Sib mating without inbreeding in the longhorn crazy ant.

Sib matings increase homozygosity and, hence, the frequency of detrimental phenotypes caused by recessive deleterious alleles. However, many species have evolved adaptations that prevent the genetic costs associated with inbreeding. We discovered that the highly invasive longhorn crazy ant, Paratrechina longicornis, has evolved an unusual mode of reproduction whereby sib mating does not result in inbreeding. A population genetic study of P. longicornis revealed dramatic differences in allele frequencies between queens, males and workers. Mother-offspring analyses demonstrated that these allele frequency differences resulted from the fact that the three castes were all produced through different means. Workers developed through normal sexual reproduction between queens and males. However, queens were produced clonally and, thus, were genetically identical to their mothers. In contrast, males never inherited maternal alleles and were genetically identical to their fathers. The outcome of this system is that genetic inbreeding is impossible because queen and male genomes remain completely separate. Moreover, the sexually produced worker offspring retain the same genotype, combining alleles from both the maternal and paternal lineage over generations. Thus, queens may mate with their brothers in the parental nest, yet their offspring are no more homozygous than if the queen mated with a male randomly chosen from the population. The complete segregation of the male and female gene pools allows the queens to circumvent the costs associated with inbreeding and therefore may act as an important pre-adaptation for the crazy ant's tremendous invasive success.

Origins and functional evolution of Y chromosomes across mammals.

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.

Functional analysis of a conserved DNA methyltransferase in caulobacter crescentus

CcrM is a DNA methyltransferase that methylates the adenine in GANTC motifs in the chromo-some of the bacterial model Caulobacter crescentus. The loss of the CcrM homolog is lethal in C. crescentus and in several other species of Alphaproteobacteria. In this research, we used different experimental and bioinformatic approaches to determine why CcrM is so critical to the physiology of C. crescentus. We first showed that CcrM is a resident orphan DNA methyltransferase in non-Rickettsiales Alphaproteobacteria and that its gene is strictly conserved in this clade (with only one ex¬ception among the genomes sequenced so far). In C. crescentus, cells depleted in CcrM in rich medium quickly lose viability and present an elongated phenotype characteristic of an im¬pairment in cell division. Using minimal medium instead of rich medium as selective and main¬tenance substrate, we could generate a AccrM mutant that presents a viability comparable to the wild type strain and only mild morphological defects. On the basis of a transcriptomic ap¬proach, we determined that several genes essential for cell division were downregulated in the AccrM strain in minimal medium. We offered decisive arguments to support that the efficient transcription of two of these genes, ftsZ and mipZ, coding respectively for the Z-ring forming GTPase FtsZ and an inhibitor of FtsZ polymerization needed for the correct positioning of the Z- ring at mid-cell, requires the methylation of an adenine in a conserved GANTC motif located in their core promoter region. We propose a model, according to which the genome of C. crescentus encodes a transcriptional activator that requires a methylated adenine in a GANTC context to bind to DNA and suggest that this transcriptional regulator might be the global cell-cycle regulator GcrA. In addition, combining a classic genetic approach and in vitro evolution experiments, we showed that the mortality and cell division defects of the AccrM strain in rich medium are mainly due to limiting intracellular levels of the FtsZ protein. We also studied the dynamics of GANTC methylation in C. crescentus using the SMRT technol¬ogy developed by Pacific Biosciences. Our findings support the commonly accepted model, accord¬ing to which the methylation state of GANTC motifs varies during the cell cycle of C. crescentus: before the initiation of DNA replication, the GANTC motifs are fully-methylated (methylated on both strands); when the DNA gets replicated, the GANTC motifs become hemi-methylated (methyl¬ated on one strand only) and this occurs at different times during replication for different loci along the chromosome depending on their position relative to the origin of replication; the GANTC mo¬tifs are only remethylated after DNA replication has finished as a consequence of the massive and short-lived expression of CcrM in predivisional cells. About 30 GANTC motifs in the C. crescentus chromosome were found to be undermethylated in most of the bacterial population; these might be protected from CcrM activity by DNA binding proteins and some of them could be involved in methylation-based bistable transcriptional switches. - CcrM est une ADN méthyltransférase qui méthyle les adénines dans le contexte GANTC dans le génome de la bactérie modèle Caulobacter crescentus. La perte de l'homologue de CcrM chez C. crescentus et chez plusieurs autres espèces d'Alphaproteobactéries est létale. Dans le courant de cette recherche, nous tentons de déterminer pourquoi la protéine CcrM est cruciale pour la survie de C. crescentus. Nous démontrons d'abord que CcrM est une adénine méthyltransférase orpheline résidente, dont le gène fait partie du génome minimal partagé par les Alphaprotéobactéries non-Rickettsiales (à une exception près). Lorsqu'une souche de C. crescentus est privée de CcrM, sa viabilité décroît rapi¬dement et ses cellules présentent une morphologie allongée qui suggère que la division cellulaire est inhibée. Nous sommes parvenus à créer une souche AccrM en utilisant un milieu minimum, au lieu du milieu riche classiquement employé, comme milieu de sélection et de maintenance pour la souche. Lorsque nous avons étudié le transcriptome de cette souche de C. crescentus privée de CcrM, nous avons pu constater que plusieurs gènes essentiels pour le bon déroulement de la division cellulaire bactérienne étaient réprimés. En particulier, l'expression adéquate des gènes ftsZ et mipZ - qui codent, respectivement, pour FtsZ, la protéine qui constitue, au milieu de la cellule, un anneau protéique qui initie le processus de division et pour MipZ, un inhibiteur de la polymérisation de FtsZ qui est indispensable pour le bon positionnement de l'anneau FtsZ - est dépendante de la présence d'une adénine méthylée dans un motif GANTC conservé situé dans leur région promotrice. Nous présentons un modèle selon lequel le génome de C. crescentus code pour un facteur de transcription qui exige la présence d'une adénine méthylée dans un contexte GANTC pour s'attacher à l'ADN et nous suggérons qu'il pourrait s'agir du régulateur global du cycle cellulaire GcrA. En outre, nous montrons, en combinant la génétique classique et une approche basée sur l'évolution expérimentale, que la mortalité et l'inhibition de la division cellulaire caractéristiques de la souche àccrMeη milieu riche sont dues à des niveaux excessivement bas de protéine FtsZ. Nous avons aussi étudié la dynamique de la méthylation du chromosome de C. crescentus sur la base de la technologie SMRT développée par Pacific Biosciences. Nous confirmons le modèle communément accepté, qui affirme que l'état de méthylation des motifs GANTC change durant le cycle cellulaire de C. crescentus: les motifs GANTC sont complètement méthylés (méthylés sur les deux brins) avant de début de la réplication de l'ADN; ils deviennent hémi-méthylés (méthylés sur un brin seulement) une fois répliqués, ce qui arrive à différents moments durant la réplication pour différents sites le long du chromosome en fonction de leur position par rapport à l'origine de répli-cation; finalement, les motifs GANTC sont reméthylés après la fin de la réplication du chromosome lorsque la protéine CcrM est massivement, mais très transitoirement, produite. Par ailleurs, nous identifions dans le chromosome de C. crescentus environ 30 motifs GANTC qui restent en perma-nence non-méthylés dans une grande partie de la population bactérienne; ces motifs sont probable-ment protégés de l'action de CcrM par des protéines qui s'attachent à l'ADN et certains d'entre eux pourraient être impliqués dans des mécanismes de régulation générant une transcription bistable.

Genomic approaches to the study of HIV‐1 acquisition.

Host genome studies are increasingly available for the study of infectious disease susceptibility. Current technologies include large-scale genotyping, genome-wide screens such as transcriptome and silencing (silencing RNA) studies, and increasingly, the possibility to sequence complete genomes. These approaches are of interest for the study of individuals who remain uninfected despite documented exposure to human immunodeficiency virus type 1. The main limitation remains the ascertainment of exposure and establishing large cohorts of informative individuals. The pattern of enrichment for CCR5 Δ32 homozygosis should serve as the standard for assessing the extent to which a given cohort (of white subjects) includes a large proportion of exposed uninfected individuals.

CNVs and genetic medicine (excitement and consequences of a rediscovery).

The extensive variability of individual human genomes contributes to phenotypic variability. Structural genomic variants, and copy number variants (CNVs) in particular, have recently been rediscovered as contributors to the genomic plasticity and evolution and as pathoetiologic elements for both monogenic and complex traits. Herein we review some of the consequences of CNVs in the context of human inherited diseases.

The genomic impact of 100 million years of social evolution in seven ant species.

Ants (Hymenoptera, Formicidae) represent one of the most successful eusocial taxa in terms of both their geographic distribution and species number. The publication of seven ant genomes within the past year was a quantum leap for socio- and ant genomics. The diversity of social organization in ants makes them excellent model organisms to study the evolution of social systems. Comparing the ant genomes with those of the honeybee, a lineage that evolved eusociality independently from ants, and solitary insects suggests that there are significant differences in key aspects of genome organization between social and solitary insects, as well as among ant species. Altogether, these seven ant genomes open exciting new research avenues and opportunities for understanding the genetic basis and regulation of social species, and adaptive complex systems in general.

Genometric analyses of the organization of circular chromosomes: a universal pressure determines the direction of ribosomal RNA genes transcription relative to chromosome replication.

Selective pressures related to gene function and chromosomal architecture are acting on genome sequences and can be revealed, for instance, by appropriate genometric methods. Cumulative nucleotide skew analyses, i.e., GC, TA, and ORF orientation skews, predict the location of the origin of DNA replication for 88 out of 100 completely sequenced bacterial chromosomes. These methods appear fully reliable for proteobacteria, Gram-positives, and spirochetes as well as for euryarchaeotes. Based on this genome architecture information, coorientation analyses reveal that in prokaryotes, ribosomal RNA (rRNA) genes encoding the small and large ribosomal subunits are all transcribed in the same direction as DNA replication; that is, they are located along the leading strand. This result offers a simple and reliable method for circumscribing the region containing the origin of the DNA replication and reveals a strong selective pressure acting on the orientation of rRNA genes similar to the weaker one acting on the orientation of ORFs. Rate of coorientation of transfer RNA (tRNA) genes with DNA replication appears to be taxon-specific. Analyzing nucleotide biases such as GC and TA skews of genes and plotting one against the other reveals a taxonomic clusterization of species. All ribosomal RNA genes are enriched in Gs and depleted in Cs, the only so far known exception being the rRNA genes of deuterostomian mitochondria. However, this exception can be explained by the fact that in the chromosome of the human mitochondrion, the model of the deuterostomian organelle genome, DNA replication, and rRNA transcription proceed in opposite directions. A general rule is deduced from prokaryotic and mitochondrial genomes: ribosomal RNA genes that are transcribed in the same direction as the DNA replication are enriched in Gs, and those transcribed in the opposite direction are depleted in Gs.

Genome Biology and Evolution of the Animal Kingdom

Estudi realitzat a partir d’una estada a la Institut J.W. Jenkinson Laboratory for Evolution and Development of the University of Oxford, Regne Unit, entre 2010 i 2012. He estat membre del laboratori del Professor Peter W.H. Holland com a becari post-doctoral Beatriu de Pinós des de setembre de 2010 al setembre de 2012. El nostre projecte de recerca se centra en l'anàlisi genòmic comparatiu del Regne Animal, tot explorant el contingut dels genomes a través de totes les branques de l'arbre dels animals. Totes les referències a les meves publicacions durant aquest post-doc es poden trobar a http://about.me/jordi_paps. Crec que el nombre i la qualitat dels resultats del meu post-doc, un total de 8 publicacions incloent dos articles a la prestigiosa revista Nature, són prova de l'èxit d'aquest post-doc. Prof Peter W. H. Holland (Departament de Zoologia de la Universitat d'Oxford) i jo som coautors de tres articles de genòmica comparativa, resultats directes d'aquest projecte: 1) comparació de families gèniques entre vertebrats invertebrats (Briefings in Functional Genomics), 2) el genoma de l'ostra (publicat a la revista Nature), i 3) els genomes de 6 platihelmints paràsits (acceptat també a Nature). A més, tenim altres 2 treballs en preparació. Un d'ells analitza l'evolució, expressió i funció dels gens Hox al a la tènia Hymenolepis. El perfil fi d'aquests gens clau del desenvolupament esclareix els canvis d'estil de vida dels organismes. A més, durant aquest últim post-doc he participat en diverses col•laboracions, incloent anàlisi de gens d'envelliment a cucs plans, un estudi sobre la filogènia del grup Gastrotricha, una revisió de l'evolució phylum Platyhelminthes, així com un capítol d'un llibre sobre l'evolució dels animals bilaterals. Finalment, gràcies a la beca Beatriu de Pinós, el Prof. Peter W.H. Holland m'ha convidat a formar part del seu equip com un investigador post-doctoral en el seu projecte ERC Advance actual sobre duplicacions genòmiques.

Network-guided analysis of genes with altered somatic copy number and gene expression reveals pathways commonly perturbed in metastatic melanoma.

Cancer genomes frequently contain somatic copy number alterations (SCNA) that can significantly perturb the expression level of affected genes and thus disrupt pathways controlling normal growth. In melanoma, many studies have focussed on the copy number and gene expression levels of the BRAF, PTEN and MITF genes, but little has been done to identify new genes using these parameters at the genome-wide scale. Using karyotyping, SNP and CGH arrays, and RNA-seq, we have identified SCNA affecting gene expression ('SCNA-genes') in seven human metastatic melanoma cell lines. We showed that the combination of these techniques is useful to identify candidate genes potentially involved in tumorigenesis. Since few of these alterations were recurrent across our samples, we used a protein network-guided approach to determine whether any pathways were enriched in SCNA-genes in one or more samples. From this unbiased genome-wide analysis, we identified 28 significantly enriched pathway modules. Comparison with two large, independent melanoma SCNA datasets showed less than 10% overlap at the individual gene level, but network-guided analysis revealed 66% shared pathways, including all but three of the pathways identified in our data. Frequently altered pathways included WNT, cadherin signalling, angiogenesis and melanogenesis. Additionally, our results emphasize the potential of the EPHA3 and FRS2 gene products, involved in angiogenesis and migration, as possible therapeutic targets in melanoma. Our study demonstrates the utility of network-guided approaches, for both large and small datasets, to identify pathways recurrently perturbed in cancer.

Pseudogenes in the ENCODE regions: consensus annotation, analysis of transcription, and evolution

Arising from either retrotransposition or genomic duplication of functional genes, pseudogenes are “genomic fossils” valuable for exploring the dynamics and evolution of genes and genomes. Pseudogene identification is an important problem in computational genomics, and is also critical for obtaining an accurate picture of a genome’s structure and function. However, no consensus computational scheme for defining and detecting pseudogenes has been developed thus far. As part of the ENCyclopedia Of DNA Elements (ENCODE) project, we have compared several distinct pseudogene annotation strategies and found that different approaches and parameters often resulted in rather distinct sets of pseudogenes. We subsequently developed a consensus approach for annotating pseudogenes (derived from protein coding genes) in the ENCODE regions, resulting in 201 pseudogenes, two-thirds of which originated from retrotransposition. A survey of orthologs for these pseudogenes in 28 vertebrate genomes showed that a significant fraction (∼80%) of the processed pseudogenes are primate-specific sequences, highlighting the increasing retrotransposition activity in primates. Analysis of sequence conservation and variation also demonstrated that most pseudogenes evolve neutrally, and processed pseudogenes appear to have lost their coding potential immediately or soon after their emergence. In order to explore the functional implication of pseudogene prevalence, we have extensively examined the transcriptional activity of the ENCODE pseudogenes. We performed systematic series of pseudogene-specific RACE analyses. These, together with complementary evidence derived from tiling microarrays and high throughput sequencing, demonstrated that at least a fifth of the 201 pseudogenes are transcribed in one or more cell lines or tissues.

Assembling genes from predicted exons in linear time with dynamic programming

In a number of programs for gene structure prediction in higher eukaryotic genomic sequences, exon prediction is decoupled from gene assembly: a large pool of candidate exons is predicted and scored from features located in the query DNA sequence, and candidate genes are assembled from such a pool as sequences of nonoverlapping frame-compatible exons. Genes are scored as a function of the scores of the assembled exons, and the highest scoring candidate gene is assumed to be the most likely gene encoded by the query DNA sequence. Considering additive gene scoring functions, currently available algorithms to determine such a highest scoring candidate gene run in time proportional to the square of the number of predicted exons. Here, we present an algorithm whose running time grows only linearly with the size of the set of predicted exons. Polynomial algorithms rely on the fact that, while scanning the set of predicted exons, the highest scoring gene ending in a given exon can be obtained by appending the exon to the highest scoring among the highest scoring genes ending at each compatible preceding exon. The algorithm here relies on the simple fact that such highest scoring gene can be stored and updated. This requires scanning the set of predicted exons simultaneously by increasing acceptor and donor position. On the other hand, the algorithm described here does not assume an underlying gene structure model. Indeed, the definition of valid gene structures is externally defined in the so-called Gene Model. The Gene Model specifies simply which gene features are allowed immediately upstream which other gene features in valid gene structures. This allows for great flexibility in formulating the gene identification problem. In particular it allows for multiple-gene two-strand predictions and for considering gene features other than coding exons (such as promoter elements) in valid gene structures.

Comparative gene prediction in human and mouse

The completion of the sequencing of the mouse genome promises to help predict human genes with greater accuracy. While current ab initio gene prediction programs are remarkably sensitive (i.e., they predict at least a fragment of most genes), their specificity is often low, predicting a large number of false-positive genes in the human genome. Sequence conservation at the protein level with the mouse genome can help eliminate some of those false positives. Here we describe SGP2, a gene prediction program that combines ab initio gene prediction with TBLASTX searches between two genome sequences to provide both sensitive and specific gene predictions. The accuracy of SGP2 when used to predict genes by comparing the human and mouse genomes is assessed on a number of data sets, including single-gene data sets, the highly curated human chromosome 22 predictions, and entire genome predictions from ENSEMBL. Results indicate that SGP2 outperforms purely ab initio gene prediction methods. Results also indicate that SGP2 works about as well with 3x shotgun data as it does with fully assembled genomes. SGP2 provides a high enough specificity that its predictions can be experimentally verified at a reasonable cost. SGP2 was used to generate a complete set of gene predictions on both the human and mouse by comparing the genomes of these two species. Our results suggest that another few thousand human and mouse genes currently not in ENSEMBL are worth verifying experimentally.

EGASP: the human ENCODE Genome Annotation Assessment Project

Background: We present the results of EGASP, a community experiment to assess the state-ofthe-art in genome annotation within the ENCODE regions, which span 1% of the human genomesequence. The experiment had two major goals: the assessment of the accuracy of computationalmethods to predict protein coding genes; and the overall assessment of the completeness of thecurrent human genome annotations as represented in the ENCODE regions. For thecomputational prediction assessment, eighteen groups contributed gene predictions. Weevaluated these submissions against each other based on a ‘reference set’ of annotationsgenerated as part of the GENCODE project. These annotations were not available to theprediction groups prior to the submission deadline, so that their predictions were blind and anexternal advisory committee could perform a fair assessment.Results: The best methods had at least one gene transcript correctly predicted for close to 70%of the annotated genes. Nevertheless, the multiple transcript accuracy, taking into accountalternative splicing, reached only approximately 40% to 50% accuracy. At the coding nucleotidelevel, the best programs reached an accuracy of 90% in both sensitivity and specificity. Programsrelying on mRNA and protein sequences were the most accurate in reproducing the manuallycurated annotations. Experimental validation shows that only a very small percentage (3.2%) of the selected 221 computationally predicted exons outside of the existing annotation could beverified.Conclusions: This is the first such experiment in human DNA, and we have followed thestandards established in a similar experiment, GASP1, in Drosophila melanogaster. We believe theresults presented here contribute to the value of ongoing large-scale annotation projects and shouldguide further experimental methods when being scaled up to the entire human genome sequence.

Comparative gene finding in chicken indicates that we are closing in on the set of multi-exonic widely-expressed human genes

The recent availability of the chicken genome sequence poses the question of whether there are human protein-coding genes conserved in chicken that are currently not included in the human gene catalog. Here, we show, using comparative gene finding followed by experimental verification of exon pairs by RT–PCR, that the addition to the multi-exonic subset of this catalog could be as little as 0.2%, suggesting that we may be closing in on the human gene set. Our protocol, however, has two shortcomings: (i) the bioinformatic screening of the predicted genes, applied to filter out false positives, cannot handle intronless genes; and (ii) the experimental verification could fail to identify expression at a specific developmental time. This highlights the importance of developing methods that could provide a reliable estimate of the number of these two types of genes.