Heterosynapsis and axial equalization of the sex chromosomes of the northern bobwhite quail.

The pairing behavior of the Z and W chromosomes in the female northern bobwhite quail (Colinus virginianus) was analyzed by electron microscopy of silver-stained synaptonemal complexes (SCs). After autosomal pairing was completed, synapsis of the sex chromosomes initiated at the short-arm end of the W chromosome and one end of the Z chromosome. Synapsis then progressed unidirectionally, producing a sex bivalent in which the entire length of the W axis was paired with an equivalent length of the Z axis. Progressive contraction and asymmetrical twisting of the Z axis ultimately resulted in a fully paired configuration with aligned axial ends. Further contraction of the Z axis reduced the extent of asymmetrical twisting such that only the nonaligned centromeric regions distinguished the SC of the ZW bivalent from SCs of similar-sized autosomes in late-pachytene nuclei. Quantitative analyses indicated that the length of the Z axis shortened significantly during the adjustment process, whereas no significant difference occurred in the length of the W axis. The nonalignment of the centromeric regions during transitional stages of ZW synapsis indicates that direct heterosynapsis of nonhomologous segments, followed by axial equalization of the length inequality, is responsible for the length adjustment during synapsis in the sex chromosomes of the bobwhite quail.     

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.     

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.     

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     

Molecular-cytogenetic analysis reveals sequence differences between the sex chromosomes of Oreochromis niloticus: evidence for an early stage of sex-chromosome differentiation

Sex determination in the Nile tilapia, Oreochromis niloticus, is primarily genetic, with XX females and XY males. A candidate sex-determining region in the terminal region of the largest chromosome pair has been identified by analysis of meiotic chromosomes. This region shows an inhibition of pairing and synapsis in the XY genotype, but not in XX or YY genotypes, suggesting that recombination is inhibited. Here we show that chromosome microdissection and subsequent amplification by degenerate oligonucleotide-primed PCR (DOP-PCR) can be used to produce in situ hybridization probes to this largest pair of O. niloticus chromosomes. Furthermore, analysis of the comparative hybridization of X and Y chromosome-derived probes to different genotypes provides the first demonstration that sequence differences exist between the sex chromosomes of O. niloticus. This provides further support for the theory that this chromosome pair is related to sex determination and further suggests that the sex chromosomes are at a very early stage of divergence.     

Meiotic chromosomes of the tree frog Smilisca baudinii (Anura: Hylidae)

The Mexican tree frog Smilisca baudinii, is a very common frog in Central America. In spite their importance to keep the ecological equilibrium of the rainforest, its biology and genetics are poorly known. In order to contribute with its biological knowledge, we described the typical meiotic karyotype based in standard cytogenetic protocols to specimens collected in Tabasco, Mexico. The study was centered in the analysis of 131 chromosome spreads at meiotic stage from two adults of the species (one female and one male). The metaphase analysis allowed the establishment of the modal haploid number of 1n = 12 bivalent chromosomes. The chromosomic formulae from the haploid bivalent karyotype was integrated by 12 biarmed chromosomes characterized by twelve pairs of metacentric-submetacentric (msm) chromosomes. The meiotic counting gives the idea that diploid chromosome number is integrated by a complement of 2n = 24 biarmed chromosomes. The presence of sex chromosomes from female and male meiotic spreads was not observed. Current results suggest that S. baudinii chromosome structure is well shared among Hylidae family and "B" chromosomes are particular structures that have very important evolutionary consequences in species diversification.     

Synaptonemal complexes of the A- and B-chromosomes of the spermatocytes from the East Asian mouse Apodemus peninsulae

The analysis of whole-mount preparations of synaptonemal complexes (SCs) from surface-spread spermatocytes of A. peninsulae (2n = 48A + 1, 2, ... 12 B) had revealed SCs of 23 autosomal bivalents, sex bivalent XY, axial cores and SCs of the B-chromosomes. The intercellular and interindividual variability of the number of B-chromosomes varied from 1 to 12 per cell. The SCs of autosomal bivalents were shown to have a typical structure. The structure and behaviour of SCs of sex bivalent throughout meiotic prophase I appeared to be similar to those observed in other species of this order. Mainly B-univalents and less frequently B-bivalents containing SCs were found to be formed in meiotic prophase I. The full homologues appear to be rarely seen among B-chromosomes of the East-Asiatic mouse. A tendency of forming clusters of B-univalents near the sex bivalent was found, in addition to B-bivalents with lateral elements, having the form of bi- and tri-stranded elements with rare synaptic fragments. Besides this, the SCs of the autosomes of pachytene cells were found to contain structures resembling the recombination nodules.     

First evidence of achiasmatic male meiosis in the water bears Richtersius coronifer and Macrobiotus richtersi (Eutardigrada, Macrobiotidae)

Chromosome behaviour during male meioses has been studied in two bisexual amphimictic populations of two tardigrade species, namely Richtersius coronifer and Macrobiotus richtersi (Eutardigrada, Macrobiotidae). Both bisexual populations exhibit a diploid chromosome number 2n=12 and no sex chromosomes were identified. DAPI staining and C-banding data indicate that all chromosomes of the bisexual population of R. coronifer are acrocentric. In both species, at male meiotic prophase, all six bivalent homologous chromosomes are aligned side by side along their length and show no evidence of chiasmata. However, in the oocytes of both species a chiasma is generally present in each bivalent at diplotene stage. Lack of recombination is previously unknown in tardigrades, but is a well known phenomenon in many other metazoans where it is always restricted to the heterogametic sex. In tardigrades there is no evidence of heterochromosomes, but it does not mean that in tardigrades, the heterogametic sex does not exist. The adaptive and evolutionary significance of achiasmatic meiosis is discussed.     

Nuclear architecture and chromosome dynamics in the search of the pairing partner in meiosis in plants

The formation of haploid gametes in organisms with sexual reproduction requires regular bivalent chromosome pairing in meiosis. In many species, homologous chromosomes occupy separate territories at the onset of meiosis. To be paired at metaphase I, they need to be brought into a close proximity for interactions that include homology recognition and the establishment of some form of bonds. How homologues find each other is one of the least understood meiotic events. Plant species with large or medium sized genomes, such as wheat or maize, are excellent materials for the cytological analysis of chromosome dynamics at early meiosis, but genes that control meiosis have been identified mainly in small genome species such as Arabidopsis thaliana. This review is focused on the contribution studies on plants are providing to the knowledge of the initial steps of the meiotic process.     

An asymmetric chromosome pair undergoes synaptic adjustment and crossover redistribution during Caenorhabditis elegans meiosis: implications for sex chromosome evolution

Heteromorphic sex chromosomes, such as the X/Y pair in mammals, differ in size and DNA sequence yet function as homologs during meiosis; this bivalent asymmetry presents special challenges for meiotic completion. In Caenorhabditis elegans males carrying mnT12, an X;IV fusion chromosome, mnT12 and IV form an asymmetric bivalent: chromosome IV sequences are capable of pairing and synapsis, while the contiguous X portion of mnT12 lacks a homologous pairing partner. Here, we investigate the meiotic behavior of this asymmetric neo-X/Y chromosome pair in C. elegans. Through immunolocalization of the axis component HIM-3, we demonstrate that the unpaired X axis has a distinct, coiled morphology while synapsed axes are linear and extended. By showing that loci at the fusion-proximal end of IV become unpaired while remaining synapsed as pachytene progresses, we directly demonstrate the occurrence of synaptic adjustment in this organism. We further demonstrate that meiotic crossover distribution is markedly altered in males with the asymmetric mnT12/+ bivalent relative to controls, resulting in greatly reduced crossover formation near the X;IV fusion point and elevated crossovers at the distal end of the bivalent. In effect, the distal end of the bivalent acts as a neo-pseudoautosomal region in these males. We discuss implications of these findings for mechanisms that ensure crossover formation during meiosis. Furthermore, we propose that redistribution of crossovers triggered by bivalent asymmetry may be an important driving force in sex chromosome evolution.     

Ultrastructure,meiotic behavior,and evolution of sex chromosomes of the genus Ellobius

The whole-mount SC preparations from males of three species of the genus Ellobius (Ellobius fuscocapillus, Ellobius lutescens), and Ellobius tancrei were studied by electron microscopy. In the males of Ellobius fuscocapillus, behavioral peculiarities of the sex bivalent (viz. the normal male heterozygosity) are characterized by early complete desynapsis of sex chromosomes (X, Y), occurring at late pachytene-early diplotene. The karyotype of species Ellobius lutescens is unique for mammals. In both sexes it is characterized by an odd number of chromosomes (2n=17). At prophase I the unpaired chromosome 9 is not involved in synapsis with other chromosomes and forms a sex body at the end of pachytene.The complete Robertsonian fan has been described for superspecies Ellobius tancrei. As shown on the basis of G-band patterns the male and female sex chromosomes are cytologically indistinguishable.Analysis of whole-mount SC preparations revealed the formation of a closed sex SC bivalent and showed some morphological differences in the axes of sex chromosomes at meiotic prophase I. A number of assumptions are made about the relationship between the behavior of sex chromosomes, their evolution and the sex determination system in the studied species of genus Ellobius.

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Insights into the meiotic behavior and evolution of multiple sex chromosome systems in spiders

A characteristic feature of spider karyotypes is the predominance of unusual multiple X chromosomes. To elucidate the evolution of spider sex chromosomes, their meiotic behavior was analyzed in 2 major clades of opisthothele spiders, namely, the entelegyne araneomorphs and the mygalomorphs. Our data support the predominance of X(1)X(2)0 systems in entelegynes, while rare X(1)X(2)X(3)X(4)0 systems were revealed in the tuberculote mygalomorphs. The spider species studied exhibited a considerable diversity of achiasmate sex chromosome pairing in male meiosis. The end-to-end pairing of sex chromosomes found in mygalomorphs was gradually replaced by the parallel attachment of sex chromosomes in entelegynes. The observed association of male X univalents with a centrosome at the first meiotic division may ensure the univalents' segregation. Spider meiotic sex chromosomes also showed other unique traits, namely, association with a chromosome pair in males and inactivation in females. Analysis of these traits supports the hypothesis that the multiple X chromosomes of spiders originated by duplications. In contrast to the homogametic sex of other animals, the homologous sex chromosomes of spider females were already paired at premeiotic interphase and were inactivated until prophase I. Furthermore, the sex chromosome pairs exhibited an end-to-end association during these stages. We suggest that the specific behavior of the female sex chromosomes may have evolved to avoid the negative effects of duplicated X chromosomes on female meiosis. The chromosome ends that ensure the association of sex chromosome pairs during meiosis may contain information for discriminating between homologous and homeologous X chromosomes and thus act to promote homologous pairing. The meiotic behavior of 4 X chromosome pairs in mygalomorph females, namely, the formation of 2 associations, each composed of 2 pairs with similar structure, suggests that the mygalomorph X(1)X(2)X(3)X(4)0 system originated by the duplication of the X(1)X(2)0 system via nondisjunctions or polyploidization.     

The program of sex chromosome pairing in meiosis is highly conserved across marsupial species: implications for sex chromosome evolution

Marsupials present a series of genetic and chromosomal features that are highly conserved in very distant species. One of these features is the absence of a homologous region between X and Y chromosomes. According to this genetic differentiation, sex chromosomes do not synapse during the first meiotic prophase in males, and a special structure, the dense plate, maintains sex chromosome association. In this report we present results on the process of meiotic sex chromosome pairing obtained from three different species, Thylamys elegans, Dromiciops gliroides, and Rhyncholestes raphanurus, representing the three orders of American marsupials. We have investigated the relationships between the axial structures organized along sex chromosomes and the formation of the dense plate. We found that in the three species the dense plate arises as a modification of sex chromosomal axial elements, but without the involvement of other meiotic axial structures, such as the cohesin axes. Considering the phylogenetic relationships among the marsupials studied here, our data reinforce the idea that the dense plate emerged early in marsupial evolution as an efficient mechanism to ensure the association of the nonhomologous sex chromosomes. This situation could have influenced the further evolution of sex chromosomes in marsupials.     

Development of the synaptonemal complex of two types of pachytene in oocytes and spermatocytes of Carausius morosus Br. (Phasmatodea)

During oogenesis of the parthenogenetic stick insect Carausius morosus (2n =61+XXX) pachytene is followed by a duplication of the desynapsed chromosomes, which results in a second type of pachytene (tetrapachytene) consisting of paired sister chromosomes (autobivalents). Electron microscopic studies on sections revealed that synaptonemal complexes (SCs) are formed during tetrapachytene only. This means that the parthenogenetically produced progeny have the genetic constitution of the mother. During spermatogenesis of rare fatherless males (2n=61 + XX) and intersexes (2n=61 +XXX) either an incomplete chromosome doubling (demonstrated by up to 10% additional DNA synthesis) or a complete chromosome doubling takes place during zygotene. EM studies on sections and spreads of germ cells of the first type of meiosis showed that unpaired lateral components (LCs), pieces of SCs and complete SCs are formed during pachytene only, the sex chromosomes being represented by unpaired thickened LCs. The incomplete SC formation reflects the complex heterozygosity of the chromosome complement. In the duplicated type SCs are found in tetrapachytene nuclei only; they are wider than the SCs in oocytes. The sex chromosome bivalents are represented by unpaired thickened LCs or partially paired LCs, in which localized chiasma formation was found. The idea is discussed that formation of SCs does not take place as long as a germ cell has been programmed either to replicate or to be able to replicate its chromosomes and that consequently SCs can be formed only once per meiosis.     

家鸡联会复合体的亚显微结构分析

本文以表面铺展——硝酸银染色技术,对家鸡的联会复合体(Syneptonemal Complex,SC)作亚显微结构分析。根据对10个精母细胞和10个卵母细胞SC的测量结果,绘制组型图。发现雌雄家鸡的常染色体的SC组型相同。在精母细胞中,性染色体(ZZ)的行为与常染色体相似。在卵母细胞中,性染色体ZW的长度不同,长轴为Z,短轴为W,两者之间只有部分配对,形成SC。从早粗线期到晚粗线期,由同源配对调整为非同源配对。另外,在一只雌鸡中,第一次观察到,有些细胞的常染色体能正常配对,而性染色体完全不配对的现象。     

Anti-topoisomerase II recognizes meiotic chromosome cores

At meiotic prophase the chromatin becomes arranged in loops on newly formed chromosome cores. The cores of homologous chromosomes become aligned in parallel and thus form the synaptonemal complex (SC), a structure found in the meiocytes of nearly all recombinationally competent, sexually reproducing organisms. We report that two polyclonal antibodies against topoisomerase II (topo II), which recognize the mitotic metaphase chromosome scaffold give, at pachytene, a positive immunocytological reaction with the chromatin and, predominantly, with the cores and centromeric regions of the paired chromosomes. It therefore appears that during meiotic prophase, topo II — a DNA-binding enzyme implicated in transient double-strand breaks, chromosome condensation, and anaphase separation — is associated with the chromatin and SCs of the pachytene and diplotene chromosomes.     

Ultrastructural characterization of the sex chromosomes during spermatogenesis of spiders having holocentric chromosomes and a long diffuse stage

An ultrastructural study has been made of spermatogenesis in two species of primitive spiders having holocentric chromosomes (Dysdera crocata, XO and Segestria florentia X₁X₂O). Analysis of the meiotic prophase shows a scarcity or absence of typical leptotene to pachytene stages. Only in D. crocata have synaptonemal complex (SC) remnants been seen, and these occurred in nuclei with an extreme chromatin decondensation. In both species typical early prophase stages have been replaced by nuclei lacking SC and with their chromatin almost completely decondensed, constituting a long and well-defined diffuse stage. Only nucleoli and the condensed sex chromosomes can be identified. — In S. florentina paired non-homologous sex chromosomes lack a junction lamina and thus clearly differ from the sex chromosomes of more evolved spiders with an X₁X₂O male sex determination mechanism. In the same species, sex chromosomes can be recognized during metaphase I due to their special structural details, while in D. crocata the X chromosome is not distinguishable from the autosomes at this stage. — The diffuse stage and particularly the structural characteristics of the sex chromosomes during meiotic prophase are reviewed and discussed in relation to the meiotic process in other arachnid groups.     

Cytogenetic analysis of Epicauta atomaria (Meloidae) and Palembus dermestoides (Tenebrionidae) with Xyp sex determination system using standard staining, C-bands, NOR and synaptonemal complex microspreading techniques

The mitotic and meiotic chromosomes of the beetles Epicauta atomaria (Meloidae) and Palembus dermestoides (Tenebrionidae) were analysed using standard staining, C-banding and silver impregnation techniques. We determine the diploid and haploid chromosome numbers, the sex determination system and describe the chromosomal morphology, the C-banding pattern and the chromosome(s) bearing NORs (nucleolar organizer regions). Both species shown 2n = 20 chromosomes, the chromosomal meioformula 9 + Xyp, and regular chromosome segregation during anaphases I and II. The chromosomes of E. atomaria are basically metacentric or submetacentric and P. dermestoides chromosomes are submetacentric or subtelocentric. In both beetles the constitutive heterochromatin is located in the pericentromeric region in all autosomes and in the Xp chromosome; additional C-bands were observed in telomeric region of the short arm in some autosomes in P. dermestoides. The yp chromosome did not show typical C-bands in these species. As for the synaptonemal complex, the nucleolar material is associated to the 7th bivalent in E. atomaria and 3rd and 7th bivalents in P. dermestoides. Strong silver impregnated material was observed in association with Xyp in light and electron microscopy preparations in these species and this material was interpreted to be related to nucleolar material.