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Andrew J Pask - One of the best experts on this subject based on the ideXlab platform.
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2016Co-Authors: Ryuichi Ono, Takanori Narita, Andrew J Pask, Changshan Wang, Amber E Alsop, Jennifer Marshall A Graves, Geoffrey Shaw, Yuji Kohara, Takashi Kohda, Marilyn B. RenfreeAbstract:Among mammals, only euTherians and marsupials are viviparous and have genomic imprinting that leads to parent-of-origin-specific differential gene expression. We used comparative analysis to investigate the origin of genomic imprinting in mammals. PEG10 (paternally expressed 10) is a retrotransposon-derived imprinted gene that has an essential role for the formation of the placenta of the mouse. Here, we show that an orthologue of PEG10 exists in another Therian mammal, the marsupial tammar wallaby (Macropus eugenii), but not in a protoTherian mammal, the egg-laying platypus (Ornithorhynchus anatinus), suggesting its close relationship to the origin of placentation in Therian mammals. We have discovered a hitherto missing link of the imprinting mechanism between euTherians and marsupials because tammar PEG10 is the first example of a differentially methylated region (DMR) associated with genomic imprinting in marsupials. Surprisingly, the marsupial DMR was strictly limited to the 59 region of PEG10, unlike the euTherian DMR, which covers the promoter regions of both PEG10 and the adjacent imprinted gene SGCE. These results not only demonstrate a common origin of the DMR-associated imprinting mechanism in Therian mammals but provide the first demonstration that DMR-associated genomic imprinting in euTherians can originat
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eggs embryos and the evolution of imprinting insights from the platypus genome
Reproduction Fertility and Development, 2009Co-Authors: Marilyn B. Renfree, Andrew J Pask, Anthony T Papenfuss, Geoff ShawAbstract:Genomic imprinting is widespread in euTherian and marsupial mammals. Although there have been many hypotheses to explain why genomic imprinting evolved in mammals, few have examined how it arose. The host defence hypothesis suggests that imprinting evolved from existing mechanisms within the cell that act to silence foreign DNA elements that insert into the genome. However, the changes to the mammalian genome that accompanied the evolution of imprinting have been hard to define due to the absence of large-scale genomic resources from all extant classes. The recent release of the platypus genome sequence has provided the first opportunity to make comparisons between protoTherian (monotreme, which show no signs of imprinting) and Therian (marsupial and euTherian, which have imprinting) mammals. We compared the distribution of repeat elements known to attract epigenetic silencing across the genome from monotremes and Therian mammals, particularly focusing on the orthologous imprinted regions. Our analyses show that the platypus has significantly fewer repeats of certain classes in the regions of the genome that have become imprinted in Therian mammals. The accumulation of repeats, especially long-terminal repeats and DNA elements, in Therian imprinted genes and gene clusters therefore appears to be coincident with, and may have been a potential driving force in, the development of mammalian genomic imprinting. Comparative platypus genome analyses of orthologous imprinted regions have provided strong support for the host defence hypothesis to explain the origin of imprinting.
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analysis of the platypus genome suggests a transposon origin for mammalian imprinting
Genome Biology, 2009Co-Authors: Andrew J Pask, Eleanor I Ager, Anthony T Papenfuss, Kaighin A Mccoll, Terence P Speed, Marilyn B. RenfreeAbstract:Background: Genomic imprinting is an epigenetic phenomenon that results in monoallelic gene expression. Many hypotheses have been advanced to explain why genomic imprinting evolved in mammals, but few have examined how it arose. The host defence hypothesis suggests that imprinting evolved from existing mechanisms within the cell that act to silence foreign DNA elements that insert into the genome. However, the changes to the mammalian genome that accompanied the evolution of imprinting have been hard to define due to the absence of large scale genomic resources between all extant classes. The recent release of the platypus genome has provided the first opportunity to perform comparisons between protoTherian (monotreme; which appear to lack imprinting) and Therian (marsupial and euTherian; which have imprinting) mammals. Results: We compared the distribution of repeat elements known to attract epigenetic silencing across the entire genome from monotremes and Therian mammals, particularly focusing on the orthologous imprinted regions. There is a significant accumulation of certain repeat elements within imprinted regions of Therian mammals compared to the platypus. Conclusions: Our analyses show that the platypus has significantly fewer repeats of certain classes in the regions of the genome that have become imprinted in Therian mammals. The accumulation of repeats, especially long terminal repeats and DNA elements, in Therian imprinted genes and gene clusters is coincident with, and may have been a potential driving force in, the development of mammalian genomic imprinting. These data provide strong support for the host defence hypothesis.
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conservation of the h19 noncoding rna and h19 igf2 imprinting mechanism in Therians
Nature Genetics, 2008Co-Authors: Guillaume Smits, Andrew J Pask, Andrew J Mungall, Sam Griffithsjones, Paul Smith, Delphine Beury, Lucy Matthews, Jane Rogers, Geoff Shaw, John L VandebergAbstract:Comparisons between euTherians and marsupials suggest limited conservation of the molecular mechanisms that control genomic imprinting in mammals. We have studied the evolution of the imprinted IGF2-H19 locus in Therians. Although marsupial orthologs of protein-coding exons were easily identified, the use of evolutionarily conserved regions and low-stringency Bl2seq comparisons was required to delineate a candidate H19 noncoding RNA sequence. The Therian H19 orthologs show miR-675 and exon structure conservation, suggesting functional selection on both features. Transcription start site sequences and poly(A) signals are also conserved. As in euTherians, marsupial H19 is maternally expressed and paternal methylation upstream of the gene originates in the male germline, encompasses a CTCF insulator, and spreads somatically into the H19 gene. The conservation in all Therians of the mechanism controlling imprinting of the IGF2-H19 locus suggests a sequential model of imprinting evolution.
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retrotransposon silencing by dna methylation can drive mammalian genomic imprinting
PLOS Genetics, 2007Co-Authors: Shunsuke Suzuki, Takanori Narita, Andrew J Pask, Changshan Wang, Amber E Alsop, Jennifer Marshall A Graves, Geoffrey Shaw, Yuji Kohara, Takashi Kohda, Fumitoshi IshinoAbstract:Among mammals, only euTherians and marsupials are viviparous and have genomic imprinting that leads to parent-of-origin-specific differential gene expression. We used comparative analysis to investigate the origin of genomic imprinting in mammals. PEG10 (paternally expressed 10) is a retrotransposon-derived imprinted gene that has an essential role for the formation of the placenta of the mouse. Here, we show that an orthologue of PEG10 exists in another Therian mammal, the marsupial tammar wallaby (Macropus eugenii), but not in a protoTherian mammal, the egg-laying platypus (Ornithorhynchus anatinus), suggesting its close relationship to the origin of placentation in Therian mammals. We have discovered a hitherto missing link of the imprinting mechanism between euTherians and marsupials because tammar PEG10 is the first example of a differentially methylated region (DMR) associated with genomic imprinting in marsupials. Surprisingly, the marsupial DMR was strictly limited to the 5′ region of PEG10, unlike the euTherian DMR, which covers the promoter regions of both PEG10 and the adjacent imprinted gene SGCE. These results not only demonstrate a common origin of the DMR-associated imprinting mechanism in Therian mammals but provide the first demonstration that DMR-associated genomic imprinting in euTherians can originate from the repression of exogenous DNA sequences and/or retrotransposons by DNA methylation.
Jennifer Marshall A Graves - One of the best experts on this subject based on the ideXlab platform.
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2016Co-Authors: Ryuichi Ono, Takanori Narita, Andrew J Pask, Changshan Wang, Amber E Alsop, Jennifer Marshall A Graves, Geoffrey Shaw, Yuji Kohara, Takashi Kohda, Marilyn B. RenfreeAbstract:Among mammals, only euTherians and marsupials are viviparous and have genomic imprinting that leads to parent-of-origin-specific differential gene expression. We used comparative analysis to investigate the origin of genomic imprinting in mammals. PEG10 (paternally expressed 10) is a retrotransposon-derived imprinted gene that has an essential role for the formation of the placenta of the mouse. Here, we show that an orthologue of PEG10 exists in another Therian mammal, the marsupial tammar wallaby (Macropus eugenii), but not in a protoTherian mammal, the egg-laying platypus (Ornithorhynchus anatinus), suggesting its close relationship to the origin of placentation in Therian mammals. We have discovered a hitherto missing link of the imprinting mechanism between euTherians and marsupials because tammar PEG10 is the first example of a differentially methylated region (DMR) associated with genomic imprinting in marsupials. Surprisingly, the marsupial DMR was strictly limited to the 59 region of PEG10, unlike the euTherian DMR, which covers the promoter regions of both PEG10 and the adjacent imprinted gene SGCE. These results not only demonstrate a common origin of the DMR-associated imprinting mechanism in Therian mammals but provide the first demonstration that DMR-associated genomic imprinting in euTherians can originat
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retrotransposon silencing by dna methylation can drive mammalian genomic imprinting
PLOS Genetics, 2007Co-Authors: Shunsuke Suzuki, Takanori Narita, Andrew J Pask, Changshan Wang, Amber E Alsop, Jennifer Marshall A Graves, Geoffrey Shaw, Yuji Kohara, Takashi Kohda, Fumitoshi IshinoAbstract:Among mammals, only euTherians and marsupials are viviparous and have genomic imprinting that leads to parent-of-origin-specific differential gene expression. We used comparative analysis to investigate the origin of genomic imprinting in mammals. PEG10 (paternally expressed 10) is a retrotransposon-derived imprinted gene that has an essential role for the formation of the placenta of the mouse. Here, we show that an orthologue of PEG10 exists in another Therian mammal, the marsupial tammar wallaby (Macropus eugenii), but not in a protoTherian mammal, the egg-laying platypus (Ornithorhynchus anatinus), suggesting its close relationship to the origin of placentation in Therian mammals. We have discovered a hitherto missing link of the imprinting mechanism between euTherians and marsupials because tammar PEG10 is the first example of a differentially methylated region (DMR) associated with genomic imprinting in marsupials. Surprisingly, the marsupial DMR was strictly limited to the 5′ region of PEG10, unlike the euTherian DMR, which covers the promoter regions of both PEG10 and the adjacent imprinted gene SGCE. These results not only demonstrate a common origin of the DMR-associated imprinting mechanism in Therian mammals but provide the first demonstration that DMR-associated genomic imprinting in euTherians can originate from the repression of exogenous DNA sequences and/or retrotransposons by DNA methylation.
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retrotransposon silencing by dna methylation can drive mammalian genomic imprinting
PLOS Genetics, 2007Co-Authors: Shunsuke Suzuki, Takanori Narita, Andrew J Pask, Changshan Wang, Amber E Alsop, Jennifer Marshall A Graves, Geoffrey Shaw, Yuji Kohara, Takashi Kohda, Fumitoshi IshinoAbstract:Among mammals, only euTherians and marsupials are viviparous and have genomic imprinting that leads to parent-of-origin-specific differential gene expression. We used comparative analysis to investigate the origin of genomic imprinting in mammals. PEG10 (paternally expressed 10) is a retrotransposon-derived imprinted gene that has an essential role for the formation of the placenta of the mouse. Here, we show that an orthologue of PEG10 exists in another Therian mammal, the marsupial tammar wallaby (Macropus eugenii), but not in a protoTherian mammal, the egg-laying platypus (Ornithorhynchus anatinus), suggesting its close relationship to the origin of placentation in Therian mammals. We have discovered a hitherto missing link of the imprinting mechanism between euTherians and marsupials because tammar PEG10 is the first example of a differentially methylated region (DMR) associated with genomic imprinting in marsupials. Surprisingly, the marsupial DMR was strictly limited to the 5′ region of PEG10, unlike the euTherian DMR, which covers the promoter regions of both PEG10 and the adjacent imprinted gene SGCE. These results not only demonstrate a common origin of the DMR-associated imprinting mechanism in Therian mammals but provide the first demonstration that DMR-associated genomic imprinting in euTherians can originate from the repression of exogenous DNA sequences and/or retrotransposons by DNA methylation.
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autosomal location of genes from the conserved mammalian x in the platypus ornithorhynchus anatinus implications for mammalian sex chromosome evolution
Chromosome Research, 2005Co-Authors: M. L. Delbridge, Paul D Waters, Janine E Deakin, Nisrine Elmogharbel, Patrick J Kirby, Denise Carvalhosilva, Jennifer Marshall A GravesAbstract: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.
Martin S. Fischer - One of the best experts on this subject based on the ideXlab platform.
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Three-dimensional kinematic analysis of the pectoral girdle during upside-down locomotion of two-toed sloths (Choloepus didactylus, Linné 1758)
Frontiers in zoology, 2010Co-Authors: John A. Nyakatura, Martin S. FischerAbstract:Background Theria (marsupials and placental mammals) are characterized by a highly mobile pectoral girdle in which the scapula has been shown to be an important propulsive element during locomotion. Shoulder function and kinematics are highly conservative during locomotion within quadrupedal Therian mammals. In order to gain insight into the functional morphology and evolution of the pectoral girdle of the two-toed sloth we here analyze the anatomy and the three-dimensional (3D) pattern of shoulder kinematics during quadrupedal suspensory ('upside-down') locomotion.
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pectoral girdle during upside-down locomotion of two-toed sloths ( Choloepus didactylus, Linné 1758)
2010Co-Authors: John A. Nyakatura, Martin S. FischerAbstract:Background: Theria (marsupials and placental mammals) are characterized by a highly mobile pectoral girdle in which the scapula has been shown to be an important propulsive element during locomotion. Shoulder function and kinematics are highly conservative during locomotion within quadrupedal Therian mammals. In order to gain insight into the functional morphology and evolution of the pectoral girdle of the two-toed sloth we here analyze the anatomy and the three-dimensional (3D) pattern of shoulder kinematics during quadrupedal suspensory ('upsidedown') locomotion. Methods: We use scientific rotoscoping, a new, non-invasive, markerless approach for x-ray reconstruction of moving morphology (XROMM), to quantify in vivo the 3D movements of all constituent skeletal elements of the shoulder girdle. Additionally we use histologic staining to analyze the configuration of the sterno-clavicular articulation (SCA). Results: Despite the inverse orientation of the body towards gravity, sloths display a 3D kinematic pattern and an orientation of the scapula relative to the thorax similar to pronograde claviculate mammalian species that differs from that of aclaviculate as well as brachiating mammals. Reduction of the relative length of the scapula alters its displacing effect on limb excursions. The configuration of the SCA maximizes mobility at this joint and demonstrates a tensile loading regime between thorax and limbs. Conclusions: The morphological characteristics of the scapula and the SCA allow maximal mobility of the forelimb to facilitate effective locomotion within a discontinuous habitat. These evolutionary changes associated with the adoption of the suspensory posture emphasized humeral influence on forelimb motion, but allowed the retention of the plesiomorphic 3D kinematic pattern.
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basic limb kinematics of small Therian mammals
The Journal of Experimental Biology, 2002Co-Authors: Martin S. Fischer, Nadja Schilling, Manuela Schmidt, Dieter Haarhaus, Hartmut WitteAbstract:A comparative study of quantitative kinematic data of fore- and hindlimb movements of eight different mammalian species leads to the recognition of basic principles in the locomotion of small Therians. The description of kinematics comprises fore- and hindlimb movements as well as sagittal spine movements including displacement patterns of limb segments, their contribution to step length, and joint movements. The comparison of the contributions of different segments to step length clearly shows the proximal parts (scapula, femur) to produce more than half of the propulsive movement of the whole limb at symmetrical gaits. Basically, a three-segmented limb with zigzag configuration of segments is mainly displaced at the scapular pivot or hip joint, both of which have the same vertical distance to the ground. Two segments operate in matched motion during retraction of the limb. While kinematic parameters of forelimbs are independent of speed and gait (with the scapula as the dominant element), fundamental changes occur in hindlimb kinematics with the change from symmetrical to in-phase gaits. Forward motion of the hindlimbs is now mainly due to sagittal lumbar spine movements contributing to half of the step length. Kinematics of small Therian mammals are independent of their systematic position, their natural habitat, and also of specific anatomical dispositions (e.g. reduction of fingers, toes, or clavicle). In contrast, the possession of a tail influences 'pelvic movements'.
Ralph Foster - One of the best experts on this subject based on the ideXlab platform.
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DO1:10.1080/10635150701477825 Calibration Choice, Rate Smoothing, and the Pattern of Tetrapod Diversification According to the Long Nuclear Gene RAG-1
2013Co-Authors: Andrew Hug F. All, Ralph Foster, Michael S. Y. LeeAbstract:Abstract. — A phylogeny of tetrapods is inferred from nearly complete sequences of the nuclear RAG-1 gene sampled across 88 taxa encompassing all major clades, analyzed via parsimony and Bayesian methods. The phylogeny provides support for Lissamphibia, Theria, Lepidosauria, a turtle-archosaur clade, as well as most traditionally accepted groupings. This tree allows simultaneous molecular clock dating for all tetrapod groups using a set of well-corroborated calibrations. Relaxed clock (PLRS) methods, using the amniote = 315 Mya (million years ago) calibration or a set of consistent calibrations, recovers reasonable divergence dates for most groups. However, the analysis systematically underestimates divergence dates within archosaurs. The bird-crocodile split, robustly documented in the fossil record as being around ~245 Mya, is estimated at only ~190 Mya, and dates for other divergences within archosaurs are similarly underestimated. Archosaurs, and particulary turtles have slow apparent rates possibly confounding rate modeling, and inclusion of calibrations within archosaurs (despite their high deviances) not only improves divergence estimates within archosaurs, but also across other groups. Notably, the monotreme-Therian split (~210 Mya) matches the fossil record; the squamate radiation (~190 Mya) is younger than suggested by some recent molecular studies and inconsistent with identification of ~220 and ~165 Myo (million-year-old) fossils as acrodont iguanians and ~95 Myo fossils colubroid snakes; the bird-lizard (reptile) split is considerably older than fossil estimates (<285 Mya); and Sphenodon is a remarkable phylogenetic relic, being the sole survivor of a lineage more than a quarter of a billion years old. Comparison with other molecular clock studies of tetrapod divergences suggests-jj^at
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calibration choice rate smoothing and the pattern of tetrapod diversification according to the long nuclear gene rag 1
Systematic Biology, 2007Co-Authors: Andrew F Hugall, Ralph FosterAbstract:A phylogeny of tetrapods is inferred from nearly complete sequences of the nuclear RAG-1 gene sampled across 88 taxa encompassing all major clades, analyzed via parsimony and Bayesian methods. The phylogeny provides support for Lissamphibia, Theria, Lepidosauria, a turtle-archosaur clade, as well as most traditionally accepted groupings. This tree allows simultaneous molecular clock dating for all tetrapod groups using a set of well-corroborated calibrations. Relaxed clock (PLRS) methods, using the amniote = 315 Mya (million years ago) calibration or a set of consistent calibrations, recovers reasonable divergence dates for most groups. However, the analysis systematically underestimates divergence dates within archosaurs. The bird-crocodile split, robustly documented in the fossil record as being around ~245 Mya, is estimated at only ~190 Mya, and dates for other divergences within archosaurs are similarly underestimated. Archosaurs, and particulary turtles have slow apparent rates possibly confounding rate modeling, and inclusion of calibrations within archosaurs (despite their high deviances) not only improves divergence estimates within archosaurs, but also across other groups. Notably, the monotreme-Therian split (~210 Mya) matches the fossil record; the squamate radiation (~190 Mya) is younger than suggested by some recent molecular studies and inconsistent with identification of ~220 and ~165 Myo (million-year-old) fossils as acrodont iguanians and ~95 Myo fossils colubroid snakes; the bird-lizard (reptile) split is considerably older than fossil estimates (<285 Mya); and Sphenodon is a remarkable phylogenetic relic, being the sole survivor of a lineage more than a quarter of a billion years old. Comparison with other molecular clock studies of tetrapod divergences suggests-jj^at . the common practice of enforcing most calibrations as minima, with a single liberal maximal constraint, will systematically-, overestimate divergence dates. Similarly, saturation of mitochondrial DNA sequences, and the resultant greater expression ^ i of basal branches means that using only external deep calibrations will also lead to inflated age estimates within the focal ; ingroup. (Amniota; cross-validation; fossil calibration; penalized likelihood rate smoothing; relaxed-clock; Reptilia; tetrapod,/ phylogeny. RAG-1.)
Paul D Waters - One of the best experts on this subject based on the ideXlab platform.
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origins and functional evolution of y chromosomes across mammals
Nature, 2014Co-Authors: Diego Cortez, Frank Grützner, Paul D Waters, Ray M Marin, Deborah Toledoflores, Laure Froidevaux, Angelica Liechti, Henrik KaessmannAbstract:Y chromosomes underlie sex determination in mammals, but their repeat-rich nature has hampered sequencing and associated evolutionary studies. Here we trace Y evolution across 15 representative mammals on the basis of high-throughput genome and transcriptome sequencing. We uncover three independent sex chromosome originations in mammals and birds (the outgroup). The original placental and marsupial (Therian) Y, containing the sex-determining gene SRY, emerged in the Therian ancestor approximately 180 million years ago, in parallel with the first of five monotreme Y chromosomes, carrying the probable sex-determining gene AMH. The avian W chromosome arose approximately 140 million years ago in the bird ancestor. The small Y/W gene repertoires, enriched in regulatory functions, were rapidly defined following stratification (recombination arrest) and erosion events and have remained considerably stable. Despite expression decreases in Therians, Y/W genes show notable conservation of proto-sex chromosome expression patterns, although various Y genes evolved testis-specificities through differential regulatory decay. Thus, although some genes evolved novel functions through spatial/temporal expression shifts, most Y genes probably endured, at least initially, because of dosage constraints. Using high-throughput genome and transcriptome sequencing, Y chromosome evolution across 15 representative mammals is explored, with results providing evidence for three independent sex chromosome originations in mammals and birds. Mammalian Y chromosomes, known for their roles in sex determination and male fertility, often contain repetitive sequences that make them harder to assemble than the rest of the genome. To counter this problem Henrik Kaessmann and colleagues have developed a new transcript assembly approach based on male-specific RNA/genomic sequencing data to explore Y evolution across 15 species representing all major mammalian lineages. They find evidence for two independent sex chromosome originations in mammals and one in birds. Their analysis of the Y/W gene repertoires suggests that although some genes evolved novel functions in sex determination/spermatogenesis as a result of temporal/spatial expression changes, most Y genes probably persisted, at least initially, as a result of dosage constraints. In a parallel study, Daniel Bellott and colleagues reconstructed the evolution of the Y chromosome, using a comprehensive comparative analysis of the genomic sequence of X–Y gene pairs from seven placental mammals and one marsupial. They conclude that evolution streamlined the gene content of the human Y chromosome through selection to maintain the ancestral dosage of homologous X–Y gene pairs that regulate gene expression throughout the body. They propose that these genes make the Y chromosome essential for male viability and contribute to differences between the sexes in health and disease.
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autosomal location of genes from the conserved mammalian x in the platypus ornithorhynchus anatinus implications for mammalian sex chromosome evolution
Chromosome Research, 2005Co-Authors: M. L. Delbridge, Paul D Waters, Janine E Deakin, Nisrine Elmogharbel, Patrick J Kirby, Denise Carvalhosilva, Jennifer Marshall A GravesAbstract: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.
