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Find video protocols related to scientific articles indexed in Pubmed.
Non-homologous sex chromosomes in two geckos (gekkonidae: gekkota) with female heterogamety.
Cytogenet. Genome Res.
PUBLISHED: 05-07-2014
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Evaluating homology between the sex chromosomes of different species is an important first step in deducing the origins and evolution of sex-determining mechanisms in a clade. Here, we describe the preparation of Z and W chromosome paints via chromosome microdissection from the Australian marbled gecko (Christinus marmoratus) and their subsequent use in evaluating sex chromosome homology with the ZW chromosomes of the Kwangsi gecko (Gekko hokouensis) from eastern Asia. We show that the ZW sex chromosomes of C. marmoratus and G. hokouensis are not homologous and represent independent origins of female heterogamety within the Gekkonidae. We also show that the C. marmoratus Z and W chromosomes are genetically similar to each other as revealed by C-banding, comparative genomic hybridization, and the reciprocal painting of Z and W chromosome probes. This implies that sex chromosomes in C. marmoratus are at an early stage of differentiation, suggesting a recent origin. © 2014 S. Karger AG, Basel.
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Tracing the evolution of amniote chromosomes.
Chromosoma
PUBLISHED: 03-04-2014
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A great deal of diversity in chromosome number and arrangement is observed across the amniote phylogeny. Understanding how this diversity is generated is important for determining the role of chromosomal rearrangements in generating phenotypic variation and speciation. Gaining this understanding is achieved by reconstructing the ancestral genome arrangement based on comparisons of genome organization of extant species. Ancestral karyotypes for several amniote lineages have been reconstructed, mainly from cross-species chromosome painting data. The availability of anchored whole genome sequences for amniote species has increased the evolutionary depth and confidence of ancestral reconstructions from those made solely from chromosome painting data. Nonetheless, there are still several key lineages where the appropriate data required for ancestral reconstructions is lacking. This review highlights the progress that has been made towards understanding the chromosomal changes that have occurred during amniote evolution and the reconstruction of ancestral karyotypes.
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Highly differentiated ZW sex microchromosomes in the Australian Varanus species evolved through rapid amplification of repetitive sequences.
PLoS ONE
PUBLISHED: 01-01-2014
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Transitions between sex determination systems have occurred in many lineages of squamates and it follows that novel sex chromosomes will also have arisen multiple times. The formation of sex chromosomes may be reinforced by inhibition of recombination and the accumulation of repetitive DNA sequences. The karyotypes of monitor lizards are known to be highly conserved yet the sex chromosomes in this family have not been fully investigated. Here, we compare male and female karyotypes of three Australian monitor lizards, Varanus acanthurus, V. gouldii and V. rosenbergi, from two different clades. V. acanthurus belongs to the acanthurus clade and the other two belong to the gouldii clade. We applied C-banding and comparative genomic hybridization to reveal that these species have ZZ/ZW sex micro-chromosomes in which the W chromosome is highly differentiated from the Z chromosome. In combination with previous reports, all six Varanus species in which sex chromosomes have been identified have ZZ/ZW sex chromosomes, spanning several clades on the varanid phylogeny, making it likely that the ZZ/ZW sex chromosome is ancestral for this family. However, repetitive sequences of these ZW chromosome pairs differed among species. In particular, an (AAT)n microsatellite repeat motif mapped by fluorescence in situ hybridization on part of W chromosome in V. acanthurus only, whereas a (CGG)n motif mapped onto the W chromosomes of V. gouldii and V. rosenbergi. Furthermore, the W chromosome probe for V. acanthurus produced hybridization signals only on the centromeric regions of W chromosomes of the other two species. These results suggest that the W chromosome sequences were not conserved between gouldii and acanthurus clades and that these repetitive sequences have been amplified rapidly and independently on the W chromosome of the two clades after their divergence.
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Karyotypic analysis and FISH mapping of microsatellite motifs reveal highly differentiated XX/XY sex chromosomes in the pink-tailed worm-lizard (Aprasia parapulchella, Pygopodidae, Squamata).
Mol Cytogenet
PUBLISHED: 10-19-2013
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The infraorder Gekkota is intriguing because it contains multiple chromosomal and environmental sex determination systems that vary even among closely related taxa. Here, we compare male and females karyotypes of the pink-tailed worm-lizard (Aprasia parapulchella), a small legless lizard belonging to the endemic Australian family Pygopodidae.
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Sequence and gene content of a large fragment of a lizard sex chromosome and evaluation of candidate sex differentiating gene R-spondin 1.
BMC Genomics
PUBLISHED: 08-19-2013
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Scant genomic information from non-avian reptile sex chromosomes is available, and for only a few lizards, several snakes and one turtle species, and it represents only a small fraction of the total sex chromosome sequences in these species.
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Transitions between sex-determining systems in reptiles and amphibians.
Annu Rev Genomics Hum Genet
PUBLISHED: 08-02-2011
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Important technological advances in genomics are driving a new understanding of the evolution of sex determination in vertebrates. In particular, comparative chromosome mapping in reptiles has shown an intriguing distribution of homology in sex chromosomes across reptile groups. When this new understanding is combined with the widespread distribution of genetic and temperature-dependent sex-determination mechanisms among reptiles, it is apparent that transitions between modes have occurred many times, as they have for amphibians (particularly between male and female heterogamety). It is also likely that thermosensitivity in sex determination is a key factor in those transitions in reptiles, and possibly in amphibians too. New models of sex determination involving temperature thresholds are providing the framework for the investigation of transitions and making possible key predictions about the homologies and sex-determination patterns expected among taxa in these groups. Molecular cytogenetics and other genomic approaches are essential to providing the fundamental material necessary to make advances in this field.
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Evolutionary transitions between mechanisms of sex determination in vertebrates.
Biol. Lett.
PUBLISHED: 01-06-2011
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Sex in many organisms is a dichotomous phenotype--individuals are either male or female. The molecular pathways underlying sex determination are governed by the genetic contribution of parents to the zygote, the environment in which the zygote develops or interaction of the two, depending on the species. Systems in which multiple interacting influences or a continuously varying influence (such as temperature) determines a dichotomous outcome have at least one threshold. We show that when sex is viewed as a threshold trait, evolution in that threshold can permit novel transitions between genotypic and temperature-dependent sex determination (TSD) and remarkably, between male (XX/XY) and female (ZZ/ZW) heterogamety. Transitions are possible without substantive genotypic innovation of novel sex-determining mutations or transpositions, so that the master sex gene and sex chromosome pair can be retained in ZW-XY transitions. We also show that evolution in the threshold can explain all observed patterns in vertebrate TSD, when coupled with evolution in embryonic survivorship limits.
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Sex chromosome evolution in amniotes: applications for bacterial artificial chromosome libraries.
J. Biomed. Biotechnol.
PUBLISHED: 07-29-2010
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Variability among sex chromosome pairs in amniotes denotes a dynamic history. Since amniotes diverged from a common ancestor, their sex chromosome pairs and, more broadly, sex-determining mechanisms have changed reversibly and frequently. These changes have been studied and characterized through the use of many tools and experimental approaches but perhaps most effectively through applications for bacterial artificial chromosome (BAC) libraries. Individual BAC clones carry 100-200?kb of sequence from one individual of a target species that can be isolated by screening, mapped onto karyotypes, and sequenced. With these techniques, researchers have identified differences and similarities in sex chromosome content and organization across amniotes and have addressed hypotheses regarding the frequency and direction of past changes. Here, we review studies of sex chromosome evolution in amniotes and the ways in which the field of research has been affected by the advent of BAC libraries.
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Non-homologous sex chromosomes of birds and snakes share repetitive sequences.
Chromosome Res.
PUBLISHED: 06-21-2010
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Snake sex chromosomes provided Susumo Ohno with the material on which he based his theory of how sex chromosomes differentiate from autosomal pairs. Like birds, snakes have a ZZ male/ZW female sex chromosome system, in which the snake Z is a macrochromosome much the same size as the bird Z. However, the gene content shows clearly that the snake and bird Z chromosomes are completely non-homologous. The molecular aspect of W chromosome degeneration in snakes remains largely unexplored. We used comparative genomic hybridization to identify the female-specific region of the W chromosome in representative species of Australian snakes. Using this approach, we show that an increasingly complex suite of repeats accompanies the evolution of W chromosome heteromorphy. In particular, we found that while the python Liasis fuscus exhibits no sex-specific repeats and indeed, no cytologically recognizable sex-specific region, the colubrid Stegonotus cucullatus shows a large domain on the short arm of the W chromosome that consists of female-specific repeats, and the large W of Notechis scutatus is composed almost entirely of repetitive sequences, including Bkm and 18S rDNA-related elements. FISH mapping of both simple and complex probes shows patterns of repeat amplification concordant with the size of the female-specific region in each species examined. Mapping of intronic sequences of genes that are sex-linked in both birds (DMRT1) and snakes (CTNNB1) reveals massive amplification in discrete domains on the W chromosome of the elapid N. scutatus. Using chicken W chromosome paint, we demonstrate that repetitive sequences are shared between the sex chromosomes of birds and derived snakes. This could be explained by ancestral but as yet undetected shared synteny of bird and snake sex chromosomes or may indicate functional homology of the repeats and suggests that degeneration is a convergent property of sex chromosome evolution. We also establish that synteny of snake Z-linked genes has been conserved for at least 166 million years and that the snake Z consists of two conserved blocks derived from the same ancestral vertebrate chromosome.
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Globin gene structure in a reptile supports the transpositional model for amniote ?- and ?-globin gene evolution.
Chromosome Res.
PUBLISHED: 04-16-2010
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The haemoglobin protein, required for oxygen transportation in the body, is encoded by ?- and ?-globin genes that are arranged in clusters. The transpositional model for the evolution of distinct ?-globin and ?-globin clusters in amniotes is much simpler than the previously proposed whole genome duplication model. According to this model, all jawed vertebrates share one ancient region containing ?- and ?-globin genes and several flanking genes in the order MPG-C16orf35-(?-?)-GBY-LUC7L that has been conserved for more than 410 million years, whereas amniotes evolved a distinct ?-globin cluster by insertion of a transposed ?-globin gene from this ancient region into a cluster of olfactory receptors flanked by CCKBR and RRM1. It could not be determined whether this organisation is conserved in all amniotes because of the paucity of information from non-avian reptiles. To fill in this gap, we examined globin gene organisation in a squamate reptile, the Australian bearded dragon lizard, Pogona vitticeps (Agamidae). We report here that the ?-globin cluster (HBK, HBA) is flanked by C16orf35 and GBY and is located on a pair of microchromosomes, whereas the ?-globin cluster is flanked by RRM1 on the 3 end and is located on the long arm of chromosome 3. However, the CCKBR gene that flanks the ?-globin cluster on the 5 end in other amniotes is located on the short arm of chromosome 5 in P. vitticeps, indicating that a chromosomal break between the ?-globin cluster and CCKBR occurred at least in the agamid lineage. Our data from a reptile species provide further evidence to support the transpositional model for the evolution of ?-globin gene cluster in amniotes.
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The ZW sex microchromosomes of an Australian dragon lizard share no homology with those of other reptiles or birds.
Chromosome Res.
PUBLISHED: 09-03-2009
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Reptiles show a diverse array of sex chromosomal systems but, remarkably, the Z sex chromosomes of chicken are homologous to the ZW sex chromosomes of a species of gecko, Gekko hokouensis, suggesting an ancient but common origin. This is in contrast to the ZW sex chromosomes of snakes and a species of soft-shelled turtle, Pelodiscus sinensis, which are nonhomologous to those of chicken or each other and appear to have been independently derived. In this paper, we determine what homology, if any, the sex chromosomes of the Australian dragon lizard Pogona vitticeps shares with those of snake and chicken by mapping the dragon homologs of five snake Z chromosome genes (WAC, KLF6, TAX1BP1, RAB5A, and CTNNB1) and five chicken Z chromosome genes (ATP5A1, GHR, DMRT1, CHD1, and APTX) to chromosomes in the dragon. The dragon homologs of snake and chicken sex chromosome genes map to chromosomes 6 and chromosome 2, respectively, in the dragon and that DMRT1, the bird sex-determining gene, is not located on the sex chromosomes of P. vitticeps. Indeed, our data show that the dragon homolog to the chicken Z chromosome is likely to be wholly contained within chromosome 2 in P. vitticeps, which suggests that the sex-determining factor in P. vitticeps is not the sex-determining gene of chicken. Homology between chicken Z chromosome and G. hokouensis ZW chromosome pairs has been interpreted as retention of ancient ZW sex chromosomes in which case the nonhomologous sex chromosomes of snake and dragons would be independently derived. Our data add another case of independently derived sex chromosomes in a squamate reptile, which makes retention of ancient sex chromosome homology in the squamates less plausible. Alternatively, the conservation between the bird Z chromosome and the G. hokouensis ZW chromosomes pairs is coincidental, may be an example of convergent evolution, its status as the Z chromosome having been independently derived in birds and G. hokouensis.
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Z and W sex chromosomes in the cane toad (Bufo marinus).
Chromosome Res.
PUBLISHED: 07-23-2009
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The cane toad (Bufo marinus) is one of the most notorious animal pests encountered in Australia. Members of the genus Bufo historically have been regarded as having genotypic sex determination with male homogamety/female heterogamety. Nevertheless, as with many toads, karyotypic analyses of the cane toad have so far failed to identify heteromorphics sex chromosomes. In this study, we used comparative genomic hybridization, reverse fluorescence staining, C-banding, and morphometric analyses of chromosomes to characterize sex chromosome dimorphism in B. marinus. We found that females consistently had a length dimorphism associated with a nucleolus organizer region (NOR) on one of the chromosome 7 pair. A strong signal over the longer NOR in females, and the absence of a signal in males indicated sex-specific DNA sequences. All females were heterozygous and all males homozygous, indicating a ZZ/ZW sex chromosomal system. Our study confirms the existence of sex chromosomes in this species. The ability to reliably identify genotypic sex of cane toads will be of value in monitoring and control efforts in Australia and abroad.
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Recombination and nucleotide diversity in the sex chromosomal pseudoautosomal region of the emu, Dromaius novaehollandiae.
J. Hered.
PUBLISHED: 06-23-2009
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Pseudoautosomal regions (PARs) shared by avian Z and W sex chromosomes are typically small homologous regions within which recombination still occurs and are hypothesized to share the properties of autosomes. We capitalized on the unusual structure of the sex chromosomes of emus, Dromaius novaehollandiae, which consist almost entirely of PAR shared by both sex chromosomes, to test this hypothesis. We compared recombination, linkage disequilibrium (LD), GC content, and nucleotide diversity between pseudoautosomal and autosomal loci derived from 11 emu bacterial artificial chromosome (BAC) clones that were mapped to chromosomes by fluorescent in situ hybridization. Nucleotide diversity (pi = 4N(e)mu) was not significantly lower in pseudoautosomal loci (14 loci, 1.9 +/- 2.4 x 10(-3)) than autosomal loci (8 loci, 4.2 +/- 6.1 x 10(-3)). By contrast, recombination per site within BAC-end sequences (rho = 4Nc) (pseudoautosomal, 3.9 +/- 6.9 x 10(-2); autosomal, 2.3 +/- 3.7 x 10(-2)) was higher and average LD (D) (pseudoautosomal, 4.2 +/- 0.2 x 10(-1); autosomal, 4.7 +/- 0.5 x 10(-1)) slightly lower in pseudoautosomal sequences. We also report evidence of deviation from a simple neutral model in the PAR and in autosomal loci, possibly caused by departures from demographic equilibrium, such as population growth. This study provides a snapshot of the population genetics of avian sex chromosomes at an early stage of differentiation.
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Isolation and development of a molecular sex marker for Bassiana duperreyi, a lizard with XX/XY sex chromosomes and temperature-induced sex reversal.
Mol. Genet. Genomics
PUBLISHED: 02-18-2009
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Sex determination in the endemic Australian lizard Bassiana duperreyi (Scincidae) is influenced by sex chromosomes and incubation temperature, challenging the traditional dichotomy in reptilian sex determination. Analysis of those interactions requires sex chromosome markers to identify temperature-induced sex reversal. Here, we report the isolation of Y chromosome DNA sequence from B. duperreyi using amplified fragment length polymorphism PCR, the conversion of that sequence to a single-locus assay, and its combination with a single-copy nuclear gene (C-mos) to form a duplex PCR test for chromosomal sex. The accuracy of the assay was tested on an independent panel of individuals with known phenotypic sex. When used on offspring from field nests, our test identified the likely occurrence of a low rate of natural sex reversal in this species. This work represents the first report of Y chromosome sequence from a reptile and one of the few reptile sex tests.
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Molecular marker suggests rapid changes of sex-determining mechanisms in Australian dragon lizards.
Chromosome Res.
PUBLISHED: 01-27-2009
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Distribution of sex-determining mechanisms across Australian agamids shows no clear phylogenetic segregation, suggesting multiple transitions between temperature-dependent (TSD) and genotypic sex determination (GSD). These taxa thus present an excellent opportunity for studying the evolution of sex chromosomes, and evolutionary transitions between TSD and GSD. Here we report the hybridization of a 3 kb genomic sequence (PvZW3) that marks the Z and W microchromosomes of the Australian central bearded dragon (Pogona vitticeps) to chromosomes of 12 species of Australian agamids from eight genera using fluorescence in-situ hybridization (FISH). The probe hybridized to a single microchromosome pair in 11 of these species, but to the tip of the long arm of chromosome pair 2 in the twelfth (Physignathus lesueurii), indicating a micro-macro chromosome rearrangement. Three TSD species shared the marked microchromosome, implying that it is a conserved autosome in related species that determine sex by temperature. C-banding identified the marked microchromosome as the heterochromatic W chromosome in two of the three GSD species. However, in Ctenophorus fordi, the probe hybridized to a different microchromosome from that shown by C-banding to be the heterochromatic W, suggesting an independent origin for the ZW chromosome pair in that species. Given the haphazard distribution of GSD and TSD in this group and the existence of at least two sets of sex microchromosomes in GSD species, we conclude that sex-determining mechanisms in this family have evolved independently, multiple times in a short evolutionary period.
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A simple non-invasive protocol to establish primary cell lines from tail and toe explants for cytogenetic studies in Australian dragon lizards (Squamata: Agamidae).
Cytotechnology
PUBLISHED: 01-20-2009
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Primary cell lines were established from cultures of tail and toe clips of five species of Australian dragon lizards: Tympanocryptis pinguicolla, Tympanocryptis sp., Ctenophorus fordi, Amphibolurus norrisi and Pogona vitticeps. The start of exponential cell growth ranged from 1 to 5 weeks. Cultures from all specimens had fibroblastic morphology. Cell lines were propagated continuously up to ten passages, cryopreserved and recovered successfully. We found no reduction in cell viability after short term (<6 months) storage at -80 degrees C. Mitotic metaphase chromosomes were harvested from these cell lines and used in differential staining, banding and fluorescent in situ hybridisation. Cell lines maintained normal diploidy in all species. This study reports a simple non-invasive method for establishing primary cell lines from Australian dragon lizards without sacrifice. The method is likely to be applicable to a range of species. Such cell lines provide a virtually unlimited source of material for cytogenetic, evolutionary and genomic studies.
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Are some chromosomes particularly good at sex? Insights from amniotes.
Chromosome Res.
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Several recent studies have produced comparative maps of genes on amniote sex chromosomes, revealing homology of gene content and arrangement across lineages as divergent as mammals and lizards. For example, the chicken Z chromosome, which shares homology with the sex chromosomes of all birds, monotremes, and a gecko, is a striking example of stability of genome organization and retention, or independent acquisition, of function in sex determination. In other lineages, such as snakes and therian mammals, well conserved but independently evolved sex chromosome systems have arisen. Among lizards, novel sex chromosomes appear frequently, even in congeneric species. Here, we review recent gene mapping data, examine the evolutionary relationships of amniote sex chromosomes and argue that gene content can predispose some chromosomes to a specialized role in sex determination.
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