Y-autosome translocation interferes with meiotic sex inactivation and expression of autosomal genes: a case study in the pig

Return of the forgotten hero: the role of Y chromosome-encoded Zfy in male reproduction

The Y-linked zinc finger gene ZFY is conserved across eutherians and is known to be a critical fertility factor in some species. The initial studies of the mouse homologues, Zfy1 and Zfy2, were performed using mice with spontaneous Y chromosome mutations and Zfy transgenes. These studies revealed that Zfy is involved in multiple processes during spermatogenesis, including removal of germ cells with unpaired chromosomes and control of meiotic sex chromosome inactivation during meiosis I, facilitating the progress of meiosis II, promoting spermiogenesis, and improving assisted reproduction outcomes. Zfy was also identified as a key gene in Y chromosome evolution, protecting this chromosome from extinction by serving as the executioner responsible for meiosis surveillance. Studies with targeted Zfy knock-outs revealed that mice lacking both homologues have severe spermatogenic defects and are infertile. Based on protein structure and in vitro assays, Zfy is expected to drive spermatogenesis as a transcriptional regulator. The combined evidence documents that the presence of at least one Zfy homologue is required for male fertility and that Zfy2 plays a more prominent role. This knowledge reinforces the importance of these factors for mouse spermatogenesis and informs our understanding of the human ZFY variants, which are homologous to the mouse Zfy1 and Zfy2.

Molecular Cytogenetic and Y Copy Number Analysis of a Reciprocal ECAY-ECA13 Translocation in a Stallion with Complete Meiotic Arrest

We present a detailed molecular cytogenetic analysis of a reciprocal translocation between horse (ECA) chromosomes Y and 13 in a Friesian stallion with complete meiotic arrest and azoospermia. We use dual-color fluorescence in situ hybridization with select ECAY and ECA13 markers and show that the translocation breakpoint in ECAY is in the multicopy region and in ECA13, at the centromere. One resulting derivative chromosome, Y;13p, comprises of ECAY heterochromatin (ETSTY7 array), a small single copy and partial Y multicopy region, and ECA13p. Another derivative chromosome 13q;Y comprises of ECA13q and most of the single copy ECAY, the pseudoautosomal region and a small part of the Y multicopy region. A copy number (CN) analysis of select ECAY multicopy genes shows that the Friesian stallion has significantly (p < 0.05) reduced CNs of TSPY, ETSTY1, and ETSTY5, suggesting that the translocation may not be completely balanced, and genetic material is lost. We discuss likely meiotic behavior of abnormal chromosomes and theorize about the possible effect of the aberration on Y regulation and the progression of meiosis. The study adds a unique case to equine clinical cytogenetics and contributes to understanding the role of the Y chromosome in male meiosis.

Bisection of the X chromosome disrupts the initiation of chromosome silencing during meiosis in Caenorhabditis elegans

During meiosis, gene expression is silenced in aberrantly unsynapsed chromatin and in heterogametic sex chromosomes. Initiation of sex chromosome silencing is disrupted in meiocytes with sex chromosome-autosome translocations. To determine whether this is due to aberrant synapsis or loss of continuity of sex chromosomes, we engineered Caenorhabditis elegans nematodes with non-translocated, bisected X chromosomes. In early meiocytes of mutant males and hermaphrodites, X segments are enriched with euchromatin assembly markers and active RNA polymerase II staining, indicating active transcription. Analysis of RNA-seq data showed that genes from the X chromosome are upregulated in gonads of mutant worms. Contrary to previous models, which predicted that any unsynapsed chromatin is silenced during meiosis, our data indicate that unsynapsed X segments are transcribed. Therefore, our results suggest that sex chromosome chromatin has a unique character that facilitates its meiotic expression when its continuity is lost, regardless of whether or not it is synapsed.

Meiotic Silencing in Pigs: A Case Study in a Translocated Azoospermic Boar

Carriers of balanced constitutional reciprocal translocations usually present a normal phenotype, but often show reproductive disorders. For the first time in pigs, we analyzed the meiotic process of an autosome-autosome translocation associated with azoospermia. Meiotic process analysis revealed the presence of unpaired autosomal segments with histone γH2AX accumulation sometimes associated with the XY body. Additionally, γH2AX signals were observed on apparently synapsed autosomes other than the SSC1 or SSC15, as previously observed in Ataxia with oculomotor apraxia type 2 patients or knock-out mice for the Senataxin gene. Gene expression showed a downregulation of genes selected on chromosomes 1 and 15, but no upregulation of SSCX genes. We hypothesized that the total meiotic arrest observed in this boar might be due to the silencing of crucial autosomal genes by the mechanism referred to as meiotic silencing of unsynapsed chromatin (MSUC).

Classical, Molecular, and Genomic Cytogenetics of the Pig, a Clinical Perspective

The chromosomes of the domestic pig (Sus scrofa domesticus) are known to be prone to reciprocal chromosome translocations and other balanced chromosome rearrangements with concomitant fertility impairment of carriers. In response to the remarkable prevalence of chromosome rearrangements in swine herds, clinical cytogenetics laboratories have been established in several countries in order to screen young boars for chromosome rearrangements prior to service. At present, clinical cytogenetics laboratories typically apply classical cytogenetics techniques such as giemsa-trypsin (GTG)-banding to produce high-quality karyotypes and reveal large-scale chromosome ectopic exchanges. Further refinements to clinical cytogenetics practices have led to the implementation of molecular cytogenetics techniques such as fluorescent in-situ hybridization (FISH), allowing for rearrangements to be visualized and breakpoints refined using fluorescently labelled painting probes. The next-generation of clinical cytogenetics include the implementation of DNA microarrays, and next-generation sequencing (NGS) technologies such as DNA sequencing to better explore tentative genome architecture changes. The implementation of these cytogenomics techniques allow the genomes of rearrangement carriers to be deciphered at the highest resolution, allowing rearrangements to be detected; breakpoints to be delineated; and, most importantly, potential gene implications of those chromosome rearrangements to be interrogated. Clinical cytogenetics has become an integral tool in the livestock industry, identifying rearrangements and allowing breeders to make informed breeding decisions.

Meiotic Executioner Genes Protect the Y from Extinction

The Y has been described as a wimpy degraded relic of the X, with imminent demise should it lose sex-determining function. Why then has it persisted in almost all mammals? Here we present a novel mechanistic explanation for its evolutionary perseverance: the persistent Y hypothesis. The Y chromosome bears genes that act as their own judge, jury, and executioner in the tightly regulated meiotic surveillance pathways. These executioners are crucial for successful meiosis, yet need to be silenced during the meiotic sex chromosome inactivation window, otherwise germ cells die. Only rare transposition events to the X, where they remain subject to obligate meiotic silencing, are heritable, posing strong evolutionary constraint for the Y chromosome to persist.

Meiotic Synapsis and Gene Expression Altered by a Balanced Y-Autosome Reciprocal Translocation in an Azoospermic Pig

Meiotic sex chromosome silencing (MSCS) has been argued as a prerequisite for normal meiotic cell division progression during the synaptic prophase I stage. Furthermore, irregular asynapsis of autosomal axes at meiosis may be encompassing the lack of transcriptional activity normally observed for the X and Y sex chromosomes. Therefore, any chromosomal rearrangement compromising the normal mechanism of MSCS and/or the contrary, the normal meiotic transcriptional activity of autosomal chromosomes, may be observed as a meiotic and concomitant spermatogenesis arrest. Previously, we have described a Y-autosome translocation t(Y;13)(p1.3;q3.3) in an azoospermic boar. Its chromosome synapsis behavior by synaptonemal complex immunostaining and FISH analyses is documented here. Histone γH2AX protein foci appeared to be located at unsynapsed chromosomal segments (e.g., X chromosome univalents or unpaired multivalent segments), although interestingly a high proportion of primary spermatocytes showed full paired synaptonemal complex-multivalent configurations which were devoid of a γH2AX focus signal, indicating meiotic chromosome silencing. RT-qPCR analysis of testicular expression showed downregulation of 3 SSC13 genes (MLH1, SOX2, UBE2B) and upregulation of SSCY genes (ZFY, SRY). The irregularity of the normal transcription pattern in case of these genes with proven roles in the testis is in agreement with the cytological observations and could contribute to the observed phenotype.

Meiotic pairing and gene expression disturbance in germ cells from an infertile boar with a balanced reciprocal autosome-autosome translocation

Individuals carrying balanced constitutional reciprocal translocations generally have a normal phenotype, but often present reproductive disorders. The aim of our research was to analyze the meiotic process in an oligoasthenoteratospermic boar carrying an asymmetric reciprocal translocation involving chromosomes 1 and 14. Different multivalent structures (quadrivalent and trivalent plus univalent) were identified during chromosome pairing analysis. Some of these multivalents were characterized by the presence of unpaired autosomal segments with histone γH2AX accumulation sometimes associated with the XY body. Gene expression in spermatocytes was studied by RNA-DNA-FISH and microarray-based testis transcriptome analysis. Our results revealed a decrease in gene expression for chromosomes 1 and 14 and an up-regulated expression of X-chromosome genes for the translocated boar compared with normal individuals. We hypothesized that the observed meiotic arrest and reproductive failure in this boar might be due to silencing of crucial autosomal genes (MSUC) and disturbance of meiotic sex chromosome inactivation (MSCI). Further analysis revealed abnormal meiotic recombination (frequency and distribution) and the production of a high rate of unbalanced spermatozoa.

Meiotic behaviour of evolutionary sex-autosome translocations in Bovidae

The recurrent occurrence of sex-autosome translocations during mammalian evolution suggests common mechanisms enabling a precise control of meiotic synapsis, recombination and inactivation of sex chromosomes. We used immunofluorescence and FISH to study the meiotic behaviour of sex chromosomes in six species of Bovidae with evolutionary sex-autosome translocations (Tragelaphus strepsiceros, Taurotragus oryx, Tragelaphus imberbis, Tragelaphus spekii, Gazella leptoceros and Nanger dama ruficollis). The autosomal regions of fused sex chromosomes showed normal synapsis with their homologous counterparts. Synapsis in the pseudoautosomal region (PAR) leads to the formation of characteristic bivalent (in T. imberbis and T. spekii with X;BTA13/Y;BTA13), trivalent (in T. strepsiceros and T. oryx with X/Y;BTA13 and G. leptoceros with X;BTA5/Y) and quadrivalent (in N. dama ruficollis with X;BTA5/Y;BTA16) structures at pachynema. However, when compared with other mammals, the number of pachynema lacking MLH1 foci in the PAR was relatively high, especially in T. imberbis and T. spekii, species with both sex chromosomes involved in sex autosome translocations. Meiotic transcriptional inactivation of the sex-autosome translocations assessed by γH2AX staining was restricted to their gonosomal regions. Despite intraspecies differences, the evolutionary fixation of sex-autosome translocations among bovids appears to involve general mechanisms ensuring sex chromosome pairing, synapsis, recombination and inactivation.

A new cytotype of the African pygmy mouse Mus minutoides in Eastern Africa. Implications for the evolution of sex-autosome translocations

The African pygmy mice (genus Mus, subgenus Nannomys) are recognized for their highly conserved morphology but extensive chromosomal diversity, particularly involving sex-autosome translocations, one of the rarest chromosomal rearrangements among mammals. It has been shown that in the absence of unambiguous diagnostic morphological traits, sex-autosome translocations offer accurate taxonomic markers. For example, in Mus minutoides, irrespective of the diploid number (which ranges from 2n = 18 to 34), all specimens possess the sex-autosome translocations (X.1) and (Y.1) that are unique to this species. In this study, we describe a new cytotype that challenges this view. Males are characterized by the translocation (Y.1) only, while females carry no sex-autosome translocation, the X chromosome being acrocentric. Hence, although sex-autosome translocations (X.1) and (Y.1) are still diagnostic when one or both are present, their absence does not rule out M. minutoides. This cytotype has a large distribution, with specimens found in Tanzania and in the eastern part of South Africa. The nonpervasive distribution of Rb(X.1) provides an opportunity to investigate different evolutionary scenarios of sex-autosome translocations using a phylogenetic framework and the distribution of telomeric repeats. The results tend to support a scenario involving a reversal event, i.e., fusion then fission of Rb(X.1), and highlighted the existence of a new X1X1X2X2/X1X2Y sex chromosome system, confirming the remarkable diversity of neo-sex chromosomes and sex determination systems in the African pygmy mice.