Patricia C. M. O'Brien

In the platypus a meiotic chain of ten sex chromosomes shares genes with the bird Z and mammal X chromosomes

Two centuries after the duck-billed platypus was discovered, monotreme chromosome systems remain deeply puzzling. Karyotypes of males, or of both sexes, were claimed to contain several unpaired chromosomes (including the X chromosome) that form a multi-chromosomal chain at meiosis. Such meiotic chains exist in plants and insects but are rare in vertebrates. How the platypus chromosome system works to determine sex and produce balanced gametes has been controversial for decades. Here we demonstrate that platypus have five male-specific chromosomes (Y chromosomes) and five chromosomes present in one copy in males and two copies in females (X chromosomes). These ten chromosomes form a multivalent chain at male meiosis, adopting an alternating pattern to segregate into XXXXX-bearing and YYYYY-bearing sperm. Which, if any, of these sex chromosomes bears one or more sex-determining genes remains unknown. The largest X chromosome, with homology to the human X chromosome, lies at one end of the chain, and a chromosome with homology to the bird Z chromosome lies near the other end. This suggests an evolutionary link between mammal and bird sex chromosome systems, which were previously thought to have evolved independently.

Cross‐species colour segmenting: A novel tool in human karyotype analysis

We used fluorescence in situ hybridization (FISH) with DNA probes derived from bivariate fluorescence activated flow sorting of primate chromosomes. In cases where human and primate karyotypes differ by chromosome rearrangements, reverse painting of primate probes resulted in a subregional delineation of the human homologous chromosomes. Probes were used from two gibbon species (Hylobates concolor and H. syndactylus) which both showed highly rearranged karyotypes. Hybridization of human chromosomes with painting probes derived from both gibbons showed that, with the exception of human chromosomes 15, 18, 21, 22 and the sex chromosomes, each chromosome was differentiated in at least two and up to six segments. These probes have been used in the analysis of various cases of constitutional chromosomal rearrangements in human pathology including complex intrachromosomal rearrangements. They were also used in a multi colour format (colour segmenting) to differentiate the entire human karyotype into 81 homologous coloured segments with probes derived from H. concolor, and 74 segments with probes derived from H. syndactylus. The addition of colours not only simplifies chromosome identification compared to the analysis of classical banding based on grey values, but colour segmenting also provides simple coloured landmarks for further fine analysis by classical banding.

Molecular Cytogenetic Definition of the Chicken Genome: The First Complete Avian Karyotype

Chicken genome mapping is important for a range of scientific disciplines. The ability to distinguish chromosomes of the chicken and other birds is thus a priority. Here we describe the molecular cytogenetic characterization of each chicken chromosome using chromosome painting and mapping of individual clones by FISH. Where possible, we have assigned the chromosomes to known linkage groups. We propose, on the basis of size, that the NOR chromosome is approximately the size of chromosome 22; however, we suggest that its original assignment of 16 should be retained. We also suggest a definitive chromosome classification system and propose that the probes developed here will find wide utility in the fields of developmental biology, DT40 studies, agriculture, vertebrate genome organization, and comparative mapping of avian species.

The multiple sex chromosomes of platypus and echidna are not completely identical and several share homology with the avian Z

BackgroundSex-determining systems have evolved independently in vertebrates. Placental mammals and marsupials have an XY system, birds have a ZW system. Reptiles and amphibians have different systems, including temperature-dependent sex determination, and XY and ZW systems that differ in origin from birds and placental mammals. Monotremes diverged early in mammalian evolution, just after the mammalian clade diverged from the sauropsid clade. Our previous studies showed that male platypus has five X and five Y chromosomes, no SRY, and DMRT1 on an X chromosome. In order to investigate monotreme sex chromosome evolution, we performed a comparative study of platypus and echidna by chromosome painting and comparative gene mapping.ResultsChromosome painting reveals a meiotic chain of nine sex chromosomes in the male echidna and establishes their order in the chain. Two of those differ from those in the platypus, three of the platypus sex chromosomes differ from those of the echidna and the order of several chromosomes is rearranged. Comparative gene mapping shows that, in addition to bird autosome regions, regions of bird Z chromosomes are homologous to regions in four platypus X chromosomes, that is, X1, X2, X3, X5, and in chromosome Y1.ConclusionMonotreme sex chromosomes are easiest to explain on the hypothesis that autosomes were added sequentially to the translocation chain, with the final additions after platypus and echidna divergence. Genome sequencing and contig anchoring show no homology yet between platypus and therian Xs; thus, monotremes have a unique XY sex chromosome system that shares some homology with the avian Z.

Deregulation of EIF4E: a novel mechanism for autism

Background: Autism is a common childhood onset neurodevelopmental disorder, characterised by severe and sustained impairment of social interaction and social communication, as well as a notably restricted repertoire of activities and interests. Its aetiology is multifactorial with a strong genetic basis. EIF4E is the rate limiting component of eukaryotic translation initiation, and plays a key role in learning and memory through its control of translation within the synapse. EIF4E mediated translation is the final common process modulated by the mammalian target of rapamycin (mTOR), PTEN and fragile X mental retardation protein (FMRP) pathways, which are implicated in autism. Linkage of autism to the EIF4E region on chromosome 4q has been found in genome wide linkage studies. Methods and results: The authors present evidence that directly implicates EIF4E in autism. In a boy with classic autism, the authors observed a de novo chromosome translocation between 4q and 5q and mapped the breakpoint site to within a proposed alternative transcript of EIF4E. They then screened 120 autism families for mutations and found two unrelated families where in each case both autistic siblings and one of the parents harboured the same single nucleotide insertion at position −25 in the basal element of the EIF4E promoter. Electrophoretic mobility shift assays and reporter gene studies show that this mutation enhances binding of a nuclear factor and EIF4E promoter activity. Conclusions: These observations implicate EIF4E, and more specifically control of EIF4E activity, directly in autism. The findings raise the exciting possibility that pharmacological manipulation of EIF4E may provide therapeutic benefit for those with autism caused by disturbance of the converging pathways controlling EIF4E activity.

Reciprocal chromosome painting illuminates the history of genome evolution of the domestic cat, dog and human

Domestic cats and dogs are important companion animals and model animals in biomedical research. The cat has a highly conserved karyotype, closely resembling the ancestral karyotype of mammals, while the dog has one of the most extensively rearranged mammalian karyotypes investigated so far. We have constructed the first detailed comparative chromosome map of the domestic dog and cat by reciprocal chromosome painting. Dog paints specific for the 38 autosomes and the X chromosomes delineated 68 conserved chromosomal segments in the cat, while reverse painting of cat probes onto red fox and dog chromosomes revealed 65 conserved segments. Most conserved segments on cat chromosomes also show a high degree of conservation in G-banding patterns compared with their canine counterparts. At least 47 chromosomal fissions (breaks), 25 fusions and one inversion are needed to convert the cat karyotype to that of the dog, confirming that extensive chromosome rearrangements differentiate the karyotypes of the cat and dog. Comparative analysis of the distribution patterns of conserved segments defined by dog paints on cat and human chromosomes has refined the human/cat comparative genome map and, most importantly, has revealed 15 cryptic inversions in seven large chromosomal regions of conserved synteny between humans and cats.

Refined genome-wide comparative map of the domestic horse, donkey and human based on cross-species chromosome painting: insight into the occasional fertility of mules

We have made a complete set of painting probes for the domestic horse by degenerate oligonucleotide-primed PCR amplification of flow-sorted horse chromosomes. The horse probes, together with a full set of those available for human, were hybridized onto metaphase chromosomes of human, horse and mule. Based on the hybridization results, we have generated genome-wide comparative chromosome maps involving the domestic horse, donkey and human. These maps define the overall distribution and boundaries of evolutionarily conserved chromosomal segments in the three genomes. Our results shed further light on the karyotypic relationships among these species and, in particular, the chromosomal rearrangements that underlie hybrid sterility and the occasional fertility of mules.

Micro- and macrochromosome paints generated by flow cytometry and microdissection: tools for mapping the chicken genome

Despite the chicken being one of the most genetically mapped of all animals, its karyotype remains poorly defined. This is primarily due to microchromosomes that belie assignment by conventional methods. To address this problem, we have developed chromosome-specific paints using flow cytometry and microdissection. For the microchromosomes it was necessary to amplify and label DNA from single microdissected chromosomes.

Spectral karyotyping, a 24-colour FISH technique for the identification of chromosomal rearrangements

Abstract Spectral karyotyping (SKY) is a new fluorescence in situ hybridisation (FISH) technique that refers to the molecular cytogenetic analysis of metaphase preparations by means of spectral microscopy. For SKY of human metaphase chromosomes, 24 chromosome-specific painting probes are used in just one FISH experiment. The probes are labelled by degenerate oligonucleotide-primed PCR using three fluorochromes and two haptens. Each probe is differentially labelled with one, two, three or four fluorescent dyes, resulting in a unique spectral signature for every chromosome. After in situ hybridisation and immunodetection, a spectral image is acquired using a conventional fluorescence light microscope equipped with a custom-designed triple-bandpass filter and the SpectraCube, which is able to retrieve spectral information for every pixel in a digital CCD image. The 24-colour display and chromosome classification are based on the unique emission spectra of the chromosomes. Together with chromosome banding information from an inverted DAPI or a G-banded metaphase, a comprehensive overview of chromosomal aberrations is presented.

Cross-species chromosome painting between human and marsupial directly demonstrates the ancient region of the mammalian X

Interspecies chromosome painting has been used to demonstrate homologies between chromosomes of human and other primates (Wienberg et al. 1990), carnivores (Rettenberger et al. 1995), and artiodactyls (Solinas-Toldo et al. 1995), which diverged about 60 million years before present (MyrBP). However, there has been no success in using the technique for wider comparisons, for instance between eutherian (“placental”) mammals and marsupials, which diverged about 130 MyrBP. We have now used a paint derived from the X Chromosome (Chr) flow-sorted from the tammar wallaby ( Macropus eugenii ), to detect homologous regions on chromosomes from cultured lymphocytes from a human male. Chromosome painting was performed under our usual conditions (Toder et al. 1997) with a 3-day hybridization time. The X Chr paint, labeled with P, was used to probe a Southern blot containing wallaby and human DNA to show that there was no DNA contaminant that could have influnced the hybridization results. Chromosome painting revealed strong signal on the long arm of the X Chr and proximally on the short arm (Fig. 1). No signal was detected on Xp distal to about Xp11.2, or on any autosomes. Paints derived from wallaby autosomes and the Y Chr detected no signal on human chromosome spreads under the same conditions.