Prasinoviruses of the marine green alga Ostreococcus tauri are mainly species specific

Prasinoviruses infecting unicellular green algae in the order Mamiellales (class Mamiellophyceae) are commonly found in coastal marine waters where their host species frequently abound. We tested 40 Ostreococcus tauri viruses on 13 independently isolated wild-type O. tauri strains, 4 wild-type O. lucimarinus strains, 1 Ostreococcus sp. ("Ostreococcus mediterraneus") clade D strain, and 1 representative species of each of two other related species of Mamiellales, Bathycoccus prasinos and Micromonas pusilla. Thirty-four out of 40 viruses infected only O. tauri, 5 could infect one other species of the Ostreococcus genus, and 1 infected two other Ostreococcus spp., but none of them infected the other genera. We observed that the overall susceptibility pattern of Ostreococcus strains to viruses was related to the size of two host chromosomes known to show intraspecific size variations, that genetically related viruses tended to infect the same host strains, and that viruses carrying inteins were strictly strain specific. Comparison of two complete O. tauri virus proteomes revealed at least three predicted proteins to be candidate viral specificity determinants.     

Molecular phylogeny and evolution of the plastid and nuclear encoded cbbX genes in the unicellular red alga Cyanidioschyzon merolae

The cbbX gene is generally encoded in proteobacterial genomes and red-algal plastid genomes. In this study, we found two distinct cbbX genes of Cyanidioschyzon merolae, a unicellular red alga, one encoded in the plastid genome and the other encoded in the cell nucleus. The phylogenetic tree inferred from cbbX genes and strongly conserved gene organization (rbcLS-cbbX) suggests that the plastid-encoded cbbX gene of C. merolae came from an ancestral proteobacterium by horizontal gene transfer. On the other hand, the nuclear-encoded cbbX gene of C. merolae was classified in another cluster together with the nucleomorph-encoded cbbX gene of Guillardia theta. Furthermore, expression of the two cbbX genes were regulated differently in response to extracellular CO(2) concentration. Our results imply that cbbX gene in the plastid genome was copied and transferred to the cell nucleus after horizontal gene transfer of RuBisCo operon from ancestral beta-proteobacteria at comparatively early stage, and that each cbbX evolved in different ways.     

A Deviant Genetic Code in the Reduced Mitochondrial Genome of the Picoplanktonic Green Alga Pycnococcus provasolii

Reduction in size of flagellated chlorophytes occurred multiple times during evolution, providing the opportunity to study the consequences of cell reduction on genome architecture. Recent investigations on the chloroplast genomes of the tiny prasinophyceans Ostreococcus tauri (Mamiellales), Micromonas sp. RCC299 (Mamiellales), and Pycnococcus provasolii (Pseudocourfieldiales) highlighted their extreme compaction and reduced gene repertoires. Genome compaction is also exemplified by the Ostreococcus and Micromonas mitochondrial DNAs (mtDNAs) although they have retained almost all of the about 65 genes presumably present in the mitochondria of ancestral prasinophyceans. In this study, the mitochondrial genome of Pycnococcus was sequenced and compared to those of previously examined chlorophytes. Our results document the first case where cellular reduction of a free-living alga was accompanied by marked reduction in gene content of both the mitochondrial and chloroplast genomes. At 24,321 bp, the intronless Pycnococcus mitochondrial genome falls within the lower size range displayed by green algal mtDNAs. The 36 conserved genes, specifying two rRNAs with conventional structures, 16 tRNAs and 18 proteins, are all encoded on the same DNA strand and represent 88% of the genome. Besides a pronounced codon bias, the protein-coding genes feature a variant genetic code characterized by the use of TGA (normally a stop codon) to code for tryptophan, and the unprecedented use of TTA and TTG (normally leucine codons) as stop codons. We conclude that substantial reduction of the mitochondrial genome occurred in at least three independent chlorophyte lineages and that this process entailed a number of convergent changes in these lineages.     

Comparative genomics of microbial pathogens and symbionts

We are interested in quantifying the contribution of gene acquisition, loss, expansion and rearrangements to the evolution of microbial genomes. Here, we discuss factors influencing microbial genome divergence based on pair-wise genome comparisons of closely related strains and species with different lifestyles. A particular focus is on intracellular pathogens and symbionts of the genera Rickettsia, Bartonella and BUCHNERA: Extensive gene loss and restricted access to phage and plasmid pools may provide an explanation for why single host pathogens are normally less successful than multihost pathogens. We note that species-specific genes tend to be shorter than orthologous genes, suggesting that a fraction of these may represent fossil-orfs, as also supported by multiple sequence alignments among species. The results of our genome comparisons are placed in the context of phylogenomic analyses of alpha and gamma proteobacteria. We highlight artefacts caused by different rates and patterns of mutations, suggesting that atypical phylogenetic placements can not a priori be taken as evidence for horizontal gene transfer events. The flexibility in genome structure among free-living microbes contrasts with the extreme stability observed for the small genomes of aphid endosymbionts, in which no rearrangements or inflow of genetic material have occurred during the past 50 millions years (1). Taken together, the results suggest that genomic stability correlate with the content of repeated sequences and mobile genetic elements, and thereby indirectly with bacterial lifestyles.     

Horizontal gene transfers as metagenomic gene duplications

While it is well accepted that horizontal gene transfer plays an important role in the evolution and the diversification of prokaryotic genomes, many questions remain open regarding its functional mechanisms of action and its interplay with the extant genome. This study addresses the relationship between proteome innovation by horizontal gene transfer and genome content in Proteobacteria. We characterize the transferred genes, focusing on the protein domain compositions and their relationships with the existing protein domain superfamilies in the genome. In agreement with previous observations, we find that the protein domain architectures of horizontally transferred genes are significantly shorter than the genomic average. Furthermore, protein domains that are more common in the total pool of genomes appear to have a proportionally higher chance to be transferred. This suggests that transfer events behave as if they were drawn randomly from a cross-genomic community gene pool, much like gene duplicates are drawn from a genomic gene pool. Finally, horizontally transferred genes carry domains of exogenous families less frequently for larger genomes, although they might do it more than expected by chance.     

Evolutionary genomics of a temperate bacteriophage in an obligate intracellular bacteria (Wolbachia)

Genome evolution of bacteria is usually influenced by ecology, such that bacteria with a free-living stage have large genomes and high rates of horizontal gene transfer, while obligate intracellular bacteria have small genomes with typically low amounts of gene exchange. However, recent studies indicate that obligate intracellular species that host-switch frequently harbor agents of horizontal transfer such as mobile elements. For example, the temperate double-stranded DNA bacteriophage WO in Wolbachia persistently transfers between bacterial coinfections in the same host. Here we show that despite the phage's rampant mobility between coinfections, the prophage's genome displays features of constraint related to its intracellular niche. First, there is always at least one intact prophage WO and usually several degenerate, independently-acquired WO prophages in each Wolbachia genome. Second, while the prophage genomes are modular in composition with genes of similar function grouping together, the modules are generally not interchangeable with other unrelated phages and thus do not evolve by the Modular Theory. Third, there is an unusual core genome that strictly consists of head and baseplate genes; other gene modules are frequently deleted. Fourth, the prophage recombinases are diverse and there is no conserved integration sequence. Finally, the molecular evolutionary forces acting on prophage WO are point mutation, intragenic recombination, deletion, and purifying selection. Taken together, these analyses indicate that while lateral transfer of phage WO is pervasive between Wolbachia with occasional new gene uptake, constraints of the intracellular niche obstruct extensive mixture between WO and the global phage population. Although the Modular Theory has long been considered the paradigm of temperate bacteriophage evolution in free-living bacteria, it appears irrelevant in phages of obligate intracellular bacteria.     

Deriving the genomic tree of life in the presence of horizontal gene transfer: conditioned reconstruction

The horizontal gene transfer (HGT) being inferred within prokaryotic genomes appears to be sufficiently massive that many scientists think it may have effectively obscured much of the history of life recorded in DNA. Here, we demonstrate that the tree of life can be reconstructed even in the presence of extensive HGT, provided the processes of genome evolution are properly modeled. We show that the dynamic deletions and insertions of genes that occur during genome evolution, including those introduced by HGT, may be modeled using techniques similar to those used to model nucleotide substitutions that occur during sequence evolution. In particular, we show that appropriately designed general Markov models are reasonable tools for reconstructing genome evolution. These studies indicate that, provided genomes contain sufficiently many genes and that the Markov assumptions are met, it is possible to reconstruct the tree of life. We also consider the fusion of genomes, a process not encountered in gene sequence evolution, and derive a method for the identification and reconstruction of genome fusion events. Genomic reconstructions of a well-defined classical four-genome problem, the root of the multicellular animals, show that the method, when used in conjunction with paralinear/logdet distances, performs remarkably well and is relatively unaffected by the recently discovered big genome artifact.     

啮总目昆虫的线粒体基因组多样性及系统发育研究进展

啮总目包括啮虫目（皮虱和书虱）和虱目（羽虱和吸虱）,是农业和医学等领域具有重要经济意义和研究价值的类群,目前已鉴定和描述的物种超过10 000个。啮总目昆虫线粒体基因组的变异性在昆虫各类群中最为剧烈,这些变异包括基因组的结构、基因排序、基因含量和链上分布等诸多方面。本文全面分析和总结了啮总目昆虫裂化线粒体基因组的进化属性,并结合两侧对称动物线粒体基因组的裂化特征重构了线粒体基因组环裂化的过程。引入“线粒体基因组核型”的概念来描述动物线粒体基因组丰富的变异程度。动物线粒体的染色体有减小的趋势,而线粒体基因组的裂化正是体现这种趋势的一种重要策略。同时,总结和探讨了目前具有争议的啮总目主要类群间的系统发育关系。本综述为啮总目昆虫线粒体基因组学、啮总目系统发生关系以及两侧对称动物线粒体基因组进化模式的研究提供一个新的视角。     

Bioinformatic analysis of the Acinetobacter baumannii phage AB1 genome

As one of the pathogens of hospital-acquired infections, Acinetobacter baumannii poses great challenges to the public health. A. baumannii phage could be an effective way to fight multi-resistant A. baumannii. Here, we completed the whole genome sequencing of the complete genome of A. baumannii phage AB1, which consists of 45,159bp and is a double-stranded DNA molecule with an average GC content of 37.7%. The genome encodes one tRNA gene and 85 open reading frames (ORFs) and the average size of the ORF is 531bp in length. Among 85 ORFs, only 14 have been identified to share significant sequence similarities to the genes with known functions, while 28 are similar in sequence to the genes with function-unknown genes in the database and 43 ORFs are uniquely present in the phage AB1 genome. Fourteen function-assigned genes with putative functions include five phage structure proteins, an RNA polymerase, a big sub-unit and a small sub-unit of a terminase, a methylase and a recombinase and the proteins involved in DNA replication and so on. Multiple sequence alignment was conducted among those homologous proteins and the phylogenetic trees were reconstructed to analyze the evolutionary courses of these essential genes. From comparative genomics analysis, it turned out clearly that the frame of the phage genome mainly consisted of genes from Xanthomonas phages, Burkholderia ambifaria phages and Enterobacteria phages and while it comprises genes of its host A. baumannii only sporadically. The mosaic feature of the phage genome suggested that the horizontal gene transfer occurred among the phage genomes and between the phages and the host bacterium genomes. Analyzing the genome sequences of the phages should lay sound foundation to investigate how phages adapt to the environment and infect their hosts, and even help to facilitate the development of biological agents to deal with pathogenic bacteria.     

P-value based visualization of codon usage data

Two important and not yet solved problems in bacterial genome research are the identification of horizontally transferred genes and the prediction of gene expression levels. Both problems can be addressed by multivariate analysis of codon usage data. In particular dimensionality reduction methods for visualization of multivariate data have shown to be effective tools for codon usage analysis. We here propose a multidimensional scaling approach using a novel similarity measure for codon usage tables. Our probabilistic similarity measure is based on P-values derived from the well-known chi-square test for comparison of two distributions. Experimental results on four microbial genomes indicate that the new method is well-suited for the analysis of horizontal gene transfer and translational selection. As compared with the widely-used correspondence analysis, our method did not suffer from outlier sensitivity and showed a better clustering of putative alien genes in most cases.     

Limitations of compositional approach to identifying horizontally transferred genes

Genes with atypical G+C content and pattern of codon usage in a certain genome are possibly of exotic origin, and this idea has been applied to identify horizontal events. In this way, it was postulated that a total of 755 genes in the E. coli genome are relics of horizontal events after the divergence of E. coli from the Salmonella lineage 100 million years ago (Lawrence and Ochman, 1998). In this paper we propose a new way to study sequence composition more thoroughly. We found that although the 755 genes differ in composition from other genes in the E. coli genome, the difference is minor. If we accepted that these genes are horizontally transferred, then (1) it would be more likely that they were transferred from genomes evolutionarily closely related to E. coli; but (2) the dating method used by Lawrence and Ochman (1997, 1998) largely underestimated the average age of introduced sequences in the E. coli genome, in particular, most of the 755 genes should be introduced into E. coli before, instead of after, the divergence of E. coli from the Salmonella lineage. Our study reveals that atypical G+C content and pattern of codon usage are not reliable indicators of horizontal gene transfer events. Received: 27 September 2000 / Accepted: 9 April 2001     

Insights into the evolutionary process of genome degradation

Studies of noncoding and pseudogene sequence diversity, particularly in Rickettsia, have begun to reveal the basic principles of genome degradation in microorganisms. Increasingly, studies of genes and genomes suggest that there has been an extensive amount of horizontal gene transfer among microorganisms. As this inflow of genetic material does not seem generally to have resulted in genome size expansions, however, degenerative processes must be at the very least as widespread as horizontal gene transfer. The basic principles of gene degradation and elimination that are being explored in Rickettsia are likely to be of major importance for our understanding of how microbial genomes evolve.     

Genomics of bacteria and archaea: the emerging dynamic view of the prokaryotic world

The first bacterial genome was sequenced in 1995, and the first archaeal genome in 1996. Soon after these breakthroughs, an exponential rate of genome sequencing was established, with a doubling time of approximately 20 months for bacteria and approximately 34 months for archaea. Comparative analysis of the hundreds of sequenced bacterial and dozens of archaeal genomes leads to several generalizations on the principles of genome organization and evolution. A crucial finding that enables functional characterization of the sequenced genomes and evolutionary reconstruction is that the majority of archaeal and bacterial genes have conserved orthologs in other, often, distant organisms. However, comparative genomics also shows that horizontal gene transfer (HGT) is a dominant force of prokaryotic evolution, along with the loss of genetic material resulting in genome contraction. A crucial component of the prokaryotic world is the mobilome, the enormous collection of viruses, plasmids and other selfish elements, which are in constant exchange with more stable chromosomes and serve as HGT vehicles. Thus, the prokaryotic genome space is a tightly connected, although compartmentalized, network, a novel notion that undermines the ‘Tree of Life’ model of evolution and requires a new conceptual framework and tools for the study of prokaryotic evolution.     

Viral component of the human genome

Relationships between viruses and their human host are traditionally described from the point of view taking into consideration hosts as victims of viral aggression, which results in infectious diseases. However, these relations are in fact two-sided and involve modifications of both the virus and host genomes. Mutations that accumulate in the populations of viruses and hosts may provide them advantages such as the ability to overcome defense barriers of host cells or to create more efficient barriers to deal with the attack of the viral agent. One of the most common ways of reinforcing anti-viral barriers is the horizontal transfer of viral genes into the host genome. Within the host genome, these genes may be modified and extensively expressed to compete with viral copies and inhibit the synthesis of their products or modulate their functions in other ways. This review summarizes the available data on the horizontal gene transfer between viral and human genomes and discusses related problems.     

Using Homolog Groups to Create a Whole-Genomic Tree of Free-Living Organisms: An Update

Genomic trees have been constructed based on the presence and absence of families of protein-encoding genes observed in 27 complete genomes, including genomes of 15 free-living organisms. This method does not rely on the identification of suspected orthologs in each genome, nor the specific alignment used to compare gene sequences because the protein-encoding gene families are formed by grouping any protein with a pairwise similarity score greater than a preset value. Because of this all inclusive grouping, this method is resilient to some effects of lateral gene transfer because transfers of genes are masked when the recipient genome already has a homolog (not necessarily an ortholog) of the incoming gene. Of 71 genes suspected to have been laterally transferred to the genome of Aeropyrum pernix, only approximately 7 to 15 represent genes where a lateral gene transfer appears to have generated homoplasy in our character dataset. The genomic tree of the 15 free-living taxa includes six different bacterial orders, six different archaeal orders, and two different eukaryotic kingdoms. The results are remarkably similar to results obtained by analysis of rRNA. Inclusion of the other 12 genomes resulted in a tree only broadly similar to that suggested by rRNA with at least some of the differences due to artifacts caused by the small genome size of many of these species. Very small genomes, such as those of the two Mycoplasma genomes included, fall to the base of the Bacterial domain, a result expected due to the substantial gene loss inherent to these lineages. Finally, artificial ``partial genomes' were generated by randomly selecting ORFs from the complete genomes in order to test our ability to recover the tree generated by the whole genome sequences when only partial data are available. The results indicated that partial genomic data, when sampled randomly, could robustly recover the tree generated by the whole genome sequences. Received: 30 May 2001 / Accepted: 10 October 2001     

Life-cycle and genome of OtV5, a large DNA virus of the pelagic marine unicellular green alga Ostreococcus tauri

Large DNA viruses are ubiquitous, infecting diverse organisms ranging from algae to man, and have probably evolved from an ancient common ancestor. In aquatic environments, such algal viruses control blooms and shape the evolution of biodiversity in phytoplankton, but little is known about their biological functions. We show that Ostreococcus tauri, the smallest known marine photosynthetic eukaryote, whose genome is completely characterized, is a host for large DNA viruses, and present an analysis of the life-cycle and 186,234 bp long linear genome of OtV5. OtV5 is a lytic phycodnavirus which unexpectedly does not degrade its host chromosomes before the host cell bursts. Analysis of its complete genome sequence confirmed that it lacks expected site-specific endonucleases, and revealed the presence of 16 genes whose predicted functions are novel to this group of viruses. OtV5 carries at least one predicted gene whose protein closely resembles its host counterpart and several other host-like sequences, suggesting that horizontal gene transfers between host and viral genomes may occur frequently on an evolutionary scale. Fifty seven percent of the 268 predicted proteins present no similarities with any known protein in Genbank, underlining the wealth of undiscovered biological diversity present in oceanic viruses, which are estimated to harbour 200Mt of carbon.     

Measuring conservation of contiguous sets of autosomal markers on bovine and porcine genomes in relation to the map of the human genome.

Based on published information, we have identified 991 genes and gene-family clusters for cattle and 764 for pigs that have orthologues in the human genome. The relative linear locations of these genes on human sequence maps were used as "rulers" to annotate bovine and porcine genomes based on a CSAM (contiguous sets of autosomal markers) approach. A CSAM is an uninterrupted set of markers in one genome (primary genome; the human genome in this study) that is syntenic in the other genome (secondary genome; the bovine and porcine genomes in this study). The analysis revealed 81 conserved syntenies and 161 CSAMs between human and bovine autosomes and 50 conserved syntenies and 95 CSAMs between human and porcine autosomes. Using the human sequence map as a reference, these 991 and 764 markers could correlate 72 and 74% of the human genome with the bovine and porcine genomes, respectively. Based on the number of contiguous markers in each CSAM, we classified these CSAMs into five size groups as follows: singletons (one marker only), small (2-4 markers), medium (5-10 markers), large (11-20 markers), and very large (> 20 markers). Several bovine and porcine chromosomes appear to be represented as di-CSAM repeats in a tandem or dispersed way on human chromosomes. The number of potential CSAMs for which no markers are currently available were estimated to be 63 between human and bovine genomes and 18 between human and porcine genomes. These results provide basic guidelines for further gene and QTL mapping of the bovine and porcine genomes, as well as insight into the evolution of mammalian genomes.     

Ancient lateral gene transfer in the evolution of Bdellovibrio bacteriovorus

The recently sequenced genome of the predatory delta-proteobacterium Bdellovibrio bacteriovorus provides many insights into its metabolism and evolution. Because its genes are reasonably uniform in G+C content, it was suggested that B. bacteriovorus actively resists recombination with foreign DNA and horizontal transfer of DNA from other bacteria. To investigate this further, we carried out a variety of phylogenetic and comparative genomics analyses using data from >200 microbial genomes, including several published delta-proteobacteria. Although there might be little evidence for the extensive recent transfer of genes, we demonstrate that ancient lateral gene acquisition has shaped the B. bacteriovorus genome to a great extent.     

A novel group of type I polyketide synthases (PKS) in animals and the complex phylogenomics of PKSs

Type I polyketide synthases (PKSs), and related fatty acid synthases (FASs), represent a large group of proteins encoded by a diverse gene family that occurs in eubacteria and eukaryotes (mainly in fungi). Collectively, enzymes encoded by this gene family produce a wide array of polyketide compounds that encompass a broad spectrum of biological activity including antibiotic, antitumor, antifungal, immunosuppressive, and predator defense functional roles. We employed a phylogenomics approach to estimate relationships among members of this gene family from eubacterial and eukaryotic genomes. Our results suggest that some animal genomes (sea urchins, birds, and fish) possess a previously unidentified group of pks genes, in addition to possessing fas genes used in fatty acid metabolism. These pks genes in the chicken, fish, and sea urchin genomes do not appear to be closely related to any other animal or fungal genes, and instead are closely related to pks genes from the slime mold Dictyostelium and eubacteria. Continued accumulation of genome sequence data from diverse animal lineages is required to clarify whether the presence of these (non-fas) pks genes in animal genomes owes their origins to horizontal gene transfer (from eubacterial or Dictostelium genomes) or to more conventional patterns of vertical inheritance coupled with massive gene loss in several animal lineages. Additionally, results of our broad-scale phylogenetic analyses bolster the support for previous hypotheses of horizontal gene transfer of pks genes from bacterial to fungal and protozoan lineages.     

Mitochondrial diversity of early-branching metazoa is revealed by the complete mt genome of a haplosclerid demosponge

The first mitochondrial (mt) genomes of demosponges have recently been sequenced and appear to be markedly different from published eumetazoan mt genomes. Here we show that the mt genome of the haplosclerid demosponge Amphimedon queenslandica has features that it shares with both demosponges and eumetazoans. Although the A. queenslandica mt genome has typical demosponge features, including size, long noncoding regions, and bacterialike rRNA genes, it lacks atp9, which is found in the other demosponges sequenced to date. We found strong evidence of a recent transposon-mediated transfer of atp9 to the nuclear genome. In addition, A. queenslandica bears an incomplete tRNA set, unusual amino acid deletion patterns, and a putative control region. Furthermore, the arrangement of mt rRNA genes differs from that of other demosponges. These genes evolve at significantly higher rates than observed in other demosponges, similar to previously observed nuclear rRNA gene rates in other haplosclerid demosponges.