Insect sex chromosomes

A presumptive mechanism of X inactivation has been investigated by using tritiated uridine-induced chromosome aberrations to distinguish active from inactive X chromosome arms in the insect Gryllotalpa fossor. Previous work on therian mammals has shown that constitutive and facultative heterochromatin are less susceptible to breakage by ³H-Urd than euchromatin (active). The present study indicates that, irrespective of the presence of two X chromosomes in females, only one of these is affected as in males and that the total number of aberrations induced by ³H-Urd in both male and female Gryllotalpa is the same. This suggests that in the female only one arm of one X chromosome is active and that a facultative heterochromatinization of the homologous arm of the other X is operative coupled with the presence of constitutive heterochromatin in the second arm of both X chromosomes.     

Sex chromatin and X chromosome replication patterns and incidence of sex chromatin in canine cell cultures

In order to provide evidence as to whether sex chromatin (SC) of interphase cells is equivalent to the late replicating X chromosome in female mammalian cells, time-lapse cinephotometric and autoradiographic methods were used to give precise data for comparison of the DNA replication patterns of SC with that of each of the X chromosomes throughout the S period. Canine kidney epithelial cells were selected because they have distinct large metacentric X chromosomes and typical SC. Time-lapse cinephotometry was used to avoid possible alteration of DNA synthesis by chemical cell synchronization agents. Determination of the incidence of SC during the stages of the cell life cycle of proliferating cells of the same origin was performed in order hopefully to clarify conflicting reports on the subject. Our results clearly show that time and intensity of the SC replication throughout S period is like that of the late replicating X chromosome and unlike that of the early replicating X chromosome. The incidence of SC in proliferating cells in culture was found to vary with the stage of the cell life cycle, increasing with increasing postmitotic interval — least in G1, greater in S, and greatest in G2. The SC incidence increased strikingly from G1 to S and a less marked increase was observed between S and G2.     

Stable versus unstable orientations of sex chromosomes in two grasshopper species

The structural basis of orientation stability was investigated. The stable unipolar orientation of the Melanoplus sanguinipes X-chromosome univalent is unique in that it is stable without tension created by forces towards opposite poles; tension is thought to be the principle component in stabilizing kinetochore orientations to a pole. Stable orientation of the X chromosome in Melanoplus sanguinipes was compared with unstable X orientation in Melanoplus differentialis. Ten cells (five of each species) were studied, firstly in living cultures where chromosome behavior was followed, then by serial-section electron microscopy where the structural basis for chromosome behavior was examined. Microtubules other than kinetochore microtubules were observed impinging on the X chromosomes. One end of these microtubules was buried in chromatin, while the other ran towards a pole. The X chromosomes of M. sanguinipes had more of these microtubules than did M. differentialis X chromosomes. It is suggested that M. sanguinipes X chromosomes are less condensed than M. differentialis X chromosomes and so allow more microtubules to penetrate the chromosome. The extra microtubules impinging on the M. sanguinipes X chromosome probably prevent reorientation by inhibiting the turning of the chromosome towards the opposite pole, i.e., more force is needed to turn a kinetochore towards the opposite pole than can be generated and attempts at reorientation fail. This may be analogous to the effect that tension has on the orientation stability of bivalents.     

Evolution of karyotype,sex chromosomes,and meiosis in mygalomorph spiders (Araneae: Mygalomorphae)

Spider diversity is partitioned into three primary clades, namely Mesothelae, Mygalomorphae, and Araneomorphae. Mygalomorph cytogenetics is largely unknown. Our study revealed a remarkable karyotype diversity of mygalomorphs. Unlike araneomorphs, they show no general trend towards a decrease of 2n, as the chromosome number was reduced in some lineages and increased in others. A biarmed karyotype is a symplesiomorphy of mygalomorphs and araneomorphs. Male meiosis of some mygalomorphs is achiasmatic, or includes the diffuse stage. The sex chromosome system X₁X₂0, which is supposedly ancestral in spiders, is uncommon in mygalomorphs. Many mygalomorphs exhibit more than two (and up to 13) X chromosomes in males. The evolution of X chromosomes proceeded via the duplication of chromosomes, fissions, X–X, and X‐autosome fusions. Spiders also exhibit a homomorphic sex chromosome pair. In the germline of mygalomorph males these chromosomes are often deactivated; their deactivation and pairing is initiated already at spermatogonia. Remarkably, pairing of sex chromosomes in mygalomorph females is also initiated at gonial cells. Some mygalomorphs have two sex chromosome pairs. The second pair presumably arose in early‐diverging mygalomorphs, probably via genome duplication. The unique behaviour of spider sex chromosomes in the germline may promote meiotic pairing of homologous sex chromosomes and structural differentiation of their duplicates, as well as the establishment of polyploid genomes. © 2013 The Linnean Society of London, Biological Journal of the Linnean Society, 2013, 109 , 377–408.     

Chromosome complements of the genus Xenopus

The oytogenetic analysis of the genus Xenopus shows that X. laevis laevis, X. laevis petersi, X. laevis victorianus, X. (laevis) borealis, X. gilli, X. muelleri, and X. fraseri have chromosome numbers 2n=36; X. tropicalis has 20 (2n), X. (laevis) bunyoniensis 72 and X. ruwenzoriensis 108. This heterogeneity of the chromosome numbers is interesting as it represents new examples of polyploidy among Anurans. There are no big morphological differences among the karyotypes of the divers species, only the chromosomes with secondary constrictions vary considerably.     

Assessment of the genotoxicity of atenolol in human peripheral blood lymphocytes: Correlation between chromosomal fragility and content of micronuclei

The antihypertensive drug atenolol was found to induce chromosome loss, detected as micronuclei in the peripheral lymphocytes of treated patients. The fundamental question which chromosomes the micronuclei were derived from remains to be answered. Analysis of structural chromosomal aberrations (CAs) and expression of fragile sites (FS) were pursued in this study. They revealed a significantly higher incidence of chromosomal aberrations (chromatid and chromosome breaks) in patients compared with controls, where 10 FS emerged as specific. Also, the band 17q12–21, where known fragile sites have not been reported, was only expressed in atenolol-treated patients. Fluorescence in situ hybridization using chromosome-specific probes revealed the preferential involvement of chromosomes 7, 11, 17 and X in the micronuclei (MN) of patients. The results also suggest a correlation between chromosomal fragility and content of MN, and support the findings for a linkage between hypertension and a locus on chromosome 17.     

ANOLIS SEX CHROMOSOMES ARE DERIVED FROM A SINGLE ANCESTRAL PAIR

To explain the frequency and distribution of heteromorphic sex chromosomes in the lizard genus Anolis, we compared the relative roles of sex chromosome conservation versus turnover of sex‐determining mechanisms. We used model‐based comparative methods to reconstruct karyotype evolution and the presence of heteromorphic sex chromosomes onto a newly generated Anolis phylogeny. We found that heteromorphic sex chromosomes evolved multiple times in the genus. Fluorescent in situ hybridization (FISH) of repetitive DNA showed variable rates of Y chromosome degeneration among Anolis species and identified previously undetected, homomorphic sex chromosomes in two species. We confirmed homology of sex chromosomes in the genus by performing FISH of an X‐linked bacterial artificial chromosome (BAC) and quantitative PCR of X‐linked genes in multiple Anolis species sampled across the phylogeny. Taken together, these results are consistent with long‐term conservation of sex chromosomes in the group. Our results pave the way to address additional questions related to Anolis sex chromosome evolution and describe a conceptual framework that can be used to evaluate the origins and evolution of heteromorphic sex chromosomes in other clades.     

An X-autosome fusion chromosome of Caenorhabditis elegans

Summary The translocation mnT12(IV;X) is a fusion of holocentric chromosomes IV and X, the breakpoints occurring near the left end of IV and the right end of X. Animals homozygous for mnT12 are viable and fertile; they contain five pairs of chromosomes rather than the normal set of six pairs. The mnT12 chromosome is larger than all wild-type chromosomes and thus identifies linkage groups IV and X cytologically. Hermaphrodites heterozygous for mnT12 show high frequency meiotic nondisjunction both between mnT12 and the X chromosome, which results in a high incidence of male self progeny (27% compared to the wild-type incidence of 0.2%), and between mnT12 and chromosome IV, which results in a high incidence of self progeny essentially trisomic for chromosome IV (karyotype IV/mnT12/mnT12). The viability of chromosome IV trisomics has been confirmed by constructing animals trisomic for only normal copies of chromosome IV; these animals are morphologically wild type. Meiotic chromosome disjunction in mnT12 homozygotes appears to be normal, although the frequency of recombination between markers that are normally X-linked is significantly reduced. Males of genotype IV/mnT12/0 are fertile. They can be thought of as having a neo-X(mnT12) neo-Y(normal IV) karyotype since it is possible to maintain a male-hermaphrodite stock of C. elegans consisting of such males and hermaphrodites carrying two neo-X chromosomes and no neo-Y; the organism is thus converted from an XO:XX type of sex determination to an XX:XX system.     

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.     

Mitotic chromosomal abnormalities and DNA polymorphism in the Nile tilapia (Oreochromis niloticus,Linnaeus, 1758) as a biomarker for water pollution by heavy metals

Mitotic chromosomal aberrations and DNA polymorphism (RAPD marker) were carried out on the Nile tilapia Oreochromis niloticus collected from five sites in Minia governorate, Egypt to test their applicability as biomonitors for heavy metal contaminants of water. The diploid chromosome number of O. niloticus population was 2 n = 44. Different types of chromosomal aberrations were recorded (e.g., deletion, ring, centromeric attenuation, end-to-end association, dicentric chromosome, stickiness chromosomes, endomitosis, fragments and chromatid gap). The chromosomal aberrations varied between O. niloticus population collected from five sites, and the most common type was ring (R) chromosomes. Samples obtained from Bahr Yousef and Irrigation drain exhibited the highest aberration frequency. The frequency of chromosomal aberration was positively correlated with the concentration of heavy metals where their concentration in the surface water of Irrigation drain and Bahr Yousef exceeded the limits defined by WHO as well as the concentration of Pb in muscles. The RAPD marker was also used to identify genetic variation among Nile tilapia samples collected from five different water sources. It created polymorphic and unique bands that can be used as genetic markers to track DNA variations. The dendrogram also revealed that exposure to heavy metal pollution causes gradual accumulation of variance, whereas areas subjected to environmental stress showed higher genetic variation and clustered together.     

Tritiated uridine induced chromosome aberrations in relation to heterochromatin and nucleolar organisation in Microtus agrestis L.

Microtus agrestis is characterised by long sex chromosomes, most of which are constitutively heterochromatic, and thus supposedly, genetically inactive. A method to assess the template activity of the chromosomes is to study the distribution of chromatid aberrations produced by H³UdR, among and within the chromosomes. In such a study, in female Microtus agrestis cells in culture, it was found that, a large number of localised chromatid aberrations was induced in the constitutively heterochromatic regions of both X chromosomes. The frequency distribution and types of aberrations were found to be cell cycle dependent. With differential staining it has been possible to demonstrate that the constitutive heterochromatin of the sex chromosomes are involved in the nucleolar organisation in this species, thus containing the ribosomal RNA cistrons.     

Sister chromatid exchanges in Drosophila melanogaster cell lines in vitro

The frequency of sister chromatid exchanges (SCEs) in two cell lines of Drosophila melanogaster with different karyotypes (XX and XY) was determined, considering (1) the distribution of SCEs within each chromosome, with reference to eu- and heterochromatin and (2) the distribution of SCEs in different chromosomes. A comparison was made between chromosome pairs within each karyotype and between the two different karyotypes. The following results were obtained. The SCEs are not randomly distributed along chromosomes, since exchanges were never observed in heterochromatin. SCEs are more frequent in XY than in XX cells; moreover, in both cell types there exists a significantly higher frequency of SCEs in the X chromosome than in the autosomes. These findings are discussed in relation to chromosome aberrations and mitotic recombination.     

Intercalary heterochromatin in Drosophila

Sites of intercalary heterochromatin (IH) in the complete set of Drosophila melanogaster polytene chromosomes were localized and studied according to the following criteria: tendency to break (weak points), ectopic pairing and late replication, the existence of repeats (in X and 2R) including those enriched with A-T bases. Correlation between these features investigated, the highest correlation coefficients found between weak point behavior, late replication, and ectopic pairing. The frequency of breaks in weak points in some IH bands was shown to be different in different tissues, strains and closely related Drosophila species. Sexual differences in morphology and manifestation of IH features were found in bands of the X chromosome: weak point behavior and participation in ectopic pairing of IH bands are an order of magnitude less frequent in male X chromosomes than in female X chromosomes. In autosomes such differences have not been observed. IH bands in male X chromosomes look more massive than the homologous ones in female X chromosomes: the DNA content of the 11A6-9 region is four times less in females than in males. The hypothesis is proposed that the specific features of intercalary heterochromatin bands are determined by tandem repetitiveness and late replication. The latter, if it occurs in a cluster of repetitions, could cause incomplete polytenization of the region and, as a consequence, breaks (or weak points) and the appearance of adhesive ends which may take part either in realization of ectopic contacts or in fixation of those occurring previously. Breaks caused by chromosome aberrations in regions with repeats may not result in a sharp decline of viability, so that break points of chromosome rearrangements in intercalary heterochromatin may be more frequent than in other regions.     

Heterochromatin and chromosomal aberration in microtus agrestis: role of chromosomal association

Mitomycin C (MC) -induced chromatid aberrations among the chromosomes of Microtus agrestis are preferentially localized in the constitutive heterochromatic regions, i.e., major part of the sex chromosomes and the centromeric regions of the autosomes. In the sex chromosomes, intrachanges predominate, while interchanges between the two X chromosomes are very rare. This pattern of distribution of different types of aberrations is interpreted as due to the individual chromocentres that are formed by the two X chromosomes in the interphase.     

Contrasting patterns of X‐chromosome divergence underlie multiple sex‐ratio polymorphisms in stalk‐eyed flies

Sex‐linked segregation distorters cause offspring sex ratios to differ from equality. Theory predicts that such selfish alleles may either go to fixation and cause extinction, reach a stable polymorphism or initiate an evolutionary arms race with genetic modifiers. The extent to which a sex ratio distorter follows any of these trajectories in nature is poorly known. Here, we used X‐linked sequence and simple tandem repeat data for three sympatric species of stalk‐eyed flies (Teleopsis whitei and two cryptic species of T. dalmanni) to infer the evolution of distorting X chromosomes. By screening large numbers of field and recently laboratory‐bred flies, we found no evidence of males with strongly female‐biased sex ratio phenotypes (SR) in one species but high frequencies of SR males in the other two species. In the two species with SR males, we find contrasting patterns of X‐chromosome evolution. T. dalmanni‐1 shows chromosome‐wide differences between sex‐ratio (X^SR) and standard (X^ST) X chromosomes consistent with a relatively old sex‐ratio haplotype based on evidence including genetic divergence, an inversion polymorphism and reduced recombination among X^SR chromosomes relative to X^ST chromosomes. In contrast, we found no evidence of genetic divergence on the X between males with female‐biased and nonbiased sex ratios in T. whitei. Taken with previous studies that found evidence of genetic suppression of sex ratio distortion in this clade, our results illustrate that sex ratio modification in these flies is undergoing recurrent evolution with diverse genomic consequences.     

Genomic conflict drives patterns of X‐linked population structure in Drosophila neotestacea

Intragenomic conflict has the potential to cause widespread changes in patterns of genetic diversity and genome evolution. In this study, we investigate the consequences of sex‐ratio (SR) drive on the population genetic patterns of the X‐chromosome in Drosophila neotestacea. An SR X‐chromosome prevents the maturation of Y‐bearing sperm during male spermatogenesis and thus is transmitted to ~100% of the offspring, nearly all of which are daughters. Selection on the rest of the genome to suppress SR can be strong, and the resulting conflict over the offspring sex ratio can result in the accumulation of multiple loci on the X‐chromosome that are necessary for the expression of drive. We surveyed variation at 12 random X‐linked microsatellites across 16 populations of D. neotestacea that range in SR frequency from 0% to 30%. First, every locus was differentiated between SR and wild‐type chromosomes, and this drives genetic structure at the X‐chromosome. Once the association with SR is accounted for, the patterns of differentiation among populations are similar to the autosomes. Second, within wild‐type chromosomes, the relative heterozygosity is reduced in populations with an increased prevalence of drive, and the heterozygosity of SR chromosomes is higher than expected based on its prevalence. The combination of the relatively high prevalence of SR drive and the structuring of polymorphism between the SR and wild‐type chromosomes suggests that genetic conflict because of SR drive has had significant consequences on the patterns of X‐linked polymorphism and thus also probably affects the tempo of X‐chromosome evolution in D. neotestacea.     

Multicolor FISH mapping of the dioecious model plant,<Emphasis Type="Italic"> Silene latifolia</Emphasis>

Silene latifolia is a key plant model in the study of sex determination and sex chromosome evolution. Current studies have been based on genetic mapping of the sequences linked to sex chromosomes with analysis of their characters and relative positions on the X and Y chromosomes. Until recently, very few DNA sequences have been physically mapped to the sex chromosomes of S. latifolia. We have carried out multicolor fluorescent in situ hybridization (FISH) analysis of S. latifolia chromosomes based on the presence and intensity of FISH signals on individual chromosomes. We have generated new markers by constructing and screening a sample bacterial artificial chromosome (BAC) library for appropriate FISH probes. Five newly isolated BAC clones yielded discrete signals on the chromosomes: two were specific for one autosome pair and three hybridized preferentially to the sex chromosomes. We present the FISH hybridization patterns of these five BAC inserts together with previously described repetitive sequences (X-43.1, 25S rDNA and 5S rDNA) and use them to analyze the S. latifolia karyotype. The autosomes of S. latifolia are difficult to distinguish based on their relative arm lengths. Using one BAC insert and the three repetitive sequences, we have constructed a standard FISH karyotype that can be used to distinguish all autosome pairs. We also analyze the hybridization patterns of these sequences on the sex chromosomes and discuss the utility of the karyotype mapping strategy presented to study sex chromosome evolution and Y chromosome degeneration.Communicated by J.S. Heslop-Harrison     

Simple GA C T A repeats characterize the X chromosomal heterochromatin of Microtus agrestis,European field vole (Rodentia,Cricetidae)

The sex chromosomes of Microtus agrestis are extremely large due to the accumulation of constitutive heterochromatin. We have identified two prominent satellite bands of 2.0 and 2.8 kb in length after HaeIII and HinfI restriction enzyme digestion of genomic DNA, respectively. These satellites are located on the heterochromatic long arm of the X chromosome as shown using Microtus x mouse somatic cell hybrids. By in-gel hybridization with oligonucleotide probes, the organization of the two satellites was studied: among the many copies of the simple tandem tetranucleotide repeat GATA are interspersed rare single GACA tetramers. One of the satellites also harbours related GGAT simple tandem repeats. In situ hybridizations with plasmid-carried or oligonucleotide GA _C ^T A probes show clustered silver grains on the long and short arm of the X chromosome. Interspersion of differently organized (GATA)_n elements is also demonstrable in the autosomal complement and on the Y chromosome. These results are discussed in the context of the evolution of vertebrate sex chromosomes in relation to heterochromatin and simple repetitive DNA sequences.