The spatial relationship of the X and Y chromosomes during meiotic prophase in mouse spermatocytes

Cohesin SMC1β promotes closed chromatin and controls TERRA expression at spermatocyte telomeres

Previous data showed that meiotic cohesin SMC1β protects spermatocyte telomeres from damage. The underlying reason, however, remained unknown as the expressions of telomerase and shelterin components were normal in Smc1β -/- spermatocytes. Here. we report that SMC1β restricts expression of the long noncoding RNA TERRA (telomeric repeat containing RNA) in spermatocytes. In somatic cell lines increased TERRA was reported to cause telomere damage through altering telomere chromatin structure. In Smc1β -/- spermatocytes, we observed strongly increased levels of TERRA which accumulate on damaged chromosomal ends, where enhanced R-loop formation was found. This suggested a more open chromatin configuration near telomeres in Smc1β -/- spermatocytes, which was confirmed by ATAC-seq. Telomere-distal regions were not affected by the absence of SMC1β but RNA-seq revealed increased transcriptional activity in telomere-proximal regions. Thus, SMC1β promotes closed chromatin specifically near telomeres and limits TERRA expression in spermatocytes.

Meiotic pairing and double-strand break formation along the heteromorphic threespine stickleback sex chromosomes

Double-strand break repair during meiosis is normally achieved using the homologous chromosome as a repair template. Heteromorphic sex chromosomes share little sequence homology, presenting unique challenges to the repair of double-strand breaks. Our understanding of how heteromorphic sex chromosomes behave during meiosis has been focused on ancient sex chromosomes, where the X and Y differ markedly in overall structure and gene content. It remains unclear how more recently evolved sex chromosomes that share considerably more sequence homology with one another pair and form double-strand breaks. One possibility is barriers to pairing evolve rapidly. Alternatively, recently evolved sex chromosomes may exhibit pairing and double-strand break repair that more closely resembles that of their autosomal ancestors. Here, we use the recently evolved X and Y chromosomes of the threespine stickleback fish (Gasterosteus aculeatus) to study patterns of pairing and double-stranded break formation using molecular cytogenetics. We found that the sex chromosomes of threespine stickleback fish did not pair exclusively in the pseudoautosomal region. Instead, the chromosomes fully paired in a non-homologous fashion. To achieve this, the X chromosome underwent synaptic adjustment during pachytene to match the axis length of the Y chromosome. Double-strand break formation and repair rate also matched that of the autosomes. Our results highlight that recently evolved sex chromosomes exhibit meiotic behavior that is reminiscent of autosomes and argues for further work to identify the homologous templates that are used to repair double-strand breaks on the X and Y chromosomes.

Meiotic Behavior of Achiasmate Sex Chromosomes in the African Pygmy Mouse Mus mattheyi Offers New Insights into the Evolution of Sex Chromosome Pairing and Segregation in Mammals

X and Y chromosomes in mammals are different in size and gene content due to an evolutionary process of differentiation and degeneration of the Y chromosome. Nevertheless, these chromosomes usually share a small region of homology, the pseudoautosomal region (PAR), which allows them to perform a partial synapsis and undergo reciprocal recombination during meiosis, which ensures their segregation. However, in some mammalian species the PAR has been lost, which challenges the pairing and segregation of sex chromosomes in meiosis. The African pygmy mouse Mus mattheyi shows completely differentiated sex chromosomes, representing an uncommon evolutionary situation among mouse species. We have performed a detailed analysis of the location of proteins involved in synaptonemal complex assembly (SYCP3), recombination (RPA, RAD51 and MLH1) and sex chromosome inactivation (γH2AX) in this species. We found that neither synapsis nor chiasmata are found between sex chromosomes and their pairing is notably delayed compared to autosomes. Interestingly, the Y chromosome only incorporates RPA and RAD51 in a reduced fraction of spermatocytes, indicating a particular DNA repair dynamic on this chromosome. The analysis of segregation revealed that sex chromosomes are associated until metaphase-I just by a chromatin contact. Unexpectedly, both sex chromosomes remain labelled with γH2AX during first meiotic division. This chromatin contact is probably enough to maintain sex chromosome association up to anaphase-I and, therefore, could be relevant to ensure their reductional segregation. The results presented suggest that the regulation of both DNA repair and epigenetic modifications in the sex chromosomes can have a great impact on the divergence of sex chromosomes and their proper transmission, widening our understanding on the relationship between meiosis and the evolution of sex chromosomes in mammals.

Sex differences in the meiotic behavior of an XX sex chromosome pair in males and females of the mole vole Ellobius tancrei: turning an X into a Y chromosome?

Sex determination in mammals is usually provided by a pair of chromosomes, XX in females and XY in males. Mole voles of the genus Ellobius are exceptions to this rule. In Ellobius tancrei, both males and females have a pair of XX chromosomes that are indistinguishable from each other in somatic cells. Nevertheless, several studies on Ellobius have reported that the two X chromosomes may have a differential organization and behavior during male meiosis. It has not yet been demonstrated if these differences also appear in female meiosis. To test this hypothesis, we have performed a comparative study of chromosome synapsis, recombination, and histone modifications during male and female meiosis in E. tancrei. We observed that synapsis between the two X chromosomes is limited to the short distal (telomeric) regions of the chromosomes in males, leaving the central region completely unsynapsed. This uneven behavior of sex chromosomes during male meiosis is accompanied by structural modifications of one of the X chromosomes, whose axial element tends to appear fragmented, accumulates the heterochromatin mark H3K9me3, and is associated with a specific nuclear body that accumulates epigenetic marks and proteins such as SUMO-1 and centromeric proteins but excludes others such as H3K4me, ubiH2A, and γH2AX. Unexpectedly, sex chromosome synapsis is delayed in female meiosis, leaving the central region unsynapsed during early pachytene. This region accumulates γH2AX up to the stage in which synapsis is completed. However, there are no structural or epigenetic differences similar to those found in males in either of the two X chromosomes. Finally, we observed that recombination in the sex chromosomes is restricted in both sexes. In males, crossover-associated MLH1 foci are located exclusively in the distal regions, indicating incipient differentiation of one of the sex chromosomes into a neo-Y. Notably, in female meiosis, the central region of the X chromosome is also devoid of MLH1 foci, revealing a lack of recombination, possibly due to insufficient homology. Overall, these results reveal new clues about the origin and evolution of sex chromosomes.

Meiotic sex chromosome cohesion and autosomal synapsis are supported by Esco2

In mitotic cells, establishment of sister chromatid cohesion requires acetylation of the cohesin subunit SMC3 (acSMC3) by ESCO1 and/or ESCO2. Meiotic cohesin plays additional but poorly understood roles in the formation of chromosome axial elements (AEs) and synaptonemal complexes. Here, we show that levels of ESCO2, acSMC3, and the pro-cohesion factor sororin increase on meiotic chromosomes as homologs synapse. These proteins are less abundant on the largely unsynapsed sex chromosomes, whose sister chromatid cohesion appears weaker throughout the meiotic prophase. Using three distinct conditional Esco2 knockout mouse strains, we demonstrate that ESCO2 is essential for male gametogenesis. Partial depletion of ESCO2 in prophase I spermatocytes delays chromosome synapsis and further weakens cohesion along sex chromosomes, which show extensive separation of AEs into single chromatids. Unsynapsed regions of autosomes are associated with the sex chromatin and also display split AEs. This study provides the first evidence for a specific role of ESCO2 in mammalian meiosis, identifies a particular ESCO2 dependence of sex chromosome cohesion and suggests support of autosomal synapsis by acSMC3-stabilized cohesion.

Distinct Roles of Meiosis-Specific Cohesin Complexes in Mammalian Spermatogenesis

Mammalian meiocytes feature four meiosis-specific cohesin proteins in addition to ubiquitous ones, but the roles of the individual cohesin complexes are incompletely understood. To decipher the functions of the two meiosis-specific kleisins, REC8 or RAD21L, together with the only meiosis-specific SMC protein SMC1β, we generated Smc1β-/-Rec8-/- and Smc1β-/-Rad21L-/- mouse mutants. Analysis of spermatocyte chromosomes revealed that besides SMC1β complexes, SMC1α/RAD21 and to a small extent SMC1α/REC8 contribute to chromosome axis length. Removal of SMC1β and RAD21L almost completely abolishes all chromosome axes. The sex chromosomes do not pair in single or double mutants, and autosomal synapsis is impaired in all mutants. Super resolution microscopy revealed synapsis-associated SYCP1 aberrantly deposited between sister chromatids and on single chromatids in Smc1β-/-Rad21L-/- cells. All mutants show telomere length reduction and structural disruptions, while wild-type telomeres feature a circular TRF2 structure reminiscent of t-loops. There is no loss of centromeric cohesion in both double mutants at leptonema/early zygonema, indicating that, at least in the mutant backgrounds, an SMC1α/RAD21 complex provides centromeric cohesion at this early stage. Thus, in early prophase I the most prominent roles of the meiosis-specific cohesins are in axis-related features such as axis length, synapsis and telomere integrity rather than centromeric cohesion.

High density of REC8 constrains sister chromatid axes and prevents illegitimate synaptonemal complex formation

During meiosis, cohesin complexes mediate sister chromatid cohesion (SCC), synaptonemal complex (SC) assembly and synapsis. Here, using super-resolution microscopy, we imaged sister chromatid axes in mouse meiocytes that have normal or reduced levels of cohesin complexes, assessing the relationship between localization of cohesin complexes, SCC and SC formation. We show that REC8 foci are separated from each other by a distance smaller than 15% of the total chromosome axis length in wild-type meiocytes. Reduced levels of cohesin complexes result in a local separation of sister chromatid axial elements (LSAEs), as well as illegitimate SC formation at these sites. REC8 but not RAD21 or RAD21L cohesin complexes flank sites of LSAEs, whereas RAD21 and RAD21L appear predominantly along the separated sister-chromatid axes. Based on these observations and a quantitative distribution analysis of REC8 along sister chromatid axes, we propose that the high density of randomly distributed REC8 cohesin complexes promotes SCC and prevents illegitimate SC formation.

The role of sex chromosomes in mammalian germ cell differentiation: can the germ cells carrying X and Y chromosomes differentiate into fertile oocytes?

The sexual differentiation of germ cells into spermatozoa or oocytes is strictly regulated by their gonadal environment, testis or ovary, which is determined by the presence or absence of the Y chromosome, respectively. Hence, in normal mammalian development, male germ cells differentiate in the presence of X and Y chromosomes, and female germ cells do so in the presence of two X chromosomes. However, gonadal sex reversal occurs in humans as well as in other mammalian species, and the resultant XX males and XY females can lead healthy lives, except for a complete or partial loss of fertility. Germ cells carrying an abnormal set of sex chromosomes are efficiently eliminated by multilayered surveillance mechanisms in the testis, and also, though more variably, in the ovary. Studying the molecular basis for sex-specific responses to a set of sex chromosomes during gametogenesis will promote our understanding of meiotic processes contributing to the evolution of sex determining mechanisms. This review discusses the fate of germ cells carrying various sex chromosomal compositions in mouse models, the limitation of which may be overcome by recent successes in the differentiation of functional germ cells from embryonic stem cells under experimental conditions.

Recombination between the mouse Y chromosome short arm and an additional Y short arm-derived chromosomal segment attached distal to the X chromosome PAR

In a male mouse, meiosis markers of processed DNA double strand breaks (DSBs) such as DMC1 and RAD51 are regularly seen in the non-PAR region of the X chromosome; these disappear late in prophase prior to entry into the first meiotic metaphase. Marker evidence for DSBs occurring in the non-PAR region of the Y chromosome is limited. Nevertheless, historically it has been documented that recombination can occur within the mouse Y short arm (Yp) when an additional Yp segment is attached distal to the X and/or the Y pseudoautosomal region (PAR). A number of recombinants identified among offsprings involved unequal exchanges involving repeated DNA segments; however, equal exchanges will have frequently been missed because of the paucity of markers to differentiate between the two Yp segments. Here, we discuss this historical data and present extensive additional data obtained for two mouse models with Yp additions to the X PAR. PCR genotyping enabled identification of a wider range of potential recombinants; the proportions of Yp exchanges identified among the recombinants were 9.7 and 22.4 %. The frequency of these exchanges suggests that the Yp segment attached to the X PAR is subject to the elevated level of recombinational DSBs that characterizes the PAR.

Identification and characterisation of synaptonemal complex genes in monotremes

The platypus and echidna are the only extant species belonging to the clade of monotremata, the most basal mammalian lineage. The platypus is particularly well known for its mix of mammalian and reptilian characteristics and work in recent years has revealed this also extends to the genetic level. Amongst the monotreme specific features is the unique multiple sex chromosome system (5X4Y in the echidna and 5X5Y in the platypus), which forms a chain in meiosis. This raises questions about sex chromosome organisation at meiosis, including whether there has been changes in genes coding for synaptonemal complex proteins which are involved in homologous synapsis. Here we investigate the key structural components of the synaptonemal complex in platypus and echidna, synaptonemal complex proteins 1, 2 and 3 (SYCP1, SYCP2 and SYCP3). SYCP1 and SYCP2 orthologues are present, conserved and expressed in platypus testis. SYCP3 in contrast is highly diverged, but key residues required for self-association are conserved, while those required for tetramer stabilisation and DNA binding are missing. We also discovered a second SYCP3-like gene (SYCP3-like) in the same region. Comparison with the recently published Y-borne SYCP3 amino acid sequences revealed that SYCP3Y is more similar to SYCP3 in other mammals than the monotreme autosomal SYCP3. It is currently unclear if these changes in the SYCP3 gene repertoire are related to meiotic organisation of the extraordinary monotreme sex chromosome system.

Incomplete meiotic sex chromosome inactivation in the domestic dog

BACKGROUND

In mammalian meiotic prophase, homologous chromosome recognition is aided by formation and repair of programmed DNA double-strand breaks (DSBs). Subsequently, stable associations form through homologous chromosome synapsis. In male mouse meiosis, the largely heterologous X and Y chromosomes synapse only in their short pseudoautosomal regions (PARs), and DSBs persist along the unsynapsed non-homologous arms of these sex chromosomes. Asynapsis of these arms and the persistent DSBs then trigger transcriptional silencing through meiotic sex chromosome inactivation (MSCI), resulting in formation of the XY body. This inactive state is partially maintained in post-meiotic haploid spermatids (postmeiotic sex chromatin repression, PSCR). For the human, establishment of MSCI and PSCR have also been reported, but X-linked gene silencing appears to be more variable compared to mouse. To gain more insight into the regulation and significance of MSCI and PSCR among different eutherian species, we have performed a global analysis of XY pairing dynamics, DSB repair, MSCI and PSCR in the domestic dog (Canis lupus familiaris), for which the complete genome sequence has recently become available, allowing a thorough comparative analyses.

RESULTS

In addition to PAR synapsis between X and Y, we observed extensive self-synapsis of part of the dog X chromosome, and rapid loss of known markers of DSB repair from that part of the X. Sequencing of RNA from purified spermatocytes and spermatids revealed establishment of MSCI. However, the self-synapsing region of the X displayed higher X-linked gene expression compared to the unsynapsed area in spermatocytes, and was post-meiotically reactivated in spermatids. In contrast, genes in the PAR, which are expected to escape MSCI, were expressed at very low levels in both spermatocytes and spermatids. Our comparative analysis was then used to identify two X-linked genes that may escape MSCI in spermatocytes, and 21 that are specifically re-activated in spermatids of human, mouse and dog.

CONCLUSIONS

Our data indicate that MSCI is incomplete in the dog. This may be partially explained by extensive, but transient, self-synapsis of the X chromosome, in association with rapid completion of meiotic DSB repair. In addition, our comparative analysis identifies novel candidate male fertility genes.

Phosphorylation of CDK2 on threonine 160 influences silencing of sex chromosome during male meiosis

In mammalian meiosis, the X and Y chromosomes are largely unsynapsed and transcriptionally silenced during the pachytene stage of meiotic prophase (meiotic sex chromosome inactivation), forming a specialized nuclear territory called sex or XY body. An increasing number of proteins and noncoding RNAs were found to localize to the sex body and take part in influencing expression of sex chromosome genes. Cyclin-dependent kinase 2 (Cdk2 (-/-)) spermatocytes show incomplete sex chromosome pairing. Here, we further showed that phosphorylation of CDK2 isoform 1 (p-CDK2(39) [39 kDa]) on threonine 160 localizes to the sites of asynapsis and the sex body, interacting with phosphorylated gamma-H2AX. Meanwhile, p-CDK2(39) is frequently mislocalized throughout the sex body, and meiotic sex chromosome inactivation is disrupted in PWK×C57BL/6J hybrid mice. Furthermore, pachytene spermatocytes treated with mevastatin (an inhibitor of p-CDK2) showed overexpression of sex chromosome-linked genes. Our results highlight an important role for p-CDK2(39) in influencing silencing of the sex chromosomes during male meiosis by interacting with gamma-H2AX.

Dissociation of the X chromosome from the synaptonemal complex in the XY body of the rodent Galea musteloides

The XY body from spermatocytes of the rodent Galea musteloides shows progressive changes of the synaptonemal complex (SC) axes and the X-chromatin during pachynema. There is a gross thickening of the X-axis and the formation of a large X chromosome loop at mid and late pachytene stages. The SC proteins synaptonemal complex protein 3 (SYCP3), synaptonemal complex protein 1, and synaptonemal complex central element protein 3 and the proteins breast cancer 1, MutL homolog 1 (MLH1), and radiation-repair 51 (related to meiotic processes), the cohesin structural maintenance of chromosome 3, the centromeric protein (with CREST antibody), and the silenced chromatin (with phosphorylated (139ph) H2A histone family, member X (γ-H2AX) antibody) were analyzed in this XY body. The thick X-axis, including the interstitial loop, becomes formed by four to six laminae showing a cross-striation with a periodicity of about 20 nm. The whole length of the gross X-axis shows no significant changes during pachynema, but the interstitial chromatin of the X chromosome and the X centromere are included in the large loop, and it becomes separated from the SC. A conventional SC formed by the Y-axis, a central region and a thin lateral element originally corresponding to the X-axis, remains undisturbed up to the end of pachynema. A single MLH1 focus develops either at the distal or the proximal region of the loop end attached to the conventional SC. The chromatin surrounding the thickened axis is labeled with γ-H2AX. It is shown that most of the SYCP3 protein associated with the X chromosome loop is not involved in the SC maintenance, but it is located with the cohesin axis separated from the SC proper.

Chromatin configuration and epigenetic landscape at the sex chromosome bivalent during equine spermatogenesis

Pairing of the sex chromosomes during mammalian meiosis is characterized by the formation of a unique heterochromatin structure at the XY body. The mechanisms underlying the formation of this nuclear domain are reportedly highly conserved from marsupials to mammals. In this study, we demonstrate that in contrast to all eutherian species studied to date, partial synapsis of the heterologous sex chromosomes during pachytene stage in the horse is not associated with the formation of a typical macrochromatin domain at the XY body. While phosphorylated histone H2AX (γH2AX) and macroH2A1.2 are present as a diffuse signal over the entire macrochromatin domain in mouse pachytene spermatocytes, γH2AX, macroH2A1.2, and the cohesin subunit SMC3 are preferentially enriched at meiotic sex chromosome cores in equine spermatocytes. Moreover, although several histone modifications associated with this nuclear domain in the mouse such as H3K4me2 and ubH2A are conspicuously absent in the equine XY body, prominent RNA polymerase II foci persist at the sex chromosomes. Thus, the localization of key marker proteins and histone modifications associated with the XY body in the horse differs significantly from all other mammalian systems described. These results demonstrate that the epigenetic landscape and heterochromatinization of the equine XY body might be regulated by alternative mechanisms and that some features of XY body formation may be evolutionary divergent in the domestic horse. We propose equine spermatogenesis as a unique model system for the study of the regulatory networks leading to the epigenetic control of gene expression during XY body formation.

Protein immunolocalization supports the presence of identical mechanisms of XY body formation in eutherians and marsupials

The meiotic sex chromosomes of the American marsupials Monodelphis dimidiata and Didelphis albiventris were studied with electron microscopy (EM) and with immunofluorescence localization of meiotic proteins SYCP1 and SYCP3, and proteins essential for meiotic sex chromosome inactivation (MSCI), gamma-H2AX and BRCA1. The chromatin of the non-synaptic X and Y chromosomes contains gamma-H2AX, first as foci and then as homogeneous staining at late stages. The thick and split X and Y axes are labelled with BRCA1 except at one terminus. The bulgings of the axes contain SYCP1 as well as the inner side of the dense plate. The evenly spaced and highly packed chromatin fibres of the conjoined XY body in these species have the same behaviour and the same components (gamma-H2AX in the chromatin, BRCA1 in the axes) as in the XY body of eutherian species. These observations and recent data from the literature suggest that XY body formation is ancestral to the metatherian-eutherian divergence.

The X and Y chromosomes assemble into H2A.Z-containing [corrected] facultative heterochromatin [corrected] following meiosis

Spermatogenesis is a complex sequential process that converts mitotically dividing spermatogonia stem cells into differentiated haploid spermatozoa. Not surprisingly, this process involves dramatic nuclear and chromatin restructuring events, but the nature of these changes are poorly understood. Here, we linked the appearance and nuclear localization of the essential histone variant H2A.Z with key steps during mouse spermatogenesis. H2A.Z cannot be detected during the early stages of spermatogenesis, when the bulk of X-linked genes are transcribed, but its expression begins to increase at pachytene, when meiotic sex chromosome inactivation (MSCI) occurs, peaking at the round spermatid stage. Strikingly, when H2A.Z is present, there is a dynamic nuclear relocalization of heterochromatic marks (HP1beta and H3 di- and tri-methyl K9), which become concentrated at chromocenters and the inactive XY body, implying that H2A.Z may substitute for the function of these marks in euchromatin. We also show that the X and the Y chromosome are assembled into facultative heterochromatic structures postmeiotically that are enriched with H2A.Z, thereby replacing macroH2A. This indicates that XY silencing continues following MSCI. These results provide new insights into the large-scale changes in the composition and organization of chromatin associated with spermatogenesis and argue that H2A.Z has a unique role in maintaining sex chromosomes in a repressed state.

Sex chromosomes, synapsis, and cohesins: a complex affair

During first meiotic prophase, homologous chromosomes are held together by the synaptonemal complex, a tripartite proteinaceous structure that extends along the entire length of meiotic bivalents. While this feature is applicable for autosomes, sex chromosomes often escape from this rule. Many species present sex chromosomes that differ between them in their morphology, length, and gene content. Moreover, in some species, sex chromosomes appear in a single dose in one of the sexes. In all of these cases, the behavior of sex chromosomes during meiosis is conspicuously affected, and this includes the assembly and dynamics of the synaptonemal complex. We review in this study the structure of the synaptonemal complex in the sex chromosomes of three groups of organisms, namely: mammals, orthopterans, and hemipterans, which present different patterns of sex chromosome structure and behavior. Of special interest is the analysis of the organization of the axial/lateral elements of the synaptonemal complex in relation to other axial structures organized along meiotic chromosomes, mainly the cohesin axis. The differences found in the behavior of both axial structures reveal that while the organization of a cohesin axis along sex chromosomes is a conserved feature in most organisms and it shows very little morphological variations, the axial/lateral elements of the synaptonemal complex present a wide range of structural modifications on these chromosomes.

Testicular expression of small ubiquitin-related modifier-1 (SUMO-1) supports multiple roles in spermatogenesis: silencing of sex chromosomes in spermatocytes, spermatid microtubule nucleation, and nuclear reshaping

SUMO-1 is a member of a ubiquitin-related family of proteins that mediates important post-translational effects affecting diverse physiological functions. Whereas SUMO-1 is detected in the testis, little is known about its reproductive role in males. Herein, cell-specific SUMO-1 was localized in freshly isolated, purified male germ cells and somatic cells of mouse and rat testes using Western analysis, high-resolution single-cell bioimaging, and in situ confocal microscopy of seminiferous tubules. During germ cell development, SUMO-1 was observed at low but detectable levels in the cytoplasm of spermatogonia and early spermatocytes. SUMO-1 appeared on gonosomal chromatin during zygotene when chromosome homologues pair and sex chromatin condensation is initiated. Striking SUMO-1 increases in the sex body of early-to-mid-pachytene spermatocytes correlated with timing of additional sex chromosome condensation. Before the completion of the first meiotic division, SUMO-1 disappeared from the sex body when X and Y chromosomal activity resumed. Together, these data indicate that sumoylation may be involved in non-homologous chromosomal synapsis, meiotic sex chromosome inactivation, and XY body formation. During spermiogenesis, SUMO-1 localized in chromocenters of certain round spermatids and perinuclear ring and centrosomes of elongating spermatids, data implicating SUMO-1 in the process of microtubule nucleation and nuclear reshaping. STAT-4, one potential target of sumoylation, was located along the spermatid nuclei, adjacent but not co-localized with SUMO-1. Androgen receptor-positive Leydig, Sertoli, and some peritubular myoepithelial cells express SUMO-1, findings suggesting a role in modulating steroid action. Testicular SUMO-1 expression supports its specific functions in inactivation of sex chromosomes during meiosis, spermatid microtubule nucleation, nuclear reshaping, and gene expression.

XMR, a dual location protein in the XY pair and in its associated nucleolus in mouse spermatocytes

Xlr and Xmr are sex-specific genes which are expressed during the meiotic prophase I in the mouse. In spermatocytes, XMR concentrates on the asynapsed regions of the XY chromosomes, suggesting that XMR plays a role in sex chromosome condensation and silencing. The present study shows that in the mouse, XMR also concentrates in the nucleolus which is closely associated with the XY chromosome pair. In this species, the formation of a large fibrillo-granular nucleolus signals the activation of the ribosomal genes, but release of pre-ribosomal particles is inhibited. Using laser confocal microscopy we characterized the distribution of XMR in the XY body relative to the XY chromatin and the nucleolus. Immunoelectron microscopy showed that XMR concentrates in the fibrillo-granular component and the granular component (GC) of the nucleolus. In (T[X;16]16H) mouse spermatocytes, the nucleolus displays little or no activity and does not associate with the XY pair. XMR concentrated only on the XY chromosomes in (T[X;16]16H) mouse spermatocytes. These data suggest that XMR could play a role both in the XY pair and the nucleolus associated to the sex chromosomes.

Quantitative study on guinea pig spermatogenesis shows a relative high percentage of early meiotic prophase stages

Meiosis is the special double cellular division characterized by the reduction of chromosome number of the final products and recombination of genetic information present in maternal and paternal homologous chromosomes. Early stages of meiotic prophase, leptotene and zygotene (L/Z), are functionally important since homologous chromosomes recognize, align, and pair during them. They are poorly represented in the seminiferous tubules of mammalian species, and this fact turns studies focused on these stages difficult to perform. As a consequence, the molecular bases of these important events are so far poorly known and understood in higher eukaryotes. The purpose of this work was to provide an advantageous experimental mammalian model (with a reasonable number of cells) for biochemical and molecular analysis of early meiotic prophase stages. Here, we present the results of our quantitative study on testes material of both immature and adult guinea pig specimens (Cavia porcellus). We show that their seminiferous tubules contain a comparatively high percentage of L/Z spermatocytes, as well as a very conspicuous chromosome bouquet at the L/Z transition, which points out this species as a well-suited one to address studies on such stages in mammals.

Telomere-Binding TRF2/MTBP Localization during Mouse Spermatogenesis and Cell Cycle of the Mouse Cells L929

Observations of the organization and distribution of telomeres (Tel) in somatic tissues still remain controversial. The Tel topography revealed by modern microscopy shows them to be associated with the nuclear envelope (NE) in a wide variety of eukaryotic cells, although not at the Rabl orientation (peripheral position at one pole of the nucleus at prophase). We used two cell types that have different nuclear architectures. The cell line L929 shows lack of any rigid Tel architecture in the nucleus. In contrast, spermatozoa have a precise architecture established during spermiogenesis. We observed Tel and membrane Tel binding protein (MTBP/TRF2) position by immunoFISH in L929 cells and by immunofluorescence and immunogold electron microscopy, using antibodies against Membrane Tel Binding Protein (MTBP/TRF2), during different stages of spermiogenesis. At all stages of the L929 cell cycle, MTBP/TRF2 is co-localized with Tel. The only Tel order found in this cell type is similar to the Rabl-orientation, probably due to fast divisions. In the mouse pachytene spermatocytes, the membrane structures abut on the synaptonemal complex (SC) attachment sites contain MTBP/TRF2. In fully formed spermatozoa and during spermiogenesis, apart from the expected MTBP/TRF2 position at the nuclear periphery, MTBP/TRF2 unexpectedly localized at the acrosomal membrane that is adjacent to the nucleus. The difference in the MTBP/TRF2 distribution in the oocyte and spermatozoa leads to the suggestion that the MTBP/TRF2 location might reflect preparation for fertilization events. The Tel distribution is not static in cultured cells throughout the cell cycle or during spermatogenesis. When the Tel are attached to the NE, as during SC formation, MTBP/TRF2 is the member of the protein complex, which appears to be responsible for this attachment.

The pairing of X and Y chromosomes during meiotic prophase in the marsupial species Thylamys elegans is maintained by a dense plate developed from their axial elements

Unlike eutherian males, pairing of the sex chromosomes in marsupial males during the first meiotic prophase is not mediated by a synaptonemal complex. Instead, a specific structure, the dense plate, develops during pachytene between the sex chromosomes. We have investigated the development and structural nature of this asynaptic association in males of the marsupial species Thylamys elegans by means of immunolabelling and electron microscopy techniques. Our results show that the behaviour of male marsupial sex chromosomes during first meiotic prophase is complex, involving modifications of their structure and/or composition. Pairing of the sex chromosomes and formation of the dense plate take place in mid pachytene, paralleling morphological changes in the sex chromosomal axial elements. Components of the central element of the synaptonemal complex were not found in the sex body, in agreement with ultrastructural studies that reported the absence of a canonical tripartite synaptonemal complex between male marsupial sex chromosomes. Interestingly, the dense plate is labelled with antibodies against the SCP3 protein of the lateral elements of the synaptonemal complex. Moreover, as sex chromosome axial elements decrease in mass throughout mid-late pachytene, the dense plate increases, suggesting that material moves from the axial elements to the dense plate. Additionally, both sex chromosome axial elements and the dense plate have proteins that are specifically phosphorylated, as revealed by their labelling with the MPM-2 antibody, indicating that they undergo a chromosome-specific regulation process throughout first meiotic prophase. We propose that the unique modifications of the composition and structure of the axial elements of the sex chromosomes in meiotic prophase may result in the prescription of synaptonemal complex formation between male marsupial sex chromosomes, where the dense plate is an extension of the axial elements of sex chromosomes. This replaces synapsis to maintain X and Y association during first meiotic prophase.

Estimation of the highest chromosome number of eukaryotes based on the minimum interaction theory

According to the minimum interaction theory, the chromosome evolution of eukaryotes proceeds as a whole toward increasing the chromosome number. This raises the following two questions: what was the starting chromosome number of eukaryotes and does the chromosome number increase infinitely? We attempted to provide a theoretical framework to resolve these questions. We propose that the species with n=2 observed in Protozoa, Platyhelminthes, Annelid, Algae, Fungi and higher plants would be chromosomal relicts conserving the karyotypes of ancestral eukaryotes. We also propose that the ideal highest number of eukaryotes (n(max)) can be given by an inverse of the minimum terminal interference distance (It(min)) in crossing-over (n(max)=100/It(min)). AsIt(min) =0.6 in mammals, n(max) approximately 166. On the other hand, the value estimated by computer simulations is somewhat lower with n(max)=133-138. Our arguments can be applied to other eukaryotes, if they have a localized centromere and the ratio of total synaptonemal complex/nuclear volume is comparable to that of mammals. We revealed that the index of gene shuffling per karyotypes (G) by means of the total number of gamete types with different gene combinations can be formulated asG =2(n+Fxi), where Fxi means interstitial chiasma frequency per cell corresponding to crossing-over mediated by the recombination nodule. The Fxi value increases in proportion to the n value in areas where n<40, but decreases gradually when n>40 and becomes zero when n>83. Therefore, in the ultimate karyotype with n(max)=166, FXi=0 andG =2(n)=2(166), where gene shuffling is guaranteed by the random orientation of chromosomes at the equatorial plate of meiotic metaphase I.

Nuclear envelope associated protein that binds telomeric DNAs

Rana temporaria oocytes at the 6th diplotene stage of maturation contain a special structure, the karyosphere capsule, with chromosomes covered and detached from the nuclear envelope (NE), though at the previous stage the telomeres were attached to the membrane, as characteristic of germ cells. The DNA-protein complexes from band shift assays with proteins extracted from oocyte NEs and telomeric DNA fragment (T(2)G(4))(130) were isolated and injected into a guinea pig. In the present paper the only protein of 70 kDa recognized by antibody (AB) in the NE is named the Membrane Telomere Binding Protein (MTBP). Western blots with guinea pig AB and AB against telobox peptide from TRF2 show that protein of 60 kDa (probably TRF1) belongs to the chromatin, but MTBP (TRF2 according to immunoprecipitation) belongs to the NE. In the somatic cell nuclei both proteins are present and recognized by AB against telobox peptide, but AB raised recognize only MTBP/TRF2 due to the epitope different from telobox. Combined in situ hybridization with the vertebrate telomeric DNA sequences (T(2)AG(3))(135) and immunocytochemistry with the MTBP AB showed them to be colocalized within the mouse nucleus. As it was shown by immunofluorescense of NE spread, MTBP is organized in a distinct pattern that looks like a network made of double-dots. Electron microscope immunogold staining with both ABs showed that the protein is localized on the outer surface of the oocyte NE within cup-like structures attached to the membrane. This is the first clear evidence of a protein, which could be responsible for the attachment of telomeres to the nuclear membrane.

Meiotic chromosomes: integrating structure and function

Meiotic chromosomes have been studied for many years, in part because of the fundamental life processes they represent, but also because meiosis involves the formation of homolog pairs, a feature which greatly facilitates the study of chromosome behavior. The complex events involved in homolog juxtaposition necessitate prolongation of prophase, thus permitting resolution of events that are temporally compressed in the mitotic cycle. Furthermore, once homologs are paired, the chromosomes are connected by a specific structure: the synaptonemal complex. Finally, interaction of homologs includes recombination at the DNA level, which is intimately linked to structural features of the chromosomes. In consequence, recombination-related events report on diverse aspects of chromosome morphogenesis, notably relationships between sisters, development of axial structure, and variations in chromatin status. The current article reviews recent information on these topics in an historical context. This juxtaposition has suggested new relationships between structure and function. Additional issues were addressed in a previous chapter (551).

Mitotic and meiotic analysis in Arctocephalus australis (Otariidae)

The karyotype with C-, G- and NOR-banding of Arctocephalus australis is reported for the first time. The chromosomal number is 2n = 36. The X chromosome, identified in G-banded metaphases from males, is metacentric and the Y chromosome is a minute chromosome, also metacentric. Pachytene spermatocytes were used for synaptonemal complexes analysis with a surface spreading technique. A total of 17 autosomal synaptonemal complexes are observed plus the XY pair. During early pachytene, the X and Y axes are thickened and remain unpaired. As pachytene advances, a short SC is formed between the gonosomes, as it is common among eutherian mammals. The particular asymmetrical appearance of the synaptonemal complex in the sex pair is described and compared to other cases among mammals.

XYbp, a novel RING-finger protein, is a component of the XY body of spermatocytes and centrosomes

RING-finger proteins participate in developmental processes, including gametogenesis. A fetal oocyte cDNA library was used to select genes expressed during male germ-cell differentiation. A novel RING-finger protein, XYbp (XY body protein), participating in mouse spermatogenesis has been identified. This novel gene generates a ubiquitously expressed transcript of 4.2 kb and a testis-specific one of 2.8 kb, processed by an alternative polyadenylation mechanism from a non-canonical polyadenylation signal. Transcription of XYbp is regulated during spermatocyte differentiation. The antiserum raised against the XYbp peptide demonstrated that XYbp is localised mainly in the XY bivalent of spermatocytes (XY body) and in the centrosomes of somatic and germ cells in all phases of the cell cycle. These studies indicate that we have identified a new member of the RING-finger family of proteins associated with the XY meiotic bivalent during spermatogenesis development and with the centrosomes of all cells.

Architecture of the nuclear periphery of rat pachytene spermatocytes: distribution of nuclear envelope proteins in relation to synaptonemal complex attachment sites

The nucleus of spermatocytes provides during the first meiotic prophase an interesting model for investigating relationships of the nuclear envelope (NE) with components of the nuclear interior. During the pachytene stage, meiotic chromosomes are synapsed via synaptonemal complexes (SCs) and attached through both ends to the nuclear periphery. This association is dynamic because chromosomes move during the process of synapsis and desynapsis that takes place during meiotic prophase. The NE of spermatocytes possesses some peculiarities (e.g., lower stability than in somatic cells, expression of short meiosis-specific lamin isoforms called C2 and B3) that could be critically involved in this process. For better understanding of the association of chromosomes with the nuclear periphery, in the present study we have investigated the distribution of NE proteins in relation to SC attachment sites. A major outcome was the finding that lamin C2 is distributed in the form of discontinuous domains at the NE of spermatocytes and that SC attachment sites are embedded in these domains. Lamin C2 appears to form part of larger structures as suggested by cell fractionation experiments. According to these results, we propose that the C2-containing domains represent local reinforcements of the NE that are involved in the proper attachment of SCs.

Immunocytology of chiasmata and chromosomal disjunction at mouse meiosis

Immunocytological and in situ hybridization evidence supports the hypothesis that at meiosis of chiasmate organisms, chromosomal disjunction and reductional segregation of sister centromeres are integrated with synaptonemal complex functions. The Mr 125,000 synaptic protein, Syn1, present between cores of paired homologous chromosomes during pachytene of meiotic prophase, is lost from synaptonemal complexes coordinately with homolog separation at diplotene. Separation is constrained by exchanges between non-sister chromatids, the chiasmata. We show that the Mr 30,000 chromosomal core protein, Cor1, associated with sister chromatid pairs, remains an axial component of post-pachytene chromosomes until metaphase I. We demonstrate that at this time the chromatin loops are still attached to their cores. A reciprocal exchange event between two homologous non-sister chromatids is therefore immobilized by anchorage of sister chromatids to their respective cores. Cores thus contribute to the sister chromatid cohesiveness required for maintenance of chiasmata and proper chromosomal disjunction. Cor1 protein accumulates in juxtaposition to pairs of sister centromeres during metaphase I. Presumably, independent movement of sister centromeres at anaphase I is restricted by Cor1 anchorage. That reductional separation of sister centromeres is mediated by Cor1, is supported by the dissociation of Cor1 from separating sister centromeres at anaphase II and by its absence from mitotic anaphases.

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.

Theoretical analyses of chiasmata using a novel chiasma graph method applied to Chinese hamsters, mice, and dog

Some basic concepts of chiasma (including chiasma distribution, chiasma frequency, interstitial and terminal chiasmata, and chiasma interference) are reexamined theoretically in the light of gene shuffling, and a new method for chiasma analysis termed the chiasma graph is proposed. Chiasma graphs are developed for three mammals with greatly different chromosome numbers: Chinese hamster (with n = 11), mice (n = 20), and a dog (n = 39). The results demonstrate that interstitial chiasmata can contribute both to gene shuffling and to the binding of bivalents, but that so-called terminal chiasmata are in fact mostly achiasmatic terminal associations, the main function of which is to bind bivalents. For this reason, terminal chiasmata should be excluded when chiasma frequency is estimated. It is also demonstrated that interstitial chiasmata distribute on bivalents randomly and uniformly, except at the centromere and telomere. Interference distance fluctuates almost randomly above a minimum value equivalent to about 1.8% of total bivalent length at diakinesis. These results indicate that chiasma formation in mammals is principally a random event. The demonstrated minimum interference distance seems consistent with the polymerization model for chiasma formation. Some cytological aspects of crossing-over are discussed with reference to the minimum interaction theory for eukaryotic chromosome evolution.

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.

Meiosis-specific protein selectively associated with sex chromosomes of rat pachytene spermatocytes

During the first meiotic prophase of mammalian spermatogenesis, the sex chromosomes X and Y show a characteristic allocyclic behavior with respect to the autosomes. This is particularly evident during pachytene stage when sex chromosomes form the so-called sex vesicle. This structure is characterized by the condensed state of chromatin, transcriptional inactivity, and the limited extension of chromosome pairing, which is usually restricted to a short segment of sex chromosome axial elements. The molecular basis and functional significance of sex vesicle formation during mammalian spermatogenesis remain obscure. Here we report on the identification of a meiosis-specific sex vesicle protein we called XY40. Immunocytochemical localization on rat testis cryosections with a XY40-specific monoclonal antibody revealed that the labeling is confined to the axial elements of sex chromosomes. Biochemical characterization showed that protein XY40 (40 kDa; pI 5.7-5.8) can be extracted from rat pachytene spermatocytes and recovered in particles of 9.5 S with a native molecular mass of approximately 152 kDa. We speculate that protein XY40 may be involved in the allocyclic behavior of sex chromosomes during male meiotic prophase.

Identification of a structural protein component of rat synaptonemal complexes

Synaptonemal complexes (SCs) are evolutionarily conserved nuclear structures of meiotic cells which form during the zygotene stage of the first meiotic prophase and are responsible for the pairing of homologous chromosomes. Their formation appears to be a prerequisite for crossing-over events and proper chromosome segregation during the first meiotic division. Despite knowledge of their central role in genetic recombination processes very little is known about the molecular composition and the mechanisms governing the assembly of the SCs. In the present study we report on the characterization of a monoclonal antibody (SC14f10) which enabled us to identify a novel SC protein termed SC48. Protein SC48 has a Mr of 48,000 and migrates in two-dimensional gels with a pH value of 6.9. By means of immunogold EM we localized this protein to the central region of the SC. In cell fractionation experiments we recovered protein SC48 together with SC-residual structures in a karyoskeletal fraction of pachytene spermatocytes. Our results indicate that SC48 is a meiosis-specific structural protein component of the SC probably involved in the pairing of homologous chromosomes.

Tissue distribution of two major components of synaptonemal complexes of the rat

In this paper we describe an analysis of the tissue distribution of two recently identified components of synaptonemal complexes (SCs), an Mr 125,000 and an Mr 190,000 protein, in the male rat by immunoblot analysis and immunocytochemical techniques. We compared the tissue distribution of these antigens with that of two earlier identified SC components, an Mr 30,000 and an Mr 33,000 polypeptide. For this purpose we used monoclonal antibodies (Mabs) that react exclusively with SCs in lysed spermatocytes, and that recognize the above mentioned antigens specifically in immunoblots of SC proteins or of nuclear proteins from spermatocytes; these were Mab IX9D5 (anti-190,000), Mab IX5B2 (anti-125,000), Mab II52F10 (anti-30,000 + 33,000), and Mab IX8G9 (anti-30,000 + 33,000). In the immunoblot experiments, we could detect the Mr 190,000 and 125,000 antigens exclusively in blots of SC proteins or nuclear proteins from spermatocytes; these antigens were not detectable in blots of nuclear proteins from liver, brain, spermatogonia or spermatids or in blots of proteins from mitotic chromosomes or nuclear laminae. With the anti- 30,000 + 33,000 Mabs we obtained essentially the same result, except that Mab IX8G9, but not II52F10, recognizes a small amount of Mr 30,000 antigen in blots of nuclear proteins from spermatids and spermatogonia. Although this might be ascribed to contamination of the isolated spermatids and spermatogonia, we cannot exclude that a small amount of Mr 30,000 antigen is present in these cells. In the immunofluorescence analysis, the testis was the only tissue that reacted detectably with the above antibodies. Within the testis, spermatocytes and some early spermatids were the only cell types that contained detectable amounts of antigen.(ABSTRACT TRUNCATED AT 250 WORDS)

Temporal comparison of recombination and synaptonemal complex formation during meiosis in S. cerevisiae

In synchronous cultures of S. cerevisiae undergoing meiosis, an early event in the meiotic recombination pathway, site-specific double strand breaks (DSBs), occurs early in prophase, in some instances well before tripartite synaptonemal complex (SC) begins to form. This observation, together with previous results, supports the view that events involving DSBs are required for SC formation. We discuss the possibility that the mitotic pathway for recombinational repair of DSBs served as the primordial mechanism for connecting homologous chromosomes during the evolution of meiosis. DSBs disappear during the period when tripartite SC structure is forming and elongating (zygotene); presumably, they are converted to another type of recombination intermediate. Neither DSBs nor mature recombinant molecules are present when SCs are full length (pachytene). Mature reciprocally recombinant molecules arise at the end of or just after pachytene. We suggest that the SC might coordinate recombinant maturation with other events of meiosis.

Genetic basis of X-Y chromosome dissociation and male sterility in interspecific hybrids

A high frequency of X-Y chromosome dissociation (95%) was found at first meiotic metaphase (MI) in spermatocytes of interspecific hybrids between laboratory mice, C57BL/6J (BL/6) and Mus spretus, compared with an X-Y dissociation frequency of only 3-4% in parental mice. The X-Y dissociation in F1 hybrids occurred before diakinesis rather than as a precocious dissociation at MI. The high X-Y dissociation was accompanied by spermatogenic breakdown after MI, resulting in male sterility. All F1 males were sterile and approximately half of the backcross males from fertile F1 females crossed with either BL/6 or M. spretus males were sterile. Male sterility was highly correlated with X-Y dissociation in both backcrosses. All of the mice with high X-Y dissociation were sterile and all of the males with low X-Y dissociation were fertile or subfertile. This correlation suggested that genetic divergence of the X-Y pairing region could contribute to the male sterile phenotype such that the BL/6 X chromosome would not pair with the M. spretus Y chromosome. The segregation of species-type alleles of amelogenin (Amelb and Amels), a distal X chromosome locus adjacent to the X-Y pairing region, was followed in backcross males that were analyzed for X-Y dissociation and sterility (we have used Amel as the designation for the mouse amelogenin locus; the current designation for this locus is Amg). A 95% concordance between Amelb with fertility and Amels with sterility was observed in backcrosses with BL/6, whereas the converse was observed in the backcross to M. spretus. These results imply that X-Y pairing plays an important role in male fertility and suggest that genetic divergence in X-Y pairing region between Mus species can contribute to the reproductive barriers between species and the process of speciation.

DNA variants with telomere probe enable genetic mapping of ends of mouse chromosomes

Dde I-digested DNA fragments from 11 inbred mouse strains were separated by electrophoresis, blotted and probed with a labeled oligomer, TELO, containing five repeats of the consensus mammalian telomere sequence, TTAGGG. Each strain produced a unique set of hybridizing fragments. Segregation analysis of TELO-hybridizing fragments from the BXD RI strains indicated that each fragment segregated as expected for a single gene. One fragment from strain DBA/2J was genetically linked to locus Xmv-9, previously mapped near the distal end of the map of chromosome (Chr) 4 and three fragments to Cck, near the distal end of Chr 9, suggesting that these fragments are telomeric and represent the ends of the chromosome maps. Confirmation of these map positions was obtained from a backcross. Fragments associated with the short arm of the Y Chr were found in DNA from strains C57BL/6J and DBA/2J. TELO-hybridizing fragments from DBA/2J were digested by the exonuclease Bal 31, under conditions in which fragments hybridizing to a cDNA probe for the metallothionein locus, located at the middle of mouse Chr 8, remained intact. Thus both biochemical and genetic tests indicate that several TELO-hybridizing fragments from Dde I-digested DNA are at the ends of chromosomes and probably derive from mouse telomeres. Using this approach should allow the mapping of genes relative to the ends of other mouse chromosomes.

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.

The histones of the insect trypanosomatid, Crithidia fasciculata

The histone-like proteins of the monogenetic parasite Crithidia fasciculata were extracted with 0.2 M sulfuric acid either from purified nuclei, or from purified chromatin, in both cases in the presence of 1 mM tosyl lysylchloromethylketone and 2 mM phenyl methyl sulfonyl fluoride as proteinase inhibitors. The presence of histones in the flagellate, nonidentical with those from calf thymus used as controls, was shown by their electrophoretic patterns in three different polyacrylamide gel systems; their staining with Alkaline fast green, specific for basic proteins; their global amino acid composition and absorption spectrum and their molecular weights. The protein showing the slower mobility in SDS gels and the fastest mobility in the urea-acetic acid-Triton gels, seems to be an H1 histone, because of its metachromatic staining with Coomassie brilliant blue, solubility characteristics, differential destaining properties and amino acid composition. Band 5 in Triton-urea-acetic acid gels is probably an HMG protein. We conclude that C. fasciculata has a complete set of histones and that the lack of chromosome condensation during mitosis is not due to lack of histone H1.

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.