Meiosis in trisomic female mice with Robertsonian translocations. I. Prophase pairing

The prophase oocytes of two murine Robertsonian translocation (Rb) trisomies of chromosomes 16 and 19 were investigated using electron microscopy and a whole-cell micro-spreading technique after silver staining. About 20% of fetuses of each type were trisomic. They were obtained by mating animals heterozygous for two Rb's, monobrachially homologous for either chromosome 16 or 19, to an entirely acrocentric stock. Because of the almost inevitable prenatal mortality of the trisomic embryos, their fetal ovaries were "rescued" by an in vitro method for prophase studies. Analysis of the recovered oocytes showed frequent, close pairing associations of the three trisomic axes and evidence suggesting that the closely apposed axes coincided with the side-by-side formation of parallel, complete, true synaptonemal complexes; hence, the cytogenetic dogma that pairing is always two-by-two was contradicted. The presence of two parallel complexes has implications for crossing-over recombination. Triple associations of axes were found in almost half the trisomy 19 (Ts19) and in about 70% of the trisomy 16 (Ts16) prophases. The extent of triple associations varied and was greater in Ts16 than in Ts19 oocytes. Other relevant observations concerned the proportions of univalents and of univalence of the trisomic axes (21% in Ts16 and 46% in Ts19) and the distinctive, thickened appearance of all univalent axes. The pairing behaviour observed in balanced heterozygotes confirms what appears to be nonhomologous pairing and synaptic adjustment within the short-arm axes of the Rb trivalents.     

The induction of tail malformations in trisomy 16 mouse fetuses heterozygous for the curly tail recessive gene

The mouse mutant curly tail is thought to be inherited as an autosomal recessive (ct/ct) with incomplete penetrance so that approximately 60% of ct/ct individuals exhibit the curly tail (CT) phenotype. By outcrossing ct/ct with mouse stock carrying specific heterozygous combinations of Robertsonian (Rb) chromosomes, trisomy 16 (Ts16) and Ts19 mouse fetuses (and their chromosomally balanced littermates) were derived which were heterozygous for the ct gene. All of the Ts16 (ct/Rb;Rb) fetuses, studied between days 14-19 gestation had tail malformations, 86% of which were tail flexion defects (TFD) apparently very similar to the curly tail phenotype. Neither Ts19 nor any of the chromosomally balanced (ct/Rb) littermates from both experimental crosses showed any type of tail or other spinal malformation. At the 27-29 somite stage of development, Ts16 (ct/Rb;Rb) fetuses did not show any significant delay in the closure of the posterior neuropore (PNP) compared with their littermate controls, suggesting that the tail malformation observed in Ts16 (ct/Rb;Rb) occur as a result of mechanisms which differ significantly from those thought to be responsible to causing the curly tail malformation.     

Absence of correlation between univalent formation and meiotic nondisjunction in aged female Chinese hamsters

The effects of maternal aging on the configuration of chiasmata, formation of univalents, and segregation of first meiotic (MI) chromosomes were investigated in young (5-8 mo) and old (16-19 mo) Chinese hamsters. Primary oocytes were collected only from mature follicles approximately 10 h before ovulation, and secondary oocytes were obtained from the oviducts 5 h after spontaneous ovulation. The average number of chiasmata per oocyte was significantly smaller in aged hamsters than in the young hamsters (P less than 0.001). Terminal chiasmata were found more frequently in the former group than in the latter one (P less than 0.001). These results coincided well with findings in the mouse. Since the 11 meiotic chromosomes could be divided into four morphologically distinguishable subgroups, it was possible to determine whether the same bivalent forming univalents at MI actually underwent nondisjunction in the following meiotic division. The incidence of both MI oocytes with a univalent pair and aneuploid MII oocytes due to first meiotic nondisjunction was significantly higher in the aged group than in the young group (P less than 0.01) and P less than 0.05, respectively). However, univalents occurred almost exclusively in the smallest metacentric chromosome group (96%), whereas nondisjunction took place nearly equally in each chromosomal subgroup. These results clearly showed that there was no correlation between the univalents seen at MI and nondisjunction during the first meiotic division.     

Chiasma frequency and position in male and female mice of chromosomes involved in homozygous and heterozygous translocations and tertiary trisomy

Chiasma frequency and position were analyzed at a predominantly late diplotene-diakinesis stage of the first meiotic division in oocytes and spermatocytes from T(1;13)70H homozygotes and heterozygotes, T(2;8)26H heterozygotes and from Ts(I¹³)70H tertiary trisomics of the mouse, Mus musculus. For T70H/T70H, the 13¹ long marker bivalent was studied and for the other karyotypes, analysis was confined to the multivalent configurations adopted by the rearranged chromosomes and their homologues. For the 13¹ bivalent, the chiasma frequency tended to be increased in the female. For the T26H and the T70H multivalents, the chiasma frequencies were higher in the female, largely due to the much higher values in the short interstitial segments. This observed enhancement has been attributed to pairing differences rather than to differences in chiasma forming capability. Both in the telomeric region of the 13¹ bivalent and in the short translocated segments of the reciprocal translocation and tertiary trisomic multivalents, females showed fewer chiasmata than males. The determinations of chiasma position in the 13¹ bivalent indicated chiasma interference with respect to position, leading to clustering of chiasmata somewhat beyond the centromere and towards the telomere of this chromosome.     

Chiasma frequency,distribution and interference maps of mouse autosomes

Chiasma frequencies were analysed and chiasma positions measured in diakinesis/metaphase I autosomal bivalents from oocytes and spermatocytes of F₁ hybrid C3H/HeH×101/H mice. Twenty chromosome size ranks, including the presumptive X bivalent, could be distinguished in oocytes, and nineteen autosomal ranks plus the XY pair spermatocytes. Overall, mean cell chiasma frequencies of the two sexes did not differ significantly once the contribution of the presumptive X bivalent and the XY pair were taken into account. Sex related differences in chiasma distribution patterns were evident, however. In monochiasmate bivalents, the chiasma was most commonly located interstitially in oocytes while in spermatocytes it could be either interstitial or distal. In dichiasmate bivalents, the chiasmata tended to be more centrally located in oocytes than in spermatocytes. Minimum inter-chiasma distances did not appear to show any great variation in chromosome pairs of different sizes, however, mean inter-chiasma distances did increase with the bivalent length. The minimum-inter chiasma distance data suggest that chiasma interference is complete over a chromosomal segment equating to approximately 60 Mb. Measurement of the positions of chiasmata along chromosome arms open up the possibility of producing chiasma-based genetic maps for all the autosomes of the mouse.     

Chiasma distribution in mouse oocytes during diakinesis

The distribution of chiasmata in the mouse was examined by measurement of a single metacentric bivalent in 173 oocytes taken from 36 mice of the Rb3Bnr stock. Frequency distribution analysis revealed a well defined pattern of chiasma formation in both arms of the metacentric and, as in other organisms, interference and localization were thought to be major factors influencing this pattern. Despite the tendency for bivalents to form terminal associations at metaphase in the mouse and reported differences in chiasma frequency between early and late stages of meiosis, analysis of bivalents at diakinesis has produced no quantitative support for the concept of terminalization of chiasmata during meiosis.     

The effects of ageing on the meiotic chromosomes of male and female mice

The effects of age on the chiasma frequencies, chiasma position and numbers of univalents at MI in males and females of three strains of mouse were examined. Males showed a slight but non significant rise in chiasma frequency in age due to an increase in bivalents with two chiasmata at the expense of single chiasmata bivalents. In contrast, females exhibited a significant decrease in chiasma frequency with age due to the loss of two chiasma bivalents with a corresponding increase in single terminal chiasmata bivalents. In both males and females there was no significant increase in univalents with age in the strains studied. Of interest was the finding of a greater degree of contraction of the MI chromosomes in the oocytes of old relative to young females, a differential contraction that was independent of culture time. This finding is discussed with regard to the production line theory and non disjunction at Anaphase in other strains of mice.     

Adoptive transfer of the hematopoietic system of trisomic mice with limited life span: stem cells from six different trisomies are capable of survival.

The life span of murine trisomies is limited to the fetal or early postnatal period. However, rescue of the hematopoietic system of fetal mice with trisomies (Ts) 12, 13, 14, 16, 18, and 19 is possible by transplanting hematopoietic stem cells from the liver into lethally irradiated adult hosts. Thus, radiation chimeras with permanent and almost complete trisomic hematopoietic and lymphocytopoietic systems were constructed. The longest documented survival of a trisomic graft was 12 months in Ts 19 chimeras. Blood counts in trisomic chimeras reveal a marked anemia in Ts 16 chimeras; lymphocytopenia in Ts 12, Ts 16, and Ts 19 chimeras; and granulocytopenia in Ts 18 chimeras. Survival rates of Ts 12, Ts 18, and Ts 19 chimeras were not different from those of the respective controls, whereas survival rates of chimeras with Ts 13 and Ts 16 hematopoiesis were markedly reduced and that of Ts 14 chimeras only slightly reduced. These results indicate that transplanted hematopoietic stem cells from Ts 13, Ts 14, and Ts 16 fetuses exhibit relevant genetically determined defects, resulting in a reduced restoration capacity of hematopoietic organs and/or deficiencies of differentiated blood cells.     

The quantitative analysis of chromosome pairing and chiasma formation based on the relative frequencies of M I configurations

In autotetraploids, chromosome pairing may be in the form of quadrivalents or bivalent pairs. Whether or not the quadrivalents are maintained until first meiotic metaphase depends on the formation of chiasmata. The relative frequencies of M I configurations thus contain information both on pairing and on chiasma formation. With distal chiasma localisation six configurations can be recognised and their relative frequencies determined: ring quadrivalents, chain quadrivalents, trivalents (with univalent), ring bivalents, open (rod) bivalents, univalent pairs. These represent five degrees of freedom permitting five parameters to be estimated: the frequency (f) of quadrivalent pairing; the frequencies of chiasmate association of the two ends (arms in metacentrics), a′, b′, after quadrivalent pairing, and a, b after bivalent pairing. — The appropriate formulae have been derived and applied to observations on Tradescantia virginiana (4n=24) which has pronounced distal chiasma localisation. Slight modifications make the model applicable to autotetraploids with interstitial in addition to distal chiasmata. In T. virginiana, chromosome pairing appeared to be random between homologues (65.8% quadrivalent pairing; 55.4% observed at M I). After quadrivalent pairing chiasmate association is frequent in the “average long” arm (95.0%) and much less so in the other arm (60.5%). This is attributed to partner exchange. After bivalent pairing chiasma frequencies are still different for the two arms (93.8% and 83.5% association respectively) but much less pronounced. Various complications are discussed.     

Adoptive transfer of the hematopoietic system of trisomic mice with limited life span: Stem cells from six different trisomies are capable of survival

The life span of murine trisomies is limited to the fetal or early postnatal period. However, rescue of the hematopoietic system of fetal mice with trisomies (Ts) 12, 13, 14, 16, 18, and 19 is possible by transplanting hematopoietic stem cells from the liver into lethally irradiated adult hosts. Thus, radiation chimeras with permanent and almost complete trisomic hematopoietic and lymphocytopoietic systems were constructed. The longest documented survival of a trisomic graft was 12 months in Ts 19 chimeras. Blood counts in trisomic chimeras reveal a marked anemia in Ts 16 chimeras; lymphocytopenia in Ts 12, Ts 16, and Ts 19 chimeras; and granulocytopenia in Ts 18 chimeras. Survival rates of Ts 12, Ts 18, and Ts 19 chimeras were not different from those of the respective controls, whereas survival rates of chimeras with Ts 13 and Ts 16 hematopoiesis were markedly reduced and that of Ts 14 chimeras only slightly reduced. These results indicate that transplanted hematopoietic stem cells from Ts 13, Ts 14, and Ts 16 fetuses exhibit relevant genetically determined defects, resulting in a reduced restoration capacity of hematopoietic organs and/or deficiencies of differentiated blood cells. © 1992 Wiley-Liss, Inc.     

Meiotic nondisjunction in oocytes from aged Djungarian hamsters correlates with an alteration in meiosis rate but not in univalent formation

Summary The effect of maternal ageing on the meiotic rate, on chiasma and univalent frequency as well as on heteroploidy in secondary oocytes from Djungarian hamsters was exammed. The frequency of hyperhaploid oocytes increased from 0.6% in young (8–14 weeks) to 2.8% in middle-aged (26–46 weeks) and reached 3.6% in the oldest females (49–75 weeks). On the basis of malsegregated bivalents per oocyte, nondisjunction occurred most often in the middle-aged group (5.42x10^-2 bivalents per oocyte). Hereby, the large meta- and submetacentric A-D chromosomes were preferentially involved. Furthermore, the pattern of nondisjunction was not different from that expected on the basis of chromosome length or induced by colchicine. The large A-D chromosomes did not show any alteration in chiasma or univalent frequency. Terminalized chiasmata were only detected in the E group and univalents increased slightly, but not significantly in the small chromosomes (G group). At higher ages, both chromosome group were not preferentially involved in nondisjunction. Presegregation slightly increased with age and affected more or less all bivalents, whereas the incidence of diploidy significantly decreased. With respect to the rate of meiosis in oocytes from aged females, the resumption was delayed at metaphase I. Our data suggest that failures in the control of oocyte proliferation are involved in nondisjunction rather than the production-line. Furthermore, a model is proposed to explain nondisjunction of specific bivalents at certain maternal ages.     

Meiotic behavior of an unequal bivalent in the grasshopper Calliptamus palaestinensis Bdhr.

Summary Unequal bivalents were found in two of four populations of the short horned grasshopper, Calliptamus palaestinensis bdhr. sampled in Israel.The inequality of the homologues was due to an extra segment which was heterochromatic and apparently terminal. Pairing configurations at pachytene and position of chiasmata at later stages revealed however that the extra segment was interstitial, and the long member terminated in a minute segment homologous to the terminal part of its normal partner. The percentage of reductional divisions at anaphase I corresponded well with the percentage of terminal chiasmata (i.e. distal to the extra segment) observed at diakinesis.The assumption of an interstitial position of the extra segment would explain the previously puzzling examples of unequal bivalents in Orthoptera whose reductional divisions have not been readily accounted for otherwise.     

Pachytene pairing and oocyte numbers in mice with two single Robertsonian translocations and the male-sterile compound with monobrachial homology

Oocyte numbers and synaptonemal complexes were studied in two Robertsonian translocations, Rb(6.15)1Ald and Rb(4.6)2Bnr, and their male-sterile compound. Oocyte numbers in the compound were lower than those of either parent, and there was a marked difference between reciprocal crosses. Synaptonemal complexes of homozygous females appeared as 19 bivalents, those of single heterozygotes as 18 bivalents and a trivalent, and those of compound heterozygotes as 17 bivalents and a quadrivalent. Most trivalents were fully paired, whereas the majority of quadrivalents exhibited terminal asynapsis. About one-half of all oocytes had other pairing abnormalities, probably reflecting reduced survivability. Whereas all fully paired quadrivalents were present in cells not showing any pairing anomalies, one-half of the quadrivalents with terminal asynapsis were seen in oocytes with other anomalies. It is suggested that in oocytes destined for atresia, there is a predisposition to synaptic failure of translocation configurations. Additional oocytes are likely to break down because of the deleterious effect of the compound translocation on gametogenesis. This effect seems to be more pronounced in Rb1Ald/Rb2Bnr spermatocytes than in oocytes.     

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.     

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.     

Reproduction and cytogenetic characterization of interspecific hybrids derived from Cucumis hystrix Chakr. x Cucumis sativus L

Interspecific hybrids between Cucumis hystrix Chakr. (2n = 2 x = 24) and Cucumis sativus L. (2n = 2 x = 14) were produced by means of F(1) (2n = 19) embryo rescue and subsequent chromosome doubling. The hybridity was confirmed by genomic in situ hybridization (GISH) and chromosome analysis. The amphidiploid (2n = 38) was self-pollinated and backcrossed to cucumber resulting in lines with improved crossability to C. sativus. Examination of shape, stainability, and germination rate of pollen grains and yield as a function of mature fruit set per ten pollinated flowers indicated a tendency for increased fertility in BC(1)S(1) progeny when compared to F(1) and amphidiploid offspring. Cytogenetic characterization of F(1) and amphidiploid progeny was performed. Generally normal meioses produced viable pollen grains, and fertilization resulted in partial fertility restoration in amphidiploid progeny. Chromosome anomalies such as "frying-pan trivalent", chromosome lagging and spindle mis-orientation were also observed. In most of the PMCs of the F(1) diploid hybrid progeny, 19 univalents were observed at diakinesis and MI. In the amphidiploid, more than 90% of the configurations at MI consisted of the predicted 19 bivalents and less than 5% contained multivalents [trivalents (2.3%) + quadrivalents (0.3%)], suggesting the presence of preferential pairing, and a distinctive parental genome as well. The chiasmata observed between homoeologous chromosomes further demonstrated the introgression of the C. hystrix genome into that of C. sativus.     

Son-sire-regression based heritability estimates of chiasma frequency, using T70H mouse translocation heterozygotes, and the relation between univalence, chiasma frequency and sperm production.

T(1;13)70H/+ translocation heretozygous mice were used for assessing heritability values for chiasma frequencies and the epididymal sperm count. The chiasma frequency estimates were based on 15 son-sire pairs, the translocation heterozygotes being maintained in a Swiss random-bred genetic background. The chiasma frequencies were scored separately for the T70H/+ derived multivalent, specific pairing segments within the multivalent and the remaining bivalents. Chiasma counts within these specified parts of the genome were positively correlated. The heritability estimates, significantly greater than zero, ranged from 0.78-0.98, depending on the chromosome segments included. These results indicate a strong genetic control on a cellular basis for the formation of chiasmata in the mouse. Despite significantly positive correlations and regressions between the various chiasma frequencies and the sperm count (for which 29 pairs of observations were available), no significant heritability estimate for the sperm count was obtained. The relation between the chiasma frequency and the sperm count was weakest when the chiasma count was confined to a region of the translocation-caused multivalent in which the absence of a chiasma almost always resulted in the production of an univalent. This indicates that in the translocation heterozygotes used, the overall chiasma frequency has a greater predictive value for the sperm count than autosomal univalence alone.     

An analysis of meiotic impairment and of sex chromosome associations throughout meiosis in XYY mice

The existing XYY meiotic data for mice present a very heterogeneous picture with respect to the relative frequencies of different sex chromosome associations, both at pachytene and diakinesis/metaphase I. Furthermore, where both pachytene and diakinesis/MI data are available for the same males, the frequencies of the different configurations at the two stages are very different. In the present paper we utilise "XYY" and "XY/XYY" mosaic mice with cytologically distinguishable Y chromosomes to investigate the factors responsible for this heterogeneity between different males and between the two meiotic stages. It is concluded (1) that the initial pattern of synapsis is driven by the relatedness of the three pseudoautosomal regions (PARs); (2) that the order and extent of PAR synapsis within radial trivalents are also affected by PAR relatedness and that this leads to chiasmata being preferentially formed between closely related PARs; (3) that trivalents with a single chiasma resolve into a bivalent + univalent by the diakinesis stage; (4) that although many spermatocytes with asynapsed sex chromosomes are eliminated between pachytene and diakinesis, those that survive this phase of elimination progress to the first meiotic metaphase (MI) and accumulate in large numbers, leading to an over-representation of those with univalents as compared to radial trivalents; and (5) that the arrested MI cells are eventually eliminated, so that very few "XYY" cells contribute products to MII.     

Synapsis and chiasma distribution in mice heterozygous for translocations in chromosomes 16 and 17

Electron microscopic analysis of synaptonemal complexes and analysis of chiasmata distribution in male mice heterozygous for Robertsonian translocation T(16; 17)7Bnr - (Rb7), for synaptonemal reciprocal translocation T(16;17)43H - (T43), in double heterozygotes for these translocations and in males with partial trisomy of the proximal region of chromosome 17 was carried out. Synaptic disturbances around the breakpoints of the translocations, such as asynapsis of homologous regions of partners and non-homologous synapsis of centromeric regions of acrocentric chromosomes, were revealed. Synaptic regularity in the proximal part of the chromosome 17 appeared to be affected by no t12 haplotype. Good coincidence between sizes of mitotic chromosomes and corresponding lateral elements of synaptonemal complexes was found for all chromosomes, with the exception of Rb7 in trisomics. In the latter karyotype, the proximal part of chromosome 17 involved in Robertsonian fusion seems to be shortened in the course of zygotene and never synapted with homologous segment of neither the acrocentric chromosome 17 nor large product of reciprocal translocation. Drastic increase in chiasmata frequency in the proximal part of chromosome 17 was revealed in heterozygotes for T43H and in trisomics, as compared with the double heterozygotes Rb7/T43. The latter finding was explained by the existence of two independent pairing segments in the former karyotypes.     

Trisomic pregnancy and earlier age at menopause

We tested the hypothesis that the connection between advanced maternal age and autosomal trisomy reflects the diminution of the oocyte pool with age. Because menopause occurs when the number of oocytes falls below some threshold, our hypothesis is that menopause occurs at an earlier age among women with trisomic pregnancies than it does among women with chromosomally normal pregnancies. To determine their menstrual status, we interviewed women from our previous study of karyotyped spontaneous abortions who, in 1993, were age >/=44 years. Premenopausal women completed interviews every 4-5 mo, until menopause or until the study ended in 1997. The primary analyses compare 111 women whose index pregnancy was a trisomic spontaneous abortion with two groups: women whose index pregnancy was a chromosomally normal loss (n=157) and women whose index pregnancy was a chromosomally normal birth (n=226). We used a parametric logistic survival analysis to compare median ages at menopause. The estimated median age at menopause was 0.96 years earlier (95% confidence interval -0.18 to 2.10) among women with trisomic losses than it was among women with chromosomally normal losses and chromosomally normal births combined. Results were unaltered by adjustment for education, ethnicity, and cigarette smoking. Our results support the hypothesis that trisomy risk is increased with decreased numbers of oocytes. Decreased numbers may indicate accelerated oocyte atresia or fewer oocytes formed during fetal development.