Heterochromatin-Mediated Association of Achiasmate Homologs Declines With Age When Cohesion Is Compromised

Normally, meiotic crossovers in conjunction with sister-chromatid cohesion establish a physical connection between homologs that is required for their accurate segregation during the first meiotic division. However, in some organisms an alternative mechanism ensures the proper segregation of bivalents that fail to recombine. In Drosophila oocytes, accurate segregation of achiasmate homologs depends on pairing that is mediated by their centromere-proximal heterochromatin. Our previous work uncovered an unexpected link between sister-chromatid cohesion and the fidelity of achiasmate segregation when Drosophila oocytes are experimentally aged. Here we show that a weak mutation in the meiotic cohesion protein ORD coupled with a reduction in centromere-proximal heterochromatin causes achiasmate chromosomes to missegregate with increased frequency when oocytes undergo aging. If ORD activity is more severely disrupted, achiasmate chromosomes with the normal amount of pericentric heterochromatin exhibit increased nondisjunction when oocytes age. Significantly, even in the absence of aging, a weak ord allele reduces heterochromatin-mediated pairing of achiasmate chromosomes. Our data suggest that sister-chromatid cohesion proteins not only maintain the association of chiasmate homologs but also play a role in promoting the physical association of achiasmate homologs in Drosophila oocytes. In addition, our data support the model that deterioration of meiotic cohesion during the aging process compromises the segregation of achiasmate as well as chiasmate bivalents.     

The Effect of Recombination-Defective Meiotic Mutants on Fourth-Chromosome Crossing over in DROSOPHILA MELANOGASTER

Crossing over was measured on the normally achiasmate fourth chromosome in females homozygous for one of our different recombination-defective meiotic mutants. Under the influence of those meiotic mutants that affect the major chromosomes by altering the spatial distribution of exchanges, meiotic fourth-chromosome recombinants were recovered irrespective of whether or not the meiotic mutant decreases crossing over on the other chromosomes. No crossing over, on the other hand, was detected on chromosome 4 in either wild type or in the presence of a meiotic mutant that decreases the frequency, but does not affect the spatial distribution, of exchange on the major chromosomes. It is concluded from these observations that (a) in wild type there are regional constraints on exchange that can be attenuated or eliminated by the defects caused by recombination-defective meiotic mutants; [b] these very constraints account for the absence of recombination on chromosome 4 in wild type; and [c] despite being normally achiasmate, chromosome 4 responds to recombination-defective meiotic mutants in the same way as do the other chromosomes.     

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.     

A deficiency screen of the major autosomes identifies a gene (matrimony) that is haplo-insufficient for achiasmate segregation in Drosophila oocytes

In Drosophila oocytes, euchromatic homolog-homolog associations are released at the end of pachytene, while heterochromatic pairings persist until metaphase I. A screen of 123 autosomal deficiencies for dominant effects on achiasmate chromosome segregation has identified a single gene that is haplo-insufficient for homologous achiasmate segregation and whose product may be required for the maintenance of such heterochromatic pairings. Of the deficiencies tested, only one exhibited a strong dominant effect on achiasmate segregation, inducing both X and fourth chromosome nondisjunction in FM7/X females. Five overlapping deficiencies showed a similar dominant effect on achiasmate chromosome disjunction and mapped the haplo-insufficient meiotic gene to a small interval within 66C7-12. A P-element insertion mutation in this interval exhibits a similar dominant effect on achiasmate segregation, inducing both high levels of X and fourth chromosome nondisjunction in FM7/X females and high levels of fourth chromosome nondisjunction in X/X females. The insertion site for this P element lies immediately upstream of CG18543, and germline expression of a UAS-CG18543 cDNA construct driven by nanos-GAL4 fully rescues the dominant meiotic defect. We conclude that CG18543 is the haplo-insufficient gene and have renamed this gene matrimony (mtrm). Cytological studies of prometaphase and metaphase I in mtrm hemizygotes demonstrate that achiasmate chromosomes are not properly positioned with respect to their homolog on the meiotic spindle. One possible, albeit speculative, interpretation of these data is that the presence of only a single copy of mtrm disrupts the function of whatever "glue" holds heterochromatically paired homologs together from the end of pachytene until metaphase I.     

Dynein promotes achiasmate segregation in Schizosaccharomyces pombe

Most organisms use crossovers (chiasmata) to maintain physical connections between homologous chromosomes that ensure their proper segregation at the first meiotic division. The fission yeast Schizosaccharomyces pombe has a residual ability to segregate homologous chromosomes in the absence of meiotic recombination (achiasmate segregation). Using cytologically tagged chromosomes, we established a role for the microtubule motor dynein in meiotic chromosome segregation. Dhc1, the motor subunit of dynein, is required for chromosome segregation in both the presence and the absence of recombination. Dlc1, a member of the Tctex-1 dynein light-chain family, preferentially affects the segregation of achiasmate chromosomes. Dlc1 is the first identified protein, outside of Drosophila, that preferentially affects achiasmate chromosome segregation. We discuss possible roles of the dynein motor in this process.     

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.     

Genetic control of mammalian meiotic recombination. I. Variation in exchange frequencies among males from inbred mouse strains

Genetic background effects on the frequency of meiotic recombination have long been suspected in mice but never demonstrated in a systematic manner, especially in inbred strains. We used a recently described immunostaining technique to assess meiotic exchange patterns in male mice. We found that among four different inbred strains--CAST/Ei, A/J, C57BL/6, and SPRET/Ei--the mean number of meiotic exchanges per cell and, thus, the recombination rates in these genetic backgrounds were significantly different. These frequencies ranged from a low of 21.5 exchanges in CAST/Ei to a high of 24.9 in SPRET/Ei. We also found that, as expected, these crossover events were nonrandomly distributed and displayed positive interference. However, we found no evidence for significant differences in the patterns of crossover positioning between strains with different exchange frequencies. From our observations of >10,000 autosomal synaptonemal complexes, we conclude that achiasmate bivalents arise in the male mouse at a frequency of 0.1%. Thus, special mechanisms that segregate achiasmate chromosomes are unlikely to be an important component of mammalian male meiosis.     

Centromere localization at meiosis and the position of chiasmata in the male and female mouse

Techniques for obtaining differential Giemsa staining of the paracentromeric (p.c.) regions of male and female mouse meiotic chromosomes (centromeric heterochromatin) were explored and standard procedures developed for the different meiotic cells in the two sexes. The best result followed the use of heat at controlled pH in Sörensen's phosphate buffer or in Standard Saline Citrate (SSC) solutions. With these techniques, morphological features of the p.c. regions and their variation were studied in normal animals (CFLP strain) and in a strain (AKR) homozygous for a centric fusion [T(11; ?)-1 Ald] between chromosomes No. 6 and No. 15 (Miller et al., 1971). The Y chromosome was often found to show distinct p. c. staining at first and apparently at second meiotic metaphase, and the X and Y chromosomes were found to associate as bivalents by their long arms. Autosomal p.c. regions showed variation in size which might indicate differences between non-homologous chromosomes but a tendency to similarity between homologues. Differences were found between males and females in respect to proportions and variation of bivalents with single and double chiasmata. The relative positions of chiasmata were different in the two sexes. The presence of the centric fusion in the males did not seem to affect the pairing behaviour of the remaining autosomes or of those taking part in the centric fusion. The possibility is discussed that the p.c. regions, to which also other functions would seem to appertain, may be important for chromosome recognition and pairing, possibly on a quantitative basis.     

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. © 1993 Wiley-Liss, Inc.     

Meiotic and sperm chromosome studies in a reciprocal translocation t(1;2)(q32;q36)

Summary Meiotic and sperm chromosomes were studied in a man heterozygous for a reciprocal translocation t(1;2)(q32; q36). Forty-five meiotic metaphase I cells were obtained from semen samples: 86.6% were 22,XY,IV and 13.3% had synaptic anomalies that affected all or some of the bivalents. The quadrivalents observed had a ring configuration (92.3%) or a chain configuration (7.7%). A total of 105 sperm chromosome complements were analyzed: 41% resulted from an alternate segregation, and the percentage of unbalanced sperm was 59%; most of them (71%) resulted from an adjacent 1 segregation. The frequency of anomalies unrelated to the translocation (5.7% numerical and 14.1% structural anomalies) were within the normal range for control donors. There was a good correspondence between the percentage of cells with a ring IV (92.3%) and the proportion of 2:2 segregations (88.6%) and between the percentage of chain IV (7.7%) and the incidence of 3:1 segregations (11.4%).     

Sex-dependent frequency and type of autosomal univalency at the first meiotic metaphase in mouse germ cells

Univalents at the first meiotic metaphase in mouse spermatocytes occur mainly in the XY pair, making it difficult to compare the amounts of univalency in males and females. In this study, the amounts of autosomal univalency in male and female meiosis were compared using the model strain CBA-T6, in which univalency of the small marker autosome pair T6 has been shown to occur very frequently in spermatocytes. Mice from inbred CBA and DBA strains were also analysed. The total frequencies of univalency (sex chromosomes plus autosomes) in metaphase I spermatocytes were 45.6% in CBA, 36.9% in CBA-T6, and 37.3% in DBA males. The aneuploidy in metaphase II spermatocytes ranged from 1.4 to 3% in these strains, which was in agreement with previous findings that most primary spermatocytes with abnormal chromosome configurations are arrested in their development before metaphase II. In the CBA-T6 strain, autosomal univalency at metaphase I mostly involved chromosome pair T6; however, its frequency differed significantly between the sexes, amounting to 18.9% in spermatocytes and 4.3% in oocytes. In the CBA strain, autosomal univalents at metaphase I were seen in 7.7% of the spermatocytes and 1.4% of the oocytes and, in DBA mice, in 4.9% of the spermatocytes and 3.8% of the oocytes. However, in DBA oocytes, when univalency occurred it usually concerned a greater number of bivalents in one cell (range: 2-19 disjoined bivalents), a phenomenon very rare in males of this strain. This study shows that univalent formation differs between the male and female types of meiosis.     

Effects of Homology, Size and Exchange on 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.     

Meiotic disjunction,sex-determination and embryonic lethality in an X-linked “Simple” translocation of the onion fly,Hylemya antiqua (Meigen)

It was shown that the translocation in study is X-linked. After testcrossing translocation heterozygous males they generally only produce translocation heterozygous daughters and normal sons. The small acrocentric chromosomes involved in the translocation appeared to be the sex-chromosomes. The X-chromosome has a secondary constriction which is missing in the (male determining) Y-chromosome. Meiotic orientation was studied in translocation heterozygous males and females. The alternate and adjacent I orientations were found in about equal frequencies. Further, numerical meiotic non-disjunction (two types) occurred in translocation heterozygous males (about 2%), but is much higher in females (18.7%). In (achiasmate) males the homologous centromeres predominantly regulate meiotic pairing, coorientation and disjunction, apparently independently of the chromosomal rearrangement. Disturbed telomere pairing in particular leading to reduced chiasma frequency most probably explains the high numerical non-disjunction in chiasmate females. A rather good relationship exists between the percentage “semi”-sterility (28%), scored as late embryonic lethals (eggs, 72 hrs.) and the percentage karyotypes (20%) in young eggs (8–16 hrs.) with a large chromosomal deficiency. The remaining sterility (8%) can be explained by the somewhat decreased viability of tertiary trisomics and duplication karyotypes at the end of the egg stage. This translocation behaves like a “simple” one.     

Spectral karyotyping (SKY) of mouse meiotic chromosomes.

The spectral karyotyping procedure of in situ hybridization with chromosome-specific probes assigns a unique colour code to each of the 21 mouse mitotic chromosomes. We have adapted this procedure to meiotic prophase chromosomes, and the results show that each of the pachytene or metaphase I bivalents can be identified. This technique has the potential to recognize synaptic anomalies and chromosome-specific structural and behavioural characteristics. We confirm these potentials by the recognition of the heterologous synapsis of the X and Y chromosomes and by the variances of synaptonemal complex lengths for each of the colour-coded bivalents in eight prophase nuclei.     

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.     

Drosophila BubR1 is essential for meiotic sister-chromatid cohesion and maintenance of synaptonemal complex

The partially conserved Mad3/BubR1 protein is required during mitosis for the spindle assembly checkpoint (SAC). In meiosis, depletion causes an accelerated transit through prophase I and missegregation of achiasmate chromosomes in yeast [1], whereas in mice, reduced dosage leads to severe chromosome missegregation [2]. These observations indicate a meiotic requirement for BubR1, but its mechanism of action remains unknown. We identified a viable bubR1 allele in Drosophila resulting from a point mutation in the kinase domain that retains mitotic SAC activity. In males, we demonstrate a dose-sensitive requirement for BubR1 in maintaining sister-chromatid cohesion at anaphase I, whereas the mutant BubR1 protein localizes correctly. In bubR1 mutant females, we find that both achiasmate and chiasmate chromosomes nondisjoin mostly equationally consistent with a defect in sister-chromatid cohesion at late anaphase I or meiosis II. Moreover, mutations in bubR1 cause a consistent increase in pericentric heterochromatin exchange frequency, and although the synaptonemal complex is set up properly during transit through the germarium, it is disassembled prematurely in prophase by stage 1. Our results demonstrate that BubR1 is essential to maintain sister-chromatid cohesion during meiotic progression in both sexes and for normal maintenance of SC in females.     

Male-Sex-Ratio Trait in Drosophila Pseudoobscura: Frequency of Autosomal Aneuploid Sperm

Males with the SR X chromosome show the "sex-ratio" (sr) phenotype in which they produce almost entirely daughters. The few sons (about 1%) are invariably sterile X/O males and result entirely from nullo-XY sperm. The "male-sex-ratio" (msr) phenotype is a modified form of sr in which SR/Y males produce a higher frequency of sterile X/O sons. The msr trait is due to the presence of the SR X-chromosome in males which are also homozygous for one or more autosomes from the L116 strain. Here the frequency of nullo-3 and diplo-3 sperm from msr males was measured by crossing to a compound-3 strain and found to be 13.8% and 3.2%, respectively, of the total viable sperm. The sr males produced very low levels of nullo-3 sperm at a frequency not different from control X/Y males and a slightly elevated frequency of diplo-3 sperm over X/Y males. The msr males were found to have only 12% the fecundity of sr males and in matings to cause a high frequency of brown inviable eggs. These results indicate that high rates of autosomal aneuploidy are not restricted to chromosome 3 but also occur for chromosomes 2, 4 and 5. The overall frequency of autosomal aneuploid sperm is estimated to be approximately 50%. Microscopic studies of meiosis in testes from msr males indicates meiotic nondisjunction and meiotic chromosome loss are responsible for the msr phenotype. Last, microscopic studies of sperm cysts from msr males reveal high levels of spermiogenic failure.     

A synaptonemal complex-derived mechanism for meiotic segregation precedes the evolutionary loss of homology between sex chromosomes in arvicolid mammals

Synapsis and reciprocal recombination between sex chromosomes are restricted to the pseudoautosomal region. In some animal species, sex chromosomes do not present this region, although they utilize alternative mechanisms that ensure meiotic pairing and segregation. The subfamily Arvicolinae (Rodentia, Cricetidae) includes numerous species with achiasmate sex chromosomes. In order to know whether the mechanism involved in achiasmate segregation is an ancient feature in arvicolid species, we have compared the sex chromosomes of both the Mediterranean vole (Microtus duodecimcostatus) and the water vole (Arvicola terrestris). By means of immunofluorescence, we have found that sex chromosomes in M. duodecimcostatus are asynaptic and develop a synaptonemal complex-derived structure that mediates pairing and facilitates segregation. In A. terrestris, sex chromosomes are synaptic and chiasmate but also exhibit a synaptonemal complex-derived filament during anaphase I. Since phylogenetic relationships indicate that the synaptic condition is ancestral in arvicolids, this finding indicates that the mechanism for achiasmate sex chromosome segregation precedes the switching to the asynaptic condition. We discuss the origin of this synaptonemal complex-derived mechanism that, in turn, could counterbalance the disruption of homology in the sex chromosomes of those species.     

No induction of interchromosomal effect in male meiosis of Chinese hamsters with reciprocal translocations

Chinese hamsters from five strains with reciprocal translocations, T(1;3)7Idr, T(1;3)8Idr, T(1;2)9Idr, T(7;9)16Idr, and T(1;5)17Idr, and a karyotypically normal strain, CHS/Idr, were used to look for an interchromosomal effect by chromosomal analysis of meiotic cells and one-cell embryos. The frequencies of nondisjunction at first meiosis in five normal (+/+) males, calculated by doubling the number of hyperhaploid cells, ranged from 0.43% to 1.33%, and there was no significant difference in frequency among individuals. On the other hand, the frequency of hyperhaploid cells in males heterozygous for each translocation ranged from 3.0% to 11.8%, and the frequency of hyperhaploid cells with an extra translocation-unrelated chromosome ranged from 0.2% to 0.4%, which is no different from that estimated from scoring of +/+ males at the second meiotic metaphase. In one-cell embryos from crosses between karyotypically normal females and male heterozygotes for T(1;2)9Idr and T(7;9)16Idr, 1.1% and 0.5% of embryos had an extra translocation-unrelated chromosome. Compared with the control, the frequency of meiotic nondisjunction showed no increase in male heterozygotes for the reciprocal translocations. Therefore, the results suggest that multivalents and rearranged chromosomes existing at first and second meiosis in male Chinese hamsters exert no influence on segregation of normal bivalents and chromosomes unrelated to the rearrangements.     

Two new X-autosome Robertsonian translocations in the mouse

The influence of X-autosome Robertsonian (Rb) translocation hemizygosity on meiotic chromosome behaviour was investigated in male mice. Two male fertile translocations [Rb(X.2)2Ad and Rb(X.9)6H] and a male sterile translocation [Rb(X.12)7H] were used. In males of all three Rb translocation types, the acrocentric homologue of the autosome involved in the rearrangement regularly failed at pachytene to pair completely with its partner in the Rb metacentric. The centric end of the acrocentric autosome was found regularly to associate either with the proximal end of the Y chromosome or with the ends of nonhomologous autosomal bivalents; the proportions of cells with such configurations varied between pachytene substages and genotypes. Various other categories of synaptic anomaly, such as nonhomologous synapsis, foldback pairing and interlocks, affected the sex chromosome multivalent in a substantial proportion of cells. In one of the Rb(X.12)7H males screened, an unusual, highly aneuploid spermatocyte that contained trivalent and bivalent configurations was found. Rb translocation hemizygosity did not appear to increase to a significant extent the incidence of X-Y pairing failure at pachytene, although the incidence was elevated at metaphase I in Rb(X.12)7H animals. Overall, a comparison of the frequencies and types of chromosome pairing anomalies did not suggest that these were important factors in the aetiology of infertility in males carrying the Rb(X.12)7H translocation.