High frequency of unequal recombination in pseudoautosomal region shown by proviral insertion in transgenic mouse

High-speed rail model reveals the gene tandem amplification mediated by short repeated sequence in eukaryote

The occurrence of gene duplication/amplification (GDA) provide potential material for adaptive evolution with environmental stress. Several molecular models have been proposed to explain GDA, recombination via short stretches of sequence similarity plays a crucial role. By screening genomes for such events, we propose a “SRS (short repeated sequence) *N + unit + SRS*N” amplified unit under USCE (unequal sister-chromatid exchange) for tandem amplification mediated by SRS with different repeat numbers in eukaryotes. The amplified units identified from 2131 well-organized amplification events that generate multi gene/element copy amplified with subsequent adaptive evolution in the respective species. Genomic data we analyzed showed dynamic changes among related species or subspecies or plants from different ecotypes/strains. This study clarifies the characteristics of variable copy number SRS on both sides of amplified unit under USCE mechanism, to explain well-organized gene tandem amplification under environmental stress mediated by SRS in all eukaryotes.

Evolution of the Autism-Associated Neuroligin-4 Gene Reveals Broad Erosion of Pseudoautosomal Regions in Rodents

Variants in genes encoding synaptic adhesion proteins of the neuroligin family, most notably neuroligin-4, are a significant cause of autism spectrum disorders in humans. Although human neuroligin-4 is encoded by two genes, NLGN4X and NLGN4Y, that are localized on the X-specific and male-specific regions of the two sex chromosomes, the chromosomal localization and full genomic sequence of the mouse Nlgn4 gene remain elusive. Here, we analyzed the neuroligin-4 genes of numerous rodent species by direct sequencing and bioinformatics, generated complete drafts of multiple rodent neuroligin-4 genes, and examined their evolution. Surprisingly, we find that the murine Nlgn4 gene is localized to the pseudoautosomal region (PAR) of the sex chromosomes, different from its human orthologs. We show that the sequence differences between various neuroligin-4 proteins are restricted to hotspots in which rodent neuroligin-4 proteins contain short repetitive sequence insertions compared with neuroligin-4 proteins from other species, whereas all other protein sequences are highly conserved. Evolutionarily, these sequence insertions initiate in the clade eumuroidea of the infraorder myomorpha and are additionally associated with dramatic changes in noncoding sequences and gene size. Importantly, these changes are not exclusively restricted to neuroligin-4 genes but reflect major evolutionary changes that substantially altered or even deleted genes from the PARs of both sex chromosomes. Our results show that despite the fact that the PAR in rodents and the neuroligin-4 genes within the rodent PAR underwent massive evolutionary changes, neuroligin-4 proteins maintained a highly conserved core structure, consistent with a substantial evolutionary pressure preserving its physiological function.

Ensuring meiotic DNA break formation in the mouse pseudoautosomal region

Sex chromosomes in males of most eutherian species share only a diminutive homologous segment, the pseudoautosomal region (PAR), wherein double-strand break (DSB) formation, pairing, and crossing over must occur for correct meiotic segregation^1,2. How cells ensure PAR recombination is unknown. Here we delineate an unexpected dynamic ultrastructure of the PAR and identify controlling cis- and trans-acting factors that make this the hottest area of DSB formation in the male mouse genome. Before break formation, multiple DSB-promoting factors hyper-accumulate in the PAR, its chromosome axes elongate, and the sister chromatids separate. These phenomena are linked to heterochromatic mo-2 minisatellite arrays and require MEI4 and ANKRD31 proteins but not axis components REC8 or HORMAD1. We propose that the repetitive PAR sequence confers unique chromatin and higher order structures crucial for recombination. Chromosome synapsis triggers collapse of the elongated PAR structure and, remarkably, oocytes can be reprogrammed to display spermatocyte-like PAR DSB levels simply by delaying or preventing synapsis. Thus, sexually dimorphic behavior of the PAR rests in part on kinetic differences between the sexes for a race between maturation of PAR structure, DSB formation, and completion of pairing and synapsis. Our findings establish a mechanistic paradigm of sex chromosome recombination during meiosis.

A pronounced evolutionary shift of the pseudoautosomal region boundary in house mice

The pseudoautosomal region (PAR) is essential for the accurate pairing and segregation of the X and Y chromosomes during meiosis. Despite its functional significance, the PAR shows substantial evolutionary divergence in structure and sequence between mammalian species. An instructive example of PAR evolution is the house mouse Mus musculus domesticus (represented by the C57BL/6J strain), which has the smallest PAR among those that have been mapped. In C57BL/6J, the PAR boundary is located just ~700 kb from the distal end of the X chromosome, whereas the boundary is found at a more proximal position in Mus spretus, a species that diverged from house mice 2-4 million years ago. In this study we used a combination of genetic and physical mapping to document a pronounced shift in the PAR boundary in a second house mouse subspecies, Mus musculus castaneus (represented by the CAST/EiJ strain), ~430 kb proximal of the M. m. domesticus boundary. We demonstrate molecular evolutionary consequences of this shift, including a marked lineage-specific increase in sequence divergence within Mid1, a gene that resides entirely within the M. m. castaneus PAR but straddles the boundary in other subspecies. Our results extend observations of structural divergence in the PAR to closely related subspecies, pointing to major evolutionary changes in this functionally important genomic region over a short time period.

Homologous illegitimate random integration of foreign DNA into the X chromosome of a transgenic mouse line

Background

It is not clear how foreign DNA molecules insert into the host genome. Recently, we have produced transgenic mice to investigate the role of the fad2 gene in the conversion of oleic acid to linoleic acid. Here we describe an integration mechanism of fad2 transgene by homologous illegitimate random integration.

Results

We confirmed that one fad2 line had a sole integration site on the X chromosome according to the inheritance patterns. Mapping of insertion sequences with thermal asymmetric interlaced and conventional PCR revealed that the foreign DNA was inserted into the XC1 region of the X chromosome by a homologous illegitimate replacement of an entire 45,556-bp endogenous genomic region, including the ovarian granulosa cell tumourigenesis-4 allele. For 5' and 3' junction sequences, there were very short (3-7 bp) common sequences in the AT-rich domains, which may mediate the recognition of the homologous arms between the transgene and the host genome. In addition, analysis of gene transcription indicated that the transgene was expressed in all tested fad2 tissues and that its transcription level in homozygous female tissues was about twice as high as in the heterozygous female (p < 0.05).

Conclusions

Taken together, the results indicated that the foreign fad2 behaved like an X-linked gene and that foreign DNA molecules were inserted into the eukaryotic genome through a homologous illegitimate random integration.

The mouse A/HeJ Y chromosome: another good Y gone bad

In both humans and mice there are numerous reports of Y chromosome abnormalities that interfere with sex determination. Recent studies in the mouse of one such mutation have identified Y chromosome nondisjunction during preimplantation development as the cause of abnormal testis determination that results in a high frequency of true hermaphroditism. We report here that the mouse Y chromosome from the A/HeJ inbred strain induces similar aberrations in sex determination. Our analyses provide evidence, however, that the mechanism underlying these aberrations is not Y chromosome nondisjunction. On the basis of our findings, we postulate that a mutation at or near the centromere affects both the segregation and sex-determining properties of the A/HeJ Y chromosome. This Y chromosome adds to the growing list of Y chromosome aberrations in humans and mice. In both species, the centromere of the Y is structurally and morphologically distinct from the centromeres of all other chromosomes. We conclude that these centromeric features make the human and mouse Y chromosomes extremely sensitive to minor structural alterations, and that our studies provide yet another example of a good Y chromosome gone 'bad.'

Rapid divergency of rodent CD99 orthologs: implications for the evolution of the pseudoautosomal region

The human pseudoautosomal region 1 (PAR1) is essential for the obligatory X-Y crossover in male meiosis. Despite its critical role, comparative studies of human and mouse pseudoautosomal genes have been limited owing to the scarcity of genes conserved between the two species. Human CD99 is a 32-kDa cell surface protein that is encoded by the MIC2 gene localized to the PAR1. Although several sequences such as CD99L2, PBDX, and CD99L1 are related to CD99, its murine ortholog, Cd99, has not yet been identified. Here we report a novel mouse Cd99, designated D4, which shows overall sequence homology to CD99, with the highest conservation between the two genes being found in the transmembrane regions. In addition, the D4 protein displays biochemical characteristics, functional homology, and expression patterns similar to those of CD99. The D4 gene is localized on an autosome, chromosome 4, reflecting a common mapping feature with other mouse orthologs of human PAR1 genes. Furthermore, a phylogenetic analysis of CD99-related genes confirmed that the D4 gene is indeed an ortholog of CD99 and exhibits the accelerated evolution pattern of CD99 orthologs, as compared to the CD99L2 orthologs. On the basis of these findings, we suggest that CD99 belongs to the ancient PAR genes, and that the rapid interspecies divergence of its present sequence and map position is due to a high recombination frequency and the occurrence of chromosomal translocation, supporting the addition-attrition hypothesis for PAR evolution.

Recombination has little effect on the rate of sequence divergence in pseudoautosomal boundary 1 among humans and great apes

Recent studies indicated that recombination is strongly mutagenic. In particular, data from the mouse pseudoautosomal boundary (PAB) suggested that locally intensive recombination increased the nucleotide substitution rate by more than 100-fold and greatly increased the GC content. Here we study the rates of nucleotide substitution in eight introns of the human and great ape XG gene, which spans the boundary between the pseudoautosomal region 1 (PAR1) and the X-specific region. Contrary to what is expected under the above hypothesis, our sequence data from humans and great apes reveal that the PAR1 introns of XG have actually evolved slightly slower than X-specific introns. Only when a New World monkey was compared with hominoids were the rates slightly increased in the PAR1 introns. In terms of base composition, although the intergenic regions of the human PAR1 show a significant increase of G and C nucleotides, the base composition of the surveyed PAR1 introns is similar to that of the X-specific introns. Direct and indirect evidence indicates that the recombination rate is, indeed, much higher in PAR1 introns than in X-specific introns, and that the present PAB has persisted since the common ancestor of hominoids. Therefore, the mutagenic effect of recombination is far weaker than previously proposed, at least in hominoid PABs.

Differential divergence of three human pseudoautosomal genes and their mouse homologs: implications for sex chromosome evolution

The human pseudoautosomal region 1 (PAR1) is essential for meiotic pairing and recombination, and its deletion causes male sterility. Comparative studies of human and mouse pseudoautosomal genes are valuable in charting the evolution of this interesting region, but have been limited by the paucity of genes conserved between the two species. We have cloned a novel human PAR1 gene, DHRSXY, encoding an oxidoreductase of the short-chain dehydrogenase/reductase family, and isolated a mouse ortholog Dhrsxy. We also searched for mouse homologs of recently reported PGPL and TRAMP genes that flank it within PAR1. We recovered a highly conserved mouse ortholog of PGPL by cross-hybridization, but found no mouse homolog of TRAMP. Like Csf2ra and Il3ra, both mouse homologs are autosomal; Pgpl on chromosome 5, and Dhrsxy subtelomeric on chromosome 4. TRAMP, like the human genes within or near PAR1, is probably very divergent or absent in the mouse genome. We interpret the rapid divergence and loss of pseudoautosomal genes in terms of a model of selection for the concentration of repetitive recombinogenic sequences that predispose to high recombination and translocation.

A short pseudoautosomal region in laboratory mice

The pseudoautosomal region (PAR) of mammalian sex chromosomes is a small region of sequence identity that is the site of an obligatory pairing and recombination event between the X and Y chromosomes during male meiosis. During female meiosis, X chromosomes can pair and recombine along their entire length; recombination in the PAR is therefore approximately 10x greater in male meiosis compared with female meiosis. A consequence of the presence of the PAR in two copies in males and females is that genes in the region escape the process of X-inactivation. Although the structure and gene content of the human PAR at Xq/Yq is well understood, the mouse PAR, which appears to be of independent evolutionary origin, is poorly characterized. Here we describe a yeast artificial chromosome (YAC) contig covering the distal part of the mouse X chromosome, which we have used to define the pseudoautosomal boundary, that is, the point of divergence of X-specific and X-Y-identical sequences. In addition, we have investigated the size of the mouse PAR by integrating a unique restriction endonuclease recognition site just proximal to the pseudoautosomal boundary by homologous recombination. Restriction digestion of this modified DNA and pulsed field gel electrophoresis reveal that the PAR in these cells is approximately 700 kb. Thus, the mouse PAR, although small in size, has retained essential sex chromosome pairing functions despite its rapid rate of evolution.

A gene spans the pseudoautosomal boundary in mice

The X and Y chromosomes of the mouse, like those of other mammals, are heteromorphic over most of their length, but at the distal ends of the chromosomes is a region of sequence identity, the pseudoautosomal region (PAR), where the chromosomes pair and recombine during male meiosis. The point at which the PAR diverges into X- and Y-specific sequences is called the pseudoautosomal boundary. We have completed a genomic walk from the X-specific Amelogenin gene to the PAR. Analysis of this region revealed that the pseudoautosomal boundary of mice is located within an intron of a transcribed gene that encodes a novel RING finger protein. The first three of the exons of the gene are located on the X chromosome whereas the 3' exons of the gene are located on both X and Y chromosomes. This unusual arrangement may indicate that the gene is in a state of transition from pseudoautosomal to X-unique and provides evidence for a process of attrition of the pseudoautosomal region on the Y chromosome.

Telomeres in the mouse have large inter-chromosomal variations in the number of T2AG3 repeats

The ultra-long telomeres that have been observed in mice are not in accordance with the concept that critical telomere shortening is related to aging and immortalization. Here, we have used quantitative fluorescence in situ hybridization to estimate (T2AG3)n lengths of individual telomeres in various mouse strains. Telomere lengths were very heterogeneous, but specific chromosomes of bone marrow cells and skin fibroblasts from individual mice had similar telomere lengths. We estimate that the shortest telomeres are around 10 kb in length, indicating that each mouse cell has a few telomeres with (T2AG3)n lengths within the range of human telomeres. These short telomeres may be critical in limiting the replicative potential of murine cells.

High frequency de novo alterations in the long-range genomic structure of the mouse pseudoautosomal region

The pseudoautosomal region (PAR) is a segment of shared homology between the X and Y chromosomes. Here we report physical linkage of three mouse PAR probes: DXYHgu1, DXYMov15 and (TTAGGG)n. Steroid sulphatase (Sts) maps distal to these probes, indicating that there is an internal array of the telomere sequence (TTAGGG)n in the PAR. Pseudoautosomal PacI restriction fragments, up to 2 Mb in size, are unstable in C57BL/6 x C57BL/6 crosses. New alleles, often several hundred kilobases different in size, occur at a sex-averaged rate of approximately 30% per allele. Such frequent large-scale germline genome arrangements are without precedent in mammals.

Structural variation of the pseudoautosomal region between and within inbred mouse strains

The pseudoautosomal region (PAR) is a segment of shared homology between the sex chromosomes. Here we report additional probes for this region of the mouse genome. Genetic and fluorescence in situ hybridization analyses indicate that one probe, PAR-4, hybridizes to the pseudoautosomal telomere and a minor locus at the telomere of chromosome 9 and that a PCR assay based on the PAR-4 sequence amplifies only the pseudoautosomal locus (DXYHgu1). The region detected by PAR-4 is structurally unstable; it shows polymorphism both between mouse strains and between animals of the same inbred strain, which implies an unusually high mutation rate. Variation occurs in the region adjacent to a (TTAGGG)n array. Two pseudoautosomal probes can also hybridize to the distal telomeres of chromosomes 9 and 13, and all three telomeres contain DXYMov15. The similarity between these telomeres may reflect ancestral telomere-telomere exchange.

Isolation and characterization of a pseudoautosomal region-specific genetic marker in C57BL/6 mice using genomic representational difference analysis

Representational difference analysis was used to identify strain-specific differences in the pseudoautosomal region (PAR) of mouse X and Y chromosomes. One second generation (C57BL/6 x Mus spretus) x Mus spretus interspecific backcross male carrying the C57BL/6 (B6) PAR was used for tester DNA. DNA from five backcross males from the same generation that were M. spretus-type for the PAR was pooled for the driver. A cloned probe designated B6-38 was recovered that is B6-specific in Southern analysis. Analysis of genomic DNA from several inbred strains of laboratory mice and diverse Mus species and subspecies identified a characteristic Pst I pattern of fragment sizes that is present only in the C57BL family of strains. Hybridization was observed with sequences in DBA/2J and to a limited extent with Mus musculus (PWK strain) and Mus castaneus DNA. No hybridization was observed in DNA of different Mus species, M. spretus, M. hortulanus, and M. caroli. Genetic analyses of B6-38 was conducted using C57BL congenic males that carry M. spretus alleles for distal X chromosome loci and the PAR and outcrosses of heterozygous congenic females with M. spretus. These analyses demonstrated that the B6-38 sequences were inherited with both the X and Y chromosome. B6-38 sequences were genetically mapped as a locus within the PAR using two interspecific backcrosses. The locus defined by B6-38 is designated DXYRp1. Preliminary analyses of recombination between the distal X chromosome gene amelogenin (Amg) and the PAR loci for either TelXY or sex chromosome association (Sxa) suggest that the locus DXYRp1 maps to the distal portion of the PAR.

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 human pseudoautosomal GM-CSF receptor alpha subunit gene is autosomal in mouse

The gene encoding the granulocyte macrophage colony stimulating factor receptor alpha subunit (CSF2RA) has previously been mapped to the pseudoautosomal region of the human sex chromosomes. In contrast, we report that the murine locus, Csf2ra, maps to an autosome in the laboratory mouse. By in situ hybridization and genetic mapping, Csf2ra maps at telomeric band D2 of mouse chromosome 19. This first instance of a pseudoautosomal locus in human being autosomal in mouse, indicates incomplete conservation between the human and mouse X chromosomes and suggests that the genetic content of the pseudoautosomal region may differ between species of eutherian mammals due to chromosomal rearrangements.

The identification of Y chromosome-linked markers with random sequence oligonucleotide primers

The polymerase chain reaction (PCR)-based technique of random amplification of polymorphic DNA (RAPD) is extremely useful for developing DNA-based markers. We previously identified a linkage group of eight unmapped RAPD markers that distinguish C57BL/6J and DBA/2J mice (Mammalian Genome 3: Woodward et al., 73-78, 1992). In this study, we report that all eight markers are Y Chromosome (Chr)-linked. One additional Y-linked RAPD was discovered serendipitously during the screening of a C3H/HeJ x (C3H/HeJ x SJL/J)F1 BC1 population. The segregation of all nine markers was analyzed with a panel of 14 independent inbred strains of male mice. The nine markers could be divided into three distinct groups: (1) DYByu2, DYByu5, DYByu6, and DYByu8 identify both the M.m. musculus and M.m. domesticus type Y Chr; (2) DYByu1, DYByu3, DYByu4, and DYByu7 are specific for the M.m. musculus type; and (3) DYByu9 is specific for the M.m. domesticus type. The results clearly indicate that the RAPD technique can be used to identify Y Chr-linked, DNA-based markers in mammalian species.

Recombination between the X and Y chromosomes and the Sxr region of the mouse

The Sxr (sex-reversed) region that carries a copy of the mouse Y chromosomal testis-determining gene can be attached to the distal end of either the Y or the X chromosome. During male meiosis, Sxr recombined freely between the X and Y chromosomes, with an estimated recombination frequency not significantly different from 50% in either direction. During female meiosis, Sxr recombined freely between the X chromosome to which it was attached and an X-autosome translocation. A male mouse carrying the original Sxra region on its Y chromosome, and the shorter Sxrb variant on the X, also showed 50% recombination between the sex chromosomes. Evidence of unequal crossing-over between the two Sxr regions was obtained: using five markers deleted from Sxrb, 3 variant Sxr regions were detected in 159 progeny (1.9%). Four other variants (one from the original cross and three from later generations) were presumed to have been derived from illegitimate pairing and crossing-over between Sxrb and the homologous region on the short arm of the Y chromosome. The generation of new variants throws light on the arrangement of gene loci and other markers within the short arm of the mouse Y chromosome.

Telomere-related markers for the pseudoautosomal region of the mouse genome

The pseudoautosomal (PA) region of the mammalian genome is the region of the X and Y chromosomes that shares extensive DNA sequence homology and is of special interest because it may play an essential role during male meiosis. We have identified three telomere-related restriction fragments from the PA region of the mouse genome, using an oligonucleotide probe composed of the mammalian telomere consensus sequence TTAGGG. PA assignment of two C57BL/6J-derived fragments was initially suggested by analysis of DNAs from progeny sired by C57BL/6J males carrying the rearranged Y chromosome, Y*: the hybridization intensity of both fragments was concordant with the sex-chromosome complement of the offspring. Further analysis indicated that both fragments were present in female and male F1, mice regardless of the sex of their C57BL/6J parent--a criterion for autosomal or PA linkage. Both fragments were closely linked to each other and located on the X chromosome distal to amelogenin (Amg)--in agreement with X or PA linkage. Confirmation of the PA derivation of these fragments was accomplished by following their segregation in a cross involving XY* males mated to DBA/2J females. A similar experiment identified a third PA-derived restriction fragment of LT/SvEi origin. Identification of PA-derived telomere-related restriction fragments will enable further genetic analysis of this region of the mouse genome.

Application of a locus-specific DNA hybridization probe in the analysis of the slow-feathering endogenous virus complex of chickens

To investigate further the sex-linked, slow-feathering (SF) locus, a DNA hybridization probe that flanks the integration site of endogenous virus ev21 was used to probe Southern blots from a variety of commercial SF White Leghorn (WL), broiler, and endangered lines. After HaeIII digestion of DNA from SF WL, the 5' and 3' provirus-cell junction fragments were seen in addition to a 2.5-kb fragment of cell sequences that was homologous with the viral integration region. The latter polymorphism, which appeared to represent sequences duplicated after integration of EV21, was designated unoccupied repeat URa. Among SF broiler crosses, the same provirus-cell junction fragments were found but the pristine region that represented the 1.6-kb proviral URb was also found. Only URb was found among rapid-feathering (RF) chickens, regardless of breed. Although there was marked (greater than 95%), agreement between the presence of ev21-cell; junction fragments and the SF phenotype among both WL and broilers, Southern blots of DNA from a few commercial SF broiler chickens lacked ev21 junction fragments but some RF revertants harbored ev21 junction fragments. These anomalies suggest that ev21 may not be the sole determinant of the SF phenotype.

Tg (9 HSA-MYC), a homozygous lethal insertion in the mouse

Transgenic mice generated with different DNA sequences were surveyed for possible homozygous mutant phenotypes. We found an embryonic lethal mutation in the transgenic mouse strain (MT-MYC12.4) containing the human c-myc gene. Embryos homozygous for the transgene die shortly after implantation. The strain MT-MYC12.4 carries approximately 50 tandem copies of the recombinant plasmid sequence. The 3' flanking sequence has been cloned and analyzed. It contains a unique sequence that has been conserved during evolution and maps to Chromosome (Chr) 9. This mutant has been designated Tg 9 (HSA-MYC).

Characterization of the central region containing the X-inactivation center and terminal region of the mouse X chromosome using irradiation and fusion gene transfer hybrids

The irradiation and fusion gene transfer (IFGT) procedure provides a means of isolating subchromosomal fragments for use in the mapping of loci and for cloning probes from a particular area of a chromosome. Using this procedure, two large panels of somatic cell hybrids that contain mouse X Chromosome (Chr) fragments have been generated. These hybrid panels were generated by irradiating the monochromosomal mouse-hamster hybrid HYBX, which retains the mouse X Chr, with either 10 K or 50 K rads of X-irradiation followed by fusion with a recipient Chinese hamster cell line. IFGT hybrids retaining mouse material were generated at high frequency. These hybrids were used to orient loci in the X-inactivation center region that had not been resolvable in our interspecies backcross panel and also to map, within the terminal region of the X Chr, repeat elements detected by the probe p15-4. These hybrids not only complement existing interspecies meiotic mapping panels for the detailed analysis of specific regions of particular chromosomes, but also provide a potential source of material for chromosome-specific probe isolation.

Unequal crossing over and heterochromatin exchange in the X-Y bivalents of the deer mouse, Peromyscus beatae

Differences in length of the heterochromatic short arms of the X and Y chromosomes in individuals of Peromyscus beatae are hypothesized to result from unequal crossing over. To test this hypothesis, we examined patterns of synapsis, chiasma formation, and segregation for male P. beatae which were either heterozygous or homozygous for the amount of short-arm sex heterochromatin. Synaptonemal complex analysis demonstrated that mitotic differences in heterochromatic short-arm lengths between the X and Y chromosomes were reflected in early pachynema as corresponding differences in axial element lengths within the pairing region of the sex bivalent. These length differences were subsequently eliminated by synaptic adjustment such that by late pachynema, the synaptonemal complex configurations of the XY bivalent of heterozygotes were not differentiable from those of homozygotes. Crossing over between the heterochromatic short arms of the XY bivalent was documented by the routine appearance of a single chiasma in this region during diakinesis/metaphase I. Sex heterochromatin heterozygotes were characterized by the presence of asymmetrical chiasma between the X and Y short arms at diakinesis/metaphase I and sex chromosomes with unequal chromatid lengths at metaphase II. These data corroborate our hypothesis on the role of unequal crossing over in the production and propagation of X and Y heterochromatin variation and suggest that, in some cases, crossing over can occur during the process of synaptic adjustment.

Nature of the genetic contribution to psychotic illness--a continuum viewpoint

The recurrent psychoses, rather than, as Kraepelin supposed, constituting 2 major entities, manic depressive illness and schizophrenia, as separate diseases, may be distributed along a continuum that extends from unipolar depressive illness through bipolar and schizoaffective psychosis to schizophrenia with increasing severities of defect state. It is proposed that this continuum rests on a genetic base, variations in the form of the gene accounting for variations in form of psychosis. The simplest interpretation of the continuum is that such variation relates to changes at a single genetic locus. Evidence from a postmortem study of brain structure in schizophrenia suggests that this is the gene that determines the development of asymmetries in the human brain, i.e., the cerebral dominance gene or right shift factor of Annett; a possible genomic location is in the pseudoautosomal region of the sex chromosomes.

Genesis by meiotic unequal crossover of a de novo deletion that contributes to steroid 21-hydroxylase deficiency

The HLA-linked human steroid 21-hydroxylase gene CYP21B and its closely homologous pseudogene CYP21A are each normally located centromeric to a fourth component of complement (C4) gene, C4B and C4A, respectively, in an organization suggesting tandem duplication of a ca. 30-kilobase DNA unit containing a CYP21 gene and a C4 gene. Such an organization has been considered to facilitate gene deletion and addition events by unequal crossover between the tandem repeats. We have identified a steroid 21-hydroxylase [steroid, hydrogen-donor:oxygen oxidoreductase (21-hydroxylating), EC 1.14.99.10] deficiency patient who has a maternally inherited disease haplotype that carries a de novo deletion of a ca. 30-kilobase repeat unit including the CYP21B gene and associated C4B gene. This disease haplotype appears to have been generated as a result of meiotic unequal crossover between maternal homologous chromosomes. One of the maternal haplotypes is the frequently occurring HLA-DR3, B8, A1 haplotype that normally carries a deletion of a ca. 30-kilobase unit including the CYP21A gene and C4A gene. Haplotypes of this type may possibly act as premutations, increasing the susceptibility of developing a 21-hydroxylase deficiency mutation by facilitating unequal chromosome pairing.

A rapidly rearranging retrotransposon within the miniexon gene locus of Crithidia fasciculata

The tandemly arrayed miniexon genes of the trypanosomatid Crithidia fasciculata are interrupted at specific sites by multiple copies of an inserted element. The element, termed Crithidia retrotransposable element 1 (CRE1), is flanked by 29-base-pair target site duplications and contains a long 3'-terminal poly(dA) stretch. A single 1,140-codon reading frame is similar in sequence to the integrase and reverse transcriptase regions of retroviral pol polyproteins. Cloned lines derived from a stock of C. fasciculata have unique arrangements of CRE1s. In different cloned lines, CRE1s, in association with miniexon genes, are located on multiple chromosomes. By examining the arrangement of CRE1s in subclones, we estimate that the element rearranges at a rate of ca. 1% per generation. These results indicate that the C. fasciculata miniexon locus is the target for a novel retrotransposon.

A rapidly rearranging retrotransposon within the miniexon gene locus of Crithidia fasciculata

The tandemly arrayed miniexon genes of the trypanosomatid Crithidia fasciculata are interrupted at specific sites by multiple copies of an inserted element. The element, termed Crithidia retrotransposable element 1 (CRE1), is flanked by 29-base-pair target site duplications and contains a long 3'-terminal poly(dA) stretch. A single 1,140-codon reading frame is similar in sequence to the integrase and reverse transcriptase regions of retroviral pol polyproteins. Cloned lines derived from a stock of C. fasciculata have unique arrangements of CRE1s. In different cloned lines, CRE1s, in association with miniexon genes, are located on multiple chromosomes. By examining the arrangement of CRE1s in subclones, we estimate that the element rearranges at a rate of ca. 1% per generation. These results indicate that the C. fasciculata miniexon locus is the target for a novel retrotransposon.

The evolution of the mammalian Y chromosome

There is a predominant theory for the evolution of the mammalian Y chromosome. This theory hypothesizes that genes for sex determination and male-specific traits, as well as sequences for X-Y meiotic pairing, are conserved on the mammalian Y chromosome across all lineages and that all other Y chromosomal genes or sequences have been or will be lost in each mammalian lineage. There are effects of mouse Y chromosomal genes on behaviors and other traits that are not male specific. Under the predominant theory, these Y chromosomal genes could be the same as the conserved genes for sex determination or male-specific traits, or they could be genes that have been lost from the Y chromosomes of other mammalian lineages and that will eventually be lost from the Y chromosome of the rodent lineage. Recently, the evolution of the primate and rodent Y chromosomes has been studied at the DNA level. These studies are summarized and reviewed in this article. The findings of these studies are not fully consistent with the predominant theory for the evolution of the mammalian Y chromosome. Also, they imply that there are other possibilities for the phylogenetic history of Y chromosomal genes of mice with effects on behavior. These are that Y chromosomal genes with effects on mouse behaviors or other traits could be conserved genes other than those for sex determination or male-specific traits or that they could be novel genes on the Y chromosome of the rodent or Mus lineage.

The human X-linked steroid sulfatase gene and a Y-encoded pseudogene: evidence for an inversion of the Y chromosome during primate evolution

The mammalian X and Y chromosomes are thought to have evolved from a common, nearly homologous chromosome pair. Although there is little sequence similarity between the mouse or the human X and Y, there are several regions in which moderate to extensive sequence homologies have been found, including, but not limited to, the so-called pseudoautosomal segment, in which X-Y pairing and recombination take place. The steroid sulfatase gene is in the pseudoautosomal region of the mouse, but not in man. We have cloned and characterized the human STS X-encoded locus and a pseudogene that is present on the long arm of the Y chromosome. Our data in humans and other primates suggest that there has been a pericentric inversion of the Y chromosome during primate evolution that has disrupted the former pseudoautosomal arrangement of these genes. These results provide additional insight into the evolution of the sex chromosomes and into the nature of this interesting portion of the human genome.

Molecular aspects of sex determination in mice: an alternative model for the origin of the Sxr region

Using a combination of in situ mapping and DNA analysis with recombinant DNA probes specific for the Sxr region of the mouse Y chromosome, we show that both the gene(s) controlling primary sex determination and the expression of the male-specific antigen H-Y (Tdy and Hya respectively) are located on the minute short arm of the mouse Y chromosome. We demonstrate that the H-Y- variant of Sxr (Sxr') arose by a partial deletion within the Sxr region and propose an alternative model for the generation of the original Sxr region.

Molecular and cytogenetic evidence for the location of Tdy and Hya on the mouse Y chromosome short arm

Using a combination of in situ mapping and DNA analysis with recombinant DNA probes specific for the Sxr region of the mouse Y chromosome, we show that both the gene(s) controlling sex determination and the expression of the male-specific antigen H-Y (Tdy and Hya, respectively) are located on the minute short arm of the mouse Y chromosome. We demonstrate that the H-Y- variant of Sxr (Sxr') arose by a partial deletion within the Sxr region. Also, we show that intrachromosomal recombination between the Y short arm and Sxr' can sometimes occur during male meiosis, restoring the deleted DNA sequences and resulting in an H-Y+ mouse (male 719 in this paper). Based on these results, we propose a model for the generation of the original Sxr region and the Sxr' and Sxr719 variants.

Transgenic animals

The ability to introduce foreign genes into the germ line and the successful expression of the inserted gene in the organism have allowed the genetic manipulation of animals on an unprecedented scale. The information gained from the use of the transgenic technology is relevant to almost any aspect of modern biology including developmental gene regulation, the action of oncogenes, the immune system, and mammalian development. Because specific mutations can be introduced into transgenic mice, it becomes feasible to generate precise animal models for human genetic diseases and to begin a systematic genetic dissection of the mammalian genome.

Heterogeneity in the Sxr (sex-reversal) locus of the mouse as revealed by synthetic GATA-GACA probes

The sex-reversal mutation, Sxr and a variant form, Sxr′ have been established on the inbred C57BL/6Mcl background by repeated backcrossing to form the CB and CB′ strains, respectively. DNAs of normal XY, XX Sxr and XX Sxr′ as well as XY Sxr and XY Sxr′ carrier male mice have been digested with the restriction enzymes Hae III and Hinf I and electrophoresed. The DNAs show many common but also differing hybridization bands with synthetic oligonucleotide probes. In XY Sxr (and XY Sxr′) carrier males, the hybridization patterns of normal XY and those of XX Sxr (and XX Sxr′) males are simply superimposed. Individual differing bands can be categorized by their differential hybridization behaviour to the (GATA)4, (GACA)4, (GATA)2 GACA (GATA)2 and (GATA)3 (GACA)2 probes. In general, the hybridization patterns are regularly inherited. In addition to the predominant pattern in each strain, one additional XX Sxr and one additional XX Sxr′ hybridization pattern was observed: the additional pattern in the CB strain was transmitted (via variant XY Sxr carriers) while the secondary XX Sxr′ pattern in the CB′ strain could only be observed once. Thus ‘DNA finger printing’ with oligonucleotide probes can successfully be used to discriminate the DNAs of normal Y chromosomes, XX Sxr and XX Sxr′ variants as well as XY Sxr and XY Sxr′ carrier mice. Implications of the comparatively high unequal recombination rate involving the murine Y chromosome are discussed, as well as possible mechanisms.

Restriction fragment polymorphism in the sex-determining region of the Y chromosomal DNA of European wild mice

Using 32P-labeled probe consisting mainly of (GATA)n we have shown that a male specific Alu1 DNA blot pattern which defines the Y chromosome sex-determining locus in inbred mice is highly polymorphic in wild mice, indicating substantial sequence evolution in this region under field conditions. In all cases examined by in situ hybridization, the region concerned is paracentromeric. In contrast, the blot pattern of another probe (M 34) which detects repeated sequences specific to the mouse Y chromosome but outside the sex-determining locus, remains constant between different isolates.

Site of unequal sister chromatid exchange contains a potential Z-DNA-forming tract

Previous studies from this laboratory have provided evidence that the duplication of the IgG2a heavy chain constant region gene (C gamma 2a) in the murine myeloma cell line MPC-11 occurred via unequal sister chromatid exchange. We now report the determination of the DNA sequences of the germ-line regions in which this exchange has occurred. The two donor sequences have long regions of virtual identity. Furthermore, both chromatids contain stretches of T-C and T-G dinucleotides; the latter has the potential to form Z-DNA. Localization of the actual breakpoints via genomic Southern blot analysis suggests a novel mechanism for the recombination and strongly implicates the simple sequence in the process.

Linkage of the murine steroid sulfatase locus, Sts, to sex reversed, Sxr: a genetic and molecular analysis

We present genetic and molecular data demonstrating linkage of the gene for steroid sulfatase (Sts) to the mutation sex reversed (Sxr) definitively showing the existance of a functional allele for Sts mapping to the pseudoautosomal region of the mouse Y chromosome. Thus, in mouse, functional Sts genes are present in the pseudoautosomal region of both the X and Y chromosomes. This is in contrast to man where Sts has been mapped to the short arm of the X just centromeric to the pseudoautosomal region. Only a single recombinant separating Sts and Sxr was found out of 103 male meioses analyzed; double recombinants were not found between sex (Tdy), Sts and Sxr. If the rate of recombination in the pseudoautosomal region in male mice is equivalent to that in man and thus 7-10X higher than normal, then our data suggest that the distance between Sts and Sxr (or the telomere of the Y) is approximately 100-200 kb in length. Our data is in contrast to a recent report of a recombination frequency separating Sts and Sxr of as high as 6.2-9.8%.

High rate of recombination and double crossovers in the mouse pseudoautosomal region during male meiosis

The recombination rate in meiosis between the mouse X and Y chromosomes was analyzed. Mice heterozygous at two pseudoautosomal alleles, the steroid sulfatase gene and the Mov-15 provirus marker, were crossed. The provirus in the Mov-15 transgenic mouse strain had been previously shown to be carried in the pseudoautosomal region of the sex chromosomes. Recombination frequencies were shown to be 7-fold higher in this region in male meiosis than in female meiosis. Three-point crosses indicated the occurrence in male meiosis of double recombination events in the pseudoautosomal region, with little or no crossover interference, which is in marked contrast to observations made on the similar region of the human sex chromosomes. This result is contrary to a previous model, which predicted a single crossover event in male meiotic pairing of mammalian sex chromosomes.

Retroviruses and insertional mutagenesis in mice: proviral integration at the Mov 34 locus leads to early embryonic death

Thirty-four transgenic mouse strains, each carrying a single proviral insert, were generated by infection of preimplantation and postimplantation embryos with retroviruses. Animals homozygous for proviral integrations were derived for all strains with the exception of Mov 24, where the provirus is inserted on the Y chromosome, and Mov 34. Embryos homozygous at the Mov 34 locus develop normally to the blastocyst stage and die shortly after implantation, indicating that virus integration resulted in a recessive lethal mutation. The provirus and flanking sequences were cloned and the virus was mapped to the 5' side of an abundantly and ubiquitously transcribed gene. Similar to the previously derived Mov 13 mutation, proviral integration at the Mov 34 locus interferes with the expression of the adjacent gene. These and our previous results indicate that of a total of 48 proviral integrations in the germ line, two resulted in transgenic mouse strains with recessive lethal mutations.

Cloning and expression of steroid sulfatase cDNA and the frequent occurrence of deletions in STS deficiency: implications for X-Y interchange

Human STS is a microsomal enzyme important in steroid metabolism. The gene encoding STS is pseudoautosomal in the mouse but not in humans, and escapes X inactivation in both species. We have prepared monoclonal and polyclonal antibodies to the protein which has been purified and from which partial amino acid sequence data have been obtained. cDNA clones containing the entire coding sequence were isolated, sequenced, and expressed in heterologous cells. Variable length transcripts have been shown to be present and due to usage of alternative poly(A) addition sites. The functional gene maps to Xp22.3-Xpter and there is a pseudogene on Yq suggesting a recent pericentric inversion. Absence of STS enzymatic activity occurs frequently in human populations and produces a visible phenotype of scaly skin or ichthyosis. Ten patients with inherited STS deficiency were studied and eight had complete gene deletions. The possibility that STS deficiency results from aberrant X-Y interchange is discussed.