic drive in the male.-Eleven mutants were found which increased nondisjunction in females. These mutants were characterized as to (1) the division at which they acted; (2) their effect on recombination; (3) their dominance; (4) their effects on disjunction of all four chromosome pairs. Five female mutants caused a nonuniform decrease in recombination, being most pronounced in distal regions, and an increase in first division nondisjunction of all chromosome pairs. Their behavior is consistent with the hypothesis that these mutants are defective in a process which is a precondition for exchange. Two female mutants were allelic and caused a uniform reduction in recombination for all intervals (though to different extents for the two alleles) and an increase in first-division nondisjunction of all chromosomes. Limited recombination data suggest that these mutants do not alter coincidence, and thus, following the arguments of Sandler et al. (1968), are defective in exchange rather than a precondiiton for exchange. A single female mutant behaves in a manner that is consistent with it being a defect in a gene whose functioning is essential for distributive pairing. Three of the female meiotic mutants cause abnormal chromosome behavior at a number of times in meiosis. Thus, nondisjunction at both meiotic divisions is increased, recombinant chromosomes nondisjoin, and there is a polarized alteration in recombination.-The striking differences between the types of control of meiosis in the two sexes is discussed and attention is drawn to the possible similarities between (1) the disjunction functions of exchange and the process specified by the chromosome-specific male mutants; and (2) the prevention of functional aneuploid gamete formation by distributive disjunction and meiotic drive.     

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 mechanism of secondary nondisjunction in Drosophila melanogaster females

Bridges (1916) observed that X chromosome nondisjunction was much more frequent in XXY females than it was in genetically normal XX females. In addition, virtually all cases of X nondisjunction in XXY females were due to XX <--> Y segregational events in oocytes in which the two X chromosomes had failed to undergo crossing over. He referred to these XX <--> Y segregation events as "secondary nondisjunction." Cooper (1948) proposed that secondary nondisjunction results from the formation of an X-Y-X trivalent, such that the Y chromosome directs the segregation of two achiasmate X chromosomes to opposite poles on the first meiotic spindle. Using in situ hybridization to X and YL chromosomal satellite sequences, we demonstrate that XX <--> Y segregations are indeed presaged by physical associations of the X and Y chromosomal heterochromatin. The physical colocalization of the three sex chromosomes is observed in virtually all oocytes in early prophase and maintained at high frequency until midprophase in all genotypes examined. Although these XXY associations are usually dissolved by late prophase in oocytes that undergo X chromosomal crossing over, they are maintained throughout prophase in oocytes with nonexchange X chromosomes. The persistence of such XXY associations in the absence of exchange presumably facilitates the segregation of the two X chromosomes and the Y chromosome to opposite poles on the developing meiotic spindle. Moreover, the observation that XXY pairings are dissolved at the end of pachytene in oocytes that do undergo X chromosomal crossing over demonstrates that exchanges can alter heterochromatic (and thus presumably centromeric) associations during meiotic prophase.     

The meiotic defects of mutants in the Drosophila mps1 gene reveal a critical role of Mps1 in the segregation of achiasmate homologs

The conserved kinase Mps1 is necessary for the proper functioning of the mitotic and meiotic spindle checkpoints (MSCs), which monitor the integrity of the spindle apparatus and prevent cells from progressing into anaphase until chromosomes are properly aligned on the metaphase plate. In Drosophila melanogaster, a null allele of the gene encoding Mps1 was recently shown to be required for the proper functioning of the MSC, but it did not appear to exhibit a defect in female meiosis. We demonstrate here that the meiotic mutant ald1 is a hypomorphic allele of the mps1 gene. Both ald1 and a P-insertion allele of mps1 exhibit defects in female meiotic chromosome segregation. The observed segregational defects are substantially more severe for pairs of achiasmate homologs, which are normally segregated by the achiasmate (or distributive) segregation system, than they are for chiasmate bivalents. Furthermore, cytological analysis of ald1 mutant oocytes reveals both a failure in the coorientation of achiasmate homologs at metaphase I and a defect in the maintenance of the chiasmate homolog associations that are normally observed at metaphase I. We conclude that Mps1 plays an important role in Drosophila female meiosis by regulating processes that are especially critical for ensuring the proper segregation of nonexchange chromosomes.     

Orientation Disruptor (ord): A Recombination-Defective and Disjunction-Defective Meiotic Mutant in DROSOPHILA MELANOGASTER

The effects of a semidominant autosomal meiotic mutant, orientation disruptor (symbol: ord), located at 2–103.5 on the genetic map and in region 59B-D of the salivary map, have been examined genetically and cytologically. The results are as follows. (1) Crossing over in homozygous females is reduced to about seven percent of controls on all chromosomes, with the reduction greatest in distal regions. (2) Crossing over on different chromosomes is independent. (3) Reductional nondisjunction of any given chromosome is increased to about thirty percent of gametes from homozygous females. The probability of such nondisjunction is the same among exchange and nonexchange tetrads with the exception that a very proximal exchange tends to regularize segregation. (4) Equational nondisjunction of each chromosome is increased to about ten percent of gametes in homozygous females; this nondisjunction is independent of exchange. (5) The distributive pairing system is operative in homozygous females. (6) In homozygous males, reductional nondisjunction of each chromosome is increased to about ten percent, and equational nondisjunction to about twenty percent, of all gametes. (7) Cytologically, two distinct meiotic divisions occur in spermatocytes of homozygous males. The first division looks normal although occasional univalents are present at prophase I and a few lagging chromosomes are seen at anaphase I. However, sister chromatids of most chromosomes have precociously separated by metaphase II. Possible functions of the ord⁺ gene are considered.     

Genetic Analysis of Hybrid Strains Trisomic for the Chromosome Containing a Fatty Acid Synthetase Gene Complex (fas1) in Yeast

Evidence of spontaneous n+1 aneuploidy has been obtained by trisomic segregation analysis of four independently maintained stocks of Saccharomyces cerevisiae defective in saturated fatty acid synthesis (fas1). In all cases tested, only the chromosome bearing the mutant fatty acid locus was disomic. Tetrad analysis of trisomic hybrids enabled the identification of chromosome XI as the one bearing the fatty acid locus and the assignment of fragment 5 to chromosome XI. Statistical analysis of tetrad frequencies generated by markers in triplex configuration provided information on the meiotic configuration of pairing of the three homologous chromosomes. The possible relationship between defective nuclear membranes and the disjunction of chromosomes in fas1 strains is discussed.     

Identification of novel Drosophila meiotic genes recovered in a P-element screen.

The segregation of homologous chromosomes from one another is the essence of meiosis. In many organisms, accurate segregation is ensured by the formation of chiasmata resulting from crossing over. Drosophila melanogaster females use this type of recombination-based system, but they also have mechanisms for segregating achiasmate chromosomes with high fidelity. We describe a P-element mutagenesis and screen in a sensitized genetic background to detect mutations that impair meiotic chromosome pairing, recombination, or segregation. Our screen identified two new recombination-deficient mutations: mei-P22, which fully eliminates meiotic recombination, and mei-P26, which decreases meiotic exchange by 70% in a polar fashion. We also recovered an unusual allele of the ncd gene, whose wild-type product is required for proper structure and function of the meiotic spindle. However, the screen yielded primarily mutants specifically defective in the segregation of achiasmate chromosomes. Although most of these are alleles of previously undescribed genes, five were in the known genes alphaTubulin67C, CycE, push, and Trl. The five mutations in known genes produce novel phenotypes for those genes.     

A Cis-Acting Locus That Promotes Crossing over between X Chromosomes in Caenorhabditis Elegans

This study reports the characterization of a cis-acting locus on the Caenorhabditis elegans X chromosome that is crucial for promoting normal levels of crossing over specifically between the X homologs and for ensuring their proper disjunction at meiosis I. The function of this locus is disrupted by the mutation me8, which maps to the extreme left end of the X chromosome within the region previously implicated by studies of X;A translocations and X duplications to contain a meiotic pairing site. Hermaphrodites homozygous for a deletion of the locus (Df/Df) or heterozygous for a deletion and the me8 mutation (me8/Df) exhibit extremely high levels of X chromosome nondisjunction at the reductional division; this is correlated with a sharp decrease in crossing over between the X homologs as evidenced both by reductions in genetic map distances and by the presence of achiasmate chromosomes in cytological preparations of oocyte nuclei. Duplications of the wild-type region that are unlinked to the X chromosome cannot complement the recombination and disjunction defects in trans, indicating that this region must be present in cis to the X chromosome to ensure normal levels of crossing over and proper homolog disjunction. me8 homozygotes exhibit an altered distribution of crossovers along the X chromosome that suggests a defect in processivity along the X chromosome of an event that initiates at the chromosome end. Models are discussed in which the cis-acting locus deleted by the Dfs functions as a meiotic pairing center that recruits trans-acting factors onto the chromosomes to nucleate assembly of a crossover-competent complex between the X homologs. This pairing center might function in the process of homolog recognition, or in the initiation of homologous synapsis.     

Separation of meiotic and mitotic effects of claret non-disjunctional on chromosome segregation in Drosophila.

The claret (ca) locus in Drosophila encodes a kinesin-related motor molecule that is required for proper distribution of chromosomes in meiosis in females and in the early mitotic divisions of the embryo. Here we demonstrate that a mutant allele of claret non-disjunctional (ca(nd)), non-claret disjunctional Dominant (ncdD), causes abnormalities in meiotic chromosome segregation, but is near wild-type with respect to early mitotic chromosome segregation. DNA sequence analysis of this mutant allele reveals two missense mutations compared with the predicted wild-type protein. One mutation lies in a proposed microtubule binding region of the motor domain and affects an amino acid residue that is conserved in all kinesin-related proteins reported to date. This region of the motor domain can be used to distinguish meiotic and mitotic motor function, defining an amino acid sequence criterion for classifying motors according to function. ncdD's mutant meiotic effect, but near wild-type mitotic effect, suggests that interactions of the ca motor protein with spindle microtubules differ in meiosis and mitosis.     

Genetic evidence that nonhomologous disjunction and meiotic drive are properties of wild-type Drosophila melanogaster male meiosis

We have followed sex and second chromosome disjunction, and the effects of these chromosomes on sperm function, in four genotypes: wild-type males, males deficient for the Y-linked crystal locus, males with an X chromosome heterochromatic deficiency that deletes all X-Y pairing sites, and males with both deficiencies. Both mutant situations provoke chromosome misbehavior, but the disjunctional defects are quite different. Deficiency of the X heterochromatin, consonant with the lack of pairing sites, mostly disrupts X-Y disjunction with a decidedly second-level effect on major autosome behavior. Deleting crystal, consonant with the cytological picture of postpairing chromatin-condensation problems, disrupts sex and autosome disjunction equally. Even when the mutant-induced nondisjunction has very different mechanics, however, and even more importantly, even in the wild type, there is strong, and similar, meiotic drive. The presence of meiotic drive when disjunction is disrupted by distinctly different mechanisms supports the notion that drive is a normal cellular response to meiotic problems rather than a direct effect of particular mutants. Most surprisingly, in both wild-type and crystal-deficient males the Y chromosome moves to the opposite pole from a pair of nondisjoined second chromosomes nearly 100% of the time. This nonhomologous interaction is, however, absent when the X heterochromatin is deleted. The nonhomologous disjunction of the sex and second chromosomes may be the genetic consequence of the chromosomal compartmentalization seen by deconvolution microscopy, and the absence of Y-2 disjunction when the X heterochromatin is deleted suggests that XY pairing itself, or a previously unrecognized heterochromatic function, is prerequisite to this macrostructural organization of the chromosomes.     

Meiotic Disjunction of Homologs in Saccharomyces Cerevisiae Is Directed by Pairing and Recombination of the Chromosome Arms but Not by Pairing of the Centromeres

We explored the behavior of meiotic chromosomes in Saccharomyces cerevisiae by examining the effects of chromosomal rearrangements on the pattern of disjunction and recombination of chromosome III during meiosis. The segregation of deletion chromosomes lacking part or all (telocentric) of one arm was analyzed in the presence of one or two copies of a normal chromosome III. In strains containing one normal and any one deletion chromosome, the two chromosomes disjoined in most meioses. In strains with one normal chromosome and both a left and right arm telocentric chromosome, the two telocentrics preferentially disjoined from the normal chromosome. Homology on one arm was sufficient to direct chromosome disjunction, and two chromosomes could be directed to disjoin from a third. In strains containing one deletion chromosome and two normal chromosomes, the two normal chromosomes preferentially disjoined, but in 4-7% of the tetrads the normal chromosomes cosegregated, disjoining from the deletion chromosome. Recombination between the two normal chromosomes or between the deletion chromosome and a normal chromosome increased the probability that these chromosomes would disjoin, although cosegregation of recombinants was observed. Finally, we observed that a derivative of chromosome III in which the centromeric region was deleted and CEN5 was integrated at another site on the chromosome disjoined from a normal chromosome III with fidelity. These studies demonstrate that it is not pairing of the centromeres, but pairing and recombination along the arms of the homologs, that directs meiotic chromosome segregation.