Chromosomes In Females Research Articles

EMQN best practice guidelines for the molecular genetic testing and reporting of fragile X syndrome and other fragile X-associated disorders.

Different mutations occurring in the unstable CGG repeat in 5' untranslated region of FMR1 gene are responsible for three fragile X-associated disorders. An expansion of over ∼200 CGG repeats when associated with abnormal methylation and inactivation of the promoter is the mutation termed ‘full mutation' and is responsible for fragile X syndrome (FXS), a neurodevelopmental disorder described as the most common cause of inherited intellectual impairment. The term ‘abnormal methylation' is used here to distinguish the DNA methylation induced by the expanded repeat from the ‘normal methylation' occurring on the inactive X chromosomes in females with normal, premutation, and full mutation alleles. All male and roughly half of the female full mutation carriers have FXS. Another anomaly termed ‘premutation' is characterized by the presence of 55 to ∼200 CGGs without abnormal methylation, and is the cause of two other diseases with incomplete penetrance. One is fragile X-associated primary ovarian insufficiency (FXPOI), which is characterized by a large spectrum of ovarian dysfunction phenotypes and possible early menopause as the end stage. The other is fragile X-associated tremor/ataxia syndrome (FXTAS), which is a late onset neurodegenerative disorder affecting males and females. Because of the particular pattern and transmission of the CGG repeat, appropriate molecular testing and reporting is very important for the optimal genetic counselling in the three fragile X-associated disorders. Here, we describe best practice guidelines for genetic analysis and reporting in FXS, FXPOI, and FXTAS, including carrier and prenatal testing.

Differentially methylated CpG island within human XIST mediates alternative P2 transcription and YY1 binding.

BackgroundX-chromosome inactivation silences one X chromosome in females to achieve dosage compensation with the single X chromosome in males. While most genes are silenced on the inactive X chromosome, the gene for the long non-coding RNA XIST is silenced on the active X chromosome and expressed from the inactive X chromosome with which the XIST RNA associates, triggering silencing of the chromosome. In mouse, an alternative Xist promoter, P2 is also the site of YY1 binding, which has been shown to serve as a tether between the Xist RNA and the DNA of the chromosome. In humans there are many differences from the initial events of mouse Xist activation, including absence of a functional antisense regulator Tsix, and absence of strictly paternal inactivation in extraembryonic tissues, prompting us to examine regulatory regions for the human XIST gene.ResultsWe demonstrate that the female-specific DNase hypersensitivity site within XIST is specific to the inactive X chromosome and correlates with transcription from an internal P2 promoter. P2 is located within a CpG island that is differentially methylated between males and females and overlaps conserved YY1 binding sites that are only bound on the inactive X chromosome where the sites are unmethylated. However, YY1 binding is insufficient to drive P2 expression or establish the DHS, which may require a development-specific factor. Furthermore, reduction of YY1 reduces XIST transcription in addition to causing delocalization of XIST.ConclusionsThe differentially methylated DNase hypersensitive site within XIST marks the location of an alternative promoter, P2, that generates a transcript of unknown function as it lacks the A repeats that are critical for silencing. In addition, this region binds YY1 on the unmethylated inactive X chromosome, and depletion of YY1 untethers the XIST RNA as well as decreasing transcription of XIST.

Variable gene expression in turner syndrome patients with bicuspid aortic valve

Turner syndrome (TS) is caused by the partial or complete loss of the second sex chromosome in females and affects 1 in 2500 live female births. Thirty percent of women with non-mosaic classic TS (45,X) have bicuspid aortic valve (BAV), as compared to 1-2% of individuals in the general population. BAV and aortic coarctation have been associated with loss of the p arm of the X chromosome in TS. Importantly, the presence of cardiac anomalies in TS greatly increases the risk of cardiac related death in women with TS who pursue donor egg pregnancy. Gene expression profiling studies may identify specific genes on the X chromosome associated with BAV, or may highlight ongoing differences in gene transcription related to the presence of BAV. Gene expression profiling analysis. Sixteen women with 45,X detected on peripheral karyotype in 50 cells underwent DNA expression array profiling of peripheral blood lymphocytes using Affymetrix GeneChip Human Genome U133 Plus 2.0 Array as part of the IRB approved NICHD protocol “Turner Syndrome: Genotype and Phenotype.” All women underwent cardiac MRI to accurately access aortic valve anatomy and all 16 had unambiguous cardiac MRI results. Eight women had BAV and eight women had tricuspid aortic valve (TAV). Analysis comparing TS with TAV and TS with BAV gene expression was performed with Partek Genomics Suite, using a 1-way ANOVA test. Comparing TS with BAV and TS with TAV, 2747 genes show a significant difference in expression of 1.2 fold or greater (p<0.05) and 155 genes were differentially expressed with p-value less than 0.001. Genes with greater than a 2 fold difference are enriched for immunoglobulin genes, with 11 immunoglobulin (Ig) genes showing greater than 2 fold up-regulation in BAV versus TAV (of 67 genes). CD38, a gene encoding for a cell surface marker, is up-regulated 1.9 fold in BAV versus TAV (p=0.0011). There are differences in gene expression comparing TS with BAV versus TAV. It is unclear why expression of Ig genes would be up-regulated with BAV; however, this finding may have potential as a biomarker in TS for BAV through measurement of quantitative Igs or Ig light chain assays. Additionally, flow cytometry may detect different levels of CD-38 expression, another possible biomarker. These findings require confirmation using a different expression profiling modality. Further study is needed to explore whether these or possibly other biomarkers suggested by transcriptomic data, may be useful clinically in TS.

Aneuploidy and skeletal health.

The normal human chromosome complement consists of 46 chromosomes comprising 22 morphologically different pairs of autosomes and one pair of sex chromosomes. Variations in either chromosome number and/or structure frequently result in significant mental impairment and/or a variety of other clinical problems, among them, altered bone mass and strength. Chromosomal syndromes associated with specific chromosomal abnormalities are classified as either numerical or structural and may involve more than one chromosome. Aneuploidy refers to the presence of an extra copy of a specific chromosome, or trisomy, as seen in Down's syndrome (trisomy 21), or the absence of a single chromosome, or monosomy, as seen in Turner syndrome (a single X chromosome in females: 45, X). Aneuploidies have diverse phenotypic consequences, ranging from severe mental retardation and developmental abnormalities to increased susceptibility to various neoplasms and premature death. In fact, trisomy 21 is the prototypical aneuploidy in humans, is the most common genetic abnormality associated with longevity, and is one of the most widespread genetic causes of intellectual disability. In this review, the impact of trisomy 21 on the bone mass, architecture, skeletal health, and quality of life of people with Down syndrome will be discussed.

Human 45,X fibroblast transcriptome reveals distinct differentially expressed genes including long noncoding RNAs potentially associated with the pathophysiology of Turner syndrome.

Turner syndrome is a chromosomal abnormality characterized by the absence of whole or part of the X chromosome in females. This X aneuploidy condition is associated with a diverse set of clinical phenotypes such as gonadal dysfunction, short stature, osteoporosis and Type II diabetes mellitus, among others. These phenotypes differ in their severity and penetrance among the affected individuals. Haploinsufficiency for a few X linked genes has been associated with some of these disease phenotypes. RNA sequencing can provide valuable insights to understand molecular mechanism of disease process. In the current study, we have analysed the transcriptome profiles of human untransformed 45,X and 46,XX fibroblast cells and identified differential expression of genes in these two karyotypes. Functional analysis revealed that these differentially expressing genes are associated with bone differentiation, glucose metabolism and gonadal development pathways. We also report differential expression of lincRNAs in X monosomic cells. Our observations provide a basis for evaluation of cellular and molecular mechanism(s) in the establishment of Turner syndrome phenotypes.

RLIM is dispensable for X-chromosome inactivation in the mouse embryonic epiblast.

SUMMARYIn female mice, two forms of X chromosome inactivation (XCI) ensure the selective silencing of female sex chromosomes during mouse embryogenesis. Beginning at the four cell stage, imprinted XCI (iXCI) exclusively silences the paternal X (Xp). Later, around implantation, epiblast cells of the ICM that give rise to the embryo reactivate the Xp and undergo a random form of XCI (rXCI)1,2. Xist, a long non-coding RNA crucial for both forms of XCI, is activated by the ubiquitin ligase Rnf12/RLIM3-5. While Rnf12/RLIM is required for triggering iXCI in mice, its importance for rXCI has been controversial. Here, we show that Rnf12/RLIM levels are downregulated in embryonic cells undergoing rXCI. Using mouse genetics we demonstrate that female cells lacking Rnf12/RLIM from pre-implantation stages onwards display hallmarks of XCI including Xist clouds and H3K27me3 foci and display full embryogenic potential. These results provide evidence that Rnf12/RLIM is dispensable for rXCI, indicating that in mice an Rnf12/RLIM-independent mechanism activates Xist in the embryo proper.

First cytogenetic characterization of the giant Amazonian catfishBrachyplatystoma filamentosum(Siluriformes, Pimelodidae)

In contrast to the enormous variability of Neotropical fish species, genetic information about many groups is not yet available. Chromosomal studies have greatly contributed to the characterization of several species, providing important data on these animals. We performed the first cytogenetic characterization of the Amazonian fish species Brachyplatystoma filamentosum (Pimelodidae), popularly known as “piraíba”. The results revealed a diploid number of 2n = 56 composed of 24 metacentric, 12 submetacentric, and 10 acrocentric chromosomes in both females and males. After silver nitrate treatment (AgNORs) it was possible to visualize the nucleolus organizer region located in the terminal portion of the short arms of subtelocentric chromosome pair 22, presenting size polymorphism. Hybridization with 18S and 5S rDNA probes confirmed the number and location of 18S marks rDNA in pair 22 and differences in the size of signals among homologs. The 5S rDNA genes were localized near the centromere on the short arms of chromosome pair 19. Constitutive heterochromatin (C-bands) were localized predominantly in the terminal regions of the chromosomes, and also occurred in some interstitial and centromeric positions. The chromosomal genetic data obtained in this study contribute to the biological characterization of B. filamentosum, which has economic and ecological importance as the largest freshwater catfish occurring in many rivers of the Amazon and Tocantins-Araguaia basins in Brazil. These results may also be used in to infer relationships among Pimelodidae species.

Complete Dosage Compensation and Sex-Biased Gene Expression in the Moth Manduca sexta

Sex chromosome dosage compensation balances homogametic sex chromosome expression with autosomal expression in the heterogametic sex, leading to sex chromosome expression parity between the sexes. If compensation is incomplete, this can lead to expression imbalance and sex-biased gene expression. Recent work has uncovered an intriguing and variable pattern of dosage compensation across species that includes a lack of complete dosage compensation in ZW species compared with XY species. This has led to the hypothesis that ZW species do not require complete compensation or that complete compensation would negatively affect their fitness. To date, only one study, a study of the moth Bombyx mori, has discovered evidence for complete dosage compensation in a ZW species. We examined another moth species, Manduca sexta, using high-throughput sequencing to survey gene expression in the head tissue of males and females. We found dosage compensation to be complete in M. sexta with average expression between the Z chromosome in males and females being equal. When genes expressed at very low levels are removed by filtering, we found that average autosome expression was highly similar to average Z expression, suggesting that the majority of genes in M. sexta are completely dosage compensated. Further, this compensation was accompanied by sex-specific gene expression associated with important sexually dimorphic traits. We suggest that complete dosage compensation in ZW species might be more common than previously appreciated and linked to additional selective processes, such as sexual selection. More ZW and lepidopteran species should now be examined in a phylogenetic framework, to understand the evolution of dosage compensation.

Spatiotemporal Expression of DNA Demethylation Enzymes and Histone Demethylases in Bovine Embryos

Fertilization in bovines causes profound changes in the epigenetic profile that affect both DNA methylation patterns and posttranslational histone modifications. These dynamic changes have a great potential for activating pluripotency genes and unfolding certain chromatin regions to recruit different transcription factors. Surprisingly, while the fundamental function of epigenetic remodeling is well understood, the bases of the process are still unknown. Recent developments in epigenetics suggest a multistep demethylation process that would imply the prior modification of the methylated cytosine or methyl group, followed by a DNA repair mechanism implicating enzymes such as activation-induced cytidine deaminase (AICDA) and ten-eleven translocation (TET) dioxygenase. Their functions seem to differ from one species to the other, and they are not yet well characterized in large mammals. Histones have, for their part, many associated and specific lysine demethylases (KDM). Their expression profile in large mammals is not well characterized. We have been interested in characterizing the spatiotemporal expression profile for each of the genes studied to increase our understanding of the molecular interactions following fertilization in early bovine embryo stages. Bovine oocytes and embryos at various preimplantation stages were collected following in vitro fertilization protocol. Total RNA for AICDA, TET1, TET2, TET3, KDM3A, KDM4A, KDM4C, and KDM5B was extracted, reverse transcribed into cDNA, and amplified by real-time PCR. Other embryo pools were collected, and protein localization of the genes studied was characterized. TET3 dioxygenase was present in the very first embryo stages, in contrast to TET1 and AICDA. Histone demethylases KDM3A, KDM4A, and KDM4C were expressed before and after embryonic genome activation, whereas KDM5B was mainly expressed during the blastocyst period. DNA demethylation following fertilization in bovines is not accomplished by AICDA but most probably by TET3. Histone demethylation is carried out by, among others, KDM3A, KDM4A, and KDM4C, which could act in sequence to demethylate histones prior to DNA demethylation of the female chromosomes.

Hermaphroditism in two dogs - pathological and cytogenetic studies: a case report

A Pit-bull and a Beagle, both one-year-old, with complaints of an enlarged clitoris were clinically suspected for hermaphroditism. The enlarged clitoris, the uteri and the gonads were removed surgically from each animal and submitted to our department for histopathological evaluation. Tissue samples were fixed with 10% buffered formalin solution and processed routinely, after which paraffin sections were obtained and stained with H&amp;amp;E. A blood sample was taken from the Beagle dog and a peripheral lymphocyte culture was prepared. While the clitoris and uteri were confirmed histopathologically, the gonads were detected as a testis instead of an ovary for both dogs. Additionally, cytogenetic evaluation revealed a normal female chromosome complement, 78, XX for the Beagle dog. According to the gonadal and phenotypic sexes, both cases were first determined as &amp;lsquo;male pseudohermaphroditism&amp;rsquo;, a phenotypic sex disorder. However, after karyotyping analysis, we concluded that the 78, XX Beagle dog should be defined as suffering from XX sex reversal syndrome, a gonadal sex disorder. &amp;nbsp;

Current view of the potential roles of proteins enriched on the inactive X chromosome.

X chromosome inactivation (X-inactivation) is triggered by X-linked noncoding Xist RNA, which is expressed asymmetrically from one of the two X chromosomes in females and coats it in cis to induce chromosome-wide silencing. Xist RNA is thought to play a role as a platform in recruiting proteins involved in gene silencing and heterochromatinization, which mediate serial changes in epigenetic modification of the chromatin. During the last two decades, many proteins have been shown to be enriched on the inactivated X chromosome in mouse and human. Although the biological significance of most of them for X-inactivation has not been fully established, extensive studies of these proteins should provide a better understanding of the molecular basis of how X-inactivation mediated by Xist RNA is regulated. Here, we review the potential roles of some of these proteins in the stepwise process of Xist RNA-mediated chromosome silencing.

Caracterização citogenética das raças bovinas Caracu, Junqueira, Pantaneira e Patuá

During the adaptive process to Brazilian conditions, the native cattle breeds acquired physiological and phenotypic characteristics wich differs them from European breeds, from which they derive. The objectives of present study were to compare the sex chromosomes morphologies between four Brazilian native cattle breeds as well as the incidence of chromosomal abnormalities. The karyotype of 332 animals from four breeds were analyzed: Caracu (115 females and 159 males), Junqueira (13 females and 3 males), Pantaneiro (8 males) and Patua (21 females and 13 males). The Giemsa coloring were used to read the chromosome. All cattle breeds studied showed the normal chromosome (2n=60) consisting of 58 acrocentric autosomes and two submetacentric X chromosomes in females, and one X and one Y in males. In Caracu breed, selected for dairy or beef, it was developed by animals dimorphism carriers of Y chromosome (acrocentric and submetacentric), and the beef bloodlines showed higher percentage of acrocentric Y chromosome. There was 1% significant difference in Chi-Square test. In two females and two male born from twin birth of Caracu breed it was observed chemerism 60, XX/ 60, XY, and in other female from same breed it was observed 60, XX/ 61 XXY. For other three breeds, no chromosomal abnormality was observed. The Y chromosome morphology of the Pantaneiro and Patua breeds showed dimorphism of Y chromosome (submetacentric an acrocentric). In the Junqueira breed, only submetacentric morphology was observed.

Função Gonadal na Síndrome de Turner

Turner syndrome is characterized by the absence, total or partial, of one X chromosome in females, being one of the most frequent chromosomal abnormalities. Diagnosis is made by karyotype. Turner syndrome manifestations include primary hypogonadism, before or after puberty (gonadal dysgenesis). The degree and extent of gonadal disfunction are variable. We intended to assess clinical, karyotype, gonadal function and pelvic ultrasound characteristics in women with Turner syndrome. Retrospective study of patients with Turner syndrome followed in Endocrinology and Human Reproduction Departments of Hospitais da Universidade de Coimbra - Centro Hospitalar e Universitário de Coimbra, E.P.E. We evaluated the entire sample and considered group 1 (with spontaneous puberty and menarche) and group 2 (without spontaneous puberty). Parameters assessed: age at initial study, puberty (Tanner stages), karyotype, FSH, pelvic ultrasound (initial and after puberty), diagnostic laparoscopy and pubertal induction. Statistical Program: SPSS (20.0). Global sample: 79 patients, 14.7 ± 6.6 years. No pubertal signs in 57.1%; 67.1% with primary amenorrhea and 6.6% with secondary amenorrhea. Karyotype: X monosomy-37.2%, mosaicism-37.2%, X structural changes-25.6%. Median FSH of 59.5 mIU/ mL. Initial ultrasound: normal uterus 34.2%, atrophic uterus 65.8%; normal ovaries 21.6%, atrophic ovaries 78.4%, ovarian follicles in 5.1%. Post-puberty ultrasound: normal uterus 67.9%, atrophic uterus 32.1%; normal ovaries 36.4%, atrophic ovaries 63.6%. Laparoscopy was performed in 16 (20.3%) patients, confirming the sonographic findings. Only two women with induced puberty became pregnant: one spontaneously, interrupted; another by donated oocytes, normal outcome. Group 1 (with spontaneous puberty and menarche): 20 (25.3%) patients, 16.1 ± 8.9 years. Tanner at baseline: M1-22.2%, M2-33.3%, M3-16.7%, M4-16.7%, M5-11.1%. Karyotype: mosaicism-65%, X structural changes-20%, X monosomy-15%. Median FSH of 7 mUI/mL. Initial ultrasound: normal uterus-72.2%, atrophic uterus 27.8%; normal ovaries 63.2%, atrophic ovaries 36.8%. Post-puberty ultrasound: normal uterus 100%; normal ovaries 72.7%, atrophic ovaries 27.3%. Group 2 (without spontaneous puberty): 59 (74.7%) patients, 14.0 ± 5.5 years. Tanner at baseline: M1-69.2%, M2-13.5%, M3-5.8%, M4-3.8%, M5-7.7%. Karyotype: X monosomy-43.9%, X structural changes-28.1% mosaicism-28.1%. Median FSH of 74 mUI/mL. Initial ultrasound: normal uterus 20.4%, atrophic uterus 79.6%; normal ovaries 7.4%, atrophic ovaries 92.6%. Post-puberty ultrasound: normal uterus 60.0%, atrophic uterus 40.0%; normal ovaries 27.3%, atrophic ovaries 72.7%. Pubertal induction at 16.1 ± 4.1 years, with bone age of 12.7 ± 1.6 years. Groups 1 and 2 differ significantly in karyotype (p = 0.010), median FSH (p < 0.001), and uterine and ovarian dimensions (p < 0.001). Most patients had gonadal dysfunction and needed pubertal induction. Spontaneous puberty with menarche occurred in 25.3% of patients (predominantly mosaics). 43.9% of patients with pubertal induction had X monosomy. These patients fertility is compromised and, in some cases, we should refer to assisted reproductive specialist for pregnancy or fertility preservation.

DNA methylation profiling in X;autosome translocations supports a role for L1 repeats in the spread of X chromosome inactivation

X chromosome inactivation (XCI) is an epigenetic mechanism that silences the majority of genes on one X chromosome in females. Previous studies have suggested that the spread of XCI might be facilitated in part by common repeats such as long interspersed nuclear elements (LINEs). However, owing to the unusual sequence content of the X and the nonrandom distribution of genes that escape XCI, it has been unclear whether the correlation between repeat elements and XCI is a functional one. To test the hypothesis that the spread of XCI shows sequence specificity, we have analyzed the pattern of XCI in autosomal chromatin by performing DNA methylation profiling in six unbalanced X;autosome translocations. Using promoter hypermethylation as an epigenetic signature of XCI, we have determined the inactivation status of 1050 autosomal genes after translocation onto an inactive derivative X. By performing a comparative sequence analysis of autosomal genes that are either subject to or escape the X inactivation signal, we identified a number of common repetitive elements, including L1 and L2 LINEs, and DNA motifs that are significantly enriched around inactive autosomal genes. We show that these same motifs predominantly map to L1P repeat elements, are significantly enriched on the X chromosome versus the autosomes and also occur at higher densities around X-linked genes that are subject to X inactivation compared with those that escape X inactivation. These results are consistent with a potential causal relationship between DNA sequence features such as L1s and the spread of XCI, lending strong support to Mary Lyon's 'repeat hypothesis'.

Spread of X-chromosome inactivation into autosomal sequences: role for DNA elements, chromatin features and chromosomal domains

X-chromosome inactivation results in dosage equivalence between the X chromosome in males and females; however, over 15% of human X-linked genes escape silencing and these genes are enriched on the evolutionarily younger short arm of the X chromosome. The spread of inactivation onto translocated autosomal material allows the study of inactivation without the confounding evolutionary history of the X chromosome. The heterogeneity and reduced extent of silencing on autosomes are evidence for the importance of DNA elements underlying the spread of silencing. We have assessed DNA methylation in six unbalanced X-autosome translocations using the Illumina Infinium HumanMethylation450 array. Two to 42% of translocated autosomal genes showed this mark of silencing, with the highest degree of inactivation observed for trisomic autosomal regions. Generally, the extent of silencing was greatest close to the translocation breakpoint; however, silencing was detected well over 100 kb into the autosomal DNA. Alu elements were found to be enriched at autosomal genes that escaped from inactivation while L1s were enriched at subject genes. In cells without the translocation, there was enrichment of heterochromatic features such as EZH2 and H3K27me3 for those genes that become silenced when translocated, suggesting that underlying chromatin structure predisposes genes towards silencing. Additionally, the analysis of topological domains indicated physical clustering of autosomal genes of common inactivation status. Overall, our analysis indicated a complex interaction between DNA sequence, chromatin features and the three-dimensional structure of the chromosome.

How to correct chromosomal trisomy

Trisomy 21 in human causes Down syndrome, a common chromosome disorder with devastating phenotypes ranging from early death in utero to intellectual disability together with an array of physical anomalies and late-onset diseases. In a recent study published in Nature, Jeanne Lawrence and her colleagues restored normal gene expression in trisomy 21 cells by silencing the extra chromosome using XIST, the non-coding RNA that normally silences one X chromosome in females; this improved growth and differentiation of neural cells, which offers hope that some deleterious effects of the trisomy could be reversed to improve this incurable disease.

Counseling parents before prenatal diagnosis: Do we need to say more about the sex chromosome aneuploidies?

Sex chromosome trisomies (SCT), an extra X chromosome in females (triple X, XXX), males with an extra X chromosome (Klinefelter syndrome, XXY) or an extra Y chromosome (XYY) occur because of errors during meiosis and are relatively frequent in humans. Their identification has never been the goal of prenatal diagnosis (PD) but they almost never escape detection by any of the methods commonly in use. Despite recommendations and guide-lines which emphasize the importance of structured counseling before and after PD, most women remain unaware that testing for serious genetic abnormalities is more likely to uncover these trisomies. With the increasing use of PD more and more prospective parents receive a diagnosis of sex chromosome trisomies and are faced with the dilemma of whether to terminate the pregnancy or to carry it to term. Despite the dramatic and emotionally devastating consequences of having to make such a decision, they have little opportunity to consider in advance the possible outcomes of such a pregnancy and, rather than relying on their own feelings and judgements, are forced to depend on the advice of counseling professionals who may or may not themselves be fully aware of what having an extra sex chromosome can mean to the development of a child. We address here the principles of reproductive autonomy together with an analysis of the major issues that ought to be discussed with the parents before a PD is carried out in order to minimize detrimental effects caused by this unexpected finding.

Comparative Sex Chromosome Genomics in Snakes: Differentiation, Evolutionary Strata, and Lack of Global Dosage Compensation

Snakes exhibit genetic sex determination, with female heterogametic sex chromosomes (ZZ males, ZW females). Extensive cytogenetic work has suggested that the level of sex chromosome heteromorphism varies among species, with Boidae having entirely homomorphic sex chromosomes, Viperidae having completely heteromorphic sex chromosomes, and Colubridae showing partial differentiation. Here, we take a genomic approach to compare sex chromosome differentiation in these three snake families. We identify homomorphic sex chromosomes in boas (Boidae), but completely heteromorphic sex chromosomes in both garter snakes (Colubridae) and pygmy rattlesnake (Viperidae). Detection of W-linked gametologs enables us to establish the presence of evolutionary strata on garter and pygmy rattlesnake sex chromosomes where recombination was abolished at different time points. Sequence analysis shows that all strata are shared between pygmy rattlesnake and garter snake, i.e., recombination was abolished between the sex chromosomes before the two lineages diverged. The sex-biased transmission of the Z and its hemizygosity in females can impact patterns of molecular evolution, and we show that rates of evolution for Z-linked genes are increased relative to their pseudoautosomal homologs, both at synonymous and amino acid sites (even after controlling for mutational biases). This demonstrates that mutation rates are male-biased in snakes (male-driven evolution), but also supports faster-Z evolution due to differential selective effects on the Z. Finally, we perform a transcriptome analysis in boa and pygmy rattlesnake to establish baseline levels of sex-biased expression in homomorphic sex chromosomes, and show that heteromorphic ZW chromosomes in rattlesnakes lack chromosome-wide dosage compensation. Our study provides the first full scale overview of the evolution of snake sex chromosomes at the genomic level, thus greatly expanding our knowledge of reptilian and vertebrate sex chromosomes evolution.

Parastrongyloides trichosurisuggests that XX/XO sex determination is ancestral in Strongyloididae (Nematoda)

The parasitic roundworms Strongyloides stercoralis (in man) and Strongyloides ratti (in rats) employ environmentally controlled XX/XO sex determination with a pair of X chromosomes and two pairs of autosomes. Strongyloides papillosus (in sheep) has only two pairs of chromosomes, one of which combines the genetic material homologous to the S. ratti chromosomes X and I. This species creates males through the elimination of one copy of the portion related to the X chromosome (chromatin diminution). It is not clear which one of these two sex-determining mechanisms is ancestral. We demonstrate that Strongyloides vituli (in cattle) has two pairs of chromosomes like its very close relative S. papillosus whereas Parastrongyloides trichosuri, a closely related out-group to Strongyloides spp. in Australian brushtail possums, has three chromosome pairs and employs XX/XO sex determination. The X chromosome of P. trichosuri is homologous to the X chromosome of S. ratti. Our data strongly suggest that the last common ancestor of Strongyloides spp. and Parastrongyloides spp. had two pairs of autosomes along with two or one X chromosome in females and males, respectively. The situation with two pairs of chromosomes is likely derived and occurred through the fusion of the X chromosome with an autosome.

A mother with variant Turner syndrome and two daughters with trisomy X: a case report

Though fertility among mosaic Turner syndrome women has been reported, spontaneous pregnancy is rare with risk of miscarriages and chromosomal abnormalities (Rizk and Deb 2003). We report an unusual family in which a mother with mosaic 45,X[13]/46,X,add(X)(?q26)[17] gave birth to two children bearing 47,XX,add(X)(q26)mat. Both children, aged 12 years old and 9 years old, presented with learning disabilities and subtle dysmorphism. The mother has premature menopause but did not have learning disability or dysmorphism. Whole chromosome painting revealed that the abnormal chromosome X in both sisters was duplicated chromosome X, which was inherited from their mother. Turner syndrome is characterized by absence of all or part of one X chromosome from all or some cell lines affecting approximately 1 in 2500 live births (Rizk and Deb 2003). Features include short stature, gonadal dysgenesis, primary amenorrhoea, decreased fertility and anomalies of cardiac, renal and endocrine origin. Mosaic Turner syndrome is associated with infertility, secondary amenorrhoea and recurrent abortions (Kammoun et al. 2008). Spontaneous conception among Turner syndrome patients is extremely rare with cases reported mainly in mosaics. About 30% of these pregnancies have a normal outcome while the rest are complicated by chromosomal abnormalities such as recurrence of X chromosome defect in female offspring and trisomy 21, stillbirths, foetal malformations and recurrent abortions (Rizk and Deb 2003). Trisomy X is a common female chromosome abnormality, occurring in approximately 1 in 1000 births. It has a variable phenotype and includes learning disabilities, speech delays,