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1.
Intersection of the RNA interference and X-inactivation pathways 总被引：1，自引：0，他引：1
In mammals, dosage compensation is achieved by X-chromosome inactivation (XCI) in the female. The noncoding Xist gene initiates silencing of the X chromosome, whereas its antisense partner Tsix blocks silencing. The complementarity of Xist and Tsix RNAs has long suggested a role for RNA interference (RNAi). Here, we report that murine Xist and Tsix form duplexes in vivo. During XCI, the duplexes are processed to small RNAs (sRNAs), most likely on the active X (Xa) in a Dicer-dependent manner. Deleting Dicer compromises sRNA production and derepresses Xist. Furthermore, without Dicer, Xist RNA cannot accumulate and histone 3 lysine 27 trimethylation is blocked on the inactive X (Xi). The defects are partially rescued by truncating Tsix. Thus, XCI and RNAi intersect, down-regulating Xist on Xa and spreading silencing on Xi. 相似文献
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Derived features of a new boreosphenidan mammal from the Lower Cretaceous Yixian Formation of China suggest that it has a closer relationship to metatherians (including extant marsupials) than to eutherians (including extant placentals). This fossil dates to 125 million years ago and extends the record of marsupial relatives with skeletal remains by 50 million years. It also has many foot structures known only from climbing and tree-living extant mammals, suggesting that early crown therians exploited diverse niches. New data from this fossil support the view that Asia was likely the center for the diversification of the earliest metatherians and eutherians during the Early Cretaceous. 相似文献
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Molecular coupling of Xist regulation and pluripotency 总被引：2，自引：0，他引：2
Navarro P Chambers I Karwacki-Neisius V Chureau C Morey C Rougeulle C Avner P 《Science (New York, N.Y.)》2008,321(5896):1693-1695
During mouse embryogenesis, reversion of imprinted X chromosome inactivation in the pluripotent inner cell mass of the female blastocyst is initiated by the repression of Xist from the paternal X chromosome. Here we report that key factors supporting pluripotency-Nanog, Oct3/4, and Sox2-bind within Xist intron 1 in undifferentiated embryonic stem (ES) cells. Whereas Nanog null ES cells display a reversible and moderate up-regulation of Xist in the absence of any apparent modification of Oct3/4 and Sox2 binding, the drastic release of all three factors from Xist intron 1 triggers rapid ectopic accumulation of Xist RNA. We conclude that the three main genetic factors underlying pluripotency cooperate to repress Xist and thus couple X inactivation reprogramming to the control of pluripotency during embryogenesis. 相似文献
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Sensing X chromosome pairs before X inactivation via a novel X-pairing region of the Xic 总被引：1，自引：0，他引：1
Augui S Filion GJ Huart S Nora E Guggiari M Maresca M Stewart AF Heard E 《Science (New York, N.Y.)》2007,318(5856):1632-1636
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To equalize X-chromosome dosages between the sexes, the female mammal inactivates one of her two X chromosomes. X-chromosome inactivation (XCI) is initiated by expression of Xist, a 17-kb noncoding RNA (ncRNA) that accumulates on the X in cis. Because interacting factors have not been isolated, the mechanism by which Xist induces silencing remains unknown. We discovered a 1.6-kilobase ncRNA (RepA) within Xist and identified the Polycomb complex, PRC2, as its direct target. PRC2 is initially recruited to the X by RepA RNA, with Ezh2 serving as the RNA binding subunit. The antisense Tsix RNA inhibits this interaction. RepA depletion abolishes full-length Xist induction and trimethylation on lysine 27 of histone H3 of the X. Likewise, PRC2 deficiency compromises Xist up-regulation. Therefore, RepA, together with PRC2, is required for the initiation and spread of XCI. We conclude that a ncRNA cofactor recruits Polycomb complexes to their target locus. 相似文献
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Fooden J 《Science (New York, N.Y.)》1972,175(4024):894-898
The composition of aboriginal land mammal faunas in Australia and New Guinea (prototherians and metatherians), South America (metatherians and eutherians) and Madagascar (eutherians only) is reconsidered in light of continental drift reconstructions of Mesozoic-Tertiary world paleogeography It is proposed that these three faunas represent successively detached samples of the evolving world mammal fauna as it existed when each of these land masses became faunally isolated from the rest of the world as a result of the progressive fragmentation of Pangaea. Isolation of aboriginal prototherians and metatherians in Australia and New Guinea may date from the Upper JurassicLower Cretaceous; isolation of aboriginal metatherians and eutherians in South America may date from the Middle Cretaceous-Upper Cretaceous; isolation of aboriginal eutherians in Madagascar may date from the Paleocene-Eocene. 相似文献
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Dermitzakis ET Reymond A Scamuffa N Ucla C Kirkness E Rossier C Antonarakis SE 《Science (New York, N.Y.)》2003,302(5647):1033-1035
Analysis of the human and mouse genomes identified an abundance of conserved non-genic sequences (CNGs). The significance and evolutionary depth of their conservation remain unanswered. We have quantified levels and patterns of conservation of 191 CNGs of human chromosome 21 in 14 mammalian species. We found that CNGs are significantly more conserved than protein-coding genes and noncoding RNAS (ncRNAs) within the mammalian class from primates to monotremes to marsupials. The pattern of substitutions in CNGs differed from that seen in protein-coding and ncRNA genes and resembled that of protein-binding regions. About 0.3% to 1% of the human genome corresponds to a previously unknown class of extremely constrained CNGs shared among mammals. 相似文献
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【目的】研究父源印记基因在牛早期胚胎中的表达模式。【方法】利用Real-time PCR技术,检测Gnas、Grb10和Xist这3个父源性印记基因在牛体外受精（In vitro Fertilization,IVF）和孤雌激活（Parthenogenetic Activation,PA）胚胎各发育阶段的表达情况。【结果】对于IVF和PA两种来源的牛胚胎,Gnas在胚胎各发育阶段的表达没有明显差异;Grb10在2细胞阶段的PA胚胎中的表达量明显高于在IVF胚胎的,但从4细胞阶段到囊胚阶段,Grb10在两种来源的牛胚胎中表达没有明显差异;从2细胞阶段到8细胞阶段,Xist在两种来源的牛胚胎中表达没有明显差异,但从16细胞期开始,Xist在PA胚胎中的表达明显高于IVF胚胎的表达量。【结论】对于牛IVF胚胎和PA胚胎,Gnas、Grb10和Xist从2细胞期到囊胚期都可以检测到表达,但表达模式不同。这3个父源性印记基因在牛胚胎发育过程中可能发挥着重要的作用。 相似文献
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Signal transduction and the control of gene expression 总被引：1，自引：0，他引：1
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Thorington RW 《Science (New York, N.Y.)》1984,225(4666):1048-1050
Seven genera of flying squirrels share five characters of wrist anatomy, which form a functional complex associated with the support of the patagium. In these characters, they differ from all genera of tree and ground squirrels examined. Among mammals, gliding membranes have evolved independently in several other groups. The manner of attachment of the patagium to the forelimb is different in each and demonstrates five morphologies differing from that of flying squirrels. This complex wrist anatomy of flying squirrels provides evidence that gliding evolved only once among squirrels and that the flying squirrels are a monophyletic group. 相似文献
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Antisense transcription in the mammalian transcriptome 总被引：2，自引：0，他引：2
Katayama S Tomaru Y Kasukawa T Waki K Nakanishi M Nakamura M Nishida H Yap CC Suzuki M Kawai J Suzuki H Carninci P Hayashizaki Y Wells C Frith M Ravasi T Pang KC Hallinan J Mattick J Hume DA Lipovich L Batalov S Engström PG Mizuno Y Faghihi MA Sandelin A Chalk AM Mottagui-Tabar S Liang Z Lenhard B Wahlestedt C;RIKEN Genome Exploration Research Group;Genome Science Group 《Science (New York, N.Y.)》2005,309(5740):1564-1566
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The unusual varl gene of yeast mitochondrial DNA 总被引：7，自引：0，他引：7
The var1 gene specifies the only mitochondrial ribosomal protein known to be encoded by yeast mitochondrial DNA. The gene is unusual in that its base composition is nearly 90 percent adenine plus thymine. It and its expression product show a strain-dependent variation in size of up to 7 percent; this variation does not detectably interfere with function. Furthermore, var1 is an expandable gene that participates in a novel recombinational event resembling gene conversion whereby shorter alleles are preferentially converted to longer ones. The remarkable features of var1 indicate that it may have evolved by a mechanism analogous to exon shuffling, although no introns are actually present. 相似文献
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Brain evolution: new light on old principles 总被引：2，自引：0，他引：2
H J Jerison 《Science (New York, N.Y.)》1970,170(963):1224-1225
The fossil evidence on the evolution of brains and bodies in mammals shows that there has been a progressive increase in relative brain size accompanied by and correlated with increased diversity among species in relative brain size. Small-brained species have also evolved, but more large-brained species have appeared in successive epochs. 相似文献
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Rich TH Hopson JA Musser AM Flannery TF Vickers-Rich P 《Science (New York, N.Y.)》2005,307(5711):910-914
A dentary of the oldest known monotreme, the Early Cretaceous Teinolophos trusleri, has an internal mandibular trough, which in outgroups to mammals houses accessory jaw bones, and probable contact facets for angular, coronoid, and splenial bones. Certain of these accessory bones were detached from the mandible to become middle ear bones in mammals. Evidence that the angular (homologous with the mammalian ectotympanic) and the articular and prearticular (homologous with the mammalian malleus) bones retained attachment to the lower jaw in a basal monotreme indicates that the definitive mammalian middle ear evolved independently in living monotremes and therians (marsupials and placentals). 相似文献
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Mammalian X inactivation turns off one female X chromosome to enact dosage compensation between XX and XY individuals. X inactivation is known to be regulated in cis by Xite, Tsix, and Xist, but in principle the two Xs must also be regulated in trans to ensure mutually exclusive silencing. Here, we demonstrate that interchromosomal pairing mediates this communication. Pairing occurs transiently at the onset of X inactivation and is specific to the X-inactivation center. Deleting Xite and Tsix perturbs pairing and counting/choice, whereas their autosomal insertion induces de novo X-autosome pairing. Ectopic X-autosome interactions inhibit endogenous X-X pairing and block the initiation of X-chromosome inactivation. Thus, Tsix and Xite function both in cis and in trans. We propose that Tsix and Xite regulate counting and mutually exclusive choice through X-X pairing. 相似文献
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Dentary-squamosal joint and the origin of mammals 总被引：1，自引：0，他引：1
The dentary-squamosal jaw joint evolved more than once in advanced cynodont therapsids or their descendants, probably as a buttress against the reaction force created at the articulation by the adductor jaw musculature. The multiple origin of this joint suggests that additional criteria are required to separate early mammals from advanced therapsids. 相似文献