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X chromosome dosage compensation ensures balanced gene dosage between the X chromosome and autosomes and between the sexes, involving divergent mechanisms among mammals. We elucidated a distinct mechanism for X chromosome inactivation (XCI) in cynomolgus monkeys, a model for human development. The trophectoderm and cytotrophoblast acquire XCI around implantation through an active intermediate bearing repressive modifications and compacted structure, whereas the amnion, epiblast, and hypoblast maintain such an intermediate protractedly, attaining XCI by a week after implantation. Males achieve X chromosome up-regulation (XCU) progressively, whereas females show XCU coincidentally with XCI, both establishing the X:autosome dosage compensation by 1 week after implantation. Conversely, primordial germ cells undergo X chromosome reactivation by reversing the XCI pathway early during their development. Our findings establish a foundation for clarifying the dosage compensation mechanisms in primates, including humans.

Diverse cell types in tissues have distinct gene expression programs, chromatin states, and nuclear architectures. To correlate such multimodal information across thousands of single cells in mouse brain tissue sections, we use integrated spatial genomics, imaging thousands of genomic loci along with RNAs and epigenetic markers simultaneously in individual cells. We reveal that cell type–specific association and scaffolding of DNA loci around nuclear bodies organize the nuclear architecture and correlate with differential expression levels in different cell types. At the submegabase level, active and inactive X chromosomes access similar domain structures in single cells despite distinct epigenetic and expression states. This work represents a major step forward in linking single-cell three-dimensional nuclear architecture, gene expression, and epigenetic modifications in a native tissue context.

Chromosome structural change has long been considered important in the evolution of post-zygotic reproductive isolation. The premise that karyotypic variation can serve as a possible barrier to gene flow is founded on the expectation that heterozygotes for structurally distinct chromosomal forms would be partially sterile (negatively heterotic) or show reduced recombination. We report the outcome of a detailed comparative molecular cytogenetic study of three antelope species, genus Raphicerus, that have undergone a rapid radiation. The species are largely conserved with respect to their euchromatic regions but the X chromosomes, in marked contrast, show distinct patterns of heterochromatic amplification and localization of repeats that have occurred independently in each lineage. We argue a novel hypothesis that postulates that the expansion of heterochromatic blocks in the homogametic sex can, with certain conditions, contribute to post-zygotic isolation. i.e., female hybrid incompatibility, the converse of Haldane's rule. This is based on the expectation that hybrids incur a selective disadvantage due to impaired meiosis resulting from the meiotic checkpoint network's surveillance of the asymmetric expansions of heterochromatic blocks in the homogametic sex. Asynapsis of these heterochromatic regions would result in meiotic silencing of unsynapsed chromatin and, if this persists, germline apoptosis and female infertility.

The genus Psammolestes within the subfamily Triatominae and tribe Rhodniini comprises the species Psammolestes arthuri , Psammolestes coreodes , and Psammolestes tertius , all potential vectors of Chagas disease. A feature of Psammolestes is their close association with birds, which makes them an interesting model for evolutionary studies. We analyzed cytogenetically Psammolestes spp., with the aim of contributing to the genetic and evolutionary knowledge of these vectors. All species of the Psammolestes showed the same chromosomal characteristics: chromocenter formed only by sex chromosomes X and Y, karyotype 2 n = 22 and constitutive heterochromatin, and AT base pairs restricted to the sex chromosome Y. These results corroborate the monophyly of the genus and lead to the hypothesis that during the derivation of P. tertius , P. coreodes , and P. arthuri from their common ancestor, there was no reorganization in the number or structure of chromosomes.

Heteromorphic sex chromosomes are common in eukaryotes and largely ubiquitous in birds and mammals. The largest number of multiple sex chromosomes in vertebrates known today is found in the monotreme platypus (Ornithorhynchus anatinus, 2n = 52) which exhibits precisely 10 sex chromosomes. Interestingly, fish, amphibians, and reptiles have sex determination mechanisms that do or do not involve morphologically differentiated sex chromosomes. Relatively few amphibian species carry heteromorphic sex chromosomes, and when present, they are frequently represented by only one pair, either XX:XY or ZZ:ZW types. Here, in contrast, with several evidences, from classical and molecular cytogenetic analyses, we found 12 sex chromosomes in a Brazilian population of the smoky jungle frog, designated as Leptodactylus pentadactylus Laurenti, 1768 (Leptodactylinae), which has a karyotype with 2n = 22 chromosomes. Males exhibited an astonishing stable ring-shaped meiotic chain composed of six X and six Y chromosomes. The number of sex chromosomes is larger than the number of autosomes found, and these data represent the largest number of multiple sex chromosomes ever found among vertebrate species. Additionally, sequence and karyotype variation data suggest that this species may represent a complex of species, in which the chromosomal rearrangements may possibly have played an important role in the evolution process.

X inactivation is a fundamental mechanism in eutherian mammals to restore a balance of X-linked gene products between XY males and XX females. However, it has never been extensively studied in a eutherian species with a sex determination system that deviates from the ubiquitous XX/XY. In this study, we explore the X inactivation process in the African pygmy mouse Mus minutoides, that harbours a polygenic sex determination with three sex chromosomes: Y, X, and a feminizing mutant X, named X*; females can thus be XX, XX*, or X*Y, and all males are XY. Using immunofluorescence, we investigated histone modification patterns between the two X chromosome types. We found that the X and X* chromosomes are randomly inactivated in XX* females, while no histone modifications were detected in X*Y females. Furthermore, in M. minutoides, X and X* chromosomes are fused to different autosomes, and we were able to show that the X inactivation never spreads into the autosomal segments. Evaluation of X inactivation by immunofluorescence is an excellent quantitative procedure, but it is only applicable when there is a structural difference between the two chromosomes that allows them to be distinguished.

A striking difference between male and female nuclei was recognized early on by the presence of a condensed chromatin body only in female cells. Mary Lyon proposed that X inactivation or silencing of one X chromosome at random in females caused this structural difference. Subsequent studies have shown that the inactive X chromosome (Xi) does indeed have a very distinctive structure compared to its active counterpart and all autosomes in female mammals. In this review, we will recap the discovery of this fascinating biological phenomenon and seminal studies in the field. We will summarize imaging studies using traditional microscopy and super-resolution technology, which revealed uneven compaction of the Xi. We will then discuss recent findings based on high-throughput sequencing techniques, which uncovered the distinct three-dimensional bipartite configuration of the Xi and the role of specific long non-coding RNAs in eliciting and maintaining this structure. The relative position of specific genomic elements, including genes that escape X inactivation, repeat elements and chromatin features, will be reviewed. Finally, we will discuss the position of the Xi, either near the nuclear periphery or the nucleolus, and the elements implicated in this positioning.This article is part of the themed issue 'X-chromosome inactivation: a tribute to Mary Lyon'., (© 2017 The Authors.)

The heteromorphic X and Y chromosomes behave in a special way in mammalian spermatocytes; they form the XY body and synapse only partially. The aim of this article was to study the origin and the role of the special differentiations in the XY pair of the domestic cat during pachytene by analyzing its fine structural characteristics and the immunolocalization of the main meiotic proteins SYCP3, SYCP1, SYCE3, SMC3, γ-H2AX, BRCA1, H3K27me3, and MLH1. The cat XY body shows particularly striking structures: an extreme degree of axial fibrillation in late pachynema and a special location of SYCP3-containing fibrils, bridging different regions of the main X axis, as well as one bridge at the inner end of the pairing region that colocalizes with the single mandatory MLH1 focus. There are sequential changes, first bullous expansions, then subdivision into fibrils, all involving axial thickening. The chromatin of the XY body presents the usual features of meiotic sex chromosome inactivation. An analysis of the XY body of many eutherians and metatherians suggests that axial thickenings are primitive features. The sequential changes in the mass and location of SYCP3-containing fibers vary among the clades because of specific processes of axial assembly/disassembly occurring in different species., (© 2017 S. Karger AG, Basel.)

We show that the geophilomorph centipede Strigamia maritima possesses an XX/XY system of sex chromosomes, with males being the heterogametic sex. This is, to our knowledge, the first report of sex chromosomes in any geophilomorph centipede. Using the recently assembled Strigamia genome sequence, we identified a set of scaffolds differentially represented in male and female DNA sequence. Using quantitative real-time PCR, we confirmed that three candidate X chromosome-derived scaffolds are present at approximately twice the copy number in females as in males. Furthermore, we confirmed that six candidate Y chromosome-derived scaffolds contain male-specific sequences. Finally, using this molecular information, we designed an X chromosome-specific DNA probe and performed fluorescent in situ hybridization against mitotic and meiotic chromosome spreads to identify the Strigamia XY sex-chromosome pair cytologically. We found that the X and Y chromosomes are recognizably different in size during the early pachytene stage of meiosis, and exhibit incomplete and delayed pairing.

Trisomy, the presence of a third copy of one chromosome, is deleterious and results in inviable or defective progeny if passed through the germ line. Random segregation of an extra chromosome is predicted to result in a high frequency of trisomic offspring from a trisomic parent. Caenorhabditis elegans with trisomy of the X chromosome, however, have far fewer trisomic offspring than expected. We found that the extra X chromosome was preferentially eliminated during anaphase I of female meiosis. We utilized a mutant with a specific defect in pairing of the X chromosome as a model to investigate the apparent bias against univalent inheritance. First, univalents lagged during anaphase I and their movement was biased toward the cortex and future polar body. Second, late-lagging univalents were frequently captured by the ingressing polar body contractile ring. The asymmetry of female meiosis can thus partially correct pre-existing trisomy.

In mammals, the process of X-chromosome inactivation ensures equivalent levels of X-linked gene expression between males and females through the silencing of one of the two X chromosomes in female cells. The process is established early in development and is initiated by a unique locus, which produces a long noncoding RNA, Xist. The Xist transcript triggers gene silencing in cis by coating the future inactive X chromosome. It also induces a cascade of chromatin changes, including posttranslational histone modifications and DNA methylation, and leads to the stable repression of all X-linked genes throughout development and adult life. We review here recent progress in our understanding of the molecular mechanisms involved in the initiation of Xist expression, the propagation of the Xist RNA along the chromosome, and the cis-elements and trans-acting factors involved in the maintenance of the repressed state. We also describe the diverse strategies used by nonplacental mammals for X-chromosome dosage compensation and highlight the common features and differences between eutherians and metatherians, in particular regarding the involvement of long noncoding RNAs.

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.

Many large noncoding RNAs (lncRNAs) regulate chromatin, but the mechanisms by which they localize to genomic targets remain unexplored. We investigated the localization mechanisms of the Xist lncRNA during X-chromosome inactivation (XCI), a paradigm of lncRNA-mediated chromatin regulation. During the maintenance of XCI, Xist binds broadly across the X chromosome. During initiation of XCI, Xist initially transfers to distal regions across the X chromosome that are not defined by specific sequences. Instead, Xist identifies these regions by exploiting the three-dimensional conformation of the X chromosome. Xist requires its silencing domain to spread across actively transcribed regions and thereby access the entire chromosome. These findings suggest a model in which Xist coats the X chromosome by searching in three dimensions, modifying chromosome structure, and spreading to newly accessible locations.

In many eutherian mammals, X-Y chromosome pairing and recombination is required for meiotic progression and correct sex chromosome disjunction. Arvicoline rodents present a notable exception to this meiotic rule, with multiple species possessing asynaptic sex chromosomes. Most asynaptic vole species belong to the genus Microtus sensu lato. However, many of the species both inside and outside the genus Microtus display normal X-Y synapsis at meiosis. These observations suggest that the synaptic condition was present in the common ancestor of all voles, but gaps in current taxonomic sampling across the arvicoline phylogeny prevent identification of the lineage(s) along which the asynaptic state arose. In this study, we use electron and immunofluorescent microscopy to assess heterogametic sex chromosome pairing in 12 additional arvicoline species. Our sample includes ten species of the tribe Microtini and two species of the tribe Lagurini. This increased breadth of sampling allowed us to identify asynaptic species in each major Microtine lineage. Evidently, the ability of the sex chromosomes to pair and recombine in male meiosis has been independently lost at least three times during the evolution of Microtine rodents. These results suggest a lack of evolutionary constraint on X-Y synapsis in Microtini, hinting at the presence of alternative molecular mechanisms for sex chromosome segregation in this large mammalian tribe.

Three xenarthrans species Chaetophractus villosus, Chaetophractus vellerosus, and Zaedyus pichiy have been used for the analysis of the structure, behavior, and immunochemical features of the XY body during pachytene. In all these species, the sex chromosomes form an XY body easily identifiable in thin sections by the special and regular packing of the chromatin fibers of the internal region of the XY body ("differential" regions) and those of the peripheral region (synaptic region). Spermatocyte spreads show a complete synapsis between the X- and the Y-axis, which lasts up to the end of pachytene. From the early pachytene substages to the late ones, the X-axis develops prominent branches, which in late pachytene span the synaptic region. Synapsis is regular as shown by SYCP1 labeling. Axial development is followed by SYCP3 labeling and in the asynaptic region of the X-axis by BRCA1. Gamma-H2AX labels exclusively the differential (asynaptic) region of the X chromosome. A single focus is labeled by MLH1 in the synaptic region. The location of this MLH1 focus spans from 0.3 to 1.6 μm from the telomere in the analyzed xenarthrans, covering approximately half of the Y-axis length. It is concluded that xenarthrans, as basal placental mammals, harbor the largest pseudoautosomal regions of presently analyzed mammals, and shows the typical features of meiotic sex chromosome inactivation (MSCI).

Fissioncytorace-1, a member of the nasuta-albomicans complex of Drosophila is an evolutionary product of centric fission, which had occurred in the chromosome X3 of Cytorace 1, a hydridization product of Drosophila nasuta nasuta male (2n=8) and Drosophila nasuta albomicans female (2n=6). Cytorace 1 (males 2n=7; females 2n=6) has inherited this chromosome from its D. n. albomicans parent. The chromosome X3 of D. n. albomicans is a derivative of a centric fusion between the acrocentric chromosome 3 and the chromosome X of D. n. nasuta. The Fissioncytorace-1 has crossed 200 generations from the time of its evolution in the laboratory environment. When this centromeric fission race was subjected to some of the morphophenotypic and fitness assessment to find its overall population fitness showed, increased body size, sternopleural bristle, ovarioles, lifetime fecundity and fertility with reduced interspecific competitive ability and hatching success when compared with its parent (Cytorace 1). These results suggest that the hybrid races must have encountered an early event of recombinational raciation during their evolution in the laboratory environment, which is a unique observation in animal system illustrating the increase in the tempo of evolution after the event of hybridization.

Karyotypes can be difficult to create due to odd chromosome behaviors and high levels of adhesion between sex chromosomes. We have used live-cell imaging and micromanipulation to determine precisely the karyotype of the orb-weaving spider Neoscona arabesca (Walckenaer). We have found that N. arabesca has a sex determination mechanism of XO (male)-XX (female) with 2n = 20 autosomes. Staining of female tissues revealed a chromosome number of 22 in mitotic cells.

The effect of ectopic expression of male specific lethal 2 (msl2) on chromatin modification and gene expression was studied in Drosophila diploid females and metafemales (3X;2A). Results show that ectopic expression of MSL2 in transgenic msl2 females and metafemales sequesters the MOF histone acetylase to the X, which occurs concordantly with an increase of histone acetylation. Gene expression studies indicate that the X-linked genes are not affected by direct targeting of the MSL complex and the resulting increased H4Lys16 acetylation on the X chromosomes, suggesting one function of the MSL complex is to nullify the effect of a high level of histone acetylation. These results are not consistent with the hypothesis that the presence of the MSL complex conditions a two-fold upregulation. Autosomal gene expression is generally decreased in ectopically expressed MSL2 females, which correlates with the reduced autosomal histone acetylation. Metafemales show dosage compensation of X-linked genes with some autosomal reductions in expression. Interestingly, in metafemales with ectopically expressed MSL2, the autosomal expression is returned to a more normal level. There is a lower autosomal level of histone acetylation compared to the normal metafemales, suggesting a nullifying effect on the negative dosage effect of the X chromosome as previously hypothesized to occur in normal males., ((c) 2009 S. Karger AG, Basel.)

Acquisition of knowledge of the nature and DNA content of B chromosomes has been triggered by a collection of molecular techniques, one of which, microdissection, has provided interesting results in a number of B chromosome systems. Here we provide the first data on the molecular composition of B chromosomes in Locusta migratoria, after microdissection of the B and X chromosomes, DNA amplification by one (B) or two (X) different methods, and chromosome painting. The results showed that B chromosomes share at least two types of repetitive DNA sequences with the A chromosomes, suggesting that Bs in this species most likely arose intraspecifically. One of these repetitive DNAs is located on the heterochromatic distal half of the B chromosome and in the pericentromeric regions of about half of the A chromosomes, including the X. The other type of repetitive DNA is located interspersedly over the non-centromeric euchromatic regions of all A chromosomes and in an interstitial part of the proximal euchromatic half of the B chromosome. Chromosome painting, however, did not provide results sufficiently reliable to determine, in this species, which A chromosome gave rise to the B; this might be done by detailed analysis of the microdissected DNA sequences.

The 3D-FISH was employed to investigate the telomere topology in polytene nuclei of salivary glands of Drosophila melanogaster. The majorities of telomeres in polytene nuclei of salivary glands in Drosophila strain y(2-717) are localized in the nuclear central area and have no contacts with nuclear membrane. In females of this strain, ectopic contacts between telomeres occur at 25 % higher frequency than in males. HeT-A DNA in y(2-717alk3-2) strain, which is a derivative of y(2-717) carrying an inversion between 1D and 13C bands, is found in region 13 of X chromosome. The frequency of ectopic contacts of telomeres in y(2-717alk3-2) males is 10 % higher than that in y(2-717) strain. The number of ectopic contacts can be significantly different in independent experiments, possibly indicating the role of random factors in the contact formation.

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.

A study of the variation in pattern and frequency of constitutive heterochromatin and nucleolar organizing regions of the X chromosomes of male Chorthippus parallelus grasshoppers in 25 populations within the Iberian peninsula requires us to revise our interpretation of the biogeography and evolutionary history of this species. Hybridization between the subspecies Cp erythropus and Cp parallelus, previously only known from populations in the Pyrenean cols, is shown to extend at least 400 km further into north-west Spain. A novel X-chromosome variant is described that appears to be close to fixation in 18 populations, mainly from the centre and south of Spain. Our findings indicate a possible independent origin for each of three distinct, nonderivative X variants present in Spain: the northern Cp erythropus and Cp parallelus variants, and a central-southern Cp erythropus variant. The first two are distinguished by interstitial and distal C bands, respectively, whereas the central-southern form has neither. This central-southern form is probably the current representative of the ancestral Iberian X variant. The pattern of variation supports the hypotheses of multiple refugia for Iberian populations and that more hybrid zones exist between these chromosomal variants.

The genus Silene is a good model for studying evolution of the sex chromosomes, since it includes species that are hermaphroditic and dioecious, while maintain a basic chromosome number of 2n = 24. For some combinations of Silene species it is possible to construct interspecific hybrids. Here, we present a detailed karyological analysis of a hybrid between the dioecious Silene latifolia as the maternal plant and a related species, hermaphroditic Silene viscosa, used as a pollen partner. Using genomic probes (the genomic in situ hybridization (GISH) technique), we were able to clearly discriminate parental genomes and to show that they are largely separated in distinct nuclear domains. Molecular GISH and fluorescence in situ hybridization (FISH) markers document that the hybrid genome of somatic cells was strictly additive and stable, and that it had 12 chromosomes originating from each parent, including the only X chromosome of S. latifolia. Meiotic analysis revealed that, although related, respective parental chromosomes did not pair or paired only partially, which resulted in frequent chromosome abnormalities such as bridges and irregular non-disjunctions. GISH and FISH markers clearly document that the larger genome of S. latifolia and its largest chromosome component, the X chromosome, were mostly employed in chromosome lagging and misdivision.

Spermatocytes from the two armadillo species, C. villosus and D. hybridus were studied in microspreads for synaptonemal complexes (SCs) and in thin sections for electron microscopy (EM). The complete SC karyotype generally agrees with previous reports on mitotic chromosomes, except for the sex chromosomes. The X chromosome is submetacentric in both species and the Y is the shortest one in C. villosus and the second shortest in D. hybridus, and an extremely acrocentric one. A SC is formed along the total length of the Y chromosome, and this SC persists along all the pachytene substages. A single recombination nodule (RN) is located in the region of the SC nearest to the attachment to the nuclear envelope. The lateral element (LE) of the X axis in the SC shows a wavy aspect in most of the SC length distant from the nuclear envelope. Nucleoli are attached to acrocentric or submetacentric bivalents, are visibly double in some cells, and in thin sections show an elaborate nucleolonema. Some differences in the XY are species-specific, as the higher degree of tangling and stronger heteropycnosis in D. hybridus. The effective, single crossover of the XY pair is highly localized, despite the permanence of a long tract of SC.

The chromosomes derived from the Japanese population of Gryllus bimaculatus were characterized by C-banding and Ag-NOR staining. The chromosome number, 2n = 28 + XX (female)/XO (male), corresponded with that of other populations of G. bimaculatus, but the chromosome configuration in idiograms varied between the populations. NORs were carried on one pair of autosomes and appeared polymorphous. The positive C-bands located at the centromere of all chromosomes and the distal regions of many chromosome pairs, and the size and the distribution pattern of the distal C-heterochromatin showed differences among the chromosomes. In addition, this paper reports on the characteristics of HindIII satellite DNA isolated from the genome of G. bimaculatus. The HindIII repetitive fragments were about 0.54 kb long, and localized at the distal C-bands of the autosomes and the interstitial C-bands of the X chromosome. Molecular analysis showed two distinct satellite DNA sequences, named the GBH535 and GBH542 families, with high AT contents of about 67 and 66%, respectively. The two repetitive families seem to be derived from a common ancestral sequence, and both families possessed the same 13-bp palindrome sequence. The results of Southern blot hybridization suggest that the sequence of the GBH535 family is conserved in the genomic DNAs of Gryllus species, whereas the GBH542 family is a species-specific sequence., (2006 S. Karger AG, Basel.)

The chromosomes and banding patterns of Steindachneridion sp., a large catfish (Pimelodidae), endemic to the Iguaçu River, Brazil, were analyzed using conventional (C-, G-banding) and restriction enzyme banding methods. The same diploid number (2n = 56) as in other members of the genus and the family was found but the karyotype displayed an XX/XY sex chromosome system. The X chromosome was the smallest submetacentric, while the Y was the largest chromosome in the karyotype. Meiotic analysis showed 27 autosomal bivalents plus one heteromorphic XY bivalent during spermatogenesis. Sex chromosomes had no particular pattern after C-banding but G- and restriction enzyme bandings showed specific banding characteristics. The present finding represents the first report of a well-differentiated and uncommon sex chromosome system in the catfish family Pimelodidae., (2006 S. Karger AG, Basel.)

Cytogenetic investigations of a phenotypically normal Mehsana river buffalo calf (Bubalusbubalis) revealed an XXY chromosome complement due to X;X-translocation in all screened metaphase plates. The chromosomal anomaly was identified by GTG-banding while CBG- and RBG-banding revealed two heterochromatin blocks and that one of the two X chromosomes was late replicating, respectively. The normal cytogenetic profiles of sire, dam and relatives of the calf suggest that the anomaly could have arisen spontaneously during oogenesis. This is the first report on a male river buffalo calf having an XXY chromosome complement with translocation between the two X chromosomes., (Copyright 2006 S. Karger AG, Basel.)

Inactivation of the X chromosome occurs in female somatic cells and in male meiosis. In both cases, the inactive X chromosome undergoes changes in histone modifications including deacetylation of core histone proteins and enrichment with histone H3 lysine 9 (H3-K9) dimethylation. In this study we show that while the inactive X in female somatic cells is largely devoid of H3-K4 dimethylation, the inactive X in male meiosis is enriched with this modification. However, the inactive X chromosome in female somatic cells and the inactive X and Y in male meiosis are devoid of H3-K4 trimethylation. Further, trimethylation of H3-K4 is present at discrete regions along most of the autosomes, while H3-K4 dimethylation shows a more homogenous staining. Also, the Y chromosome is largely devoid of H3-K4 di- and trimethylation in somatic cells of both humans and mice, however, the Y chromosome is enriched with H3-K4 di- but not trimethylation throughout spermatogenesis. Our results provide insights into the differences between female somatic cells and male germ cells in inactivating the X chromosome, and suggest that trimethylation, and not dimethylation, of H3-K4 is a more robust indicator of the active regions of the genome., (Copyright 2006 S. Karger AG, Basel.)

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.

We report molecular genetic studies of three genes involved in early germ-line proliferation in Caenorhabditis elegans that lend unexpected insight into a germ-line/soma functional separation of autosomal/X-linked duplicated gene pairs. In a genetic screen for germ-line proliferation-defective mutants, we identified mutations in rpl-11.1 (L11 protein of the large ribosomal subunit), pab-1 [a poly(A)-binding protein], and glp-3/eft-3 (an elongation factor 1-alpha homolog). All three are members of autosome/X gene pairs. Consistent with a germ-line-restricted function of rpl-11.1 and pab-1, mutations in these genes extend life span and cause gigantism. We further examined the RNAi phenotypes of the three sets of rpl genes (rpl-11, rpl-24, and rpl-25) and found that for the two rpl genes with autosomal/X-linked pairs (rpl-11 and rpl-25), zygotic germ-line function is carried by the autosomal copy. Available RNAi results for highly conserved autosomal/X-linked gene pairs suggest that other duplicated genes may follow a similar trend. The three rpl and the pab-1/2 duplications predate the divergence between C. elegans and C. briggsae, while the eft-3/4 duplication appears to have occurred in the lineage to C. elegans after it diverged from C. briggsae. The duplicated C. briggsae orthologs of the three C. elegans autosomal/X-linked gene pairs also display functional differences between paralogs. We present hypotheses for evolutionary mechanisms that may underlie germ-line/soma subfunctionalization of duplicated genes, taking into account the role of X chromosome silencing in the germ line and analogous mammalian phenomena.

Many eukaryotic taxa inherit a heteromorphic sex chromosome pair. It is a generally accepted hypothesis that the sex chromosome pair is derived from a pair of homologous autosomes that has developed after the occurrence of a sex differentiator in an evolutionary process into two structurally and functionally different partners. In most of the analyzed systems the occurrence of the dominant sex differentiator is paralleled by the suppression of recombination within and close by that region. The recombinational isolation can spread in an evolutionary selection process from neighboring regions finally over the whole chromosome. Suppression of recombination strongly biases the distribution of retrotransposons in the genome. Our results and that from others indicate that the major force driving the evolution of Y chromosomes are retrotransposons, remodeling euchromatic chromosome structures into heterochromatic ones. In our model, intact or already eroded retrotransposons become trapped due to their inherent transposition mechanisms in non-recombining regions. The massive accumulation of retrotransposons interferes strongly with the activity of genes. We hypothesize that Y chromosome degeneration is a stepwise evolutionary process: (1) Massive accumulation of retrotransposons occurs in the non-recombining regions. (2) Heterochromatic nucleation centers are formed as a consequence of genomic defense against invasive parasitic elements; the established nucleation centers become epigenetically inherited. (3) Spreading of heterochromatin from the nucleation centers into flanking regions induces in an adaptive process gene silencing of neighbored genes that could either be still intact or in an already eroded condition, e.g., showing point mutations, deletions, insertions; the retroelements should be subjects to the same forces of deterioration as the genes themselves. (4) Constitutive silenced genes are not committed to the same genetic selection pressure as active genes and therefore more exposed to the decay process. (5) Gene dosage balance is reestablished by the parallel evolution of dosage compensation mechanisms. The evolving secondary sex chromosomes, neo-X and neo-Y, of Drosophila miranda are revealed to be a unique and potent model system to catch the evolutionary Y deterioration process in progress.

To help understand the evolution of suppressed recombination between sex chromosomes, and its consequences for evolution of the sequences of Y-linked genes, we have studied four X-Y gene pairs, including one gene not previously characterized, in plants in a group of closely related dioecious species of Silene which have an X-Y sex-determining system (S. latifolia, S. dioica, and S. diclinis). We used the X-linked copies to build a genetic map of the X chromosomes, with a marker in the pseudoautosomal region (PAR) to orient the map. The map covers a large part of the X chromosomes--at least 50 centimorgans. Except for a recent rearrangement in S. dioica, the gene order is the same in the X chromosomes of all three species. Silent site divergence between the DNA sequences of the X and Y copies of the different genes increases with the genes' distances from the PAR, suggesting progressive restriction of recombination between the X and Y chromosomes. This was confirmed by phylogenetic analyses of the four genes, which also revealed that the least-diverged X-Y pair could have ceased recombining independently in the dioecious species after their split. Analysis of amino acid replacements vs. synonymous changes showed that, with one possible exception, the Y-linked copies appear to be functional in all three species, but there are nevertheless some signs of degenerative processes affecting the genes that have been Y-linked for the longest times. Although the X-Y system evolved quite recently in Silene (less than 10 million years ago) compared to mammals (about 320 million years ago), our results suggest that similar processes have been at work in the evolution of sex chromosomes in plants and mammals, and shed some light on the molecular mechanisms suppressing recombination between X and Y chromosomes.

Spermidine is essential for viability in eukaryotes but the importance of the longer polyamine spermine has not been established. Spermine is formed from spermidine by the action of spermine synthase, an aminopropyltransferase, whose gene (SpmS) is located on the X chromosome. Deletion of part of the X chromosome that include SpmS in Gy mice leads to a striking phenotype in affected males that includes altered phosphate metabolism and symptoms of hypophosphatemic rickets, circling behavior, hyperactivity, head shaking, inner ear abnormalities, deafness, sterility, a profound postnatal growth retardation, and a propensity to sudden death. It was not clear to what extent these alterations were due to the loss of spermine synthase activity, since this chromosomal deletion extends well beyond the SpmS gene and includes at least one other gene termed Phex. We have bred the Gy carrier female mice with transgenic mice (CAG/SpmS mice) that express spermine synthase from the ubiquitous CAG promoter. The resulting Gy-CAG/SpmS mice had extremely high levels of spermine synthase and contained spermine in all tissues examined. These mice had a normal life span and fertility and a normal growth rate except for a reduction in body weight due to a loss of bone mass that was consistent with the observation that the derangement in phosphate metabolism is due to the loss of the Phex gene and was not restored. These results show that spermine synthesis is needed for normal growth, viability, and fertility in male mice and that regulation of spermine synthase content is not required.

The Su(var)3-7 protein is essential for fly viability, and several lines of evidence support its key importance in heterochromatin formation: it binds to pericentric heterochromatin, it potently suppresses variegation and it interacts with HP1. However, the mode of action of Su(var)3-7 is poorly understood. Here we investigate in vivo the consequences of increased Su(var)3-7 expression on fly viability and chromatin structure. A large excess of Su(var)3-7 induces lethality, whereas lower doses permit survival and cause spectacular changes in the morphology of polytene chromosomes in males, and to a lesser extent in females. The male X is always the most affected chromosome: it becomes highly condensed and shortened, and its characteristic banding pattern is modified. In addition, Su(var)3-7 was found over the complete length of all chromosomes. This event coincides with the appearance of heterochromatin markers such as histone H3K9 dimethylation and HP1 at many sites on autosomes and, more strikingly, on the male X chromosome. These two features are strictly dependent on the histone-methyltransferase Su(var)3-9, whereas the generalised localisation of Su(var)3-7 is not. These data provide evidence for a dose-dependent regulatory role of Su(var)3-7 in chromosome morphology and heterochromatin formation. Moreover they show that Su(var)3-7 expression is sufficient to induce Su(var)3-9-dependent ectopic heterochromatinisation and suggest a functional link between Su(var)3-7 and the histone-methyltransferase Su(var)3-9.

It has been proposed that dosage compensation in Drosophila males occurs by binding of two core proteins, MSL-1 and MSL-2, to a set of 35-40 X chromosome "entry sites" that serve to nucleate mature complexes, termed compensasomes, which then spread to neighboring sequences to double expression of most X-linked genes. Here we show that any piece of the X chromosome with which compensasomes are associated in wild-type displays a normal pattern of compensasome binding when inserted into an autosome, independently of the presence of an entry site. Furthermore, in chromosomal rearrangements in which a piece of X chromosome is inserted into an autosome, or a piece of autosome is translocated to the X chromosome, we do not observe spreading of compensasomes to regions of autosomes that have been juxtaposed to X chromosomal material. Taken together these results suggest that spreading is not involved in dosage compensation and that nothing distinguishes an entry site from the other X chromosome sites occupied by compensasomes beyond their relative affinities for compensasomes. We propose a new model in which the distribution of compensasomes along the X chromosome is achieved according to the hierarchical affinities of individual binding sites., Competing Interests: The authors have declared that no conflicts of interest exist.

Three new female ES cell lines (GLM1, GLP1 and GLP2) were established from mouse embryos carrying GFP (green fluorescent protein) and HMG-lacZ transgenes on one of two X chromosomes in cis. Using these cell lines, we studied the temporal relationships among three events relevant to X-chromosome inactivation: replication asynchrony of the X chromosome, and quenching of GFP fluorescence and beta-galactosidase (beta-gal) activity, during cell differentiation induced by embryoid body (EB) formation and retinoic acid (RA) treatment. In embryoid bodies adhering to the bottom of culture dishes, GFP-negative cells appeared first in the peripheral outgrowths 4 days after the initiation of EB formation, followed about 24 hours later by the appearance of cells negative for beta-gal and those having a single allocyclic X chromosome. Although the frequency of cells with an allocyclic X chromosome could reach 80% in adherent embryoid bodies, it tended to remain low and variable in embryoid bodies maintained in suspension. In spite of apparently parallel extinction of GFP and lacZ in embryoid bodies, their concurrent occurrence did not always characterize RA-induced differentiation. Moreover, an allocyclic X chromosome was identified in not more than 20 percent of informative metaphase cells up to 10 days after initiation of RA treatment. These findings suggest that RA-induced differentiation of female ES cells does not always accompany X-inactivation.

The hypoxanthine phosphoribosyltransferase (Hprt) locus has been shown to have minimal influence on transgene expression when used as a surrogate site in the mouse genome. We have developed a method to transfer bacterial artificial chromosomes (BACs) as a single copy into the partially deleted Hprt locus of embryonic stem cells. BACs were modified by Cre/loxP recombination to contain the sequences necessary for homologous recombination into and complementation of the partially deleted Hprt locus. Modified BACs were shown to undergo homologous recombination into the genome intact, to be stably transmitted through the germ line of transgenic mice, and to be expressed in the proper tissue-specific manner. This technology will facilitate many studies in which correct interpretation of data depends on developmentally appropriate transgene expression in the absence of rearrangements or deletions of endogenous DNA.

Sex chromosomes in species of the genus Microtus present some characteristic features that make them a very interesting group to study sex chromosome composition and evolution. M. cabrerae and M. agrestis have enlarged sex chromosomes (known as 'giant sex chromosomes') due to the presence of large heterochromatic blocks. By chromosome microdissection, we have generated probes from the X chromosome of both species and hybridized on chromosomes from six Microtus and one Arvicola species. Our results demonstrated that euchromatic regions of X chromosomes in Microtus are highly conserved, as occurs in other mammalian groups. The sex chromosomes heterochromatic blocks are probably originated by fast amplification of different sequences, each with an independent origin and evolution in each species. For this reason, the sex heterochromatin in Microtus species is highly heterogeneous within species (with different composition for the Y and X heterochromatic regions in M. cabrerae) and between species (as the composition of M. agrestis and M. cabrerae sex heterochromatin is different). In addition, the X chromosome painting results on autosomes of several species suggest that, during karyotypic evolution of the genus Microtus, some rearrangements have probably occurred between sex chromosomes and autosomes.

SORB (selected observed residual breakpoints) induced by ionizing radiation or endonucleases are often non-randomly distributed in mammalian chromosomes. However, the role played by chromatin structure in the localization of chromosome SORB is not well understood. Anti-topoisomerase drugs such as etoposide are potent clastogens and unlike endonucleases or ionizing radiation, induce DNA double-strand breaks (DSB) by an indirect mechanism. Topoisomerase II (Topo II) is a main component of the nuclear matrix and the chromosome scaffold. Since etoposide leads to DSB by influencing the activity of Topo II, this compound may be a useful tool to study the influence of the chromatin organization on the distribution of induced SORB in mammalian chromosomes. In the present work, we compared the distribution of SORB induced during S-phase by etoposide or X-rays in the short euchromatic and long heterochromatic arms of the CHO9 X chromosome. The S-phase stage (early, mid or late) at which CHO9 cells were exposed to etoposide or X-rays was marked by incorporation of BrdU during treatments and later determined by immunolabeling of metaphase chromosomes with an anti-BrdU FITC-coupled antibody. The majority of treated cells were in late S-phase during treatment either with etoposide or X-rays. SORB induced by etoposide mapped preferentially to Xq but random localization was observed for SORB produced by X-rays. Possible explanations for the uneven distribution of etoposide-induced breakpoints along Xq are discussed., (Copyright 2003 S. Karger AG, Basel)

Acetic orcein staining of polytene chromosomes was introduced in 1941 shortly after the initial studies on aceto-carmine-stained chromosomes by Bridges (2) and has remained a standard method of preparation. Orcein dye can be purchased in both its natural form as extracted from two species of lichens, Rocella tinctoria and Lecanora parella, and a synthetic form. The mechanism of staining is not clearly understood because the stain itself is a variety of phenazones, which may interact at an acid pH with negatively charged groups or possibly interact hydrophobically with chromatin. Acetic acid fixation accommodates stretching of the chromosomes in the interband regions during a squash, thus providing for a higher resolution of the banding structure. The later addition of lactic acid to aceto-orcein (3) kept the glands softer in the fix and allowed for easier spreading of chromosomes. The method and its variations have appeared more recently in several publications (4,5). Drosophila polytene chromosomes are found in a number of larval tissues, including the midgut, hindgut, and the fat body, but the largest chromosomes are found in the salivary glands of the third instar. They are referred to as interphase chromosomes and are structurally more comparable to highly amplified interphase chromatin than to mitotic chromosomes because the gland grows by endoreplication of DNA, thus increasing cell size rather than cell number. Each of the homologs is tightly synapsed in this somatic tissue and undergoes approx 10 rounds of endoreplication, producing 1024 chromatids closely associated in parallel arrays.

ATM/ATR kinases act as signal transducers in eukaryotic DNA damage and replication checkpoints. Mutations in ATM/ATR homologs have pleiotropic effects that range from sterility to increased killing by genotoxins in humans, mice, and Drosophila. Here we report the generation of a null allele of mei-41, Drosophila ATM/ATR homolog, and the use of it to document a semidominant effect on a larval mitotic checkpoint and methyl methanesulfonate (MMS) sensitivity. We also tested the role of mei-41 in a recently characterized checkpoint that delays metaphase/anaphase transition after DNA damage in cellular embryos. We then compare five existing mei-41 alleles to the null with respect to known phenotypes (female sterility, cell cycle checkpoints, and MMS resistance). We find that not all phenotypes are affected equally by each allele, i.e., the functions of MEI-41 in ensuring fertility, cell cycle regulation, and resistance to genotoxins are genetically separable. We propose that MEI-41 acts not in a single rigid signal transduction pathway, but in multiple molecular contexts to carry out its many functions. Sequence analysis identified mutations, which, for most alleles, fall in the poorly characterized region outside the kinase domain; this allowed us to tentatively identify additional functional domains of MEI-41 that could be subjected to future structure-function studies of this key molecule.

Background: Retroviral transduction of human hematopoietic stem cells is an attractive strategy in gene therapy; however, transduction efficiency and duration of transgene expression may not be satisfactory in current protocols. Co-expression of a human multidrug resistance gene (MDR1) with a therapeutic gene affords selectable growth advantage to genetically modified cells., Methods: A bicistronic retrovirus vector, Ha-MDR-IRES-gp91, was constructed for the co-expression of MDR1 and gp91, a gene responsible for X-linked chronic granulomatous disease (X-CGD). Drug-selected co-expression of P-glycoprotein and gp91 was evaluated in transduced cells., Results: Epstein-Barr virus-transformed B cells from X-CGD patients transduced with Ha-MDR-IRES-gp91 co-expressed human P-glycoprotein and gp91, and acquired superoxide-generating activity. Human CD34-positive cells from an X-CGD patient were transduced with Ha-MDR-IRES-gp91 and subsequently treated with 2 ng/ml vincristine. After 13 days, 20% of Ha-MDR-IRES-gp91-transduced cells were P-glycoprotein- and gp91-positive by FACS analysis. The superoxide-generating activity of the transduced population was 27% of that of normal cells. Mice transplanted with Ha-MDR-IRES-gp91-transduced bone marrow cells showed co-expression of P-glycoprotein and gp91 in peripheral blood mononuclear cells. By administering paclitaxel, the proportions of P-glycoprotein- and gp91-positive cells were increased in all the four mice examined. When mice transplanted with Ha-MDR-IRES-gp91-transduced cells were repeatedly administered paclitaxel, the ratios of P-glycoprotein- and gp91-positive cells were maintained for over 1 year., Conclusions: These results suggest that MDR1-bicistronic vectors may be useful to select the transduced hematopoietic cells in vivo. This may lead to the sustained expression of transgenes in the blood cells of patients treated with stem cell gene therapy., (Copyright 2003 John Wiley & Sons, Ltd.)

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.

The intersexual phenotypes of marsupials with XXY and XO chromosome constitutions imply that not all sexual dimorphisms are under the control of testicular hormones and, ultimately, the SRY gene on the Y chromosome. It has been hypothesised that there is a gene on the X chromosome that determines whether either a scrotum will form (one copy of the gene) or a pouch with teats (two copies of the gene). Here, we describe the anatomy and chromosomes of two intersexual dasyurid marsupials. One, a Dasyuroides byrnei, had a pouch, but the reproductive tract was essentially male. The other, a Sminthopsis douglasi, had a hemipouch and a hemiscrotum and the reproductive tract was essentially female. The S. douglasi was a mosaic for cells with an apparently normal 2n = 14, XX female karyotype and cells with 2n = 14 plus (usually) two dot-like supernumerary elements 2n = 14, XX + 2B. The D. byrnei cells examined also had a 2n = 14, XX + 2B karyotype. In fibroblasts from the male and female sides of the S. douglasi, it was possible to assign the 2n = 14, XX karyotype to the male side and the 2n = 14, XX + 2B to the female side.

Artificial life is a tool which is used for simulation of peculiar cases of evolutionary events. The main characteristic of artificial life is that with this technique it is possible to simulate for a high number of generations the evolution of a population of individuals. Each individual is characterised by a small number of parameters, but each individual has its own evolutive story. So far it is possible to simulate the evolution of a population of some thousands specimens, for a high number of generations. The realistic aspect of the simulation is that each specimen is taken individually. In our opinion this instrument is very useful to simulate the evolution of the hybrids barrier during speciation. For this reason it is applied to a peculiar case of speciation, that of the Savi pine vole (Microtus savii) whose experimental data were recently investigated.

More than a century ago, a densely stained area inside the nucleus of male meiotic cells was described. It was later shown to harbor the sex chromosomes which undergo transcriptional inactivation in conjunction with heterochromatinisation and synapsis to form the XY body. Formation of the XY body is conserved throughout the mammalian phylogenetic tree and is thought to be essential for successful spermatogenesis. However, its biological role as well as the molecular mechanisms underlying XY body formation are still far from being understood. A lot of effort has already been undertaken to characterize components of the XY body and to investigate their functional implications in sex chromatin heterochromatinisation and meiotic sex chromosome inactivation (MSCI). This review gives an overview of those components and their possible implications in XY body formation and function., (Copyright 2003 S. Karger AG, Basel)

Using gold labeled antibodies, we developed and tested an immunoelectron microscope (IEM) method for detection of protein localization in Drosophila melanogaster polytene chromosomes. This method is based on procedures widely used for indirect immunofluorescent (IF) staining of salivary gland polytene chromosome squashes. The application of IEM was evaluated by using specific antibodies against proteins earlier localized in both decondensed (interbands and puffs) and compact (bands) regions of polytene chromosomes. In all the experiments, IEM and IF images for homologous chromosome regions were compared. When applied to regions of loose structures, IEM enabled us to localize, with high precision, signals in fine bands, interbands and puffs. There was a good correspondence between immunogold EM and IF data. However, there was no correspondence for dense bands: gold particles were distributed at their boundaries, while the entire bands showed bright fluorescence. This discrepancy probably resulted from a poor penetration of antibodies conjugated to gold particles in the tightly packaged structures. From the results obtained it may by concluded that the IEM method is advantageous for studying the fine protein topography of loose decompacted regions of polytene chromosomes. And this must be taken into consideration when protein localization in polytene chromosomes is performed.

PD strain male rats that carry an autosomal recessive gene, preaxial duplication (gene symbol: pd), are sterile in the homozygous condition (pd/pd) due to a spermatogenic breakdown in the process of spermatogenesis at the spermatocyte and/or spermatid stage(s), although heterozygotes (pd/+) are normal. In this study, pd/pd males were examined for the presence of abnormal association of the sex chromosomes that might lead to spermatogenic breakdown. Light and electron microscopic observations of the chromosomes at meiotic prophase and metaphase in primary spermatocytes revealed several types of abnormal X-Y association and configurations in pd/pd males. However, the incidences of the abnormal configuration were comparable to those in pd/+ males. These results suggest that abnormal X-Y chromosome association in the germ cells is not a significant cause of spermatogenic breakdown in pd/pd males.

Photographic polytene chromosome maps from trichogen cells of pharate adult Glossina morsitans submorsitans were constructed. Using the standard system employed to map polytene chromosomes of Drosophila, the characteristic landmarks were described for the X chromosome and the two autosomes (L1 and L2). Sex-ratio distortion, which is expressed in male G. m. submorsitans, was found to be associated with an X chromosome (X8) that contains three inversions in each arm. Preliminary data indicate no differences in the fecundity of X(A)X(A) and X(A)X(B) females, but there are indications that G. m. submorsitans in colonies originating from Burkina Faso and Nigeria have genes on the autosomes and (or) the Y chromosome that suppress expression of sex-ratio distortion.