Le génome de l'ornithorynque a été séquencé (2008).

[JacquesL]

Le génome de l'ornithorynque a été séquencé.

http://www.nature.com/nature/journal/v453/n7192/full/nature06936.html

Pour une fois qu'un article de Nature est en accès gratuit, profitez-en !
Ou s'agit-il seulement d'une correction ?

Le résumé est affreusement vague, mais heureusement le corps de l'article est disponible.

We present a draft genome sequence of the platypus, Ornithorhynchus anatinus. This monotreme exhibits a fascinating combination of reptilian and mammalian characters. For example, platypuses have a coat of fur adapted to an aquatic lifestyle; platypus females lactate, yet lay eggs; and males are equipped with venom similar to that of reptiles. Analysis of the first monotreme genome aligned these features with genetic innovations. We find that reptile and platypus venom proteins have been co-opted independently from the same gene families; milk protein genes are conserved despite platypuses laying eggs; and immune gene family expansions are directly related to platypus biology. Expansions of protein, non-protein-coding RNA and microRNA families, as well as repeat elements, are identified. Sequencing of this genome now provides a valuable resource for deep mammalian comparative analyses, as well as for monotreme biology and conservation.

Amniotes split into the sauropsids (leading to birds and reptiles) and synapsids (leading to mammal-like reptiles). These small early mammals developed hair, homeothermy and lactation (red lines). Monotremes diverged from the therian mammal lineage approx166 Myr ago2 and developed a unique suite of characters (dark-red text). Therian mammals with common characters split into marsupials and eutherians around 148 Myr ago2 (dark-red text). Geological eras and periods with relative times (Myr ago) are indicated on the left. Mammal lineages are in red; diapsid reptiles, shown as archosaurs (birds, crocodilians and dinosaurs), are in blue; and lepidosaurs (snakes, lizards and relatives) are in green.



a, Platypus has miRNAs shared with eutherians and chickens, and a set that is platypus-specific. miRNAs cloned from six platypus tissues were assigned to families based on seed conservation. Platypus miRNAs and families were divided into classes (indicated) based on their conservation patterns with eutherian mammals (mouse/human) and with chicken.
b, Expression of platypus miRNAs. The cloning frequency of each platypus mature miRNA sequenced more than once is represented by a vertical bar and clustered by conservation pattern. miRNAs from a set of monotreme-specific miRNA clusters that are expressed in testis are shaded in red.



a, b, The platypus genome contains only few olfactory receptor genes from olfactory receptor families that are greatly expanded among therians (three other mammals and a reptile shown), but many genes in olfactory receptor family 14 (a), and relatively numerous vomeronasal type 1 (V1R) receptors (b). These schematic phylogenetic trees show relative family sizes and pseudogene contents of different gene families (enumerated beside internal branches) and the V1R repertoire in platypus. Pie charts illustrate the proportions of intact genes (heavily shaded) versus disrupted pseudogenes (lightly shaded).


Parentés des protéines des venins :


The diagram illustrates separate gene duplications in different parts of the phylogeny for platypus venom defensin-like peptides (vDLPs), for lizard venom crotamine-like peptides (vCLPs) and for snake venom crotamines. These venom proteins have thus been co-opted from pre-existing non-toxin homologues independently in platypus and in lizards and snakes48.


Comparaison des contenus globaux des génomes :


The gene arrangement is conserved between mammals. However, non-coding regions are expanded in therians. Arrows indicate genes and the direction of transcription; the scale shows base pairs. b, Summary of repeat distribution for the PEG1/MEST cluster. Histograms represent the sequence (%) masked by each repeat element within the MEST cluster; black bars represent repeat distribution across the entire genome. With the exception of SINEs, platypus has fewer repeats of LINEs, LTRs, DNA and simple repeats (Simple) than eutherian mammals. Low comp., low complexity; sRNAs, small RNAs.

[JacquesL]

La compréhension de l'article suivant nécessite quelques notions de base sur les Gènes soumis à empreinte :

http://fr.wikipedia.org/wiki/G%C3%A8ne_soumis_%C3%A0_empreinte

Un gène soumis à empreinte est un gène dont l'activité dépend de son origine maternelle ou paternelle alors que la plupart des gènes sont actifs quelle que soit leur origine.

Le bon fonctionnement du gène nécessite à la fois le gène maternel et paternel. La plupart des gènes soumis à empreinte sont localisés sur les chromosomes 6, 7, 11, 14, 15.

L'activité du gène dépend de sa méthylation : un gène actif est hypométhylé, un gène inactif est hyperméthylé.

La connaissance des gènes soumis à empreinte permet de comprendre certaines maladies génétiques et permet un conseil génétique pertinent. Ainsi, la disparition du gène actif par délétion ou disomie uniparentale avec présence de deux gènes inactifs est responsable du :

   * Syndrome de Prader-Willi
   * Syndrome d'Angelman
   * Syndrome de Silver-Russel
   * Diabète néonatal
   * Syndrome de Beckwith-Wiedemann
   * Ostéodystrophie héréditaire d'Albright

http://en.wikipedia.org/wiki/Genomic_imprinting

Genomic imprinting is a genetic phenomenon by which certain genes are expressed in a parent-of-origin-specific manner. It is an inheritance process independent of the classical Mendelian inheritance. Imprinted genes are either expressed only from the allele inherited from the mother (eg. H19 or CDKN1C), or in other instances from the allele inherited from the father (eg. IGF-2). Forms of genomic imprinting have been demonstrated in insects, mammals and flowering plants.

Genomic imprinting is an epigenetic process that involves methylation and histone modifications in order to achieve monoallelic gene expression without altering the genetic sequence. These epigenetic marks are established in the germline and are maintained throughout all somatic cells of an organism.

Appropriate expression of imprinted genes is important for normal development, with numerous genetic diseases associated with imprinting defects including Beckwith-Wiedemann syndrome, Silver-Russell Syndrome, Angelman Syndrome and Prader-Willi Syndrome.

In diploid organisms, somatic cells possess two copies of the genome. Each autosomal gene is therefore represented by two copies, or alleles, with one copy inherited from each parent at fertilisation. For the vast majority of autosomal genes, expression occurs from both alleles simultaneously. In mammals however, a small proportion (<1%) of genes are imprinted, meaning that gene expression occurs from only one allele.[1] The expressed allele is dependent upon its parental origin. For example, the gene encoding Insulin-like growth factor 2 (IGF2/Igf2) is only expressed from the allele inherited from the father.[2]

The phrase "imprinting" was first used to describe events in the insect Pseudococcus nipae.[3] In Pseudococcids or mealybugs (Homoptera, Coccoidea) both the male and female develop from a fertilised egg. In females, all chromosomes remain euchromatic and functional. In embryos destined to become males, one haploid set of chromosomes becomes heterochromatinised after the sixth cleavage division and remains so in most tissues; males are thus functionally haploid.[4][5][6] In insects, imprinting describes the silencing of the paternal genome in males, and thus is involved in sex determination. In mammals, genomic imprinting describes the processes involved in introducing functional inequality between two parental alleles of a gene.[7]
[edit] Imprinted genes in mammals

That imprinting might be a feature of mammalian development was suggested in breeding experiments in mice carrying reciprocal translocations.[8] Nucleus transplantation experiments in mouse zygotes in the early 1980s confirmed that normal development requires the contribution of both the maternal and paternal genomes. The vast majority of mouse parthenogenones/gynogenones (with two maternal or egg genomes) and androgenones (with two paternal or sperm genomes) die at, or before, the blastocyst/implantation stage. In the rare instances that they develop to postimplantation stages, gynogenetic embryos show better embryonic development relative to placental development, while for androgenones, the reverse is true. Nevertheless, for the latter, only a few have been described.[9][10][11]

Parthenogenetic/gynogenetic embryos have twice the normal expression level of maternally derived genes, and lack expression of paternally expressed genes, while the reverse is true for androgenetic embryos. It is now known that there are at least 80 imprinted genes in humans and mice, many of which are involved in embryonic and placental growth and development.[12][13][14][15]

No naturally occurring cases of parthenogenesis exist in mammals because of imprinted genes. Experimental manipulation of a paternal methylation imprint controlling the Igf2 gene has, however, recently allowed the creation of rare individual mice with two maternal sets of chromosomes - but this is not a true parthenogenone. Hybrid offspring of two species may exhibit unusual growth due to the novel combination of imprinted genes.[16]
[edit] Genetic mapping of imprinted genes

At the same time as the generation of the gynogenetic and androgenetic embryos discussed above, mouse embryos were also being generated that contained only small regions that were derived from either a paternal or maternal source.[17][18] The generation of a series of such uniparental disomies, which together span the entire genome, allowed the creation of an imprinting map.[19] Those regions which when inherited from a single parent result in a discernible phenotype contain imprinted gene(s). Further research showed that within these regions there were often numerous imprinted genes.[20] Around 80% of imprinted genes are found in clusters such as these, called imprinted domains, suggesting a level of co-ordinated control.[21]
...

Et voilà l'article, qui s'intéresse à cette divergence Jurassique entre monotrèmes et thériens.

http://www.epidna.com/showabstract.php?pmid=19294344
http://www.springerlink.com/content/c1506q608p8335vt/fulltext.pdf

Sci China C Life Sci (2009) 52: 195-204.

Non-coding RNAs and the acquisition of genomic imprinting in mammals.

Y Zhang, L Qu

Genomic imprinting, representing parent-specific expression of alleles at a locus, is mainly evident in flowering plants and placental mammals. Most imprinted genes, including numerous non-coding RNAs, are located in clusters regulated by imprinting control regions (ICRs). The acquisition and evolution of genomic imprinting is among the most fundamental genetic questions. Discoveries about the transition of mammalian imprinted gene domains from their non-imprinted ancestors, especially recent studies undertaken on the most ancient mammalian clades - the marsupials and monotremes from which model species genomes have recently been sequenced, are of high value. By reviewing and analyzing these studies, a close connection between non-coding RNAs and the acquisition of genomic imprinting in mammals is demonstrated. The evidence comes from two observations accompanied with the acquisition of the imprinting: (i) many novel non-coding RNA genes emerged in imprinted regions; (ii) the expressions of some conserved non-coding RNAs have changed dramatically. Furthermore, a systematical analysis of imprinted snoRNA (small nucleolar RNA) genes from 15 vertebrates suggests that the origination of imprinted snoRNAs occurred after the divergence between eutherians and marsupials, followed by a rapid expansion leading to the fixation of major gene families in the eutherian ancestor prior to the radiation of modern placental mammals. Involved in the regulation of imprinted silencing and mediating the chromatins epigenetic modification may be the major roles that non-coding RNAs play during the acquisition of genomic imprinting in mammals.