On the homology between the X and the Y chromosomes of the Chinese hamster

The chiasmatic association of the heteromorphic sex chromosomes in the spermatocytes of the Chinese hamster was observed in squash and/or air-dried preparations. The pairing arm of the Y was invariably its short arm. Although the X in diakinesis did not show distinct long and short arm as in mitotic metaphase, the DNA replication patterns of the sex chromosomes in spermatogonia suggested that the distal segment of the long arm of the X is homologous to the short arm of the Y.     

Shared DNA sequences between the X and Y chromosomes in the tammar wallaby – evidence for independent additions to eutherian and marsupial sex chromosomes

Marsupial sex chromosomes are smaller than their eutherian counterparts and are thought to reflect an ancestral mammalian X and Y. The gene content of this original X is represented largely by the long arm of the human X chromosome. Genes on the short arm of the human X are autosomal in marsupials and monotremes, and represent a recent addition to the eutherian X and Y. The marsupial X and Y apparently lack a pseudoautosomal region and show only end-to-end pairing at meiosis. However, the sex chromosomes of macropodid marsupials (kangaroos and wallabies) are larger than the sex chromosomes of other groups, and a nucleolus organizer is present on the X and occasionally the Y. Chromosome painting using DNA from sorted and microdissected wallaby X and Y chromosomes reveals homologous sequences on the tammar X and Y chromosomes, concentrated on the long arm of the Y chromosome and short arm of the X. Ribosomal DNA sequences were detected by fluorescence in situ hybridization on the wallaby Xp but not the Y. Since no chiasmata have been observed in marsupial sex chromosomes, it is unlikely that these shared sequences act as a pseudoautosomal region within which crossing over may occur, but they may be required for end-to-end associations. The shared region of wallaby X and Y chromosomes bears no homology with the recently added region of the eutherian sex chromosomes, so we conclude that independent additions occurred to both sex chromosomes in a eutherian and macropodid ancestor, as predicted by the addition-attrition hypothesis of sex chromosome evolution. Received: 18 October 1996 / Accepted: 21 February 1997     

The origin of the genetical diversity of Microtus mandarinus chromosomes

Here we describe our comparative studies on two types of X chromosomes, namely X(M) and X(SM,) of the mandarin vole (Microtus mandarinus). By chromosome G- and C-banding analysis, we have found that two different types of X chromosomes exist in mandarin voles. The two types of X chromosomes present two different G- and C-banding patterns: the X(M) chromosome is a longer metacentric X chromosome which is C-band negative; and the X(SM) is a shorter submetacentric X chromosome which has one C-band at the centromere and another one at the middle part of the short arm. The X(SM) has 6 G-bands including one on the kinetochore, one in the middle of the short arm, and four on the long arm. The X(M) has 7 G-bands including one on the kinetochore, two on the short arm, and four on the long arm. We have further found that female voles can be grouped into three types based on the composition of the X chromosome but the male voles have only one type. The three female groups are: (1) female voles (X(M)X(SM)), in which the two X chromosomes are different, the longer one is metacentric and the shorter is submetacentric; (2) female vole (X(SM)X(SM)), in which the two X chromosomes are both submetacentric; (3) female vole (X(M)O), in which there is only one X chromosome that is metacentric. Surprisingly, we have never found female voles with X(M)X(M), females with X(SM)O or males with X(M)Y. We hypothesize that the X(SM) chromosome is derived from the X(M) through its breakage and re-joining. The paper also discusses the formation of X(M)O females.     

MAMMALIAN CHROMOSOMES IN VITRO : XVIII. DNA Replication Sequence in the Chinese Hamster

The complete DNA replication sequence of the entire complement of chromosomes in the Chinese hamster may be studied by using the method of continuous H³-thymidine labeling and the method of 5-fluorodeoxyuridine block with H³-thymidine pulse labeling as relief. Many chromosomes start DNA synthesis simultaneously at multiple sites, but the sex chromosomes (the Y and the long arm of the X) begin DNA replication approximately 4.5 hours later and are the last members of the complement to finish replication. Generally, chromosomes or segments of chromosomes that begin replication early complete it early, and those which begin late, complete it late. Many chromosomes bear characteristically late replicating regions. During the last hour of the S phase, the entire Y, the long arm of the X, and chromosomes 10 and 11 are heavily labeled. The short arm of chromosome 1, long arm of chromosome 2, distal portion of chromosome 6, and short arms of chromosomes 7, 8, and 9 are moderately labeled. The long arm of chromosome 1 and the short arm of chromosome 2 also have late replicating zones or bands. The centromeres of chromosomes 4 and 5, and occasionally a band on the short arm of the X are lightly labeled.     

Mammalian sex chromosomes. II. Pairing and alignment of the X and Y chromosomes of the pygmy mouse, Mus dunni

In the pygmy mouse, Mus dunni, the entire Y chromosome and the short arm of the X and distal region of its long arm are constitutively heterochromatic. Different banding studies on somatic chromosomes revealed the GC nature of the distally located heterochromatin of the long arms of both the X and Y chromosomes. The short arm of the X and the rest of the Y are AT-rich. During meiosis, the long arms of the X and Y paired extensively, sometimes more than half of the Y pairing with the X. This observation is in disagreement with that of Pathak and Hsu (1976) who reported end-to-end pairing between the long arm of the X and the short arm of the Y. The orientation observed by us is favourable to a successful meiotic recombination but whether this takes place remains to be demonstrated.     

Replication pattern of the X and Y chromosomes in partially synchronized human lymphocyte cultures

The replication pattern of the X and Y chromosomes at the beginning of the synthetic phase was studied in human lymphocyte cultures partially synchronized by the addition of 5-fluoro-2-deoxyuridine (FUdR). The data were evaluated statistically by an analysis of the distribution of silver grain counts over the X and Y chromosomes. —In cells from normal females, one of the X chromosomes began replication later than any other chromosomes of the complement. The short arm of the late replicating X chromosome started replication earlier than the long arm. The telomeric region of the short arm was a preferential site of DNA synthesis at the beginning of replication. —In partially synchronized lymphocyte cultures from a patient with the XXY syndrome, the Y chromosome started replication together with the late replicating X chromosome. The Y chromosome most frequently replicated synchronously with the short arm of the X. The centromeric region of the Y chromosome initiated synthesis before the telomeric region and appeared to replicate synchronously with the telomeric region of the short arm of the X. These findings are discussed with reference to the pairing of the X and Y chromosomes at meiosis.Supported in part by the National Institute of Health Research Grant HD-01979 and National Foundation Birth Defects Research Grant CRCS-40. Dr. Knight was a predoctoral fellow under National Institute of Health Training Program HD-00049-09.     

Translocation(X;Y)(p22.33;p11.2) in XX males: Etiology of male phenotype

Summary Analysis of G-banded prometaphase chromosomes from three XX males revealed extra bands on the distal end of one X short arm. These bands were similar both in size and staining properties to the distal Y short arm of their fathers (in the two cases examined) and also to other chromosomally normal males. The extra material on the abnormal X chromosomes was not C-or G-11 positive in the two cases examined, suggesting that the proximal Y long arm was not present.Previous karyotype-phenotype correlations with structurally altered Y chromosomes provided evidence for localization of male determinants on the Y short arm. The present findings in XX males provide support for more precise localization, to bands p11.2pter of Y short arm.     

The behavior and morphology of the X and Y chromosomes during prophase I in the Sitka deer mouse (Peromyscus sitkensis)

Surface-spread, silver-stained primary spermatocytes from individuals of the Sitka deer mouse (Peromyscus sitkensis) were analyzed by electron microscopy. Pairing of the X and Y chromosomes is initiated at early pachynema and is complete by mid pachynema. The pattern of sex chromosome pairing is unique in that it is initiated at an interstitial position, with subsequent synapsis proceeding in a unidirectional fashion towards the telomeres of the homologous segments. One-third the length of the X and two-thirds the length of the Y are involved in the synaptonemal complex of the sex bivalent. Various morphological complexities develop in the heteropycnotic (unpaired) segments as pachynema progresses, but desynapsis is not initiated until diplonema. Analysis of C-banded diakinetic nuclei indicated that sex chromosome pairing involves the heterochromatic short arm of the X and the long arm of the heterochromatic Y. An interstitial chiasma between the X and Y was observed in the majority of the diakinetic nuclei. The observation of a substantial pairing region and chiasma formation between the sex chromosomes of these deer mice is interpreted as indicating homology between the short arm of the X and the long arm of the Y.     

Heteromorphic X chromosomes in 46,XX males: Evidence for the involvement of X-Y interchange

Summary G- and R-banded chromosome preparations from eight of twelve 46,XX males, with no evidence of mosaicism or a free Y chromosome, were distinguished in blind trials from preparations from normal 46,XX females by virtue of heteromorphism of the short arm of one X chromosome. Photographic measurements on X chromosomes and on chromosome pair 7 in cells from twelve 46,XX males, eight 46,XX females, and four 46,XY males revealed a significant increase in the size of the p arm of one X chromosome in the group of XX males, independently characterised as being heteromorphic for Xp. No such differences were observed between X chromosomes of normal males and females or between homologues of chromosome pair 7 in all groups. The heteromorphism in XX males is a consequence of an alteration in shape (banding profile) and length of the tip of the short arm of one X chromosome, and the difference in size of the two Xp arms in these 46,XXp+ males ranged from 0.4% to 22.9%. From various considerations, including the demonstration of a Y-specific DNA fragment in DNA digests from nuclei of one of three XX males tested, it is concluded that the Xp+ chromosome is a product of Xp-Yp exchange. These exchanges are assumed to originate at meiosis in the male parent and may involve an exchange of different amounts of material. The consequences of such unequal exchange are considered in terms of the inheritance of genes located on Yp and distal Xp. No obvious phenotypic difference was associated with the presence or absence of Xp+. Thus, some males diagnosed as 46,XX are mosaic for a cryptic Y-containing cell line, and there is now excellent evidence that maleness in others may be a consequence of an autosomal recessive gene. The present data imply that in around 70% of 46,XX males, maleness is a consequence of the inheritance of a paternal X-Y interchange product.     

X chromosomes attached by their long arm: replication autonomy of the short arm adjacent to the inactive centromere.

A 16 years old girl with Turner syndrome was found to have a 45,X/46,X,t(XqXq)?(q27q23) constitution. The two X chromosomes are attached by their long arms with loss of chromosome material and have one active and one inactive centromere. Analysis of replication patterns with autoradiography and BrdU treatment showed that the abnormal X is always the late replicating one and that the short arm of the second X which is adjacent to the inactive centromere maintains a degree of replication autonomy from the rest of the long arm.     

Differential response of constitutive and facultative heterochromatin in the manifestation of mitomycin induced chromosome aberrations in Chinese hamster cells in vitro

The distribution of chromatid aberrations induced by mitomycin C among the individual chromosomes of female and male Chinese hamster cells in vitro was studied. The aberrations were found to be non-randomly distributed. Among the autosomes, the chromosomes possessing constitutive heterochromatin were more often involved in aberrations as well as in homologous exchanges. The inactivated X chromosomes in the female cells offer a situation where the short arm is facultatively heterochromatic and the long arm constitutively heterochromatic, thus enabling an analysis of their response for aberration formation. The short arm was seldom found to be involved in the aberration. The long arm of the inactivated X was more often affected (5 to 10 times) than the long arm of the functional X though both are constitutively heterochromatic. The possible role of (a) structure of heterochromatin, (b) the chromocenter formation and their association, (c) allocycly, and (d) the qualitative differences in the DNA of different types of heterochromatin are discussed in relation to the formation of chromatid aberrations.     

Does differential selection on the 5S rDNA explain why the rainbow trout sex chromosome heteromorphism is not linked to the SEX locus?

Many but not all rainbow trout strains have morphologically distinguishable sex chromosomes. In these strains, the short arm of the X has multiple copies of 5S rDNA and a bright DAPI band near the centromere, both of which are missing from the Y chromosome, which has a very small short arm. We examined the presence of these markers using fluorescence in situ hybridization (FISH) in four different YY clonal lines derived from different strains and compared the results with sexed fish of the Donaldson strain with the normal X/Y heteromorphism. The Y chromosome in two of the YY clonal lines (Arlee and Swanson) is indistinguishable from the X chromosome and it is positive for 5S rDNA and the DAPI bright band. On the other hand, both 5S rDNA sequences and the DAPI band were not found on the Y chromosome in Hot Creek and Clearwater which have the normal Y. Thus the presence of these two cytogenetic markers may account for the size difference between the short arm of the X and Y chromosome found in most rainbow trout strains. In fishes the expression of one type of 5S rRNA is restricted to oocytes and previous work suggests that although XX males are fairly common, XY females are rare, implying a selective disadvantage for XY females. A hypothesis is presented to explain why this sex chromosome heteromorphism is not closely linked to the SEX locus, which is found on the long arm of the Y chromosome in rainbow trout.     

The chromosomes of Micromys minutus (Rodentia, Murinae). II. Pairing pattern of X and Y chromosomes in meiotic prophase

Both light and electron microscopy were used to study the pairing behavior of the sex chromosomes of the harvest mouse, Micromys minutus, in surface-spread pachytene spermatocytes. The XY pairing pattern is very exceptional in that the site of synaptic initiation is located interstitially in the short arms of the X and the Y, next to their centromeric regions. From this tiny euchromatic site, synapsis proceeds unidirectionally along the homologous heterochromatic short arms of the X and the Y toward the ends of the chromosomes. After pairing of the short arm is concluded, synapsis begins between the nonhomologous long arms of the X and the Y in the immediate vicinity of the centromeres and progresses unidirectionally toward the end of the long arm of the Y. A synaptic complex develops between the constitutive heterochromatin of the long arm of the Y and the euchromatin of the long arm of the X. Analysis of C-banded and distamycin A/DAPI-stained diakineses revealed a trefoil-like XY bivalent, which was interpreted to be the result of an interstitial chiasma occurring in the paired short arms of the X and the Y. A conspicuous, electron-dense body, about 1 micron in diameter, was found closely associated with the centromeres of the X and the Y in numerous pachytene spermatocytes. A review of the literature showed that comparable XY-associated bodies have been found in only eight other mammals to date.     

Sex chromosome-autosome translocations in the leaf-nosed bats, family Phyllostomidae. I. Mitotic analyses of the subfamilies Stenodermatinae and Phyllostominae

Mitotic analyses using RBA- and C-banding were performed on Stenodermatine bats with X-autosome (XY1Y2) and X- and Y- autosome (neo-XY) translocations. RBA-banded metaphases of females revealed differential replication of the inactive X chromosome. An early replicating band comprises the short arm of the X, and an intermediate replicating band is located interstitially on the long arm. The early replicating short arm has a homologous counterpart either in the form of a free autosome (the Y2) or as part of the Y. Both the "autosomal" short arm of the X and its homologue fused to the Y are C-band negative and behave autonomously from the remainder of the sex chromosomes. They are separated from X and Y chromatin by centromeric heterochromatin which presumably acts as a barrier. The intermediate replicating region of the long arm of the X is also present in the subfamily Phyllostominae. In both subfamilies this region lacks a homologous counterpart. However, it may also represent a translocated autosome which, unlike the short arm of the X, is not separated from the inactive X by centromeric heterochromatin. Its intermediate replication time may represent a retarded replication due to its juxtaposition to late replicating X chromatin. These data are discussed in light of the theory of the evolution of sex chromosome heteromorphism, specifically as it applies to mammals.     

Polymorphic patterns of heterochromatin distribution in guinea pig chromosomes

The chromosome complement and patterns of heterochromatin distribution (as demonstrated by the DNA d-r method) were studied from three different guinea pigs. Karyotype analyses showed that one of the females had a heteromorphic sex pair formed by a submetacentric X chromosome and a subterminal X chromosome originated by a shortening of the short arm (x-chromosome). The heterochromatin was mainly found in the pericentromeric areas of the autosomes and X chromosomes and in the short arm of pair 7. The Y chromosome exhibited a degree of heterochromatinization different from that of pericentromeric areas.—The analysis of the heterochromatin distribution in the X chromosomes showed that the smaller size of the heteromorphic x-chromosoine was probably due to a lack of heterochromatin in its short arm. Moreover, two out of the three animals studied had a heteromorphic pattern of heterochromatinization in the pair 21 characterized by heterochromatinization of the pericentromeric area in one chromosome and almost complete heterochromatinization of the other homologue.—It is suggested that most of the heterochromatin disclosed by the DNA d-r method is formed by repetitious DNA; and that the Y chromosome and perhaps some autosome regions in guinea pigs are formed by a type of heterochromatin with properties different from those of the constitutive and facultative heterochromatin (intermediate heterochromatin).Supported in part by NIH Grant 5-501-RR05672-02 and by NIH contract 70-2299.     

Purification of the chromosomes of the Indian muntjac by flow sorting.

Metaphase chromosomes were isolated from a male Indian muntjac cell line, were stained with ethidium bromide and were analyzed by flow microfluorometry to establish a deoxyribonucleic acid (DNA)-based karyotype. Five major peaks were evident on the chromosomal DNA distribution corresponding to the five chromosome types in this species. The amount of DNA in each chromosome was confirmed by cytophotometric measurements of intact metaphase spreads. The five chromosome types were separated by flow sorting at rates up to several hundred chromosomes per second. The sorted chromosomes were identified by morphology and by Giemsa banding patterns. The automsomes, Numbers 1, 2 and 3, and the X + 3 composite chromosome were separated with a high degree of purity (90%). The centromere region of the X + 3 chromosome was fragile to mechanical shearing, and during isolation a small proportion of these chromosomes broke into four segiments: the long arm, the short arm, the short arm plus centromere and the centromere region. A large fraction of the constitutive heterochromatin of this species is present in the centromere region of the X + 3 chromosome and in the Y chromosome; these two regions possess similar amounts of DNA and therefore sort together. Chromosome flow sorting is rapid, reproducible and precise; it allows the collection of microgram quantities of purified chromosomes.     

Characteristics of the distribution of DNA repetitive sequences in the sex chromosomes of 4 species of rodent

Four rodent species with very large heterochromatic regions on the sex chromosomes have been studied using in situ DNA/DNA hybridization techniques. Repetitious DNA fractions were obtained at C0t 0-0.01. Heterochromatic regions of X and X chromosomes of Cricetulus barabensis and Phodopus sungorus, and the heterochromatic long arm of the Y chromosome of Mesocricetus auratus do not contain disproportionately high amounts of repeated DNA sequences. Heterochromatic regions on sex chromosomes of Microtus subarvalis contain high amounts of repeated DNA sequences. Additional heterochromatic autosomal arms, a heterochromatic arm of the X chromosome, and a short arm of the Y chromosome of Mesocricetus auratus contain high amounts of repeated DNA sequences too.     

Chromosomes and DNA of Microtus. II. Confirmation of deletion of constitutive heterochromatin in M. agrestis cells in vitro

The distribution of constitutive heterochromatin has been examined by C-banding in two somatic cell lines, grown in vitro, from a female Microtus agrestis. One line retains one intact X chromosome together with the short arm of the other X chromosome, while the other cell line retains only the short arm of one X chromosome. Thus, each cell line has lost substantial amounts of heterochromatin from the sex chromosomes, but this material has been deleted from the cells, and not translocated to other chromosomes. Nonetheless, both cell lines continue to propagate well in vitro.     

正常中国人和DMD患者X染色体Xp~(21)区的RFLP研究

本文应用从人类X柒色体Xp~(21)区不同部位分离得到的9种DNA探针,分析了100名正常中国人,38名DMD患者及其母亲X柒色体Xp~(21)区的14个限制性位点多态性(RSP;又称限制性片段长度多态性,RFLP)。发现正常的X染色体与携带DMD基因的X染色体Xp~(21)区的RFLP频率没有明显差别;在38例DMD患者中有7例的X染色体有DNA片段缺失;在本文分析的24例患者母杀中有17例是DMD基因携带者,她们在Xp~(21)区的RFLP均存在杂合的多态性,因此可以应用RFLP连锁分析对这些家系进行DMD的产前诊断。     

Identification of sex chromosomes in lake trout (Salvelinus namaycush)

In the male trout there is a difference in the quinacrine banding and C-banding patterns between the two homologs of the second largest chromosome pair. This chromosome is the only large submetacentric in the karyotype, making it easy to identify and suggesting that the sex chromosomes have become differentiated since the time of tetraploidization. In males one homolog has a medium-to-large quinacrine bright heterochromatic band on the end of the short arm, while the other lacks it completely. In females both homologs have medium-to-large quinacrine bright heterochromatic bands. Approximately half the progeny from every lake trout cross studied and half the eggs from every lake trout population examined were heteromorphic for a difference in this chromosome band. Results from sexed fish, reciprocal F1 hybrids between brook trout and lake trout, and gynogenetic haploids are all consistent with the interpretation that chromosome 2 is the sex chromosome. These results suggest that the addition of heterochromatin to the X can be the first step in the inhibition of crossing over between the X and Y chromosomes required for sex chromosome differentiation.