Structural analysis of meiotic chromosomes and synaptonemal complexes in higher vertebrates.
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Animals↗
Biomedical subjects
A J Solari
Publications and source records attributed to A J Solari.
Extreme axial equalization and wide distribution of recombination nodules in the primitive ZW pair of Rhea americana (Aves, Ratitae).
Pachytene oocytes from the ratite bird Rhea americana were used for synaptonemal complex analysis with a surface spreading technique and phosphotungstic acid staining. The ZW bivalent is slightly smaller than the fourth autosomal bivalent and clearly shows unequal W and Z axes only in 27% of the bivalents. Most of the ZW pairs are completely adjusted and thus the W and Z axes are almost equal in length. A sample of 134 recombination nodules (RNs) from 63 ZW pairs showed a striking departure of number and location of these nodules compared with those of carinate birds. The average number of RNs in the ZW pair of R. americana is 2.13, and the average SC length per RN is 4.2 microm. The locations of the RNs along most of the long arms of the Z and W are not random, and the distances between pairs of RNs show interference. Thus, the pattern of RNs in this mostly euchromatic ZW pair is identical to that of autosomes. From the present and previous data, it is concluded that the ZW pair of R. americana is in a primitive stage of chromosomal differentiation, in which recombination is restricted only in the small short arm and in the pericentromeric region.
Animals↗
The prevalence of a YY synaptonemal complex over XY synapsis in an XYY man with exclusive XYY spermatocytes.
An infertile XYY man was studied by synaptonemal complex analysis of microspread spermatocytes and by quantitation of germ cells in semithin sections. All the 74 spermatocytes micrographed have an XYY constitution, and the biopsy shows a homogeneous arrest of spermatogenesis at the spermatocyte/young spermatid stages. The overwhelming majority (86%) of spermatocytes showed a Y-Y bivalent plus a univalent X. The Y-Y bivalent is totally synapsed in 48% of the cells. In the remaining cells, the YY bivalent has an average synaptic segment covering 43% of its length that always includes Yp. Another 9% of the spermatocytes showed an XYY trivalent and 4% of the spermatocytes showed univalence of the three gonosomes. Progression through all the pachytene substages was observed in cells with the two main synaptic configurations, but a high level of germ cell death was observed at or immediately after the meiotic divisions. The prevalence of Y-Y synapsis arises from the longer homologous region and the higher speed of pairing between the two Y chromosomes. Germ cell death is probably related to the univalence of the X chromosome. Synaptic competition between three gonosomes seems to be similar to that found in triploid birds but is somewhat different from that of XYY mice.
Adult↗
Meiotic behavior of the X1X2Y1Y2 quadrivalent of the primate Alouatta caraya.
A multiple sex chromosome system was found in three unrelated individuals of the primate Alouatta caraya. This mechanism is originated by a translocation between the Y chromosome and one of the autosomes (A7). Mitotic karyotypes show two small, acrocentric chromosomes (AY and YA), which are the translocation products. In metaphase I of male meiosis, there is a very long chain quadrivalent in which the order of the element is: X-YA-A7-AY. Segregation in the quadrivalent is alternate and gives balanced products. Synaptonemal complex karyotypes at pachytene show the structure of the quadrivalent made by the four axes. There is a slight difference in the relative length of AY and YA and the kinetochore of A7 aligns with that of AY. The synaptic pattern and changes in the quadrivalent during pachytene are described. Thin sections of the quadrivalent body show that the chromatin packing in the sex chromosome region is different from that of the autosomal region. This X1X2Y1Y2/X1X1XX2 sex chromosome system may be extended among other members of the genus Alouatta.
Alouatta↗
Dynamic changes in Rad51 distribution on chromatin during meiosis in male and female vertebrates.
Antibodies against human Rad51 protein were used to examine the distribution of Rad51 on meiotic chromatin in mouse spermatocytes and oocytes as well as chicken oocytes during sequential stages of meiosis. We observed the following dynamic changes in distribution of Rad51 during meiosis: (1) in early leptotene nuclei there are multiple, apparently randomly distributed, foci that by late leptonema become organized into tracks of foci. (2) These foci persist into zygonema, but most foci are now localized on Rad51-positive axes that correspond to lateral elements of the synaptonemal complex. As homologs synapse foci from homologous axes fuse. The distribution and involvement of Rad51 foci as contact points between homologs suggest that they may be components to early recombination nodules. (3) As pachynema progresses the number of foci drops dramatically; the temporal occurrence (mice) and physical and numerical distribution of foci on axes (chickens) suggest that they may be a component of late recombination nodules. (4) In early pachynema there are numerous Rad51 foci on the single axis of the X (mouse spermatocytes) or the Z (chicken oocytes) chromosomes that neither pair, nor recombine. (5) In late pachynema in mouse spermatocytes, but not oocytes, the Rad51 signal is preferentially enhanced at both ends of all the bivalents. As bivalents in spermatocytes, but not oocytes, begin to desynapse at diplonema they are often held together at these Rad51-positive termini. These observations parallel observations that recombination rates are exceptionally high near chromosome ends in male but not female eutherian mammals. (6) From diakinesis through metaphase I, Rad51 protein is detected as low-intensity fluorescent doublets that localize with CREST-specific antigens (kinetochores), suggesting that Rad51 participates, at least as a structural component of the materials involved, in sister kinetochore cohesiveness. Finally, the changes in Rad51 distribution during meiosis do not appear to be species specific, but intrinsic to the meiotic process.
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High-resolution cytological localization of the XhoI and EcoRI repeat sequences in the pachytene ZW bivalent of the chicken.
The pachytene ZW pair of the chicken has been studied with a novel combination of fluorescent in situ hybridization and silver staining of the synaptonemal complexes, and with electron microscopic in situ hybridization. Probes for the EcoRI and the XhoI repeat sequences were used for light microscopy and probes for XhoI for electron microscopy. XhoI repeats are pericentromeric and correspond to 27.3% of the W axis length. EcoRI repeats form three distinct domains: domain I covers the distal 19.4% of the late-synapsing arm, and domains II and III cover respectively 21.7% and 8.5% of the W axis. The early-synapsing end containing the recombination nodule is free from any of both signals. This non-hybridizing region accounts for 25.9% of the W axis. It is suggested that this region is composed of a proximal region containing other repeat types and a terminal region which is the recombining or pseudoautosomal region. The successful combination of silver staining and fluorescent in situ hybridization will be generally useful for high-resolution localization of DNA sequences in meiotic chromosomes.
Animals↗
Mitosis and genome partition in trypanosomes.
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Animals↗
Fine structure of the XY body in the XY1Y2 trivalent of the bat Artibeus lituratus.
Electron microscopy of spread spermatocytes and thin sections has been used to study the sex trivalent (XY1Y2) of the bat Artibeus lituratus. Pachytene spermatocytes in thin sections show an XY body with typical chromatin condensation that is connected to autosomal chromatin through a synaptonemal complex (SC). Microspread spermatocytes show three axes and two SC segments (a short SC and a long one) in the sex trivalent. The short paired region corresponds to synapsis between the 'original' X and Y pieces, while the long paired region corresponds to synapsis between the Y2 element and the homologous, autosomal piece of the compound X-chromosome. The length ratios of the three axes correspond to those of the three mitotic chromosomes, X, Y1 and Y2. The high packing of chromatin corresponds exclusively to the 'original' pieces of the X and Y elements, while the autosomal regions of the X and the Y2 axes are surrounded by autosomal-like chromatin. Thus, in this trivalent the formation of an XY body in the 'original' sex chromosomes is not inhibited by the presence of the autosomal pieces, and typical partial synapsis between the original X and Y elements is conserved. C-banding heterochromatin seems not to be the barrier preventing the spreading of heterochromatinization towards the autosomal piece in this trivalent.
Animals↗
Recombination nodules and axial equalization in the ZW pairs of the Peking duck and the guinea fowl.
The meiotic Z and W chromosomes of the Guinea fowl (Numida meleagris) and the Peking duck (Anas platyrhynchos) show features similar to those described previously for 3 species of Phasianidae: chicken, Japanese quail and bobwhite quail. Axial equalization in the gonosomes of the Peking duck involves lengthening of the W axis along with shortening of the Z axis. A single recombination nodule is constantly found in the synaptic, terminal region of the ZW pairs stained with phosphotungstic acid. The localization of this nodule is significantly different in these two species, but the length of synaptonemal complex harboring the nodules is not significantly different. Based on the phylogenetic distance between the observed species, the present data support the hypothesis that the presence of a pseudoautosomal region is a general property of ZW pairs in carinate birds.
Animals↗
Polyamines and cell wall organization in Saccharomyces cerevisiae.
Cells of Saccharomyces cerevisiae 179-5, an ornithine decarboxylase mutant (spe-1), showed several ultrastructural abnormalities when cultivated in the absence of polyamines. Besides the appearance of microvacuole-like spaces in the cytoplasm and of deformed nuclei, the most important alterations seemed to be located in the cell wall, which was thicker and of heterogeneous texture, and in the cell membrane, of irregular contour. These modifications could not be evoked by general stress conditions elicited by lack of nutrients. The relative levels of cell wall polysaccharides were altered in polyamine-deprived organisms, giving an envelope with increased mannan and decreased glucan content; this cell wall was incompletely attacked by the lytic enzyme zymolyase. Polyamine depletion led also to some abnormalities in the budding pattern. The above observations suggest the involvement of polyamines in the correct structure and organization of the yeast cell.
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Equalization of Z and W axes in chicken and quail oocytes.
The different morphological types of ZW pairs have been classified in three main types according to the relative extension of the free segment of the Z axis: 1, "long asynaptic segment;" 2, "medium asynaptic segment;" and 3, "equalized." Pre- and post-pairing types have also been defined. Frequencies of each type were determined at day 20 and day 21 of incubation, and one and three days after hatching. The changing frequencies and the morphological transitions observed show a definite sequence of ZW types that can be used as a timetable for pachytene substaging. Measurements made on each ZW type show that the Z axis of the chicken shortens from 20.6 microns to 13.1 microns. This shortening occurs both in the free segment (at a higher rate) and in the paired segment (at a lower rate). The synaptonemal complex becomes elongated while adjustment occurs. The equalized Z axis makes many twists around the W axis. However, a segment 1 micron long from the synaptic terminus is free from twists and is assumed to be the homologously paired region. The ZW pair of the quail shows a similar behavior but equalization of the Z and W axes ends earlier and forms a straighter synaptonemal complex as compared with the chicken. In both species a recombination nodule is strictly localized near the synaptic terminus. In the ZW pair of the quail the average location of this nodule is 0.14 microns from the synaptic terminus. The meiotic behavior of ZW pairs in birds may be conserved.
Animals↗
An 'axis-like' material in the centromeric region of metaphase-I chromosomes from mouse spermatocytes.
This study reports the persistence of axis-like structures in the centromeric region of both homologues during the metaphase-I and anaphase-I stages of meiotic division of mouse spermatocytes. A novel type of silver 'argentaffin' technique (NH4-Ag) is employed. This technique includes the treatment of glutaraldehyde-fixed tissues with dilute ammonium hydroxide followed by a reduction of aldehyde groups with sodium borohydride. Staining is accomplished with ammoniacal silver nitrate in darkness followed by sulfite washing. The lateral elements of synaptonemal complexes and the single chromosomal axes of diplotene spermatocytes show a prominent reactivity with this technique. The pattern of very small grains over condensed chromatin is uniform and gives only a light opacity to the electron beam. The presence of an axis-like structure is seen in every centromeric end of meiotic chromosomes at metaphase I and anaphase I. The chromatin (heterochromatin) that surrounds the centromeric filament and some material distributed in irregular linear arrays along some of the homologues also showed a higher electron opacity than the bulk of deoxyribonucleoprotein. While the former is related to C+ heterochromatin, the latter could represent dispersed material of diplotene axes. It is suggested that the disposal of axial material is differentially delayed at the centromeric regions. The present evidence supports the hypothesis that axial fragments or lateral-element segments persisting at these regions contribute to the cohesiveness of centromeres of sister chromatids during normal disjunction.
Animals↗
Presence of a centromeric filament during meiosis.
Spermatocytes at meiotic metaphase I and anaphase I have a characteristic centromeric filament in a variety of vertebrate organisms. This centromeric filament was first demonstrated on mouse spermatocytes and its presence is now extended to spermatocytes from the human, rat, golden hamster, bull, and chicken. The visualization of this filament was possible through the use of a novel silver-staining technique, which allows a high contrast between the filament and the centromeric chromatin. In the species cited, the centromeric filament shares an intense staining, a short (0.2-0.6 micron) length, a curved and branched shape, and location inside the centromeric chromatin of seemingly every homologue of the complement. The similarity of staining reactivity and the observation of transitional structures during first meiotic prophase strongly suggest that the centromeric filament is a remnant of a lateral element of the synaptonemal complex, which stays specifically at both centromeric regions of each bivalent. This filament is not found at the second meiotic division or at the centromeres of mitotic chromosomes. It is assumed that this centromeric filament joins the two sister chromatids of each homologue at the centromere and thus ensures the proper coorientation of sister kinetochores at metaphase I. Further testable assumptions on the functions of this filament are presented.
Animals↗
Synaptonemal complex karyotyping in an oligospermic patient with heterochromatin duplication in chromosome n. 9.
Synaptonemal complex karyotyping has been performed in an oligospermia of unknown etiology in a patient carrying a 9qh+ chromosomal polymorphism. The testicular histology showed hypospermatogenesis at the spermatid level and an abnormal pattern of chromatin condensation. Spermatocytes at early pachytene showed a large, asymmetric loop in SC #9, which disappeared at late pachytene, probably because of synaptic adjustment. The loop was formed by a lateral element 7.02% longer than the average normal one. The loop exceeded the centromere towards the short arm, and it is interpreted as a tandem duplication of about 50% of the paracentromeric heterochromatin. The present observation and the previously reported asynaptic loops in carriers of pericentric inversions and showing severe oligospermia suggest that chromosomal variants producing asynaptic loops may be associated with germ cell loss. Further meiotic studies in infertile carriers of such variants are indicated.
Adult↗
Meiotic behavior of gonosomically variant females of Akodon azarae (Rodentia, Cricetidae).
The meiotic behavior of sex chromosomes has been investigated in variant females of Akodon azarae, both in pachytene oocytes and metaphase I. In somatic cells, these females have a heteromorphic sex pair, in which the minor chromosome has been previously interpreted as a major deletion of the long arm of the X chromosome (dX). After microspreading for synaptonemal complex analysis, pachytene oocytes show two axes of very different lengths (100:17.1), which correspond to the sex chromosomes X and dX. True synapsis is abnormally restricted (43.3%) between these sex chromosomes; on the other hand, self-synapsis of both the X and dX chromosomes is frequent (60%). Single, nonsynapsed axes or axial segments are thickened. Strong chromatin condensation occurs around nonsynapsed axes or axial segments, giving many of these sex pairs an appearance similar to an XY body ("sex vesicle"). The minor gonosome axis differs from that of the Y chromosome of male meiosis, as the former is shorter (relative to the X) and has a different synaptic behavior. In 17 metaphases I from XdX variant females, only heteromorphic, end-to-end joined sex pairs were observed. These variant females differ from the variant females of the wood lemming Myopus schisticolor in several respects, but a similar mechanism seems to be prevalent in other species of the genus Akodon. Self-synapsis of unequal gonosomes in oocytes is assumed as an escape from functional deterioration, following the hypothesis put forward by others.
Animals↗
Synaptic behaviour and recombination nodules in the human XY pair.
A sample of 90 XY pairs from men with normal karyotypes has been analyzed by measuring their morphological features in electron micrographs of microspread spermatocytes. The classification of human XY types (Solari, 1980) has been given stricter definitions. Stepwise splitting of the axes is seen in types 1 and 2. The development of axial branches and lengthening of the X axis is seen in type 3. In the two subtypes a and b of type 4 the net-like filamentous array grows in length to a maximum (average = 59.7 microns) in subtype b. The location of the putative Y kinetochore defines a short arm that measures 22.34% of Y axis length, and the kinetochore of the X axis defines a short arm of 38.15% of the axial length. The average number of excrescences in the X axis is 19.9 and in the Y is 4.3. The frequency of a non-homologous, distal end-joining grows steadily from type 0 to type 3. The average length of the synaptonemal complex (SC) in 51 XY pairs of types 1 and 2 is 1.33 microns (SD = 0.65) and it corresponds to 25.54% of the Y axis length. Thus, the average SC covers the short arm of the Y and the pericentromeric region. Maximum lengths of this SC may reach up to 81.8% of the Y axis. 30 recombination nodules (RNs) were located in 26 XY pairs, and 90% of the nodules are located in the distal half of the short arm of the Y axis. Thus, RNs are restricted to a segment much shorter than the length of the average SC. A gradient of decreasing probability of recombination may reach up to the centromeric region of the Y chromosome. Some possible consequences of these facts are discussed.
Humans↗
Quadrivalent formation in a tetraploid chicken oocyte.
Synaptonemal complex analysis of an exceptional tetraploid oocyte from a diploid chicken heterozygous for the MN t (Z;1) rearrangement was performed by electron microscopy of a spread preparation. Ten separate quadrivalents (26% of the chromosomal axes) were analyzed, as well as 50 autosomal bivalents. All the axes less than 2.5 microns in length formed bivalents (38) only, while axes in the 2.5-4.2 micron range formed 5 quadrivalents and 12 bivalents. The longer, separate axes formed quadrivalents only. Partner switches in excess of one were documented. The two identical W chromosomes paired only at the ends of their short arms. Quadrivalent formation may require a threshold length (2.5 microns), at least in this species. The tip of the short arm of the W chromosome may be a pairing initiation point, and it corresponds to the region associated with a localized recombination nodule previously described in diploid oocytes.
Animals↗
Pairing of ZW gonosomes and the localized recombination nodule in two Z-autosome translocations in Gallus domesticus.
Electron microscopic observations of synaptonemal complexes of oocytes from chickens heterozygous for two Z-autosome translocations have been used to identify and study the pairing region of the Z and W chromosomes. The two translocations, MN t(Z;1) and t(OH 10), have breakpoints in opposite arms of the Z, and the arm having the breakpoint of MN t(Z;1) is marked by the terminal C+ band. In both translocations the short arm of the W was specifically paired with the euchromatic short arm of the Z. In MN t(Z;1) only open quadrivalents (74%) and trivalents plus W univalents (26%) were observed, whereas t(OH 10) exhibited, in addition to the prevalent quadrivalents (62%), III + I (19%) and II + II (19%) configurations. The extent of W pairing was slightly decreased in MN t(Z;1) (68.4% of the W chromosomes paired) and considerably decreased in t(OH 10) (25.3% of the W chromosomes paired). Nonhomologous synapsis occurred regularly at the quadrivalent crosspoint in MN t(Z;1) and also in bivalents from t(OH 10). The recombination nodule normally located in the terminus of the pairing region in normal ZW pairs is present in both translocations without any alteration of its frequency or its strict terminal position. Based on these data and previous observations (Rahn and Solari, 1986), it is proposed that an obligatory recombination event occurs at a locus between 0.7 microns and 0.15 microns of the paired ZW telomeres, establishing a recombinational region and a pseudoautosomal region which determine partial sex-linkage and no sex-linkage, respectively. Most of the pairing region of the ZW pair is nonhomologously paired.
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