Synaptonemal complex analysis of B-chromosome behavior in meiotic prophase I in the East-Asiatic mouse Apodemus peninsulae (Muridae, Rodentia)

The mitotic and meiotic chromosomes of four male East-Asiatic mice, Apodemus peninsulae, having three to seven chromosomes in addition to the standard karyotype (2n = 48), were investigated. B-chromosomes were represented by medium-sized metacentric and dotlike chromosomes. Mosaicism of bone marrow cells due to a numerical variation of accessory chromosomes was established for the males examined. Capacity of B-chromosomes to form axial elements and synaptonemal complexes in meiotic prophase I was revealed by electron microscopy. The occurrence of univalents of different morphology, bivalents, and multivalents, corresponding to B-chromosomes, was demonstrated. An increase in the number of B-chromosomes was found in spermatocytes at zygotene-pachytene relative to the number in bone marrow cells, which may be evidence of B-chromosome accumulation in the germ cell line of the East-Asiatic mouse.     

The meiotic structure and behavior of the strongly heteromorphic X/Y sex chromosomes of neotropical plethodontid salamanders of the genus Oedipina

Plethodontid salamanders in the genus Oedipina are characterized by a strongly heteromorphic sex-determining pair of X/Y chromosomes. The telocentric X chromosome and the subtelocentric Y chromosome are clearly distinguished from the autosomes and their behavior during meiosis can be sequentially followed in squash preparations of spermatocytes. In Oedipina the sex chromosomes are not obscured by an opaque sex vesicle during early meiotic stages, making it possible to observe details of sex bivalent structure and behavior not directly visible in other vertebrate groups. The sex chromosomes can first be distinguished from autosomal bivalents at the conclusion of zygotene, with X and Y synapsed only along a short segment at their non-centromeric ends, forming a bivalent that contrasts sharply with the completely synapsed autosomes. During pachytene, the XY bivalent becomes progressively shortened and more compact, disappearing as a visible structure when pachytene progresses into the diffuse stage of male meiosis. Diplotene bivalents gradually emerge from the diffuse nuclei, presumably by the return of the loops of chromatin into their respective chromomeres. During early diplotene, the X/Y bivalent is clearly visible with a single chiasma within the synapsed segment. This chiasma is terminalized by first meiotic metaphase with the X and Y appearing either in end-to-end synaptic contact or as univalents separated at opposite poles relative to the equatorially distributed autosomal bivalents. In C-banded preparations, the Y is entirely heterochromatic while the X contains a large centromeric C-band and another block of heterochromatin located at the telomeric end, in the region of synapsis with the Y. We find no cytological evidence of dosage compensation, such as differential staining of the X chromosomes or Barr bodies, in mitotic or interphase cells from female animals.     

On the structure of the XY bivalent in Mus musculus L.

Summary The structure and behavior of XY bivalent in mice is discussed. The view that XY bivalent in pachytene is embedded within the sex vesicle was fully confirmed. X and Y are paired end-to-end by a nonchiasmatic connection, which is established already in pachytene and persists until first meiotic metaphase. The pachytene complement in mice consists of 19 rod-shaped autosomal bivalents and the XY bivalent embedded within the sex vesicle. A satisfactory identification of individual autosomes in male pachytene has not been found possible.This paper is dedicated to Professor L. C. Dunn on the occasion of his retirement as Professor from Columbia University in recognition of his deep humanity as a scientist and a man.Research carried out under a fellowship of the Rockefeller Foundation, and later under fellowship support of the U. S. Department of Health, Education and Welfare through a training grant to the Department of Zoology, Columbia University. The experiments were supported financially by Contract AT/30-1/1804 with the U. S. Atomic Energy Commission.     

Differential immunolocalization of a putative Rec8p in meiotic autosomes and sex chromosomes of triatomine bugs

Hemipteran chromosomes are holocentric and show regular, special behavior at meiosis. While the autosomes pair at pachytene, have synaptonemal complexes (SCs) and recombination nodules (RNs) and segregate at anaphase I, the sex chromosomes do not form an SC or RNs, divide equationally at anaphase I, and their chromatids segregate at anaphase II. Here we show that this behavior is shared by the X and Y chromosomes of Triatoma infestans and the X(1)X(2)Y chromosomes of Triatoma pallidipennis. As Rec8p is a widely occurring component of meiotic cohesin, involved in meiotic homolog segregation, we used an antibody against Rec8p of Caenorhabditis elegans for immunolocalization in these triatomines. We show that while Rec8p is colocalized with SCs in the autosomes, no Rec8p can be found by immunolabeling in the sex chromosomes at any stage of meiosis. Furthermore, Rec8p labeling is lost from autosomal bivalents prior to metaphase I. In both triatomine species the sex chromosomes conjoin with each other during prophase I, and lack any SC, but they form "fuzzy cores", which are observed with silver staining and with light and electron microscopy during pachytene. Thin, serial sectioning and electron microscopy of spermatocytes at metaphases I and II reveals differential behavior of the sex chromosomes. At metaphase I the sex chromosomes form separate entities, each surrounded by a membranous sheath. On the other hand, at metaphase II the sex chromatids are closely tied and surrounded by a shared membranous sheath. The peculiar features of meiosis in these hemipterans suggest that they depart from the standard meiotic mechanisms proposed for other organisms.     

Autosomal synaptonemal complexes and sex chromosomes without axes in Triatoma infestans (Reduviidae; Hemiptera)

Meiotic and somatic cells at interphase in Triatoma infestans are characterized by the formation of a large chromocenter, which was assumed to contain the whole of the three large pairs of autosomes and the sex chromosomes. Observations with C-banding techniques show that the chromocenter is formed only by the terminal and subterminal heterochromatic blocks of the three large pairs of autosomes and the sex chromosomes. During pachytene the two largest autosomal pairs loop on themselves and their condensed ends form the chromocenter, together with the single heterochromatic end of the third autosomal pair. The X and Y chromosomes seem to associate with these condensed ends by their affinity for C-heterochromatin. During a very short pachytene stage, bivalents and synaptonemal complexes (SCs) are observed. Pachytene is followed by a very long diffuse stage, during which SCs are disassembled, multiple complexes aggregate on the inner face of the chromocenter and finally all complexes disappear and a dense material is extruded to the cytoplasm through the annuli. The 3-dimensional reconstruction of early pachytene chromocenters show 3 SCs entering and tunnelling the chromocenter, while during mid-pachytene 4 SCs enter this mass and a 5th SC is in a separate small mass. The looping of a whole SC which has both ends in the chromocenter was shown by the reconstructions. These data are interpreted as the progressive looping of the two largest bivalents during pachytene, forming finally the association of 5 bivalent ends corresponding to the 5 C-banding blocks of the large autosomal pairs. No single axis or SC that could be ascribed to the sex chromosomes was found. This agrees with the pachytene microspreads, which show only 10 SCs corresponding to the autosomal bivalents. The X and Y chromosomes are enclosed in the chromocenter, as shown by the unravelling chromocenters at diplotene-diakinesis. Thus the sex chromosomes do not form axial condensations, and this fact may be related to the ability of the X and Y chromosomes to divide equationally at metaphase I. SCsThis paper is dedicated to the memory of the late Professor Francisco A. Saez     

Chromosomal synapsis and the meiotic process in male mesquite lizards, Sceloporus grammicus complex.

Meiosis in males of the F5 cytotype of Sceloporus grammicus was examined through the analysis of synaptonemal complexes (SCs), diakinetic (metaphase I) nuclei, and secondary spermatocytes (metaphase II configurations). These data allowed the establishment of criteria for substaging of zygonema and pachynema, morphological characterization of the SC complement, and comparison of the orientation and segregation of the autosomes and sex chromosomes. The analysis of nuclei from all stages of meiotic prophase I (leptonema through diakinesis) provided a useful means of partitioning the temporal sequence of early meiotic events. Three substages of zygonema (Z1-Z3) were established, based on the extent of synapsis of the microchromosomal and macrochromosomal elements. Synaptic initiation of the autosomes and sex chromosomes was synchronous. Two patterns of macrochromosomal synapsis were observed. Whereas synapsis of the biarmed elements was biterminal (i.e., progressing from both ends of the homologs), synapsis of the acrocentric elements was uniterminal involving only the distal (noncentromeric) ends of the homologs. Unique sex-chromosomal characteristics were not observed in S. grammicus and, therefore, the substaging of pachynema was based on subjective criteria. Examination of diakinesis--metaphase I and metaphase II configurations indicated low levels of diakinetic irregularities with balanced segregation of the autosomal bivalents and the sex-chromosomal trivalent.     

Meiosis in gray voles of the subgenus microtus (rodentia, arvicolinae) and in their hybrids

The results of light and electron microscopic (EM) studies of meiosis in Microtus arvalis males of the karyoform "arvalis" (2n = 46, NFa = 80), in hybrids between the chromosomal forms arvalis and obscurus (2n = 46, NFa = 68), in M. rossiaemeridionalis voles (2n = 54, NFa = 54), and in a hybrid between the species M. rossiaemeridionalis and M. kermanensis (2n = 54, NFa = 54) are presented. SC (synaptonemal complex) karyotypes of the parental forms and the hybrids were constructed on the basis of measurements of the length ofautosomal SCs revealed by the EM analysis in spermatocytes at the stage of middle pachytene. The SC karyotypes of M. arvalis and the hybrids female obscurus x male arvalis consist of 22 synaptonemal complexes of autosomal bivalents and the axial elements of the synaptonemal complexes of the sex chromosomes X and Y. The SC karyotypes of M. rossiaemeridionalis and the hybrid M. rossiaemeridionalis x M. kermanensis consist of 26 synaptonemal complexes of autosomal bivalents and a sex bivalent; they differ only in the length of the Y chromosome axis (Y chromosome in the hybrid was inherited from M. kermanensis). Asynaptic configurations of the autosomal SCs were not observed in the hybrids. The SC axial elements of the X and Y chromosomes in the parental forms and in the hybrids were located close to each other throughout pachytene, but they did not form a synaptic region. The normal synapsis in sterile hybrids (M. rossiaemeridionalis x M. kermanensis) and the behavior of the sex chromosomes in meiosis in fertile and sterile hybrids are discussed in the context of specific features of meiosis and reproductive isolation.     

The chromomere map of the pachytene spermatocyte of the Turkish hamster (Mesocricetus brandti).

The chromomere map of the early to mid pachytene spermatocyte of the Turkish hamster (Mesocricetus brandti) is described. Each autosomal bivalent was identified and a total of 304 chromomeres was found. A sex bivalent with a despiralized Xq protruding from the sex vesicle and a small number of the polymorphic 16q bivalents were observed.     

Synaptonemal complex analysis in spermatocytes of tilapia, Oreochromis niloticus (Pisces, Cichlidae).

Some adaptations of the synaptonemal complex (SC) whole-mounting technique first used in plants permitted its application to meiotic studies in tilapia, Oreochromis niloticus. Direct observation of the chromosome pairing process and bivalent structure during the meiotic prophase of this fish species by light and electron microscopy permitted the analysis of SCs in autosomes and the possible identification of sex chromosomes. The analysis of SCs in spermatocytes of O. niloticus revealed that all 22 bivalent chromosomes completely paired, except for the occurrence of a size heteromorphism in the terminal region of the largest bivalent associated with the presence of an incompletely paired segment during the synapsis process, which may be the cytological visualization of an XX/XY sex chromosome system in this species.     

Morphology of the axial structures in the neo-XY sex bivalent of Pycnogaster cucullata (Orthoptera) by silver impregnation

A simple method, using silver impregnation after 2 x SSC pretreatment allowed us to demonstrate axial structures (cores) in the metaphase I bivalents of the neo-XY race of Pycnogaster cucullata under bright-field microscopy. Axial structures can also be shown in DNA-depleted metaphase I bivalents, suggesting that DNA is not essential to demonstrate these elements. Specifically differentiated regions, which coincide with kinetochores, secondary constrictions (including the NOR), and chiasmata were also found. These regions have a characteristic morphology and therefore may be utilized for cytogenetic analysis. The simultaneous visualization of all these regions allowed us to establish their spatial relationships and hence the basic structural organization of the neo-XY sex bivalent in this species.     

Non-relationship between nucleolus and sex chromosome system Xyp in Chelymorpha variabilis boheman (Coleoptera: Chrysomelidae)

Nucleolar-organizer region, nucleolus and mode of association of the sex bivalent were analyzed in spermatecytes of Chelymorpha variabilis Boheman. This species (2n=10II+Xy_p) shows the typical sex chromosome system of the group Polyphaga. The results of silver staining techniques showed the nucleolar organizer region localized in a subterminal position of an autosomal bivalent. During meiotic prophase the nucleolus was distinguished with the silver staining and acridine orange fluorescence technique up to diakinesis. The independence of nucleolus and sex bivalent Xy_p during meiosis is demonstrated. The positively silver staining but negatively orange-red material found within the parachute could be involved in the regular co-orientation of both sex chromosomes. After a longer hypotonic treatment, sex bivalents were observed elongated and paired only at one end during the pachytene stage. Along these sex chromosomes, C-bands showed positive blocks located in the pericentromeric and telomeric regions. Heterochromatic association of both sex chromosomes was suggested.     

Sequential study of the synaptonemal complex in Syrian hamster (Mesocricetus auratus) and mouse (Mus musculus) oocytes by light and electron microscopy

We describe the behaviour of synaptonemal complexes (SCs) in Syrian hamster and mouse oocytes. InMesocricetus auratus, synaptonemal complexes can be observed from birth up to 7 days of life. In foetuses fromMus musculus, synaptonemal complexes can be observed from the 14th day of gestation up to the first day post-partum, when the cells enter the dictyotene stage. In both species, synaptonemal complexes show, in general, the same morphology described in male cells by light and electron microscopy, with the exception that the axes of the sex bivalent are not identifiable. The leptotene stage can be identified although it is probably of short duration. Only one type of zygotene (zygote ne II of Dietrich and Mulder(Chromosoma 88: 377), 1983) has been observed. In the hamster we also describe a desynaptic diplotene stage previous to the desintegration of the SCs. In oocytes from both species late pairing (or precocious separation) of a single bivalent can be seen in a few cells. Interlocking of some bivalents with delayed pairing of the affected region is rather frequent. Furthermore, hamster oocytes may show heterosynapsis of the telomeres of autosomal bivalents by pachytene.     

Localized chiasmata and meiotic nodules in the tetraploid onion Allium porrum.

Allium porrum L. (cultivated leek) (2n = 4x = 32) is a fertile tetraploid that forms bivalents with pericentric chiasmata at metaphase I. To investigate the basis of this unusual behavior for a tetraploid, we describe the karyotype, axial cores, synaptonemal complexes (SCs), and meiotic nodules of A. porrum. The karyotype appears to be autotetraploid. This conclusion is also supported by presynaptic alignment of axial cores in groups of four and partner trades between pairs of SCs. Numerous early nodules are distributed all along axial cores and SCs during zygonema, but they are lost by late zygonema - early pachynema. Late (recombination) nodules (RNs) are present on SCs near kinetochores throughout the remainder of pachynema. This pattern of RNs corresponds to the pattern of pericentric chiasmata. Pachytene quadrivalents usually are resolved into bivalents because partner trades between SC lateral elements rarely occur between RNs on the same segment of SC. Thus, the patterns of crossing-over and partner trades promote balanced disjunction and high fertility in autotetraploid A. porrum. Rare quadrivalents observed at metaphase I must be due to infrequent partner trades between RNs. Polycomplexes, unusual in their number and size, were observed during zygonema. Key words : synaptonemal complex, recombination nodules, localized chiasmata, polycomplex, Allium porrum.     

Nature and role of accessory chromosomes in silver-gray foxes. IV. The behavior of accessory chromosomes in meiosis

Comparison of chromosome number at somatic and spermatogonial mitoses has demonstrated the increase in the number of additional chromosomes in cells of germinal tissue. This may evidence a mechanism of B-chromosomes accumulation in foxes. B-chromosomes may lag as univalents, may form bivalent associations, or occasionally form trivalents at the stage of diakinesis-metaphase I, and they may associate with macrobivalents (A-chromosome bivalents). The analysis of metaphase II has shown that the distribution of B-chromosomes in the second metaphase is random resulting in gametes with various numbers of B-chromosomes.     

A pachytene map of the mouse spermatocyte

Pachytene chromomere maps of early and mid/late mouse spermatocytes have been prepared which permit exact identification of each bivalent. The average total number of chromomeres on the autosomal bivalents was 248 in the early cells and 184 in the mid/late. There was close correspondence between early and mid/late chromomeres in 122 locations. Comparisons of early pachytene chromomeres with published prometaphase dark G bands revealed 1.6 more chromomeres in the meiotic autosomal bivalents, with close correspondence of larger chromomeres and major mitotic bands. Fewer chromomeres were found in pachytene spermatocytes than had been seen in a previous study of pachytene oocytes. Comparisons of chromomeres of spermatocytes and oocytes revealed several differences.     

Meiotic characterization and sex determination system of neotropical primates: Bolivian squirrel monkey Saimiri boliviensis (primates: Cebidae)

The chromosomal sex determination system differs among platyrrhine monkeys more than any other group of primates. Although a number of studies have investigated mitotic chromosomes across platyrrhine species, the meiotic chromosomes of many genera have not yet been described. The goal of this study was to characterize the sex determination system of Saimiri boliviensis. We described for the first time the meiotic cycle, confirming the sexual system in germ cells from testicular biopsies of four adult male S. boliviensis. All specimens were weighed and testicular volume was measured. We observed 22 bivalents corresponding to 2N = 44, and a "human-like" XY bivalent was found in diakinesis/metaphase I. In addition, mitotic studies from blood samples of both sexes were performed and G- and C-banding patterns agreed with previously reported karylogy of S. boliviensis boliviensis. Further meiotic studies should be performed in New World primates based on the great value of those studies for systematic evolutionary biology and conservation programs.     

Meiotic sister chromatid cohesion in holocentric sex chromosomes of three heteropteran species is maintained in absence of axial elements

It has been suggested that in species with monocentric chromosomes axial element (AE) components may be responsible for sister chromatid cohesion during meiosis. To test this hypothesis in species with holocentric chromosomes we selected three heteropteran species with different sex-determining mechanisms. We observed in surface-spreads and sections using transmission electron microscopy that the univalent sex chromosomes form neither AEs nor synaptonemal complexes (SCs) during pachytene. We also found that a polyclonal antibody recognizing SCP3/Cor1, a protein present at AEs and SC lateral elements of rodents, labels the autosomal SCs but not AEs or SC stretches corresponding to the sex chromosomes. Cytological analysis of the segregational behaviour of the sex univalents demonstrates that although these chromosomes segregate equationally during anaphase I they never show precocious separation of sister chromatids during late prophase I or metaphase I. These results suggest that AEs are not responsible for sister cohesion in sex chromosomes. The segregational behaviour of these chromosomes during both meiotic divisions also indicates that different achiasmate modes of chromosome association exist in heteropteran species. Received: 22 September 1999; in revised form: 20 December 1999 / Accepted: 21 December 1999     

Evidence of B-chromosomes in the karyotype of Barypeithes pellucidus Boheman 1834 (Coleoptera, Curculionidae, Entiminae) from Central Europe

B-chromosomes were observed in spermatogonial mitotic metaphases, meiotic metaphases I and II of Barpeithes pellucidus from one population in Slovakia. The number of B-chromosomes ranged from one to six per cell and they paired with the sex heterochromosomes in the first meiotic metaphase and rarely with the autosomes. In metaphase I one B-chromosome was always associated with X chromosome forming a tripartite complex. The XyBp was easily recognizable as a complex of three chromosomes in a parachute association The size of the B-chromosomes was approximately the same or a little smaller than that of the y heterochromosome which was the smallest element of the regular chromosome set. Their staining intensity seems to be similar to that of the autosomes and sex chromosomes, respectively. The behaviour of B-chromosomes during mitosis and meiosis in weevils is briefly discussed.     

Sex chromosome-autosome translocations in the leaf-nosed bats, family Phyllostomidae. II. Meiotic analyses of the subfamilies Stenodermatinae and Phyllostominae

Analyses of meiotic pairing and synaptonemal complexes of the composite sex chromosomes of male phyllostomid bats with X-autosome or X- and Y-autosome translocations were performed using Giemsa and silver staining procedures. Typical mammalian sex vesicles were absent in all species analyzed. Stenodermatine species with X-autosome translocations possessed an open ring and tail configuration of the XY1Y2 trivalent. Species with both X- and Y-autosome translocations possessed a closed ring and tail configuration of the neo-XY bivalent. In both cases, the tail represented the autosomal short arm of the X paired with its homologue, either the Y2 in XY1Y2 species or the autosomal arm of the composite Y in neo-XY species. Autosomal pairing of the composite sex bivalent in neo-XY species replaced an association between the original X and Y in late prophase I. The absence of a sex vesicle, the unusual pairing configurations of the composite sex chromosomes, and the presumed absence of meiotic nondisjunction in these species is discussed in light of current hypotheses of sex chromosome behavior in male gametogenesis in mammals.