Interaction of B chromosomes with A or B chromosomes in segregation in insects

Additional or B chromosomes not belonging to the regular karyotype of a species are found in many animal and plant groups. They form a highly heterogeneous group with respect to their morphology and behaviour both in mitosis and meiosis. Achiasmatic mechanisms that ensure the segregation of a B chromosome from another B chromosome or from an A chromosome are reviewed. An achiasmatic mechanism characterized by the "distance pairing" of segregating univalents at metaphase I was found to be responsible for the preferential segregation of B chromosome univalents in Hemerobius marginatus L. (Neuroptera), and a mechanism characterized by the "touch and go pairing" of segregating univalents was responsible for the highly regular segregation of a B chromosome and the X chromosome in Rhinocola aceris (L.) (Psylloidea, Homoptera). The latter mechanism resulted in the integration of a B chromosome to the A chromosome set as a Y chromosome in a psyllid species Cacopsylla peregrina (Frst.). Furthermore, B chromosomes can disturb the regular segregation of the achiasmatic X and Y chromosomes resulting in the formation of X0/XY polymorphism in a population, which might precede the loss of the Y chromosome. The absence of observations on accurately functioning achiasmatic segregation mechanisms in grasshoppers (Orthoptera) was attributed to the X and B chromosomes, which re-orient one or several times during metaphase I. Apparently, these re-orientations mask any achiasmatic segregation mechanism that might operate during meiotic prophase in these insects.     

Low levels of chromosomal differentiation between the grasshoppers Chorthippus brunneus and Chorthippusjacobsi (Orthoptera; Acrididae) in northern Spain

The grasshopper species Chorthippus brunneus and C. jacobsi (Orthoptera: Acrididae) form a hybrid zone in northern Spain. These species probably diverged while isolated in southern refugia during one of the recent ice ages, and are clearly distinguished by morphology and male calling song. However, in contrast to other Chorthippus taxa that form hybrid zones in Europe, these two species cannot be reliably distinguished on the basis of characteristics of the karyotype such as heterochromatin banding patterns and composition, as revealed by C-banding and fluorochrome staining. Silver staining also reveals the presence of two autosomal nucleolar organiser regions (NORs) in both species. However, differentiation between C. brunneus and C. jacobsi was revealed on the X chromosome using fluorescent in situ hybridisation (FISH). C. brunneus individuals showed additional rDNA sequences on the X chromosome that were not observed in any C. jacobsi individuals. These sequences are not transcribed, indicating either mutational silencing of an ancestral NOR on the X chromosome, or the transposition of non-functional sequences from the autosomes. The implications of these results for the evolution of NOR number in Chorthippus are discussed.     

X chromosome inactivation and micronuclei in normal and Turner individuals

Studies on aneuploidy have shown that the X is the most frequently lost chromosome in females, and that the number of X chromosome-positive micronuclei increases with age in women. Recently, we showed that the inactive X chromosome is incorporated preferentially in micronuclei. The objectives of the current study were, firstly, to determine the incidence of X chromosome incorporation into micronuclei in males and, secondly, to determine the incidence of X chromosome incorporation into micronuclei of females with Turner syndrome. Blood samples were obtained from 18 male newborns and 35 normal adult males ranging in age from 22 to 79 years and from seven women with non-mosaic Turner syndrome aged 11–39 years. Isolated lymphocytes were cultured in the presence of cytochalasin B and 2000 binucleated cells per subject were scored for micronuclei. Cells were then hybridized with the biotinylated X centromere-specific probe, pBamX7, and visualized with fluorescein-conjugated avidin. All micronucleated cells were relocated and evaluated for the presence or absence of the X chromosome. Of the 335 micronuclei observed, 6.6% (22/335) contained an X chromosome. Analysis of variance shows a statistically significant increase, for both males and Turner females, in the number of X chromosome-positive micronuclei with age (P < 0.001). These data also show that the X chromosome is included in micronuclei from males more often than would be expected by chance (P < 0.005; χ² analysis, 15 df). Here we show that there is a tenfold difference in the frequency of X chromosome-positive micronuclei in 46,XX females compared to 46,XY males and 45,X females, providing further support to our previous finding that the X chromosome in micronuclei is the inactive chromosome. Received: 29 April 1997 / Accepted: 9 May 1997     

Accelerated Adaptive Evolution on a Newly Formed X Chromosome

Sex chromosomes originated from ordinary autosomes, and their evolution is characterized by continuous gene loss from the ancestral Y chromosome. Here, we document a new feature of sex chromosome evolution: bursts of adaptive fixations on a newly formed X chromosome. Taking advantage of the recently formed neo-X chromosome of Drosophila miranda, we compare patterns of DNA sequence variation at genes located on the neo-X to genes on the ancestral X chromosome. This contrast allows us to draw inferences of selection on a newly formed X chromosome relative to background levels of adaptation in the genome while controlling for demographic effects. Chromosome-wide synonymous diversity on the neo-X is reduced 2-fold relative to the ancestral X, as expected under recent and recurrent directional selection. Several statistical tests employing various features of the data consistently identify 10%–15% of neo-X genes as targets of recent adaptive evolution but only 1%–3% of genes on the ancestral X. In addition, both the rate of adaptation and the fitness effects of adaptive substitutions are estimated to be roughly an order of magnitude higher for neo-X genes relative to genes on the ancestral X. Thus, newly formed X chromosomes are not passive players in the evolutionary process of sex chromosome differentiation, but respond adaptively to both their sex-biased transmission and to Y chromosome degeneration, possibly through demasculinization of their gene content and the evolution of dosage compensation.     

The grasshopper X chromosome

The X chromosome can be identified with the light microscope throughout all stages of the gonial cell cycle (including interphase) in the grasshopper Brachystola magna. At gonial mitotic stages the X chromosome gives the appearance of being undercondensed or negatively heteropycnotic. At interphase the X projects out from the body of the nucleus. — Examination with the electron microscope reveals that the X is compartmentalized at least two gonial cell cycles prior to the entry of the cells into meiotic prophase. The membrane layers that envelope the X chromatin at interphase remain associated with the X chromosome throughout gonial mitotic stages providing the ultrastructural basis for the apparent negative heteropycnosis observed with the light microscope. — The X chromosome is inactive in RNA synthesis during gonial mitotic stages but is hyperactive in RNA synthesis when compared to autosomes at gonial interphase. — X chromosome condensation which reaches its maximum at premieotic interphase is initiated at or prior to the pre-pentultimate gonial division.     

Large-scale population study of human cell lines indicates that dosage compensation is virtually complete

X chromosome inactivation in female mammals results in dosage compensation of X-linked gene products between the sexes. In humans there is evidence that a substantial proportion of genes escape from silencing. We have carried out a large-scale analysis of gene expression in lymphoblastoid cell lines from four human populations to determine the extent to which escape from X chromosome inactivation disrupts dosage compensation. We conclude that dosage compensation is virtually complete. Overall expression from the X chromosome is only slightly higher in females and can largely be accounted for by elevated female expression of approximately 5% of X-linked genes. We suggest that the potential contribution of escape from X chromosome inactivation to phenotypic differences between the sexes is more limited than previously believed.     

Heterozygous expression of X-linked chondrodysplasia punctata

Summary Two females showing partial expression of X-linked chondrodysplasia punctata were identified in a family. Bone dysplasia was caused by an aberrant X chromosome that had an inverse duplication of the segment Xp21.2–Xp22.2 and a deletion of Xp22.3-Xpter. To characterise the aberrant X chromosome, dosage blots were performed on genomic DNA from a carrier using a number of X-linked probes. Anonymous sequences from Xp21.2–Xp22.2 to which probes D2, 99.61, C7, pERT87-15, and 754 bind were duplicated on the aberrant X chromosome. The proposita was heterozygous for all these markers. Dosage blots also showed that the loci for steroid sulfatase and the cell surface antigen 12E7 (MIC2) were deleted as expected from the cytogenetic results. Mouse human cell hybrids were constructed that retained the normal X in the active state. Analysis of these hybrid clones for the markers from Xp21.2–Xp22.2 revealed that all the alleles of the informative markers, present in a single dosage in the genomic DNA, were carried on the normal X chromosome of the proposita. The duplicated X chromosome therefore had two identical alleles, indicating that the aberration resulted from an intrachromosomal rearrangement.     

Replication variants of the human inactive X chromosome

Replication variants of the inactive X chromosome were investigated in lymphocytes from six donors by means of terminal BrdU or thymidine incorporation. There were interindividual differences in the incidence of particular variants. In endoreduplicated and tetraploid cells both allocyclic X chromosomes showed the same replication sequence. The Xp22 band of the allocyclic X chromosome seemed to replicate later than the homologous material in some cells. Initiation time of DNA synthesis within the inactive X chromosome was found to be stable; termination time, however, varied greatly relative to the other chromosomes. Early completion of replication within the heterochromatic X chromosome could be demonstrated preferentially for the Xq25–27 terminal sequence, but other variants expressed the phenomenon also. A variable replication rate of the inactive X chromosome is believed to be responsible for its asynchronous, independent replication. The biological significance of the phenomenon is discussed with respect to cell differentiation.     

DESCRIPTIONS OF TWO NEW SPECIES OF XIPHIDIOPSIS REDTENBACHER (ORTHOPTERA: MECONEMATIDAE) FROM GUIZHOU PROVINCE, CHINA

Abstract In this paper, two new species of the genus Xiphidiopsis Redtenbacher (Orthoptera: Meconematidae) were described from Guizhou, China, namely X. tonicosa sp. nov. and X. forcipa sp. nov.     

贵州剑螽属Xiphidiopsis Redtenbacher（直翅目：蛩螽科）两新种记述

记述了贵州剑螽属二新种,即扩板剑螽Xiphidiopsis tonikosa sp.n ov.和钳尾剑螽X.forcipa sp.nov.扩板剑冬X.tonikosa sp.nov与四川剑螽X.szechwanensis Tinkham,1944和双突剑螽X.biprocera Shi et Zheng,1996较相似,主要区别：1）头部背面具4个分离的纵斑;2）雄性尾须分2支,水平支扩展;3）雌性下生殖板延长,后缘两侧突出。钳尾剑螽X.forcipa sp.nov与双叶剑螽X.biolba B.-Bienko,1962相似,但体较小,雄性第10腹节背板稍向后延伸,端部为2个向腹面弯曲的三角形片,不呈圆柱形。     

Molecular karyotyping of an isolated partial trisomy 11q patient with additional findings

Isolated partial duplication of the long arm of chromosome 11 is very rare. The main features are dysmorphic facial features, pre/postnatal growth retardation, speech delay, mental retardation, hypotonia, microcephaly, and cardiac, vertebral, limb and genital anomalies. In this case, we report a patient with partial trisomy of 11q13.5 → qter due to a de novo rearrangement consisting of the whole X chromosome and part of chromosome 11; 46,X,der(X)(Xqter → Xp22.33::11q13.5 → 11qter). Additional findings were a separated clavicle, lacrimal duct stenosis and prenatally detected renal hypoplasia. SNP array results revealed a duplication between 11q13.5 and 11qter, measuring 58 Mb, from nucleotide 76,601,607 to 134,926,021. As a result, molecular karyotyping could be performed in such cases in order to establish a definite phenotype–genotype correlation using conventional or molecular cytogenetics techniques.     

Choroideremia associated with an X-autosomal translocation

Summary A patient with mild choroideremia has been shown to carry a balanced translocation between chromosome X and 13 – 46,X,t(X;13)(q21.2;p12). Loci (DXY21, DX232, DX233) shown to map to this region on the X chromosome and in some cases to be deleted in other patients with choroideremia are intact in the DNA from this patient. To our knowledge this is the first report of a translocation associated with choroideremia. One of the translocation chromosomes, derivative 13, free of the derivative X and normal X, has been isolated in a somatic cell hybrid. Because of the clinical association of the eye findings with chromosome interchange, we suggest that the breakpoint on the X is at or near the choroideremia locus. Further analysis of this translocation may be useful in cloning the choroideremia gene.     

X Chromosome Sites Autonomously Recruit the Dosage Compensation Complex in Drosophila Males

It has been proposed that dosage compensation in Drosophila males occurs by binding of two core proteins, MSL-1 and MSL-2, to a set of 35–40 X chromosome “entry sites” that serve to nucleate mature complexes, termed compensasomes, which then spread to neighboring sequences to double expression of most X-linked genes. Here we show that any piece of the X chromosome with which compensasomes are associated in wild-type displays a normal pattern of compensasome binding when inserted into an autosome, independently of the presence of an entry site. Furthermore, in chromosomal rearrangements in which a piece of X chromosome is inserted into an autosome, or a piece of autosome is translocated to the X chromosome, we do not observe spreading of compensasomes to regions of autosomes that have been juxtaposed to X chromosomal material. Taken together these results suggest that spreading is not involved in dosage compensation and that nothing distinguishes an entry site from the other X chromosome sites occupied by compensasomes beyond their relative affinities for compensasomes. We propose a new model in which the distribution of compensasomes along the X chromosome is achieved according to the hierarchical affinities of individual binding sites.     

The chromosome complement of the Rhesus monkey (Macaca mulatta) determined in kidney cells cultivated in vitro

Summary The chromosome complement of male and female Rhesus monkey has been investigated in kidney cells cultivatedin vitro for 3 to 6 days. The chromosome number is 42. The Y chromosome of the heterogametic male is the smallest element in the complement, and it is acrocentric. The X chromosome ranks eigth in decreasing order of size and typically has an arm ratio of 1.4. The autosomes form a graded size series of metacentric chromosomes, 3–15μ long in early metaphase, and with arm ratios from 1.1 to 3.3. Chromosome IX carries a large secondary constriction near the centromere; it is presumed to be the main nucleolar chromosome. A smaller secondary constriction is found consistently in the long arm of chromosome I. The X chromosome and chromosome XXI appear to be dimorphic in the limited population studied, the alternative forms differing in arm ratios but not in total length. An idiogram of the haploid chromosome complement is presented incorporating measurements of 10 completely analyzed nuclei, five from male monkeys and five from females. On the basis of relative length, arm ratio, and occurrence of secondary constrictions, most chromosomes of the complement can be individually identified. Supported in part by grants from the National Cancer Institute of Canada; the National Institutes of Health of the United States, Public Health Service; and the National Foundation for Infantile Paralysis.     

Generation of a human X-derived minichromosome using telomere-associated chromosome fragmentation.

A linear mammalian artificial chromosome vector will require at least three functional elements: a centromere, two telomeres and replication origins. One route to generate such a vector is by the fragmentation of an existing chromosome. We have previously described the use of cloned telomeric DNA to generate and stably rescue truncated derivatives of a human X chromosome in a somatic cell hybrid. Further rounds of telomere-associated chromosome fragmentation have now been used to engineer a human X-derived minichromosome. This minichromosome is estimated to be < 10 Mb in size. In situ hybridization and molecular analysis reveal that the minichromosome has a linear structure, with two introduced telomere constructs flanking a 2.5 Mb alpha-satellite array. The highly truncated chromosome also retains some chromosome-specific DNA, originating from Xp11.21. There is no significant change in the mitotic stability of the minichromosome as compared with the X chromosome from which it was derived.     

Transformation of the Hprt gene with DNA from spermatogenic cells

DNA-mediated transformation of hypoxanthine guanine phosphoribosyl transferase (HPRT)-deficient cells was used to assess the state of the X chromosome Hprt gene in spermatogenic cells. It had been shown previously that DNA from the inactive X chromosome of somatic cells functions poorly or not at all in HPRT transformation, indicating that DNA modification is involved in somatic cell X chromosome inactivation (XCI). In contrast, DNA from mature sperm does function in HPRT transformation suggesting that DNA modification may not be the basis of XCI in mature sperm. In this paper, transformation of HPRT^– mouse and hamster cells has been performed to test the nature of XCI during earlier stages of spermatogenesis. DNA from these developing murine germ cells was shown to be capable of HPRT transformation, extending the observation that XCI in sperm does not appear to involve a DNA modification. We also show here that DNA from mature sperm of marsupials functions in HPRT transformation, a result consistent with a role for sperm XCI in the evolution of somatic X inactivation.     

X chromosome sites autonomously recruit the dosage compensation complex in Drosophila males

It has been proposed that dosage compensation in Drosophila males occurs by binding of two core proteins, MSL-1 and MSL-2, to a set of 35–40 X chromosome “entry sites” that serve to nucleate mature complexes, termed compensasomes, which then spread to neighboring sequences to double expression of most X-linked genes. Here we show that any piece of the X chromosome with which compensasomes are associated in wild-type displays a normal pattern of compensasome binding when inserted into an autosome, independently of the presence of an entry site. Furthermore, in chromosomal rearrangements in which a piece of X chromosome is inserted into an autosome, or a piece of autosome is translocated to the X chromosome, we do not observe spreading of compensasomes to regions of autosomes that have been juxtaposed to X chromosomal material. Taken together these results suggest that spreading is not involved in dosage compensation and that nothing distinguishes an entry site from the other X chromosome sites occupied by compensasomes beyond their relative affinities for compensasomes. We propose a new model in which the distribution of compensasomes along the X chromosome is achieved according to the hierarchical affinities of individual binding sites.     

Construction and characterization of band-specific DNA libraries

Summary A universally primed polymerase chain reaction was developed to amplify DNA dissected from GTG-banded human chromosomes. The amplification products are cloned into plasmid vectors, which allow the rapid characterization of recombinant clones. Starting from 20–40 chromosome fragments, several thousand independent clones detecting single-copy sequences can be obtained. Although these libraries comprise only a few percent of the dissected DNA, they provide narrowly spaced anchor clones for the molecular characterization of chromosome bands and the identification of gene sequences. Here we describe the construction and characterization of DNA libraries for the Langer-Giedion syndrome chromosome region (LGCR, 8q23–24.1), Wilms tumor chromosome region 1 (WT1, 11p13), Prader-Willi syndrome/Angelman syndrome chromosome region (PWCR/ANCR, 15q11.2–12), meningioma chromosome region (MGCR, 22q12–13), and fragile X chromosome region (FRAXA, Xq27.3).     

DNase I sensitivity of Microtus agrestis active,inactive and reactivated X chromosomes in mouse-Microtus cell hybrids

We isolated Microtus agrestis-mouse somatic cell hybrid clones which had retained either the active or the inactive M. agrestis X chromosome. In both hybrid clones the X chromosomes retained their original chromatin conformation as studied by the in situ nick translation technique — the active X chromosome retained its high sensitivity to DNase I while the inactive one remained insensitive. A clone in which the hypoxanthine guanine phosphoribosyltransferase (HPRT) gene had been spontaneously reactivated was isolated from the hybrid containing the inactive X chromosome. The in situ nick translation technique was used to study possible DNA conformation changes in the euchromatin of the inactive X chromosome with special reference to the reactivated HPRT locus. We found that the euchromatin in this X chromosome exhibited the same low sensitivity to DNase I as is characteristic of the inactive X chromosome.Professor Marcus passed away on 2 January 1987