The Status of Dosage Compensation in the Multiple X Chromosomes of the Platypus

Dosage compensation has been thought to be a ubiquitous property of sex chromosomes that are represented differently in males and females. The expression of most X-borne genes is equalized between XX females and XY males in therian mammals (marsupials and “placentals”) by inactivating one X chromosome in female somatic cells. However, compensation seems not to be strictly required to equalize the expression of most Z-borne genes between ZZ male and ZW female birds. Whether dosage compensation operates in the third mammal lineage, the egg-laying monotremes, is of considerable interest, since the platypus has a complex sex chromosome system in which five X and five Y chromosomes share considerable genetic homology with the chicken ZW sex chromosome pair, but not with therian XY chromosomes. The assignment of genes to four platypus X chromosomes allowed us to examine X dosage compensation in this unique species. Quantitative PCR showed a range of compensation, but SNP analysis of several X-borne genes showed that both alleles are transcribed in a heterozygous female. Transcription of 14 BACs representing 19 X-borne genes was examined by RNA-FISH in female and male fibroblasts. An autosomal control gene was expressed from both alleles in nearly all nuclei, and four pseudoautosomal BACs were usually expressed from both alleles in male as well as female nuclei, showing that their Y loci are active. However, nine X-specific BACs were usually transcribed from only one allele. This suggests that while some genes on the platypus X are not dosage compensated, other genes do show some form of compensation via stochastic transcriptional inhibition, perhaps representing an ancestral system that evolved to be more tightly controlled in placental mammals such as human and mouse

Assignment1 of the DMRT1 gene to tammar wallaby chromosome 3p by fluorescence in situ hybridization

N. El-Mogharbel, J. Deakin, E. Tsend-Ayush, A. Pask and J.A.M. Grave

The monotreme genome: A patchwork of reptile, mammal and unique features?

http://www.elsevier.com/wps/locate/issn/03009629 Copyright © 2003 ElsevierThe first specimen of platypus (Ornithorhynchus anatinus) that reached Britain in the late 18th century was regarded a scientific hoax. Over decades the anatomical characteristics of these unique mammals, such as egg laying and the existence of mammary glands, were hotly debated before they were accepted. Within the last 40 years, more and more details of monotreme physiology, histology, reproduction and genetics have been revealed. Some show similarities with birds or reptiles, some with therian mammals, but many are very specific to monotremes. The genome is no exception to monotreme uniqueness. An early opinion was that the karyotype, composed of a few large chromosomes and many small ones, resembled bird and reptile macro- and micro-chromosomes. However, the platypus genome also features characteristics that are not present in other mammals, such as a complex translocation system. The sex chromosome system is still not resolved. Nothing is known about dosage compensation and, unlike in therian mammals, there seems to be no genomic imprinting. In this article we will recount the mysteries of the monotreme genome and describe how we are using recently developed technology to identify chromosomes in mitosis, meiosis and sperm, to map genes to chromosomes, to unravel the sex chromosome system and the translocation chain and investigate X inactivation and genomic imprinting in monotremes

Autosomal location of genes from the conserved mammalian X in the platypus (Ornithorhynchus anatinus): Implications for mammalian sex chromosome evolution

Mammalian sex chromosomes evolved from an ancient autosomal pair. Mapping of human X- and Y-borne genes in distantly related mammals and non-mammalian vertebrates has proved valuable to help deduce the evolution of this unique part of the genome. The platypus, a monotreme mammal distantly related to eutherians and marsupials, has an extraordinary sex chromosome system comprising five X and five Y chromosomes that form a translocation chain at male meiosis. The largest X chromosome (X1), which lies at one end of the chain, has considerable homology to the human X. Using comparative mapping and the emerging chicken database, we demonstrate that part of the therian X chromosome, previously thought to be conserved across all mammals, was lost from the platypus X1 to an autosome. This region included genes flanking the XIST locus, and also genes with Y-linked homologues that are important to male reproduction in therians. Since these genes lie on the X in marsupials and eutherians, and also on the homologous region of chicken chromosome 4, this represents a loss from the monotreme X rather than an additional evolutionary stratum of the human X. © 2005 Springer.Articl

Evolution and organization of the ten sex chromosome system in platypus

Platypus sex chromosomes have been controversial for over three decades. To solve the platypus chromosome puzzle we have generated whole chromosome paints for all platypus chromosomes. Hybridization on mitotic chromosomes of males and females and male meiotic cells revealed that platypus have five Ys and five X chromosomes, which form the meiotic chain adopting an XY alternating pattern. With remarkable accuracy the chain segregates into XXXXX-female determining and YYYYY-male determining sperm. The largest X chromosome, with homology to the human X, lies at one end of the chain (X1). At the other end of the chain we mapped the gene DMRT1 onto X5. DMRT1 is a Z-linked candidate sex-determining gene in chicken. This suggests an unexpected link between mammal and bird sex chromosome systems that were thought to have evolved independently. Moreover we found that Y1 is not very degraded and shares significant homology with the short arm of X1, indicating that X1Y1 is the most recent addition to the chain. In contrast, the tiny Y5 at the other end of the chain is very degraded and X5 showed no detectable homology with Y5. Therefore we conclude that X5Y5 is the original sex chromosome pair. Using the paints and sex chromosome specific BAC clones we have analyzed meiotic chromatin condensation and chromosome pairing of the ten sex chromosomes in platypus. Our results show that, as in other mammals, platypus sex chromosomes are condensed and pair late at pachytene, indicating that meiotic sex chromosome inactivation may occur as in other mammals

Characterizing the chromosomes of the platypus (Ornithorhynchus anatinus)

The definitive version may be found at www.springerlink.comLike the unique platypus itself, the platypus genome is extraordinary because of its complex sex chromosome system, and is controversial because of difficulties in identification of small autosomes and sex chromosomes. A 6-fold shotgun sequence of the platypus genome is now available and is being assembled with the help of physical mapping. It is therefore essential to characterize the chromosomes and resolve the ambiguities and inconsistencies in identifying autosomes and sex chromosomes. We have used chromosome paints and DAPI banding to identify and classify pairs of autosomes and sex chromosomes. We have established an agreed nomenclature and identified anchor BAC clones for each chromosome that will ensure unambiguous gene localizations.Daniel McMillan, Pat Miethke, Amber E. Alsop, Willem Rens, Patricia O’Brien, Vladimir Trifonov, Frederic Veyrunes, Kyriena Schatzkamer, Colin L. Kremitzki, Tina Graves, Wesley Warren, Frank Grützner, Malcolm A. Ferguson-Smith and Jennifer A. Marshall Grave