Adding the heterochromatic YL arm to an X chromosome reduces reproductive fitnesses in Drosophila melanogaster: implications for the evolution of rDNA, heterochromatin, and reproductive isolation

For X-Y exchange to be of importance in the coevolution of X and Y rDNA, there must be a mechanism to maintain cytologically normal X chromosomes in the face of continual infusions of X.YL chromosomes produced by X-Y exchanges. Replicated populations were founded with different frequencies of isogenic X and X.YL chromosomes. The X.YL chromosome declined in frequency over time in all lines. Relative fitnesses, estimated from chromosome frequency trajectories, were 0.40, 1.01, and 1.0 for X.YL/X.YL, X.YL/X, and X/X females and 0.75 and 1.0 for X.YL/Y and X/Y males, respectively. The equilibrium frequency for the X.YL chromosome due to the balance between X-Y exchange and selection was predicted to be 4-16 x 10(-4). The results strengthen the evidence for the involvement of X-Y exchange in the coevolution of X and Y rDNA arrays. Conditions for the evolution of reproductive isolation by sex-chromosome translocation are much less probable than previously supposed since the X.YL translocation chromosome is at a selective disadvantage to cytologically normal X chromosomes. Additional heterochromatin was not neutral but was only deleterious beyond a threshold, as one dose of the heterochromatic XL arm did not reduce female reproductive fitness, but two doses did.     

The genetic architecture of selection response. Inferences from fine-scale mapping of bristle number quantitative trait loci in Drosophila melanogaster.

Quantitative trait loci (QTL) affecting responses and correlated responses to selection for abdominal and sternopleural bristle number have been mapped with high resolution to the X and third chromosomes. Advanced intercross recombinant isogenic chromosomes were constructed from high and low selection lines in an unselected inbred background, and QTL were detected using composite interval mapping and high density transposable element marker maps. We mapped a total of 26 bristle number QTL with large effects, which were in or immediately adjacent to intervals previously inferred to contain bristle number QTL on these chromosomes. The QTL contributing to response to selection for high bristle number were not the same as those contributing to response to selection for low bristle number, suggesting that distributions of allelic effects per locus may be asymmetrical. Correlated responses were more often attributable to loose linkage than pleiotropy or close linkage. Bristle number QTL mapping to the same locations have been inferred in studies with different parental strains. Of the 26 QTL, 20 mapped to locations consistent with candidate genes affecting peripheral nervous system development and/or bristle number. This facilitates determining the molecular basis of quantitative variation and allele frequencies by associating molecular variation at the candidate genes with phenotypic variation in bristle number in samples of alleles from nature.     

Unequal exchanges and the coevolution of X and Y rDNA arrays in Drosophila melanogaster

We have examined the molecular basis of the response of individuals of D. melanogaster to artificial selection for high and low abdominal bristles. By monitoring the fate of particular rDNA spacer length variants associated with individually isolated X and Y chromosomes, we show that flies from the low bristle number selection lines have undergone an unequal exchange between the X and Y rDNA arrays. Such exchanges result in translocations between X and Y chromosomes, visualised as X.Y compound chromosomes at mitosis. Transfer of few copies of a length variant between X and Y indicates a clustering of variants. Flies that have reverted back to wild-type seemingly have undergone a second unequal exchange, giving rise to a compound X.Y chromosome containing Y rDNA of normal amounts. Unequal exchanges between X and Y rDNA arrays could contribute to the observed coevolution of rDNA sequences on these chromosomes. The biological significance of this outcome is discussed.     

Genomic imprinting: male mice with uniparentally derived sex chromosomes

Although it has been known that there is an X-chromosome imprinting effect during early embryogenesis in female mammals, it remains unknown if parental origin of the X chromosome has an effect in males. Furthermore, it has not been possible to produce animals with normal sex chromosomes of uniparental origin to further evaluate such imprinting effects. We have devised a breeding scheme to produce male mice, designated XPYP males, in which both the X and Y chromosomes are paternally inherited. To our knowledge, these are the first mammals produced that have a normal sex chromosome constitution but with both sex chromosomes derived from one parent. Development and reproduction in these XPYP males and the sex ratio and chromosome constitution of their offspring appeared normal; thus there is no apparent effect in males of having both sex chromosomes derive from one parent or of having the X chromosome derived from an inappropriate parent. Although we have detected no X-chromosome imprinting effect in these males, evidence from other sources suggest that the X chromosome is parentally imprinted. Thus detection and definition of an imprint can depend on the assay used.     

The effect of isogenization on the phenotypic manifestation of quantitative traits in Drosophila melanogaster

A comparative analysis of the phenotypic values of the proximal and distal fragments of the radial wing vein was carried out in heterogeneous lines of Drosophila melanogaster and in isogenic lines derived from them with the help of a balancer line. The mean values of the traits in the isogenic lines were shown to significantly differ from the corresponding values in the "parental" heterogeneous lines. Apparently, the change in the trait values was caused by a double recombination exchange between the inverted and the "normal" chromosomes, which suggests partial crossing over suppression in the balancer lines.     

Nonhomologous synapsis of the XY during early pachynema in In(X)1H male mice

It has been previously supposed that meiotic synapsis is restricted to homology during early, but not late pachynema. The synaptic begavior of an inverted X chromosome, In(X)1H as reflected in the synaptonemal complexes of the sex chromosomes has been examined in microspread spermatocytes by electron microscopy and evidence of extensive nonhomologus synapsis between the X and Y during early pachynema has been obtained.     

High Resolution Mapping of Genetic Factors Affecting Abdominal Bristle Number in Drosophila Melanogaster

Factors responsible for selection response for abdominal bristle number and correlated responses in sternopleural bristle number were mapped to the X and third chromosome of Drosophila melanogaster. Lines divergent for high and low abdominal bristle number were created by 25 generations of artificial selection from a large base population, with an intensity of 25 individuals of each sex selected from 100 individuals of each sex scored per generation. Isogenic chromosome substitution lines in which the high (H) X or third chromosome were placed in an isogenic low (L) background were derived from the selection lines and from the 93 recombinant isogenic (RI) HL X and 67 RI chromosome 3 lines constructed from them. Highly polymorphic neutral r00 transposable elements were hybridized in situ to the polytene chromosomes of the RI lines to create a set of cytogenetic markers. These techniques yielded a dense map with an average spacing of 4 cM between informative markers. Factors affecting bristle number, and relative viability of the chromosome 3 RI lines, were mapped using a multiple regression interval mapping approach, conditioning on all markers >/=10 cM from the tested interval. Two factors with large effects on abdominal bristle number were mapped on the X chromosome and five factors on the third chromosome. One factor with a large effect on sternopleural bristle number was mapped to the X and two were mapped to the third chromosome; all factors with sternopleural effects corresponded to those with effects on abdominal bristle number. Two of the chromosome 3 factors with large effects on abdominal bristle number were also associated with reduced viability. Significant sex-specific effects and epistatic interactions between mapped factors of the same order of magnitude as the additive effects were observed. All factors mapped to the approximate positions of likely candidate loci (ASC, bb, emc, h, mab, Dl and E(spl)), previously characterized by mutations with large effects on bristle number.     

Low frequencies of apparently fragile X chromosomes in normal control cultures: a possible explanation

Low frequencies of apparently fragile X [fra(X)] chromosomes have been reported in normal control, short-term, whole blood cultures, and they have been noted in both amniocyte and fetal blood cultures. However, there is currently no universal agreement on the lowest frequency for fra(X)(q27) that is diagnostic for the fragile X syndrome. Here, we present our observations on low levels of apparently fra(X) chromosomes in normal samples. We observed frequencies of 0.5% in short-term whole blood cultures and 0.9% in amniotic fluid cell cultures. In 1982, Steinbach et al. described nonspecific telomeric structural changes (TSC) and suggested that such low frequencies of apparently fra(X) chromosomes in normal material may be occurring by the same mechanism that is responsible for TSC formation. To determine if TSC formation can explain the significant baseline frequencies of fra(X) in normal controls, 10,457 cells were screened from 178 individuals referred for fra(X) analysis. Our findings indicated that TSC are not randomly distributed across chromosomes but tend to occur at specific sites. Based on our observations, we offer the hypothesis that the low frequency of apparent fra(X) in normal individuals may be due to nonrandom TSC distribution.     

Different patterns of X chromosome inactivity in lymphocytes and fibroblasts of a human balanced X; autosome translocation

Summary In lymphocytes of a human female carrier of a balanced X;3 translocation, 46,X,t(X;3)(q28;q21), late replication of the structurally normal X chromosome only was previously described (de la Chapelle and Schröder 1973). We have now confirmed this finding using a fresh blood sample. Examining the chromosomes of this individual in fibroblasts we observed that either the normal X or the Xq+ chromosome could replicate late and show inactivity after fusion with heteroploid mouse cells. The replication patterns of chromosomes in human X;autosome translocations have so far almost exclusively been analyzed in lymphocytes. Our findings stress that results based on these cells are not representative for all cell types.     

A Case of High Rate of Spontaneous Mutation Affecting Viability in DROSOPHILA MELANOGASTER

A spontaneous lethal mutation rate approximately twenty to thirty times greater than normal has been discovered in second and third chromosomes derived from an irradiated isogenic line and paired with marked inversion chromosomes. Mutations resulting in reductions of viability of varying magnitude short of complete lethality apparently also occur at a very high rate in the third but not in the second chromosome. The pattern of accumulation of lethal mutations over several generations and viability frequency distributions within generations have been studied in a number of independent experiments. High mutation rate occurs in heterozygous isogenic-derived second and third chromosomes, either together or apart, irrespective of the genetic constitution of nonhomologous chromosomes. High mutation rates were not observed using the same methods with chromosomes of an inbred line from a different source. The possible mechanisms responsible for these results are discussed.     

Steroid sulfatase gene in XX males.

The human X and Y chromosomes pair and recombine at their distal short arms during male meiosis. Recent studies indicate that the majority of XX males arise as a result of an aberrant exchange between X and Y chromosomes such that the testis-determining factor gene (TDF) is transferred from a Y chromatid to an X chromatid. It has been shown that X-specific loci such as that coding for the red cell surface antigen, Xg, are sometimes lost from the X chromosome in this aberrant exchange. The steroid sulfatase functional gene (STS) maps to the distal short arm of the X chromosome proximal to XG. We have asked whether STS is affected in the aberrant X-Y interchange leading to XX males. DNA extracted from fibroblasts of seven XX males known to contain Y-specific sequences in their genomic DNA was tested for dosage of the STS gene by using a specific genomic probe. Densitometry of the autoradiograms showed that these XX males have two copies of the STS gene, suggesting that the breakpoint on the X chromosome in the aberrant X-Y interchange is distal to STS. To obtain more definitive evidence, cell hybrids were derived from the fusion of mouse cells, deficient in hypoxanthine phosphoribosyltransferase, and fibroblasts of the seven XX males. The X chromosomes in these patients could be distinguished from each other when one of three X-linked restriction-fragment-length polymorphisms was used. Hybrid clones retaining a human X chromosome containing Y-specific sequences in the absence of the normal X chromosome could be identified in six of the seven cases of XX males.(ABSTRACT TRUNCATED AT 250 WORDS)     

Comparisons of recombination frequencies in hybrids involving telocentric and bibrachial wheat chromosomes

Telosomic stocks have been extensively used to map genes to chromosome arms and to determine gene-to-centromere genetic distances. It has been suggested that if a chromosome arm is present as a telosome, recombination frequencies will be drastically reduced in the centromeric region. However, previous studies have not considered the bias in recombination estimates due to selection against aneuploid gametes produced by failure of pairing at the first meiotic division. Formulas are derived here for adjusting recombination estimates for this bias. Adjusted recombination frequencies between markers located on both sides of the centromeres are analyzed in three different pairs of wheat (Triticum aestivum) isogenic segregating populations involving bibrachial and telocentric chromosomes. Recombination frequencies estimated from crosses involving telocentric chromosomes were not significantly different from recombination frequencies estimated from isogenic crosses involving bibrachial chromosomes. The implications of the present findings for karyotype evolution, and specifically for Robertsonian fissions and fusions, are discussed. Received: 10 March 1999 / Accepted: 17 June 1999     

The pseudoautosomal regions of the human sex chromosomes

In human females, both X chromosomes are equivalent in size and genetic content, and pairing and recombination can theoretically occur anywhere along their entire length. In human males, however, only small regions of sequence identity exist between the sex chromosomes. Recombination and genetic exchange is restricted to these regions of identity, which cover 2.6 and 0.4 Mbp, respectively, and are located at the tips of the short and the long arm of the X and Y chromosome. The unique biology of these regions has attracted considerable interest, and complete long-range restriction maps as well as comprehensive physical maps of overlapping YAC clones are already available. A dense genetic linkage map has disclosed a high rate of recombination at the short arm telomere. A consequence of the obligatory recombination within the pseudoautosomal region is that genes show only partial sex linkage. Pseudoautosomal genes are also predicted to escape X-inactivation, thus guaranteeing an equal dosage of expressed sequences between the X and Y chromosomes. Gene pairs that are active on the X and Y chromosomes are suggested as candidates for the phenotypes seen in numerical X chromosome disorders, such as Klinefelter's (47,XXY) and Turner's syndrome (45,X). Several new genes have been assigned to the Xp/Yp pseudoautosomal region. Potential associations with clinical disorders such as short stature, one of the Turner features, and psychiatric diseases are discussed. Genes in the Xq/Yq pseudoautosomal region have not been identified to date.     

The number of x chromosomes causes sex differences in adiposity in mice

Sexual dimorphism in body weight, fat distribution, and metabolic disease has been attributed largely to differential effects of male and female gonadal hormones. Here, we report that the number of X chromosomes within cells also contributes to these sex differences. We employed a unique mouse model, known as the "four core genotypes," to distinguish between effects of gonadal sex (testes or ovaries) and sex chromosomes (XX or XY). With this model, we produced gonadal male and female mice carrying XX or XY sex chromosome complements. Mice were gonadectomized to remove the acute effects of gonadal hormones and to uncover effects of sex chromosome complement on obesity. Mice with XX sex chromosomes (relative to XY), regardless of their type of gonad, had up to 2-fold increased adiposity and greater food intake during daylight hours, when mice are normally inactive. Mice with two X chromosomes also had accelerated weight gain on a high fat diet and developed fatty liver and elevated lipid and insulin levels. Further genetic studies with mice carrying XO and XXY chromosome complements revealed that the differences between XX and XY mice are attributable to dosage of the X chromosome, rather than effects of the Y chromosome. A subset of genes that escape X chromosome inactivation exhibited higher expression levels in adipose tissue and liver of XX compared to XY mice, and may contribute to the sex differences in obesity. Overall, our study is the first to identify sex chromosome complement, a factor distinguishing all male and female cells, as a cause of sex differences in obesity and metabolism.     

A Cis-Acting Locus That Promotes Crossing over between X Chromosomes in Caenorhabditis Elegans

This study reports the characterization of a cis-acting locus on the Caenorhabditis elegans X chromosome that is crucial for promoting normal levels of crossing over specifically between the X homologs and for ensuring their proper disjunction at meiosis I. The function of this locus is disrupted by the mutation me8, which maps to the extreme left end of the X chromosome within the region previously implicated by studies of X;A translocations and X duplications to contain a meiotic pairing site. Hermaphrodites homozygous for a deletion of the locus (Df/Df) or heterozygous for a deletion and the me8 mutation (me8/Df) exhibit extremely high levels of X chromosome nondisjunction at the reductional division; this is correlated with a sharp decrease in crossing over between the X homologs as evidenced both by reductions in genetic map distances and by the presence of achiasmate chromosomes in cytological preparations of oocyte nuclei. Duplications of the wild-type region that are unlinked to the X chromosome cannot complement the recombination and disjunction defects in trans, indicating that this region must be present in cis to the X chromosome to ensure normal levels of crossing over and proper homolog disjunction. me8 homozygotes exhibit an altered distribution of crossovers along the X chromosome that suggests a defect in processivity along the X chromosome of an event that initiates at the chromosome end. Models are discussed in which the cis-acting locus deleted by the Dfs functions as a meiotic pairing center that recruits trans-acting factors onto the chromosomes to nucleate assembly of a crossover-competent complex between the X homologs. This pairing center might function in the process of homolog recognition, or in the initiation of homologous synapsis.     

The Genetic Analysis of Distributive Segregation in Drosophila Melanogaster. I. Isolation and Characterization of Aberrant X Segregation (Axs), a Mutation Defective in Chromosome Partner Choice

We describe the isolation and characterization of Aberrant X segregation (Axs), a dominant female-specific meiotic mutation. Although Axs has little or no effect on the frequency or distribution of exchange, or on the disjunction of exchange bivalents, nonexchange X chromosomes undergo nondisjunction at high frequencies in Axs/+ and Axs/Axs females. This increased X chromosome nondisjunction is shown to be a consequence of an Axs-induced defect in distributive segregation. In Axs-bearing females, fourth chromosome nondisjunction is observed only in the presence of nonexchange X chromosomes and is argued to be the result of improper X and fourth chromosome associations within the distributive system. In XX females bearing a compound fourth chromosome, the frequency of nonhomologous disjunction of the X chromosomes from the compound fourth chromosome is shown to account for at least 80% of the total X nondisjunction observed. In addition, Axs diminishes or ablates the capacity of nonexchange X chromosomes to form trivalents in females bearing either a Y chromosome or a small free duplication for the X. Axs also impairs compound X from Y segregation. The effect of Axs on these segregations parallels the defects observed for homologous nonexchange X chromosome disjunction in Axs females. In addition to its dramatic effects on the X chromosome, Axs exerts a similar effect on the segregation of a major autosome. We conclude that Axs defines a locus required for proper homolog disjunction within the distributive system.     

Paarungsverhalten der Geschlechtschromosomen in der männlichen Meiose von Microtus agrestis

In premeiotic stages of the male, the entire Y chromosome and the heterochromatio 3/4 of the X chromosome remain heavily condensed. Pairing of the sex chromosomes does not occur during zygotene. The sex vesicle stage lasts from middle pachytene to the beginning of diplotene. At the more advanced diplotene stages, X and Y lie again separate; chiasma formation has not been observed. Thus, it seems improbable that any pairing occurs at all between X and Y during meiosis. The findings support the hypothesis that heterochromatin does not participate in meiotic exchange, independent of possible homologies between the chromosome segments.     

Exchange of genetic information between therian X and Y chromosome gametologs in old evolutionary strata

Therian X and Y sex chromosomes arose from a pair of autosomes. Y chromosomes consist of a pseudoautosomal region that crosses over with the X chromosome and a male‐specific Y‐chromosomal region that does not. The X chromosome can be structured into “evolutionary strata”. Divergence of X‐chromosomal genes from their gametologs is similar within a stratum, but differs among strata, likely caused by a different onset of suppression of crossing over between gametologs. After stratum formation, exchange of information between gametologs has long been believed absent; however, recent studies have shown limited exchange, likely through gene conversion. Herein we investigate exchange of genetic information between gametologs in old strata that formed before the split of Laurasiatheria (cattle) from Euarchontoglires (primates and rodents) with a new phylogenetic approach. A prerequisite for our test is an overall preradiative topology, that is, all X‐chromosomal gametologs are more similar among themselves than to Y‐chromosomal sequences. Screening multiple sequence alignments of the coding sequences of genes from cattle, mice, and humans identified four genes, DDX3X/Y, RBMX/Y, USP9X/Y, and UTX/Y, exhibiting a preradiation topology. Applying our test, we detected exchange of genetic information between all four X and Y gametologs after stratum formation.     

Illegitimate pairing of the X and Y chromosomes in Sxr mice

X/Y male mice carrying the sex reversal factor, Sxr, on their Y chromosomes typically produce 4 classes of progeny (recombinant X/X Sxr male male and X/Y non-Sxr male male, and non-recombinant X/X female female and X/Y Sxr male male) in equal frequencies, these deriving from obligatory crossing over between the chromatids of the X and Y during meiosis. Here we show that X/Y males that, exceptionally, carry Sxr on their X chromosome, rather than their Y, produce fewer recombinants than expected. Cytological studies confirmed that X-Y univalence is frequent (58%) at diakinesis as in X/Y Sxr males, but among those cells with X-Y bivalents only 38% showed normal X-Y pseudo-autosomal pairing. The majority of such cells (62%) instead showed an illegitimate pairing between the short arms of the Y and the Sxr region located at the distal end of the X, and this can be understood in terms of the known homology between the testis-determining region of the Y short arm and that of the Sxr region. This pairing was sufficiently tenacious to suggest that crossing over took place between the 2 regions, and misalignment and unequal exchange were suggested by indications of bivalent asymmetry. Metaphase II cells deriving from meiosis I divisions in which the normal X-Y exchange had not occurred were also found. The cytological data are therefore consistent with the breeding results and suggest that normal pseudo-autosomal pairing and crossing over is not a prerequisite for functional germ cell formation.(ABSTRACT TRUNCATED AT 250 WORDS)     

Chronology of the replication of sex chromosome bands in lymphocytes of normal subjects and patients]

The replication sequence of the bands carried by chromosomes X and Y has been studied in normal individuals and in patients with structural abnormalities of the X. By comparing the segment with that of the autosomal bands (which had been previously studied), it was shown that the normal early X replicates in early X-phase for its R-bands and in late S-phase for its Q bands. The late X replicates entirely in late S-phase, and the sequence of band replication is not as stringent as for the early X and the autosomes. The study of fourteen cases of anomalies of chromosome X in females showed the following: in balanced reciprocal X-autosome translocations the rearranged X most often replicates early and the normal X late. Both show a normal replication sequence of their bands. In non-balanced X-autosome translocations, inactivation of the autosome fragment attached to the AUTOSOME FRAGMENT ATTACHED TO THE X may take place. In Xq- or in ter rea (X;X) (pter;pter), band p22 has a delayed replication. In iso-Xor Xp-, the long-arm-band sequence of replication shows a variation comparable to that of the late X in fibroblasts. These replication modifications are likely to induce partial inactivations or changes in activity which correspond to the so-called position effect in Drosophila.