Synaptonemal complex analysis of hybrid cattle. III. Meiotic pairing mechanisms in F1 Brahman x Hereford hybrids

The mechanisms of homologous chromosome pairing were studied in synaptonemal complex (SC) spreads of F1 Brahman (Bos indicus) x Hereford (Bos taurus) cattle. The most common SC abnormalities were bivalents with partial pairing failure and interlocks. While C-band polymorphisms could underlie most of the SC abnormalities observed in the full-blood cattle, other causes seem also to be contributing in the hybrids. The pattern of the abnormalities indicates that genic differences between the species were probably involved. Pachytene substaging data suggest that in some spreads, early pachytene bivalents with partial pairing failure may achieve complete synapsis or may be converted to interlocks by late pachytene.     

Synaptonemal complex analysis of hybrid cattle. II. Bos indicus x Bos taurus F1 and backcross hybrids

The levels of meiotic chromosome pairing abnormalities observed in six Australian F1 Bos indicus x Bos taurus cattle crosses (mean = 23%) were significantly higher than those of the full-blood breeds (9%). The abnormal configurations in the F1 hybrids included partial pairing failure, multivalents, interlocks, and inversion pairing. Abnormal configurations were also present, but at lower frequency, in backcross hybrid bulls. The main types of abnormal configurations and the levels of XY-autosomal associations and autosomal asynapsis observed were unlikely to cause significant fertility problems in the hybrids.     

Meiotic analysis of two human reciprocal X-autosome translocations

Two cases of human reciprocal X-autosome translocation, t(X;12) and t(X;2), are described in sterile males, along with meiotic findings. Each carrier had inherited the translocation from his mother. Both showed azoospermia and germ-cell maturation arrest at the primary spermatocyte level, with most cells being arrested at the pachytene stage. A few metaphase I (MI) divisions were found, with occasional metaphase II cells being seen in the t(X;2) carrier. MI air-dried preparations gave clear evidence of chain quadrivalent formation. In the t(X;2) heterozygote, the pairing characteristics of the quadrivalent at pachytene were also analyzed in electron microscopic spreads. Disturbance of pairing around the breakpoints characterized most quadrivalents, and there was evidence in about 20% of the cells that nonhomologous pairing had taken place between the translocated chromosomes and the normal chromosome 2. Comparisons are made with similar nonhomologous pairing configurations seen at pachytene in quadrivalents of male reciprocal X-autosome translocations of the mouse.     

Synaptic abnormalities in spread nuclei of Secale. II. Secale vavilovii.

Secale vavilovii PMCs have more univalents and a lower frequency of bound arms at metaphase I than other diploid Secale species. The spreading technique applied at prophase I showed that the nuclei were able to complete synapsis at pachytene. However, 25% of the nuclei analyzed, which had more than 90% of their total length paired, showed two abnormalities: long fold-back loops, which were located mainly on the nucleolar organizer bivalent, and pairing-partner switches, probably involving all the chromosome complement. These synaptic abnormalities are unusual in diploid species and give rise to a high frequency of nonhomologous pairing regions and, therefore, could produce desynapsis, which could explain the data obtained from metaphase I. The possible origin of the unusual synaptic abnormalities of S. vavilovii is discussed.     

Immunocytochemical labelling of the kinetochore of human synaptonemal complexes,and the extent of pairing of the X and Y chromosomes

An immunocytochemical method was used to label the kinetochores on human synaptonemal complexes. Synaptonemal complex spreads were labelled with autoimmune CREST serum, followed by a second antibody labelled with colloidal gold, and examined by electron microscopy. Clusters of gold particles were found at discrete sites which were identified as kinetochores on the autosomal synaptonemal complexes, as well as on the XY pair. This method was used to investigate the extent of pairing of the human X and Y chromosomes at pachytene. Our observations confirm earlier work, based purely on measurements, that the pairing of the sex chromosomes sometimes extends beyond the centromere of the Y chromosome into the long arm. At the same time we showed that the centromeric indices of the X and Y at pachytene are highly variable, so that measurements alone are not sufficient to estimate the degree of pairing of the sex chromosomes.     

Ultrastructure of meiotic pairing in B chromosomes of Crepis capillaris

The meiotic pairing behaviour of four B isochromosomes of Crepis capillaris was studied by synaptonemal complex (SC) surface spreading of pollen mother cells. The four B chromosomes form a tightly associated group, separate from the standard chromosomes, throughout zygotene and pachytene. All four B chromosomes are also folded around their axis of symmetry, the centromere, and the eight homologous arms are closely aligned from the earliest prophase I stages. A high frequency of multivalent pairing of the four B chromosomes is observed at pachytene, in excess of 90%, mirroring the situation observed at metaphase I but exceeding the frequency expected (76.2%) on the assumption of random pairing among the eight B isochromosome arms with a single distal pairing initiation site per arm. The higher than expected frequency of multivalents is due to the occurrence of multiple pairing initiations along the B isochromosome arms, resulting in high frequencies of pairing partner switches. Pairing of the standard chromosome set is frequently incomplete in the presence of four B chromosomes, and abnormalities of SC structure such as thickening and splitting of axes and lateral elements are also frequently seen. Similarly, B chromosomes show partial pairing failure, the extent of which is correlated with pairing failure in the standard chromosome set. The B chromosomes themselves also show abnormalities of SC structure. Both standard and B chromosomes show non-homologous foldback pairing of regions that have failed to pair homologously.by D. Schweizer     

Synaptic patterns of rye B chromosomes. II. The effect of the standard B chromosomes on the pairing of the A set

In order to elucidate the possible effects of rye B chromosomes (Bs) on synapsis and metaphase-I associations of the A set, a comparative study between pachytene and metaphase-I-cells of rye plants carrying different numbers of Bs (0–8) has been carried out. The number of Bs was found to be positively correlated with the frequency of synaptic irregularities of the A set, i.e. multivalents and foldback pairing, and with the frequency of pachytene interlockings. It is proposed that interlockings are the origin of these irregularities because both appeared in close proximity in many nuclei. Examples of A-B pairing are described. The frequency of synaptic abnormalities seems to be unrelated to the mean of A chromosome-bound arms at metaphase I.     

Spermatogenesis in two patients with the fragile X syndrome

Summary Chromosomes at first meiosis from two males with the fra(X) form of mental retardation were studied using pachytene surface spreads and air-dried preparations. The pachytene sex bivalents showed no discontinuation of the synaptonemal complex in the terminal part of Xq corresponding to band Xq27–28 of the mitotic chromosomes. In both cases the frequency of a secondary association of Xq and Yq appeared to be increased compared with controls. The pairing behavior of autosomal bivalents in pachytene and the frequency and distribution of chiasmata in diakinesis were normal. The impairment of spermatogenesis found in these males may not be caused by a meiotic disorder, but could be related to peritubular or intratubular pressure effects on germ cells.     

Two different XY-quadrivalent associations and impairment of fertility in men

One sterile and one subfertile man with balanced reciprocal autosomal translocations, a t(9;15), and a t(14;21), were analyzed using whole mount pachytene spreads, histology, and semen analyses. In both probands with different types of quadrivalent complexes lack of pairing near the translocation breakpoints and significant associations with XY bivalents were found. Variability in the frequency of XY-quadrivalent contacts and an increase in late pachytene to 52% in t(9;15) and 90% in t(14;21) could be observed. The lower rate was associated with reduced postmeiotic spermatogenesis and the higher rate with complete spermatogenic arrest. In both translocation carriers the XY-quadrivalent association is considered to be the main cause for testicular malfunction rather than nonpaired segments in the multivalent complexes.     

The effect of multiple simple Robertsonian heterozygosity on chromosome pairing and fertility of wild-stock house mice (Mus musculus domesticus)

The influence of Robertsonian (Rb) heterozygosity on fertility has been the subject of much study in the house mouse. However, these studies have been largely directed at single simple heterozygotes (heterozygous for a single Rb metacentric) or complex heterozygotes (heterozygous for several to many metacentrics which share common chromosome arms). In this paper we describe studies on male multiple simple heterozygotes, specifically the F(1) products of crosses between wild-stock mice homozygous for four or seven metacentrics and wild-stock mice with a standard all-acrocentric karyotype; these F(1) products were characterized by four and seven trivalents at meiosis I, respectively. Mice with the same karyotype, but two different genetic backgrounds were examined. Although a range of meiotic and fertility studies were conducted, particular emphasis was paid to analysis of chromosome pairing, previously not well-described in multiple simple heterozygous mice. The progression of spermatocytes through prophase I was followed by electron microscopy of surface spread material. As previously shown for single simple Rb heterozygotes, the trivalents that characterize multiple simple heterozygotes initially showed delayed pairing of the centromeric region and later showed side arm formation, resulting from non-homologous pairing by the centromeric ends of the acrocentric chromosomes. In the four trivalent groups of mice, 15 and 32% of trivalents showed unpairing in the centromeric region at mid pachytene; equivalent values were 29 and 39% for the seven trivalent groups. Pairing abnormalities (largely attachments and interlocks between trivalents and between a trivalent and the XY configuration) were observed in 18 and 23% of mid pachytene cells in the four trivalent groups and 36 and 49% of cells in the seven trivalent groups. The greater level of pachytene irregularity (unpairing and pairing abnormalities) in seven versus four trivalent heterozygotes was mirrored in terms of higher anaphase I nondisjunction frequency and lower germ cell counts. However, while pachytene irregularities appear to contribute to germ cell death, examples of male sterility in our material undoubtedly also involve genic incompatibilities.     

Pachytene pairing and metaphase I configurations in a tetraploid somatic Lycopersicon esculentum x L. peruvianum hybrid.

In the tetraploid somatic hybrid between the diploid Lycopersicon species L. esculentum (tomato) and L. peruvianum, synaptonemal complexes formed quadrivalents in 73 of the 120 sets of four chromosomes (60.8%) in 10 cells studied in detail at pachytene. Of these, 43 had one pairing partner exchange, 22 had two, and 8 had three, very close to a Poisson distribution. The points of pairing partner exchange were concentrated at the middle of the two arms. The frequency per arm corresponded with physical arm length. There was a sharp drop around the centromere, and pericentric heterochromatin had a slightly lower probability of being involved in pairing partner exchange than euchromatin. The chromosomes align before pairing and there are several points of pairing initiation, with concentrations at or near the ends and the centromere. From zygotene to late pachytene the quadrivalent frequency decreased considerably. At late pachytene it was lower than expected with the observed high frequency of pairing partner exchange. Pairing affinity between species was only slightly lower than affinity within species, in spite of considerable genetic differentiation. The frequency of recombination nodules increased from early to late zygotene and then decreased strongly to full pachytene. There is a highly significant negative correlation between percent pairing and SC length. At metaphase I the frequency of quadrivalents was 0.444, and branched quadrivalents were rare, probably caused by interference and restriction of chiasma formation to distal euchromatin. Metaphase I quadrivalent frequency is a relatively good indication of pairing affinity in this material.     

Synaptonemal complex analysis of the Holstein-Friesian, Piemontese and Simmental breeds of Bos taurus taurus

The synaptonemal complex (SC) of specimens of Bos taurus taurus from the Holstein-Friesian, Piemontese, and Simmental breeds, was analysed. The analysis included quantification of the frequency of various types of abnormalities in the SC, and the frequency of cells with SC abnormalities. All animals had 29 autosomal bivalents and one sexual bivalent and the most frequently recorded abnormality was pairing failure. The number of cells with abnormalities in the Holstein-Friesian breed was 29.41%, in the Piemontese breed was 30.00% and in the Simmental breed it was 29.54%. The subspecies Bos taurus taurus had 29.63% of cells showing abnormalities with 57.33% of these abnormalities occurring in zygotene and 42.67% occurring in pachytene. Statistical analyses showed that there were no significant differences in the number of cells with SC abnormalities among the breeds studied. The frequency of cells with abnormalities, and the efect on the fertility of the Holstein-Friesian, Piemontese and Simmental breeds are discussed.     

Spreading synaptonemal complexes from Zea mays

Four different inversion heterozygotes of maize were examined for the occurrence of synaptic adjustment. Three substages of pachytene were identified in synaptonemal complex (SC) spreads using side-by-side comparisons of chromosome squashes with two-dimensional spreads of SCs. In SC spreads, inversion loop frequency did not change substantially from early through late pachytene for any of the four inversion heterozygotes examined. In addition, the position and size of the inversion loops remained essentially constant throughout pachytene. These results indicate that synaptic adjustment of inversion loops does not occur during pachytene in Zea mays.     

THE EFFECT OF X-RAYS ON CHROMOSOMES IN DIFFERENT STAGES OF MEIOSIS

1. Pollen mother cells exposed to low dosages of x-rays at various stages show different frequencies of chromosome abnormalities in the first meiotic anaphase. 2. Maximum frequencies of abnormalities were obtained in buds irradiated in the pachytene stage of the meiotic prophase and in the preceding mitosis. 3. These results are taken to indicate that the x-ray-sensitive portions of the chromonemata are closely approximated in pairs in pachytene and in the early mitotic prophase. The significance of this in relation to non-homologous pairing is indicated. 4. From the nature of the chromosome configurations observed it is concluded that chromonemata are two-parted when they synapse and that a chromonematic division occurs between pachytene and anaphase and during the mitotic prophase. 5. The frequencies of abnormalities show a linear relationship to dosage. 6. The diameter of the sensitive volume of the chromonema is calculated and found to approximate the diameter of some known protein molecules. 7. The linkage mechanism is found to make up about 90 per cent of the total sensitive volume which corresponds with the approximate reduction in length of the chromonema from pachytene to anaphase. 8. The relation of these sensitive volumes to the gene is discussed.     

Centric-fusion translocation and whole-arm heterochromatin in the karyotype of the blue fox (Alopex lagopus L.): synaptonemal complex analysis.

Conventional observations of mitotic chromosomes from two male blue foxes, revealing a centric-fusion translocation and whole-arm heterochromatin, were verified by synaptonemal complex analysis. This analysis revealed that the centric fusion had been preceded by a conspicuous loss of chromosome material in the two one-armed chromosomes involved, but the chromosomal origin of the centric-fusion kinetochore could not be established. The nontranslocated chromosomes of the trivalent, which in all cells but one were in cis configuration, had reached by early pachytene a stage in which almost complete homologous pairing and nonhomologous association or pairing of the free ends of the chromosomes could be observed. In later stages, complete pairing of the nontranslocated chromosomes with the corresponding arms of the centric-fusion translocation was seen occasionally. One to six autosomal bivalents demonstrated unpaired heterochromatic arms in early pachytene, and the heterochromatic chromosome arms were sometimes unpaired even in late pachytene. Some of them showed a distinct size heteromorphism in late zygotene and early pachytene. In most late-pachytene cells, however, the heteromorphic chromosomes were completely length-adjusted. Only a small fraction of the cells showed pairing interference between nonhomologous chromosomes.     

Colchicine effects on meiosis in the male mouse

Antimitotic agents administered at the time of synapsis (leptotene/zygotene) have been shown to induce synaptic abnormalities visible during pachytene in the male mouse. The object of this study was to test the hypothesis that cells with relatively large amounts of colchicine-induced damage to the synaptonemal complex (SC) are eliminated from prophase whereas cells with relatively small amounts of SC damage proceed through to the end of prophase. Male mice were injected with tritiated thymidine to mark a cohort of spermatocytes at premeiotic S-phase for tracking through pachytene. Forty-eight hours later, when those cells were at leptotene/zygotene, colchicine was administered intratesticularly. Whole-mount SC spreads were made from animals sacrificed at various times following colchicine administration, and prepared for autoradiography. The marked cells were examined by light and electron microscopy and the kind and number of synaptic abnormalities were scored throughout pachytene. Colchicine-induced SC damage included single axial elements (univalents), together with partially synapsed and nonhomologously synapsed SCs. The amount of SC damage (amount and type per cell and frequency of cells with damage) scored at early pachytene exceeded by three- to fivefold the amount at late pachytene. This is consistent with spermatogenic cell loss from the seminiferous tubule via colchicine-induced destruction of Sertoli cell microtubules. The presence of spermatocytes with no more than four autosomal univalents at late pachytene indicates that some cells with low amounts of synaptic damage progress to the end of pachytene. The loss of the most severely damaged cells may represent a meiotic checkpoint at early pachytene in the male mouse. Received: 24 April 1996; in revised form: 29 August 1996 / Accepted: 11 March 1997     

普通小麦联会复合体发育过程的电镜观察

以改进的去污剂微铺展技术制备普通小麦减数分裂联会复合体标本,并对联会复合体发育的全过程作了详细的电镜观察和描述。结果表明,小麦SC以多点式起始方式于偶线期开始形成;随SC的发育,新的SC形成和已有SC片断的延伸并存;此外,在同一核内不同二价体之间,染色体配对和SC形成并不同步;SC成熟于粗线期,而以破碎方式解体,消失于双线期。在偶线期还观察到由同祖配对形成的多价体,但在随后阶段中这些多价体消失。对Ph基因的可能作用机制作了分析和讨论。     

Maize meiotic mutants with improper or non-homologous synapsis due to problems in pairing or synaptonemal complex formation

During meiotic prophase homologous chromosomes find each other and pair. Then they synapse, as the linear protein core (axial element or lateral element) of each homologous chromosome is joined together by a transverse central element, forming the tripartite synaptonemal complex (SC). Ten uncloned Zea mays mutants in our collection were surveyed by transmission electron microscopy by making silver-stained spreads of SCs to identify mutants with non-homologous synapsis or improper synapsis. To analyse the mutants further, zyp1, the maize orthologue of the Arabidopsis central element component ZYP1 was cloned and an antibody was made against it. Using antibodies against ZYP1 and the lateral element components AFD1 and ASY1, it was found that most mutants form normal SCs but are defective in pairing. The large number of non-homologous synapsis mutants defective in pairing illustrates that synapsis and pairing can be uncoupled. Of the ten mutants studied, only dsy2 undergoes normal homologous chromosome recognition needed for homologous pairing. The dsy2 mutation fails to maintain the SC. ZYP1 elongation is blocked at zygotene, and only dots of ZYP1 are seen at prophase I. Another mutant, mei*N2415 showed incomplete but homologous synapsis and ASY1 and AFD1 have a normal distribution. Although installation of ZYP1 is initiated at zygotene, its progression is slowed down and not completed by pachytene in some cells and ZYP1 is not retained on pachytene chromosomes. The mutants described here are now available through the Maize Genetics Cooperation Stock Center (http://maizecoop.cropsci.uiuc.edu/).     

EM studies of female meiosis in wood lemmings with different sex chromosome constitutions

The chromosomes were studied throughout meiotic prophase by electron microscopy of surface-spread oocytes from one XX, four X*X, and three X*Y female wood lemmings, Myopus schisticolor. The X* chromosome had originated from X by a deletion and an inversion in the short arm. The deletion was confirmed in pachytene cells from X*X females; a D-loop was present in the sex bivalent in 16.8% of the cells, and asynapsis of unequal ends was seen in 9.1% of other cells. At late pachytene the D-loop underwent synaptic adjustment. The breakpoints of the deletion are in G-light bands. No inversion loop was seen, which also is in agreement with Ashley's ('88) hypothesis; at least one of the presumed breakpoints of the inversion is in G-dark chromatin. Various types of synaptic abnormalities, such as nonhomologous pairing (triple pairing, interchange, self-synapsis), univalents, foldbacks, and broken lateral elements, were encountered in all types of female. X*Y females showed a high frequency of abnormal oocytes (70.7%), which significantly exceeded that of X*X (23.1%) and XX (8.1%). Univalents were particularly common in the X*Y females. J. Exp. Zool. 290:504-516, 2001.     

A complex three breakpoint translocation involving chromosomes 2, 4, and 9 identified by meiotic investigations of a human male ascertained for subfertility

Summary Whole mount pachytene spreads were used to investigate the pairing of a supposed balanced reciprocal t(4;9) translocation in a human male ascertained for subfertility. All well spread pachytene spermatocytes analysed by light microscopy and electron microscopy contained a hexavalent instead of the expected quadrivalent this suggesting that a third chromosome was involved. The hexavalent showed a high efficiency of synapsis with the six arms fully paired except for the proximal segments adjacent to the breakpoints. Further meiotic investigations by the air-drying technique and the reassessment of the mitotic karyotype using stretched chromosomes revealed that the rearrangement is indeed a complex three breakpoint translocation t(2;4;9)(p13;q25;p12). There was an indication of a reduced chiasma frequency of the hexavalent but no interchromosomal effect on chiasma pattern could be detected. No selective association between the hexavalent and the XY configuration was found at any stage, and unless the central lack of pairing is of relevance we have no explanation for the subfertility and reduced testicular size. Except for the hexavalent the most impressive feature of the meiosis of this complex translocation was in fact its normality including the end product with repeated spermiograms being indistinguishable from the normal. Karyotyping of individual spermatozoa has, however, not been performed.