PGE2 alters chromatin through H2A.Z-variant enhancer nucleosome modification to promote hematopoietic stem cell fate.

Prostaglandin E2 (PGE2) and 16,16-dimethyl-PGE2 (dmPGE2) are important regulators of hematopoietic stem and progenitor cell (HSPC) fate and offer potential to enhance stem cell therapies [C. Cutler et al. Blood 122, 3074-3081(2013); W. Goessling et al. Cell Stem Cell 8, 445-458 (2011); W. Goessling et al. Cell 136, 1136-1147 (2009)]. Here, we report that PGE2-induced changes in chromatin at enhancer regions through histone-variant H2A.Z permit acute inflammatory gene induction to promote HSPC fate. We found that dmPGE2-inducible enhancers retain MNase-accessible, H2A.Z-variant nucleosomes permissive of CREB transcription factor (TF) binding. CREB binding to enhancer nucleosomes following dmPGE2 stimulation is concomitant with deposition of histone acetyltransferases p300 and Tip60 on chromatin. Subsequent H2A.Z acetylation improves chromatin accessibility at stimuli-responsive enhancers. Our findings support a model where histone-variant nucleosomes retained within inducible enhancers facilitate TF binding. Histone-variant acetylation by TF-associated nucleosome remodelers creates the accessible nucleosome landscape required for immediate enhancer activation and gene induction. Our work provides a mechanism through which inflammatory mediators, such as dmPGE2, lead to acute transcriptional changes and modify HSPC behavior to improve stem cell transplantation.

Prior metabolite extraction fully preserves RNAseq quality and enables integrative multi-'omics analysis of the liver metabolic response to viral infection.

Here, we provide an in-depth analysis of the usefulness of single-sample metabolite/RNA extraction for multi-'omics readout. Using pulverized frozen livers of mice injected with lymphocytic choriomeningitis virus (LCMV) or vehicle (Veh), we isolated RNA prior (RNA) or following metabolite extraction (MetRNA). RNA sequencing (RNAseq) data were evaluated for differential expression analysis and dispersion, and differential metabolite abundance was determined. Both RNA and MetRNA clustered together by principal component analysis, indicating that inter-individual differences were the largest source of variance. Over 85% of LCMV versus Veh differentially expressed genes were shared between extraction methods, with the remaining 15% evenly and randomly divided between groups. Differentially expressed genes unique to the extraction method were attributed to randomness around the 0.05 FDR cut-off and stochastic changes in variance and mean expression. In addition, analysis using the mean absolute difference showed no difference in the dispersion of transcripts between extraction methods. Altogether, our data show that prior metabolite extraction preserves RNAseq data quality, which enables us to confidently perform integrated pathway enrichment analysis on metabolomics and RNAseq data from a single sample. This analysis revealed pyrimidine metabolism as the most LCMV-impacted pathway. Combined analysis of genes and metabolites in the pathway exposed a pattern in the degradation of pyrimidine nucleotides leading to uracil generation. In support of this, uracil was among the most differentially abundant metabolites in serum upon LCMV infection. Our data suggest that hepatic uracil export is a novel phenotypic feature of acute infection and highlight the usefulness of our integrated single-sample multi-'omics approach.

Excess dietary carbohydrate affects mitochondrial integrity as observed in brown adipose tissue.

Hyperglycemia affects over 400 million individuals worldwide. The detrimental health effects are well studied at the tissue level, but the in vivo effects at the organelle level are poorly understood. To establish such an in vivo model, we used mice lacking TXNIP, a negative regulator of glucose uptake. Examining mitochondrial function in brown adipose tissue, we find that TXNIP KO mice have a lower content of polyunsaturated fatty acids (PUFAs) in their membrane lipids, which affects mitochondrial integrity and electron transport chain efficiency and ultimately results in lower mitochondrial heat output. This phenotype can be rescued by a ketogenic diet, confirming the usefulness of this model and highlighting one facet of early cellular damage caused by excess glucose influx.

Alterations in chromatin at antigen receptor loci define lineage progression during B lymphopoiesis.

Developing lymphocytes diversify their antigen receptor (AgR) loci by variable (diversity) joining (V[D]J) recombination. Here, using the micrococcal nuclease (MNase)-based chromatin accessibility (MACC) assay with low-cell count input, we profile both small-scale (kilobase) and large-scale (megabase) changes in chromatin accessibility and nucleosome occupancy in primary cells during lymphoid development, tracking the changes as different AgR loci become primed for recombination. The three distinct chromatin structures identified in this work define unique features of immunoglobulin H (IgH), Igκ, and T cell receptor-α (TCRα) loci during B lymphopoiesis. In particular, we find locus-specific temporal changes in accessibility both across megabase-long AgR loci and locally at the recombination signal sequences (RSSs). These changes seem to be regulated independently and can occur prior to lineage commitment. Large-scale changes in chromatin accessibility occur without significant change in nucleosome density and represent key features of AgR loci not previously described. We further identify local dynamic repositioning of individual RSS-associated nucleosomes at IgH and Igκ loci while they become primed for recombination during B cell commitment. These changes in chromatin at AgR loci are regulated in a locus-, lineage-, and stage-specific manner during B lymphopoiesis, serving either to facilitate or to impose a barrier to V(D)J recombination. We suggest that local and global changes in chromatin openness in concert with nucleosome occupancy and placement of histone modifications facilitate the temporal order of AgR recombination. Our data have implications for the organizing principles that govern assembly of these large loci as well as for mechanisms that might contribute to aberrant V(D)J recombination and the development of lymphoid tumors.

Methionine Metabolism Shapes T Helper Cell Responses through Regulation of Epigenetic Reprogramming.

Epigenetic modifications on DNA and histones regulate gene expression by modulating chromatin accessibility to transcription machinery. Here we identify methionine as a key nutrient affecting epigenetic reprogramming in CD4+ T helper (Th) cells. Using metabolomics, we showed that methionine is rapidly taken up by activated T cells and serves as the major substrate for biosynthesis of the universal methyl donor S-adenosyl-L-methionine (SAM). Methionine was required to maintain intracellular SAM pools in T cells. Methionine restriction reduced histone H3K4 methylation (H3K4me3) at the promoter regions of key genes involved in Th17 cell proliferation and cytokine production. Applied to the mouse model of multiple sclerosis (experimental autoimmune encephalomyelitis), dietary methionine restriction reduced the expansion of pathogenic Th17 cells in vivo, leading to reduced T cell-mediated neuroinflammation and disease onset. Our data identify methionine as a key nutritional factor shaping Th cell proliferation and function in part through regulation of histone methylation.

Non-neutral evolution of H3.3-encoding genes occurs without alterations in protein sequence.

Histone H3.3 is a developmentally essential variant encoded by two independent genes in human (H3F3A and H3F3B). While this two-gene arrangement is evolutionarily conserved, its origins and function remain unknown. Phylogenetics, synteny and gene structure analyses of H3.3 genes from 32 metazoan genomes indicate independent evolutionary paths for H3F3A and H3F3B. While H3F3B bears similarities with H3.3 genes in distant organisms and with canonical H3 genes, H3F3A is sarcopterygian-specific and evolves under strong purifying selection. Additionally, H3F3B codon-usage preferences resemble those of broadly expressed genes and 'cell differentiation-induced' genes, while codon-usage of H3F3A resembles that of 'cell proliferation-induced' genes. We infer that H3F3B is more similar to the ancestral H3.3 gene and likely evolutionarily adapted for a broad expression pattern in diverse cellular programs, while H3F3A adapted for a subset of gene expression programs. Thus, the arrangement of two independent H3.3 genes facilitates fine-tuning of H3.3 expression across cellular programs.

Chromatin restriction by the nucleosome remodeler Mi-2ß and functional interplay with lineage-specific transcription regulators control B-cell differentiation.

Coordinated induction, but also repression, of genes are key to normal differentiation. Although the role of lineage-specific transcription regulators has been studied extensively, their functional integration with chromatin remodelers, one of the key enzymatic machineries that control chromatin accessibility, remains ill-defined. Here we investigate the role of Mi-2ß, a SNF-2-like nucleosome remodeler and key component of the nucleosome remodeling and histone deacetylase (NuRD) complex in early B cells. Inactivation of Mi-2ß arrested differentiation at the large pre-B-cell stage and caused derepression of cell adhesion and cell migration signaling factors by increasing chromatin access at poised enhancers and chromosome architectural elements. Mi-2ß also supported IL-7R signaling, survival, and proliferation by repressing negative effectors of this pathway. Importantly, overexpression of Bcl2, a mitochondrial prosurvival gene and target of IL-7R signaling, partly rescued the differentiation block caused by Mi-2ß loss. Mi-2ß stably associated with chromatin sites that harbor binding motifs for IKAROS and EBF1 and physically associated with these transcription factors both on and off chromatin. Notably, Mi-2ß shared loss-of-function cellular and molecular phenotypes with IKAROS and EBF1, albeit in a distinct fashion. Thus, the nucleosome remodeler Mi-2ß promotes pre-B-cell differentiation by providing repression capabilities to distinct lineage-specific transcription factor-based regulatory networks.

Pervasive tertiary structure in the dengue virus RNA genome.

RNA virus genomes are efficient and compact carriers of biological information, encoding information required for replication both in their primary sequences and in higher-order RNA structures. However, the ubiquity of RNA elements with higher-order folds-in which helices pack together to form complex 3D structures-and the extent to which these elements affect viral fitness are largely unknown. Here we used single-molecule correlated chemical probing to define secondary and tertiary structures across the RNA genome of dengue virus serotype 2 (DENV2). Higher-order RNA structures are pervasive and involve more than one-third of nucleotides in the DENV2 genomic RNA. These 3D structures promote a compact overall architecture and contribute to viral fitness. Disrupting RNA regions with higher-order structures leads to stable, nonreverting mutants and could guide the development of vaccines based on attenuated RNA viruses. The existence of extensive regions of functional RNA elements with tertiary folds in viral RNAs, and likely many other messenger and noncoding RNAs, means that there are significant regions with pocket-containing surfaces that may serve as novel RNA-directed drug targets.

Recombinant Goose Circoviruses Circulating in Domesticated and Wild Geese in Poland.

Circoviruses are circular single-stranded DNA (ssDNA) viruses that infect a variety of animals, both domestic and wild. Circovirus infection in birds is associated with immunosuppression and this in turn predisposes the infected animals to secondary infections that can lead to mortality. Farmed geese (Anser anser) in many parts of the world are infected with circoviruses. The majority of the current genomic information for goose circoviruses (GoCVs) (n = 40) are from birds sampled in China and Taiwan, and only two genome sequences are available from Europe (Germany and Poland). In this study, we sampled 23 wild and 19 domestic geese from the Goplo Lake area in Poland. We determined the genomes of GoCV from 21 geese; 14 domestic Greylag geese (Anser anser), three wild Greylag geese (A. anser), three bean geese (A. fabalis), and one white fronted goose (A. albifrons). These genomes share 83-95% nucleotide pairwise identities with previously identified GoCV genomes, most are recombinants with exchanged fragment sizes up to 50% of the genome. Higher diversity levels can be seen within the genomes from domestic geese compared with those from wild geese. In the GoCV capsid protein (cp) and replication associated protein (rep) gene sequences we found that episodic positive selection appears to largely mirror those of beak and feather disease virus and pigeon circovirus. Analysis of the secondary structure of the ssDNA genome revealed a conserved stem-loop structure with the G-C rich stem having a high degree of negative selection on these nucleotides.

Detecting and Analyzing Genetic Recombination Using RDP4.

Recombination between nucleotide sequences is a major process influencing the evolution of most species on Earth. The evolutionary value of recombination has been widely debated and so too has its influence on evolutionary analysis methods that assume nucleotide sequences replicate without recombining. When nucleic acids recombine, the evolution of the daughter or recombinant molecule cannot be accurately described by a single phylogeny. This simple fact can seriously undermine the accuracy of any phylogenetics-based analytical approach which assumes that the evolutionary history of a set of recombining sequences can be adequately described by a single phylogenetic tree. There are presently a large number of available methods and associated computer programs for analyzing and characterizing recombination in various classes of nucleotide sequence datasets. Here we examine the use of some of these methods to derive and test recombination hypotheses using multiple sequence alignments.

Molecular characterization and prevalence of two capulaviruses: Alfalfa leaf curl virus from France and Euphorbia caput-medusae latent virus from South Africa.

Little is known about the prevalence, diversity, evolutionary processes, genomic structures and population dynamics of viruses in the divergent geminivirus lineage known as the capulaviruses. We determined and analyzed full genome sequences of 13 Euphorbia caput-medusae latent virus (EcmLV) and 26 Alfalfa leaf curl virus (ALCV) isolates, and partial genome sequences of 23 EcmLV and 37 ALCV isolates. While EcmLV was asymptomatic in uncultivated southern African Euphorbia caput-medusae, severe alfalfa disease symptoms were associated with ALCV in southern France. The prevalence of both viruses exceeded 10% in their respective hosts. Besides using patterns of detectable negative selection to identify ORFs that are probably functionally expressed, we show that ALCV and EcmLV both display evidence of inter-species recombination and biologically functional genomic secondary structures. Finally, we show that whereas the EcmLV populations likely experience restricted geographical dispersion, ALCV is probably freely moving across the French Mediterranean region.

RDP4: Detection and analysis of recombination patterns in virus genomes.

RDP4 is the latest version of recombination detection program (RDP), a Windows computer program that implements an extensive array of methods for detecting and visualising recombination in, and stripping evidence of recombination from, virus genome sequence alignments. RDP4 is capable of analysing twice as many sequences (up to 2,500) that are up to three times longer (up to 10 Mb) than those that could be analysed by older versions of the program. RDP4 is therefore also applicable to the analysis of bacterial full-genome sequence datasets. Other novelties in RDP4 include (1) the capacity to differentiate between recombination and genome segment reassortment, (2) the estimation of recombination breakpoint confidence intervals, (3) a variety of 'recombination aware' phylogenetic tree construction and comparison tools, (4) new matrix-based visualisation tools for examining both individual recombination events and the overall phylogenetic impacts of multiple recombination events and (5) new tests to detect the influences of gene arrangements, encoded protein structure, nucleic acid secondary structure, nucleotide composition, and nucleotide diversity on recombination breakpoint patterns. The key feature of RDP4 that differentiates it from other recombination detection tools is its flexibility. It can be run either in fully automated mode from the command line interface or with a graphically rich user interface that enables detailed exploration of both individual recombination events and overall recombination patterns.

The global distribution of Banana bunchy top virus reveals little evidence for frequent recent, human-mediated long distance dispersal events.

Banana bunchy top virus (BBTV; family Nanoviridae, genus Babuvirus) is a multi-component single-stranded DNA virus, which infects banana plants in many regions of the world, often resulting in large-scale crop losses. We analyzed 171 banana leaf samples from fourteen countries and recovered, cloned, and sequenced 855 complete BBTV components including ninety-four full genomes. Importantly, full genomes were determined from eight countries, where previously no full genomes were available (Samoa, Burundi, Republic of Congo, Democratic Republic of Congo, Egypt, Indonesia, the Philippines, and the USA [HI]). Accounting for recombination and genome component reassortment, we examined the geographic structuring of global BBTV populations to reveal that BBTV likely originated in Southeast Asia, that the current global hotspots of BBTV diversity are Southeast Asia/Far East and India, and that BBTV populations circulating elsewhere in the world have all potentially originated from infrequent introductions. Most importantly, we find that rather than the current global BBTV distribution being due to increases in human-mediated movements of bananas over the past few decades, it is more consistent with a pattern of infrequent introductions of the virus to different parts of the world over the past 1,000 years.

Molecular diversity of Chickpea chlorotic dwarf virus in Sudan: high rates of intra-species recombination - a driving force in the emergence of new strains.

In Sudan Chickpea chlorotic dwarf virus (CpCDV, genus Mastrevirus, family Geminiviridae) is an important pathogen of pulses that are grown both for local consumption, and for export. Although a few studies have characterised CpCDV genomes from countries in the Middle East, Africa and the Indian subcontinent, little is known about CpCDV diversity in any of the major chickpea production areas in these regions. Here we analyse the diversity of 146 CpCDV isolates characterised from pulses collected across the chickpea growing regions of Sudan. Although we find that seven of the twelve known CpCDV strains are present within the country, strain CpCDV-H alone accounted for ∼73% of the infections analysed. Additionally we identified four new strains (CpCDV-M, -N, -O and -P) and show that recombination has played a significant role in the diversification of CpCDV, at least in this region. Accounting for observed recombination events, we use the large amounts of data generated here to compare patterns of natural selection within protein coding regions of CpCDV and other dicot-infecting mastrevirus species.

The influence of secondary structure, selection and recombination on rubella virus nucleotide substitution rate estimates.

BACKGROUND: Annually, rubella virus (RV) still causes severe congenital defects in around 100 000 children globally. An attempt to eradicate RV is currently underway and analytical tools to monitor the global decline of the last remaining RV lineages will be useful for assessing the effectiveness of this endeavour. RV evolves rapidly enough that much of this information might be inferable from RV genomic sequence data. METHODS: Using BEASTv1.8.0, we analysed publically available RV sequence data to estimate genome-wide and gene-specific nucleotide substitution rates to test whether current estimates of RV substitution rates are representative of the entire RV genome. We specifically accounted for possible confounders of nucleotide substitution rate estimates, such as temporally biased sampling, sporadic recombination, and natural selection favouring either increased or decreased genetic diversity (estimated by the PARRIS and FUBAR methods), at nucleotide sites within the genomic secondary structures (predicted by the NASP method). RESULTS: We determine that RV nucleotide substitution rates range from 1.19 × 10(-3) substitutions/site/year in the E1 region to 7.52 × 10(-4) substitutions/site/year in the P150 region. We find that differences between substitution rate estimates in different RV genome regions are largely attributable to temporal sampling biases such that datasets containing higher proportions of recently sampled sequences, will tend to have inflated estimates of mean substitution rates. Although there exists little evidence of positive selection or natural genetic recombination in RV, we show that RV genomes possess pervasive biologically functional nucleic acid secondary structure and that purifying selection acting to maintain this structure contributes substantially to variations in estimated nucleotide substitution rates across RV genomes. CONCLUSION: Both temporal sampling biases and purifying selection favouring the conservation of RV nucleic acid secondary structures have an appreciable impact on substitution rate estimates but do not preclude the use of RV sequence data to date ancestral sequences. The combination of uniformly high substitution rates across the RV genome and strong temporal structure within the available sequence data, suggests that such data should be suitable for tracking the demographic, epidemiological and movement dynamics of this virus during eradication attempts.

SDT: a virus classification tool based on pairwise sequence alignment and identity calculation.

The perpetually increasing rate at which viral full-genome sequences are being determined is creating a pressing demand for computational tools that will aid the objective classification of these genome sequences. Taxonomic classification approaches that are based on pairwise genetic identity measures are potentially highly automatable and are progressively gaining favour with the International Committee on Taxonomy of Viruses (ICTV). There are, however, various issues with the calculation of such measures that could potentially undermine the accuracy and consistency with which they can be applied to virus classification. Firstly, pairwise sequence identities computed based on multiple sequence alignments rather than on multiple independent pairwise alignments can lead to the deflation of identity scores with increasing dataset sizes. Also, when gap-characters need to be introduced during sequence alignments to account for insertions and deletions, methodological variations in the way that these characters are introduced and handled during pairwise genetic identity calculations can cause high degrees of inconsistency in the way that different methods classify the same sets of sequences. Here we present Sequence Demarcation Tool (SDT), a free user-friendly computer program that aims to provide a robust and highly reproducible means of objectively using pairwise genetic identity calculations to classify any set of nucleotide or amino acid sequences. SDT can produce publication quality pairwise identity plots and colour-coded distance matrices to further aid the classification of sequences according to ICTV approved taxonomic demarcation criteria. Besides a graphical interface version of the program for Windows computers, command-line versions of the program are available for a variety of different operating systems (including a parallel version for cluster computing platforms).

Appearances can be deceptive: revealing a hidden viral infection with deep sequencing in a plant quarantine context.

Comprehensive inventories of plant viral diversity are essential for effective quarantine and sanitation efforts. The safety of regulated plant material exchanges presently relies heavily on techniques such as PCR or nucleic acid hybridisation, which are only suited to the detection and characterisation of specific, well characterised pathogens. Here, we demonstrate the utility of sequence-independent next generation sequencing (NGS) of both virus-derived small interfering RNAs (siRNAs) and virion-associated nucleic acids (VANA) for the detailed identification and characterisation of viruses infecting two quarantined sugarcane plants. Both plants originated from Egypt and were known to be infected with Sugarcane streak Egypt Virus (SSEV; Genus Mastrevirus, Family Geminiviridae), but were revealed by the NGS approaches to also be infected by a second highly divergent mastrevirus, here named Sugarcane white streak Virus (SWSV). This novel virus had escaped detection by all routine quarantine detection assays and was found to also be present in sugarcane plants originating from Sudan. Complete SWSV genomes were cloned and sequenced from six plants and all were found to share >91% genome-wide identity. With the exception of two SWSV variants, which potentially express unusually large RepA proteins, the SWSV isolates display genome characteristics very typical to those of all other previously described mastreviruses. An analysis of virus-derived siRNAs for SWSV and SSEV showed them to be strongly influenced by secondary structures within both genomic single stranded DNA and mRNA transcripts. In addition, the distribution of siRNA size frequencies indicates that these mastreviruses are likely subject to both transcriptional and post-transcriptional gene silencing. Our study stresses the potential advantages of NGS-based virus metagenomic screening in a plant quarantine setting and indicates that such techniques could dramatically reduce the numbers of non-intercepted virus pathogens passing through plant quarantine stations.

Patterns of recombination in HIV-1M are influenced by selection disfavouring the survival of recombinants with disrupted genomic RNA and protein structures.

Genetic recombination is a major contributor to the ongoing diversification of HIV. It is clearly apparent that across the HIV-genome there are defined recombination hot and cold spots which tend to co-localise both with genomic secondary structures and with either inter-gene boundaries or intra-gene domain boundaries. There is also good evidence that most recombination breakpoints that are detectable within the genes of natural HIV recombinants are likely to be minimally disruptive of intra-protein amino acid contacts and that these breakpoints should therefore have little impact on protein folding. Here we further investigate the impact on patterns of genetic recombination in HIV of selection favouring the maintenance of functional RNA and protein structures. We confirm that chimaeric Gag p24, reverse transcriptase, integrase, gp120 and Nef proteins that are expressed by natural HIV-1 recombinants have significantly lower degrees of predicted folding disruption than randomly generated recombinants. Similarly, we use a novel single-stranded RNA folding disruption test to show that there is significant, albeit weak, evidence that natural HIV recombinants tend to have genomic secondary structures that more closely resemble parental structures than do randomly generated recombinants. These results are consistent with the hypothesis that natural selection has acted both in the short term to purge recombinants with disrupted RNA and protein folds, and in the longer term to modify the genome architecture of HIV to ensure that recombination prone sites correspond with those where recombination will be minimally deleterious.

Extensive recombination-induced disruption of genetic interactions is highly deleterious but can be partially reversed by small numbers of secondary recombination events.

Although homologous recombination can potentially provide viruses with vastly more evolutionary options than are available through mutation alone, there are considerable limits on the adaptive potential of this important evolutionary process. Primary among these is the disruption of favorable coevolved genetic interactions that can occur following the transfer of foreign genetic material into a genome. Although the fitness costs of such disruptions can be severe, in some cases they can be rapidly recouped by either compensatory mutations or secondary recombination events. Here, we used a maize streak virus (MSV) experimental model to explore both the extremes of recombination-induced genetic disruption and the capacity of secondary recombination to adaptively reverse almost lethal recombination events. Starting with two naturally occurring parental viruses, we synthesized two of the most extreme conceivable MSV chimeras, each effectively carrying 182 recombination breakpoints and containing thorough reciprocal mixtures of parental polymorphisms. Although both chimeras were severely defective and apparently noninfectious, neither had individual movement-, encapsidation-, or replication-associated genome regions that were on their own "lethally recombinant." Surprisingly, mixed inoculations of the chimeras yielded symptomatic infections with viruses with secondary recombination events. These recombinants had only 2 to 6 breakpoints, had predominantly inherited the least defective of the chimeric parental genome fragments, and were obviously far more fit than their synthetic parents. It is clearly evident, therefore, that even when recombinationally disrupted virus genomes have extremely low fitness and there are no easily accessible routes to full recovery, small numbers of secondary recombination events can still yield tremendous fitness gains. Importance: Recombination between viruses can generate strains with enhanced pathological properties but also runs the risk of producing hybrid genomes with decreased fitness due to the disruption of favorable genetic interactions. Using two synthetic maize streak virus genome chimeras containing alternating genome segments derived from two natural viral strains, we examined both the fitness costs of extreme degrees of recombination (both chimeras had 182 recombination breakpoints) and the capacity of secondary recombination events to recoup these costs. After the severely defective chimeras were introduced together into a suitable host, viruses with between 1 and 3 secondary recombination events arose, which had greatly increased replication and infective capacities. This indicates that even in extreme cases where recombination-induced genetic disruptions are almost lethal, and 91 consecutive secondary recombination events would be required to reconstitute either one of the parental viruses, moderate degrees of fitness recovery can be achieved through relatively small numbers of secondary recombination events.

Pigeon circoviruses display patterns of recombination, genomic secondary structure and selection similar to those of beak and feather disease viruses.

Pigeon circovirus (PiCV) has a ~2 kb genome circular ssDNA genome. All but one of the known PiCV isolates have been found infecting pigeons in various parts of the world. In this study, we screened 324 swab and tissue samples from Polish pigeons and recovered 30 complete genomes, 16 of which came from birds displaying no obvious pathology. Together with 17 other publicly available PiCV complete genomes sampled throughout the Northern Hemisphere and Australia, we find that PiCV displays a similar degree of genetic diversity to that of the related psittacine-infecting circovirus species, beak and feather disease virus (BFDV). We show that, as is the case with its pathology and epidemiology, PiCV also displays patterns of recombination, genomic secondary structure and natural selection that are generally very similar to those of BFDV. It is likely that breeding facilities play a significant role in the emergence of new recombinant PiCV variants and given that ~50 % of the domestic pigeon population is infected subclinically, all pigeon breeding stocks should be screened routinely for this virus.