The sex-ratio trait in Drosophila simulans: genetic analysis of distortion and suppression

The sex-ratio trait described in several Drosophila species is a type of naturally occurring X-linked meiotic drive that causes males bearing a sex-ratio X chromosome to produce progenies with a large excess of females. We have previously reported the occurrence of sex-ratio X chromosomes in Drosophila simulans. In this species, because of the co-occurrence of drive suppressors, the natural populations and the derived laboratory strains show an equal sex-ratio even when sex-ratio X chromosomes are present at a high frequency. The presence of sex-ratio X chromosomes is established via crosses with a standard strain that is devoid of drive suppressors. In this article, we show first that the sex-ratio trait in D. simulans results from the action of several X-linked loci. Second we describe drive suppressors on each major autosome as well as on the Y chromosome. The Y-linked factors suppress the drive partially whereas the autosomal suppression can be complete.     

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

An ultrastructural study has been made of spermatogenesis in two species of primitive spiders having holocentric chromosomes (Dysdera crocata, male X0 and Sergestria florentia X1X2O). Analysis of the meiotic prophase shows a scarcity or absence of typical leptotene to pachytene stages. Only in D. crocata have synaptonemal complex (SC) remnants been seen, and these occurred in nuclei with an extreme chromatin decondensation. In both species typical early prophase stages have been replaced by nuclei lacking SC and with their chromatin almost completely decondensed, constituting a long and well-defined diffuse stage. Only nucleoli and the condensed sex chromosomes can be identified. - In S. florentina paired non-homologous sex chromosomes lack a junction lamina and thus clearly differ from the sex chromosomes of more evolved spiders with an X1X20 male sex determination mechanism. In the same species, sex chromosomes can be recognized during metaphase I due to their special structural details, while in D. crocata the X chromosome is not distinguishable from the autosomes at this stage. - The diffuse stage and particularly the structural characteristics of the sex chromosomes during meiotic prophase are reviewed and discussed in relation to the meiotic process in other arachnid goups.     

A proposed path by which genes common to mammalian X and Y chromosomes evolve to become X inactivated

Mammalian X and Y chromosomes evolved from an autosomal pair; the X retained and the Y gradually lost most ancestral genes. In females, one X chromosome is silenced by X inactivation, a process that is often assumed to have evolved on a broadly regional or chromosomal basis. Here we propose that genes or clusters common to both the X and Y chromosomes (X-Y genes) evolved independently along a multistep path, eventually acquiring dosage compensation on the X chromosome. Three genes studied here, and other extant genes, appear to be intermediates. ZFX, RPS4X and SMCX were monitored for X inactivation in diverse species by assaying CpG-island methylation, which mirrors X inactivation in many eutherians. ZFX evidently escaped X inactivation in proto-eutherians, which also possessed a very similar Y-linked gene; both characteristics were retained in most extant orders, but not in myomorph rodents. For RPS4X, escape from X inactivation seems unique to primates. SMCX escapes inactivation in primates and myomorphs but not in several other lineages. Thus, X inactivation can evolve independently for each of these genes. We propose that it is an adaptation to the decay of a homologous, Y-linked gene.     

The curious normality of the synaptic association between the sex chromosomes of two arvicoline rodents: Microtus oeconomus and Clethrionomys glareolus

In all eight species of arvicoline (microtine) rodents previously described, the X and Y chromosomes have remained asynaptic throughout pachynema. Since synapsis is presumed to be a prerequisite for crossing over, it has been concluded that the sex chromosomes in these species are also achiasmatic, but the mechanism(s) of their disjunction remains an enigma. Their asynaptic, achiasmatic condition has been attributed to loss of the pseudoautosomal region (Borodin et al. 1991; Carnero et al. 1991; Jiménez et al. 1991). This loss has been postulated to include all arvicoline rodents. We describe here the sex chromosome behavior during meiotic prophase of two additional species in this group: Microtus oeconomus and Clethrionomys glareolus. In both species there is extensive synapsis between the X and Y, providing the usual opportunity for XY recombination. These findings challenge the concept of the pseudoautosomal region as an evolutionarily conserved region of homology, at least within the arvicoline rodents. The unexpected finding of synapsis in two very different species, one with a derived and one with a primitive karyotype is discussed within its phylogenetic context.     

Assessment of sex chromosome ratio and aneuploidy rate in motile spermatozoa selected by three different methods

Using three-colour fluorescence in-situ hybridization, sex chromosome ratios and frequencies of diploidy and disomy for chromosomes X, Y and 18 were compared in spermatozoa of good and poor motility after separation by swim-up, glass-wool and two-layer discontinuous Percoll methods. Semen samples were collected from seven normal males aged 26-31 years. A minimum of 6000 sperm nuclei per sample were evaluated for each chromosome for a total of 308,432 sperm nuclei. Hybridization efficiency was 99.8%. A slight change in the ratio of X- to Y-bearing spermatozoa was noted after Percoll separation (from 49.3:49.5 to 50.0:48.9; P = 0.036 and P = 0.046), but not after separation by the other two methods. We did not observe significant differences in the disomy rates for sex chromosomes or chromosome 18 or in the diploidy rate between spermatozoa with good and poor motility after separation by any of the three methods. Our data indicate that separation of motile spermatozoa does not alter the ratio of X- to Y-bearing spermatozoa to a degree that represents sex chromosome selection.     

Anti-seminal plasma antibodies associated with infertility: II. Comparative investigation of human antibody binding to normo-, astheno-, and azoospermic seminal plasma antigens

We present a new method to detect epididymal sperm aneuploidy (ESA) in mice using simultaneous fluorescence in situ hybridization (FISH) with DNA probes specific for mouse chromosomes X, Y and 8. The method was applied to Robertsonian (Rb) translocation (8.14) heterozygotes and homozygotes as well as the chromosomally normal B6C3F1. The sex ratios of sperm did not differ from the expected 1:1 and the hybridization efficiencies were approximately 99.7% for over 60 000 sperm analyzed. Mice heterozygous for Rb (8.14) produced about tenfold higher rates of sperm with chromosome 8 hyperhaploidy than did Rb (8.14) homozygotes or chromosomally normal mice, while frequencies of sperm with hyperhaploidies for chromosomes X and Y were unaffected in all three lines of mice. Hyperhaploid frequencies obtained with the ESA method were consistent with those of the previous testicular FISH method and were validated by published data obtained by conventional cytogenetic analyses (meiotic metaphase II and first cleavage). Thus, the mouse three-chromosome ESA assay together with the previously developed aneuploidy assay for human sperm constitute a promising pair of interspecific biomarkers for comparative studies of the genetic and physiologic mechanisms of the induction and persistence of aneuploidy in male germ cells.     

Mutational analysis of the Drosophila sister-chromatid cohesion protein ORD and its role in the maintenance of centromeric cohesion

The ord gene is required for proper segregation of all chromosomes in both male and female Drosophila meiosis. Here we describe the isolation of a null ord allele and examine the consequences of ablating ord function. Cytologically, meiotic sister-chromatid cohesion is severely disrupted in flies lacking ORD protein. Moreover, the frequency of missegregation in genetic tests is consistent with random segregation of chromosomes through both meiotic divisions, suggesting that sister cohesion may be completely abolished. However, only a slight decrease in viability is observed for ord null flies, indicating that ORD function is not essential for cohesion during somatic mitosis. In addition, we do not observe perturbation of germ-line mitotic divisions in flies lacking ORD activity. Our analysis of weaker ord alleles suggests that ORD is required for proper centromeric cohesion after arm cohesion is released at the metaphase I/anaphase I transition. Finally, although meiotic cohesion is abolished in the ord null fly, chromosome loss is not appreciable. Therefore, ORD activity appears to promote centromeric cohesion during meiosis II but is not essential for kinetochore function during anaphase.     

Polytene chromosomes of monogenic and amphogenic Chrysomya species (Calliphoridae, Diptera): analysis of banding patterns and in situ hybridization with Drosophila sex determining gene sequences

Standard maps for the five banded polytene chromosomes found in trichogen cell nuclei of the monogenic blowfly Chrysomya rufifacies and the amphogenic Chrysomya pinguis are presented. The chromosomes are highly homologous in the two species; differences in banding patterns are predominantly caused by one pericentric and ten paracentric inversions. In chromosome 5 of the amphogenic Chrysomya phaonis, also analysed in this paper, an additional paracentric inversion was observed. The distribution of species specific inversions indicates that the monogenic C. rufifacies is phylogenetically older than the amphogenic species. The maternal sex realizer locus F'/f on polytene chromosome 5 of C. rufifacies is not associated with a structural heterozygosity. Chromosome pair 6 of C. rufifacies and the sex chromosome pair of C. pinguis are under-replicated in polytene nuclei; they consist of irregular chromatin granules, frequently associated with nucleolus material. Evolution of heteromorphic sex chromosomes in Chrysomya is probably correlated with heterochromatin accumulation. A search for sex determining genes in Chrysomya was initiated using sex determining sequences from Drosophila melanogaster for in situ hybridization. The polytene band 41A1 on chromosome 5 of monogenic and amphogenic Chrysomya species contains sequences homologous to the maternal sex determining gene daughterless (da). Homology to the zygotic gene Sex-lethal (Sxl) of Drosophila is detected in band 39A1 on chromosome 5 of C. rufifacies. The findings reported here are the first evidence for a possible homology between the da gene of Drosophila and the maternal sex realizer F' of C. rufifacies. An hypothesis for the evolution of the maternal effect sex determination of C. rufifacies is proposed.     

Fertility in mice requires X-Y pairing and a Y-chromosomal "spermiogenesis" gene mapping to the long arm

There is accumulating evidence that the mammalian Y chromosome, in addition to its testis-determining function, may have other male limited functions, particularly in spermatogenesis. We have previously shown that the short arm of the mouse Y carries information needed for spermatogonial proliferation. This information, together with the testis-determining gene Sry, is contained within the Y-derived sex reversal factor Sxra. XO males carrying a copy of Sxra attached to the X chromosome are nevertheless sterile owing to an almost complete arrest during the meiotic metaphase stages. Here we show that this meiotic block can be overcome by providing a meiotic pairing partner (with no Y-specific DNA) for the XSxra chromosome. However, this does not restore fertility because the sperm produced all have abnormal heads. It is concluded that the Y-specific region of the mouse Y chromosome long arm includes information essential for the normal development of the sperm head.     

Cytogenetic effects of cryopreservation on human sperm: assessment using an improved method for analyzing human sperm chromosomes

OBJECTIVE: To evaluate any cytogenetic effects of cryopreservation on human sperm by comparing the frequencies of sperm chromosome anomalies and sex ratios before and after freezing. METHODS: Using in vitro fertilization of zona-free hamster oocytes, analysis of sperm chromosomes was first performed on portions of fresh human semen samples. The residual semen was then analyzed for sperm chromosomes after cryopreservation for several weeks. Sperm donors were 5 healthy men aged 26-38 years. RESULTS: A total of 166 sperm karyotypes were analyzed, 94 before freezing and 72 after freezing. The results indicated no significant differences between fresh and frozen sperm in either frequencies of aneuploidy (fresh: 0%, frozen: 2.8%) or structural anomalies (fresh: 7.5%, frozen: 9.7%). The sex ratios did not differ from the expected 1:1 ratio under either condition. CONCLUSIONS: The results of these studies indicate that cryopreservation does not exert any cytogenetic mutagenicity on human spermatozoa or alter X/Y ratio of human sperm.     

Genetic interactions between mei-S332 and ord in the control of sister-chromatid cohesion

The Drosophila mei-S332 and ord gene products are essential for proper sister-chromatid cohesion during meiosis in both males and females. We have constructed flies that contain null mutations for both genes. Double-mutant flies are viable and fertile. Therefore, the lack of an essential role for either gene in mitotic cohesion cannot be explained by compensatory activity of the two proteins during mitotic divisions. Analysis of sex chromosome segregation in the double mutant indicates that ord is epistatic to mei-S332. We demonstrate that ord is not required for MEI-S332 protein to localize to meiotic centromeres. Although overexpression of either protein in a wild-type background does not interfere with normal meiotic chromosome segregation, extra ORD+ protein in mei-S332 mutant males enhances nondisjunction at meiosis II. Our results suggest that a balance between the activity of mei-S332 and ord is required for proper regulation of meiotic cohesion and demonstrate that additional proteins must be functioning to ensure mitotic sister-chromatid cohesion.     

The role of chromosome ends during meiosis in Caenorhabditis elegans

Chromosome ends have been implicated in the meiotic processes of the nematode Caenorhabditis elegans. Cytological observations have shown that chromosome ends attach to the nuclear membrane and adopt kinetochore functions. In this organism, centromeric activity is highly regulated, switching from multiple spindle attachments all along the chromosome during mitotic division to a single attachment during meiosis. C. elegans chromosomes are functionally monocentric during meiosis. Earlier genetic studies demonstrated that the terminal regions of the chromosomes are not equivalent in their meiotic potentials. There are asymmetries in the abilities of the ends to recombine when duplicated or deleted. In addition, mutations in single genes have been identified that mimic the meiotic effects of a terminal truncation of the X chromosome. The recent cloning and characterization of the C. elegans telomeres has provided a starting point for the study of chromosomal elements mediating the meiotic process.     

Telomere-mediated chromosome pairing during meiosis in budding yeast

Certain haploid strains of Saccharomyces cerevisiae can undergo meiosis, but meiotic prophase progression and subsequent nuclear division are delayed if these haploids carry an extra chromosome (i. e., are disomic). Observations indicate that interactions between homologous chromosomes cause a delay in meiotic prophase, perhaps to allow time for interhomolog interactions to be completed. Analysis of meiotic mutants demonstrates that the relevant aspect of homolog recognition is independent of meiotic recombination and synaptonemal complex formation. A disome in which the extra chromosome is circular sporulates without a delay, indicating that telomeres are important for homolog recognition. Consistent with this hypothesis, fluorescent in situ hybridization demonstrates that a circular chromosome has a reduced capacity to pair with its homolog, and a telomere-associated meiotic protein (Ndj1) is required to delay sporulation in disomes. A circular dimer containing two copies of the same chromosome delays meiosis to the same extent as two linear homologs, implying that physical proximity bypasses the requirement for telomeres in homolog pairing. Analysis of a disome carrying two linear permuted chromosomes suggests that even nonhomologous chromosome ends can promote homolog pairing to a limited extent. We speculate that telomere-mediated chromosome movement and/or telomere clustering promote homolog pairing.     

Analysis of the sex chromosomal equipment in spermatozoa of a 47,XYY male using two-colour fluorescence in-situ hybridization

The sex chromosomes in spermatozoa of a 47,XYY fertile male were analysed simultaneously by dual fluorescence in-situ hybridization (FISH), with two probes (pHY2.1 and pXBR). Of the 100000 cells analysed, 95179 spermatozoa (95.18%) exhibited one or more hybridization signals. Of the hybridized nuclei, 85.37% showed a normal sex chromosome constitution (37.37% X-bearing cells and 48.00% Y-bearing cells), with an X:Y ratio of 0.78:1. A total of 14.63% of the hybridized nuclei exhibited sex chromosome aneuploidy with a majority of XY- and YY-bearing spermatozoa (9.37 and 4.65% respectively). Even if the majority of spermatozoa have chromosomal haploidy, a large proportion of them exhibits numerical errors for the sex chromosomes. These observations raise questions about the commonly-admitted notions concerning the absence of chromosomal risk for XYY male offspring.     

Chromosomal strategies for adaptation to univalency

The orientation and segregation behaviour of different types of univalents, namely sex chromosomes, B chromosomes and autosomal univalents, were analysed in living spermatocytes of eight evolutionarily distant grasshopper species. The meiotic behaviour of each univalent was characterized in terms of velocity of prometaphase movements, frequency of reorientations, types of final orientation at metaphase I and modes of segregation at anaphase I. All these features were found to vary between different univalents. Certain combinations of these traits, defining a 'chromosomal strategy', appear commonly together in certain chromosome types, indicating that they are the result of selection acting on the chromosomes to increase transmission effectiveness. The sex univalents show in general a strategy in which all the features favouring an eventual equational segregation at anaphase I tend to be minimized. There is much more variation in behaviour among B chromosomes than among X chromosomes, which is a reflection of their heterogeneous nature. Induced autosomal univalents are studied in Locusta migratoria. They show a very irregular behaviour, indicating their lack of adaptation to univalency.     

Enigma of Y chromosome degeneration: neo-Y and neo-X chromosomes of Drosophila miranda a model for sex chromosome evolution

Y chromosome degeneration is characterized by structural changes in the chromosome architecture and expansion of genetic inertness along the Y chromosome. It is generally assumed that the heteromorphic sex chromosome pair has developed from a pair of homologues. Several models have been suggested. We use the unique situation of the secondary sex chromosome pair, neo-Y and neo-X (X2), in Drosophila miranda to analyze molecular mechanisms involved in the evolutionary processes of Y chromosome degeneration. Due to the fusion of one of the autosomes to the Y chromosome (about 2 Mya), a neo-Y chromosome and a neo-X chromosome, designated X2, were formed. Thus, formerly autosomal genes are inherited now on a pair of sex chromosomes in D. miranda. Analyzing DNA sequences from the X2 and neo-Y region, we observed a massive accumulation of DNA insertions on the neo-Y chromosome. From the analysis of several insertion elements, we present compelling evidence that the first step in Y chromosome degeneration is driven by the accumulation of transposable elements, especially retrotransposons. An enrichment of these elements along an evolving Y chromosome could account for the switch from a euchromatic into a heterochromatic chromatin structure.     

Assessment of General Dental Services orthodontic standards: the Dental Practice Board''s gradings compared to PAR and IOTN

Sexual isolation between the sibling species D. simulans and D. mauritiana is due largely to the rejection of D. simulans males by D. mauritiana females. Genetic analysis shows that genes on the X and third chromosomes contribute to the differences between males causing sexual isolation, while the Y chromosome, second chromosome and cytoplasm have no effect. These chromosome effects differ from those observed in a previous analysis of sexual isolation in hybrid females, implying that different genes cause sexual isolation in the two sexes.     

The chromosomal basis of sexual isolation in two sibling species of Drosophila: D. arizonensis and D. mojavensis

The chromosomal determination of interspecific differences in mating behavior was studied in the interfertile pair, Drosophila arizonensis and Drosophila mojavensis, by means of chromosomal substitutions. Interspecific crossing over was avoided by crossing hybrid males to parental females, and identification of the origin of each chromosome in backcrossed hybrids was possibly by means of allozyme markers. It was found that male mating behavior is controlled by factors located in the PGM-marked chromosome (which, in other Drosophila species, is part of the X chromosome) and in the Y chromosome. The other chromosomes influence male sexual behavior through their interactions with each other and with the PGM-marked chromosome, but their overall effect is minor. Female mating behavior is controlled by factors located in the ODH-marked and AMY-marked chromosomes, with the other chromosomes exercising a small additive effect. Hence, the two sex-specific behaviors are under different genetic control. Cytoplasmic origin has no effect on the mating behavior of either sex. There appears to be no correlation between a chromosome's structural diversity (i.e., amounts of inversion polymorphism within a species or numbers of fixed inversions across species) and its contribution to sexual isolation. These findings are in general agreement with those from similar Drosophila studies and may not be specific to the species studied here.