Orthologous Gene Groups Research Articles

New Insights on Leucine-Rich Repeats Receptor-Like Kinase Orthologous Relationships in Angiosperms.

Leucine-Rich Repeats Receptor-Like Kinase (LRR-RLK) genes represent a large and complex gene family in plants, mainly involved in development and stress responses. These receptors are composed of an LRR-containing extracellular domain (ECD), a transmembrane domain (TM) and an intracellular kinase domain (KD). To provide new perspectives on functional analyses of these genes in model and non-model plant species, we performed a phylogenetic analysis on 8,360 LRR-RLK receptors in 31 angiosperm genomes (8 monocots and 23 dicots). We identified 101 orthologous groups (OGs) of genes being conserved among almost all monocot and dicot species analyzed. We observed that more than 10% of these OGs are absent in the Brassicaceae species studied. We show that the ECD structural features are not always conserved among orthologs, suggesting that functions may have diverged in some OG sets. Moreover, we looked at targets of positive selection footprints in 12 pairs of OGs and noticed that depending on the subgroups, positive selection occurred more frequently either in the ECDs or in the KDs.

Increased rates of protein evolution and asymmetric deceleration after the whole-genome duplication in yeasts

BackgroundWhole-genome duplication (WGD) events have shaped the genomes of eukaryotic organisms. Relaxed selection after duplication along with inherent functional constraints are thought to determine the fate of the paralogs and, ultimately, the evolution of gene function. Here, we investigated the rate of protein evolution (as measured by dN/dS ratios) before and after the WGD in the hemiascomycete yeasts, and the way in which changes in such rates relate to molecular and biological function.ResultsFor most groups of orthologous genes (81%) we observed a change in the rates of evolution after genome duplication. Genes with atypically-low dN/dS ratio before the WGD were prone to increase their rates of evolution after duplication. Importantly, the paralogs were often different in their rates of evolution after the WGD (50% cases), however, this was more consistent with an asymmetric deceleration in the protein-evolution rates, rather than an asymmetric increase of the initial rates. Functional-category analysis showed that regulatory proteins such as protein kinases and transcription factors were enriched in genes that increase their rates of evolution after the WGD. While changes in the rate of protein-sequence evolution were associated to protein abundance, content of disordered regions, and contribution to fitness, these features were an attribute of specific functional classes.ConclusionsOur results indicate that strong purifying selection in ancestral pre-duplication sequences is a strong predictor of increased rates after the duplication in yeasts and that asymmetry in evolution rate is established during the deceleration phase. In addition, changes in the rates at which paralogous sequences evolve before and after WGD are different for specific protein functions; increased rates of protein evolution after duplication occur preferentially in specific protein functions.

Phylogenomics Using Transcriptome Data.

This chapter presents a generalized protocol for conducting phylogenetic analyses using large-scale molecular datasets, specifically using transcriptome data from the Illumina sequencing platform. The general molecular lab bench protocol consists of RNA extraction, cDNA synthesis, and sequencing, in this case via Illumina. After sequences have been obtained, bioinformatics methods are used to assemble raw reads, identify coding regions, and categorize sequences from different species into groups of orthologous genes (OGs). The specific OGs to be used for phylogenetic inference are selected using a custom shell script. Finally, the selected orthologous groups are concatenated into a supermatrix. Generalized methods for phylogenomic inference using maximum likelihood and Bayesian inference software are presented.

Differences in the Early Development of Human and Mouse Embryonic Stem Cells.

We performed a systematic analysis of gene expression features in early (10–21 days) development of human vs mouse embryonic cells (hESCs vs mESCs). Many development features were found to be conserved, and a majority of differentially regulated genes have similar expression change in both organisms. The similarity is especially evident, when gene expression profiles are clustered together and properties of clustered groups of genes are compared. First 10 days of mESC development match the features of hESC development within 21 days, in accordance with the differences in population doubling time in human and mouse ESCs. At the same time, several important differences are seen. There is a clear difference in initial expression change of transcription factors and stimulus responsive genes, which may be caused by the difference in experimental procedures. However, we also found that some biological processes develop differently; this can clearly be shown, for example, for neuron and sensory organ development. Some groups of genes show peaks of the expression levels during the development and these peaks cannot be claimed to happen at the same time points in the two organisms, as well as for the same groups of (orthologous) genes. We also detected a larger number of upregulated genes during development of mESCs as compared to hESCs. The differences were quantified by comparing promoters of related genes. Most of gene groups behave similarly and have similar transcription factor (TF) binding sites on their promoters. A few groups of genes have similar promoters, but are expressed differently in two species. Interestingly, there are groups of genes expressed similarly, although they have different promoters, which can be shown by comparing their TF binding sites. Namely, a large group of similarly expressed cell cycle-related genes is found to have discrepant TF binding properties in mouse vs human.

Comparative genomics of Steinernema reveals deeply conserved gene regulatory networks.

BackgroundParasitism is a major ecological niche for a variety of nematodes. Multiple nematode lineages have specialized as pathogens, including deadly parasites of insects that are used in biological control. We have sequenced and analyzed the draft genomes and transcriptomes of the entomopathogenic nematode Steinernema carpocapsae and four congeners (S. scapterisci, S. monticolum, S. feltiae, and S. glaseri).ResultsWe used these genomes to establish phylogenetic relationships, explore gene conservation across species, and identify genes uniquely expanded in insect parasites. Protein domain analysis in Steinernema revealed a striking expansion of numerous putative parasitism genes, including certain protease and protease inhibitor families, as well as fatty acid- and retinol-binding proteins. Stage-specific gene expression of some of these expanded families further supports the notion that they are involved in insect parasitism by Steinernema. We show that sets of novel conserved non-coding regulatory motifs are associated with orthologous genes in Steinernema and Caenorhabditis.ConclusionsWe have identified a set of expanded gene families that are likely to be involved in parasitism. We have also identified a set of non-coding motifs associated with groups of orthologous genes in Steinernema and Caenorhabditis involved in neurogenesis and embryonic development that are likely part of conserved protein–DNA relationships shared between these two genera.Electronic supplementary materialThe online version of this article (doi:10.1186/s13059-015-0746-6) contains supplementary material, which is available to authorized users.

Genome-wide identification, classification, and expression analysis of sHSP genes in Chinese cabbage (Brassica rapa ssp pekinensis).

Small heat shock proteins (sHSPs) are essential for the plant's normal development and stress responses, especially the heat stress response. The information regarding sHSP genes in Chinese cabbage (Brassica rapa ssp pekinensis) is sparse, hence we performed a genome-wide analysis to identify sHSP genes in this species. We identified 26 non-redundant sHSP genes distributed on all chromosomes, except chromosome A7, with one additional sHSP gene identified from an expressed sequence tag library. Chinese cabbage was found to contain more sHSP genes than Arabidopsis. The 27 sHSP genes were classified into 11 subfamilies. We identified 22 groups of sHSP syntenic orthologous genes between Chinese cabbage and Arabidopsis. In addition, eight groups of paralogous genes were uncovered in Chinese cabbage. Protein structures of the 27 Chinese cabbage sHSPs were modeled using Phyre2, which revealed that all of them contain several conserved β strands across different subfamilies. In general, gene structure was conserved within each subfamily between Chinese cabbage and Arabidopsis, except for peroxisome sHSP. Analysis of promoter motifs showed that most sHSP genes contain heat shock elements or variants. We also found that biased gene loss has occurred during the evolution of the sHSP subfamily in Chinese cabbage. Expression analysis indicated that the greatest transcript abundance of most Chinese cabbage sHSP genes was found in siliques and early cotyledon embryos. Thus, genome-wide identification and characterization of sHSP genes is a first and important step in the investigation of sHSPs in Chinese cabbage.

A meta-approach for improving the prediction and the functional annotation of ortholog groups.

BackgroundIn comparative genomics, orthologs are used to transfer annotation from genes already characterized to newly sequenced genomes. Many methods have been developed for finding orthologs in sets of genomes. However, the application of different methods on the same proteome set can lead to distinct orthology predictions.MethodsWe developed a method based on a meta-approach that is able to combine the results of several methods for orthologous group prediction. The purpose of this method is to produce better quality results by using the overlapping results obtained from several individual orthologous gene prediction procedures. Our method proceeds in two steps. The first aims to construct seeds for groups of orthologous genes; these seeds correspond to the exact overlaps between the results of all or several methods. In the second step, these seed groups are expanded by using HMM profiles.ResultsWe evaluated our method on two standard reference benchmarks, OrthoBench and Orthology Benchmark Service. Our method presents a higher level of accurately predicted groups than the individual input methods of orthologous group prediction. Moreover, our method increases the number of annotated orthologous pairs without decreasing the annotation quality compared to twelve state-of-the-art methods.ConclusionsThe meta-approach based method appears to be a reliable procedure for predicting orthologous groups. Since a large number of methods for predicting groups of orthologous genes exist, it is quite conceivable to apply this meta-approach to several combinations of different methods.

The transcriptome of an amphioxus, Asymmetron lucayanum, from the Bahamas: a window into chordate evolution.

Cephalochordates, the sister group of tunicates plus vertebrates, have been called “living fossils” due to their resemblance to fossil chordates from Cambrian strata. The genome of the cephalochordate Branchiostoma floridae shares remarkable synteny with vertebrates and is free from whole-genome duplication. We performed RNA sequencing from larvae and adults of Asymmetron lucayanum, a cephalochordate distantly related to B. floridae. Comparisons of about 430 orthologous gene groups among both cephalochordates and 10 vertebrates using an echinoderm, a hemichordate, and a mollusk as outgroups showed that cephalochordates are evolving more slowly than the slowest evolving vertebrate known (the elephant shark), with A. lucayanum evolving even more slowly than B. floridae. Against this background of slow evolution, some genes, notably several involved in innate immunity, stand out as evolving relatively quickly. This may be due to the lack of an adaptive immune system and the relatively high levels of bacteria in the inshore waters cephalochordates inhabit. Molecular dating analysis including several time constraints revealed a divergence time of ∼120 Ma for A. lucayanum and B. floridae. The divisions between cephalochordates and vertebrates, and that between chordates and the hemichordate plus echinoderm clade likely occurred before the Cambrian.

Extreme expansion of the olfactory receptor gene repertoire in African elephants and evolutionary dynamics of orthologous gene groups in 13 placental mammals.

Olfactory receptors (ORs) detect odors in the environment, and OR genes constitute the largest multigene family in mammals. Numbers of OR genes vary greatly among species—reflecting the respective species' lifestyles—and this variation is caused by frequent gene gains and losses during evolution. However, whether the extent of gene gains/losses varies among individual gene lineages and what might generate such variation is unknown. To answer these questions, we used a newly developed phylogeny-based method to classify >10,000 intact OR genes from 13 placental mammal species into 781 orthologous gene groups (OGGs); we then compared the OGGs. Interestingly, African elephants had a surprisingly large repertoire (∼2000) of functional OR genes encoded in enlarged gene clusters. Additionally, OR gene lineages that experienced more gene duplication had weaker purifying selection, and Class II OR genes have evolved more dynamically than those in Class I. Some OGGs were highly expanded in a lineage-specific manner, while only three OGGs showed complete one-to-one orthology among the 13 species without any gene gains/losses. These three OGGs also exhibited highly conserved amino acid sequences; therefore, ORs in these OGGs may have physiologically important functions common to every placental mammal. This study provides a basis for inferring OR functions from evolutionary trajectory.

Nanoliter qPCR platform for highly parallel, quantitative assessment of reductive dehalogenase genes and populations of dehalogenating microorganisms in complex environments.

Idiosyncratic combinations of reductive dehalogenase (rdh) genes are a distinguishing genomic feature of closely related organohalogen-respiring bacteria. This feature can be used to deconvolute the population structure of organohalogen-respiring bacteria in complex environments and to identify relevant subpopulations, which is important for tracking interspecies dynamics needed for successful site remediation. Here we report the development of a nanoliter qPCR platform to identify organohalogen-respiring bacteria and populations by quantifying major orthologous reductive dehalogenase gene groups. The qPCR assays can be operated in parallel within a 5184-well nanoliter qPCR (nL-qPCR) chip at a single annealing temperature and buffer condition. We developed a robust bioinformatics approach to select from thousands of computationally proposed primer pairs those that are specific to individual rdh gene groups and compatible with a single amplification condition. We validated hundreds of the most selective qPCR assays and examined their performance in a trichloroethene-degrading bioreactor, revealing population structures as well as their unexpected shifts in abundance and community dynamics.

Genome-wide identification and characterization of aquaporin genes (AQPs) in Chinese cabbage (Brassica rapa ssp. pekinensis).

Aquaporins (AQPs) are members of a superfamily of integral membrane proteins and play a significant role in the transportation of small molecules across membranes. However, currently little is known about the AQP genes in Chinese cabbage (Brassica rapa ssp. pekinensis). In this study, a genome-wide analysis was carried out to identify the AQP genes in Chinese cabbage. In total, 53 non-redundant AQP genes were identified that were located on all of the 10 chromosomes. The number of AQP genes in Chinese cabbage was greater than in Arabidopsis. They were classified into four subfamilies, including PIP, TIP, NIP, and SIP. Thirty-three groups of AQP orthologous genes were identified between Chinese cabbage and Arabidopsis, but orthologs corresponding to AtNIP1;1 and AtPIP2;8 were not detected. Seventeen groups of paralogous genes were identified in Chinese cabbage. Three-dimensional models of the AQPs of Chinese cabbage were constructed using Phyre2, and ar/R selectivity filters were analyzed comparatively between Chinese cabbage and Arabidopsis. Generally, gene structure was conserved within each subfamily, especially in the SIP subfamily. Intron loss events have occurred during the evolution of the PIP, TIP, and NIP subfamilies. The expression of AQP genes in Chinese cabbage was analyzed in different organs. Most AQP genes were downregulated in response to salt stress. This work shows that the AQP genes of Chinese cabbage have undergone triplication and subsequent biased gene loss.

New Insights into Clostridia Through Comparative Analyses of Their 40 Genomes

The Clostridium genus of bacteria contains the most widely studied biofuel-producing organisms such as Clostridium thermocellum and also some human pathogens, plus a few less characterized strains. Here, we present a comparative genomic analysis of 40 fully sequenced clostridial genomes, paying a particular attention to the biomass degradation ones. Our analysis indicates that some of the Clostridium botulinum strains may have been incorrectly classified in the current taxonomy and hence should be renamed according to the 16S ribosomal RNA (rRNA) phylogeny. A core-genome analysis suggests that only 169 orthologous gene groups are shared by all the strains, and the strain-specific gene pool consists of 22,668 genes, which is consistent with the fact that these bacteria live in very diverse environments and have evolved a very large number of strain-specific genes to adapt to different environments. Across the 40 genomes, 1.4–5.8 % of genes fall into the carbohydrate active enzyme (CAZyme) families, and 20 out of the 40 genomes may encode cellulosomes with each genome having 1 to 76 genes bearing the cellulosome-related modules such as dockerins and cohesins. A phylogenetic footprinting analysis identified cis-regulatory motifs that are enriched in the promoters of the CAZyme genes, giving rise to 32 statistically significant motif candidates.

Gene Duplication and Fragment Recombination Drive Functional Diversification of a Superfamily of Cytoplasmic Effectors in Phytophthora sojae

Phytophthora and other oomycetes secrete a large number of putative host cytoplasmic effectors with conserved FLAK motifs following signal peptides, termed crinkling and necrosis inducing proteins (CRN), or Crinkler. Here, we first investigated the evolutionary patterns and mechanisms of CRN effectors in Phytophthora sojae and compared them to two other Phytophthora species. The genes encoding CRN effectors could be divided into 45 orthologous gene groups (OGG), and most OGGs unequally distributed in the three species, in which each underwent large number of gene gains or losses, indicating that the CRN genes expanded after species evolution in Phytophthora and evolved through pathoadaptation. The 134 expanded genes in P. sojae encoded family proteins including 82 functional genes and expressed at higher levels while the other 68 genes encoding orphan proteins were less expressed and contained 50 pseudogenes. Furthermore, we demonstrated that most expanded genes underwent gene duplication or/and fragment recombination. Three different mechanisms that drove gene duplication or recombination were identified. Finally, the expanded CRN effectors exhibited varying pathogenic functions, including induction of programmed cell death (PCD) and suppression of PCD through PAMP-triggered immunity or/and effector-triggered immunity. Overall, these results suggest that gene duplication and fragment recombination may be two mechanisms that drive the expansion and neofunctionalization of the CRN family in P. sojae, which aids in understanding the roles of CRN effectors within each oomycete pathogen.

Expression of sulfatases in Rhodopirellula baltica and the diversity of sulfatases in the genus Rhodopirellula

The whole genome sequence of Rhodopirellula baltica SH1T, published nearly 10years ago, already revealed a high amount of sulfatase genes. So far, little is known about the diversity and potential functions mediated by sulfatases in Planctomycetes. We combined in vivo and in silico techniques to gain insights into the ecophysiology of planktomycetal sulfatases. Comparative genomics of nine recently sequenced Rhodopirellula strains detected 1120 open reading frames annotated as sulfatases (Enzyme Commission number (EC) 3.1.6.*). These were clustered into 173 groups of orthologous and paralogous genes. To analyze the functional aspects, 708 sulfatase protein sequences from these strains were aligned with 67 sulfatase reference sequences of reviewed functionality. Our analysis yielded 22 major similarity clusters, but only five of these clusters contained Rhodopirellula sequences homologous to reference sequences, indicating a surprisingly high diversity. Exemplarily, R. baltica SH1T was grown on different sulfated polysaccharides, chondroitin sulfate, λ-carrageenan and fucoidan. Subsequent gene expression analyses using whole genome microarrays revealed distinct sulfatase expression profiles based on substrates tested. This might be indicative for a high structural diversity of sulfated polysaccharides as potential substrates. The pattern of sulfatases in individual planctomycete species may reflect ecological niche adaptation.

PIECE: a database for plant gene structure comparison and evolution

Gene families often show degrees of differences in terms of exon–intron structures depending on their distinct evolutionary histories. Comparative analysis of gene structures is important for understanding their evolutionary and functional relationships within plant species. Here, we present a comparative genomics database named PIECE (http://wheat.pw.usda.gov/piece) for Plant Intron and Exon Comparison and Evolution studies. The database contains all the annotated genes extracted from 25 sequenced plant genomes. These genes were classified based on Pfam motifs. Phylogenetic trees were pre-constructed for each gene category. PIECE provides a user-friendly interface for different types of searches and a graphical viewer for displaying a gene structure pattern diagram linked to the resulting bootstrapped dendrogram for each gene family. The gene structure evolution of orthologous gene groups was determined using the GLOOME, Exalign and GECA software programs that can be accessed within the database. PIECE also provides a web server version of the software, GSDraw, for drawing schematic diagrams of gene structures. PIECE is a powerful tool for comparing gene sequences and provides valuable insights into the evolution of gene structure in plant genomes.

A CELLULOSE SYNTHASE (CESA) gene essential for gametophore morphogenesis in the moss Physcomitrella patens

In seed plants, different groups of orthologous genes encode the CELLULOSE SYNTHASE (CESA) proteins that are responsible for cellulose biosynthesis in primary and secondary cell walls. The seven CESA sequences of the moss Physcomitrella patens (Hedw.) B. S. G. form a monophyletic sister group to seed plant CESAs, consistent with independent CESA diversification and specialization in moss and seed plant lines. The role of PpCESA5 in the development of P. patens was investigated by targeted mutagenesis. The cesa5 knockout lines were tested for cellulose deficiency using carbohydrate-binding module affinity cytochemistry and the morphology of the leafy gametophores was analyzed by 3D reconstruction of confocal images. No defects were identified in the development of the filamentous protonema or in production of bud initials that normally give rise to the leafy gametophores. However, the gametophore buds were cellulose deficient and defects in subsequent cell expansion, cytokinesis, and leaf initiation resulted in the formation of irregular cell clumps instead of leafy shoots. Analysis of the cesa5 knockout phenotype indicates that a biophysical model of organogenesis can be extended to the moss gametophore shoot apical meristem.

Phylogenetic comparative and expression analysis of genes encoding dof transcription factors from Eucalyptus grandis

Dof proteins are a family of transcription factors specific to the plant kingdom that contain a particular class of zinc finger DNA binding domain. Members of this family are involved in the regulation of genes related to a plethora of metabolic processes including stress or hormone response, seed and endosperm development, flowering, carbohydrate metabolism, and cell or tissue specificity. Dof proteins and encoding genes were characterized in several plant species. Nevertheless poplar is the only woody species whose Dof genes were better characterized. The recent availability of the Eucalyptus grandis genome and transcriptome, along with transcription factor databases of several plant species allowed us to identify and run a valuable comparative analysis of the Dof protein family in this tree. These species included Arabidopsis thaliana, Arabidopsis lyrata, Carica papaya, Populus trichocarpa, Vitis vinifera, Sorghum bicolor, Chlamydomonas reinhardtii, Oryza sativa indica and Zea mays.The phylogenetic relationships among Dof proteins from E. grandis and Arabidopsis thaliana - a popular model for studying the genomics of many plants - is a fundamental step to unravel functionality of new Dof genes not yet characterized. Twenty-three distinct DNA sequences were predicted to belong to the Dof gene family after the analysis of the complete available genome of E. grandis. The deduced protein sequences of 22 members do contain a conserved Dof domain. One sequence seemed to have lost the conserved Dof domain, suggesting it to be a pseudogene or to present an activity not directly linked to the Dof family. Gene structures, including exon/intron positions, and amino acid sequences were predicted for each gene based on the available E. grandis transcriptome. In order to determine the relationship and function of the genes putatively encoding Dof proteins, we carried out a phylogenetic analysis with 43 Dof protein sequences from A. thaliana. Our analysis allowed us to classify the E. grandis Dof sequences into five groups of orthologous genes. Gene expression analysis via real time, quantitative PCR was also conducted with ten of the E. grandis Dof genes, using samples obtained from flowers, leaves and vascular tissue. Generally, Dof steady-state mRNA levels were higher in E. grandis vascular tissues, with more reduced levels in flowers. Dof genes showed an increase in steady-state mRNA levels after hormone signaling, and reduced levels following abiotic stress. This is the first study that aimed the identification of Dof genes in E. grandis which are possibly involved in numerous plant metabolic processes. The phylogenetic relationship to A. thaliana counterparts and the patterns of mRNA accumulation in E. grandis allowed us to speculate on possible roles for some of the Dof-encoding genes. Financial support: Brazilian Ministry of Agriculture, Farming & Supply (MAPA), and The National Council for the Development of Science & Technology (CNPq), Brazilian Ministry of Science & Technology (MCT).

Functional Genomic Analysis of Taxonomic and Typing Loci for Pathogenic Bacteria Based on Orthologous Genes

Approaches based on gene sequences of pathogenic microbes have become increasingly popular to distinguish infectious diseases. According to the Library of Pathogen Infecting in Human which is established by the Ministry of Health of China, our research covered 103 species of bacterial pathogens in China. In general, we have grouped orthologous genes for each taxonomic rank, and picked out the genes which are fitted for the identification and typing of pathogenic bacteria. In total, we aligned and analyzed 836415 genes of 275 pathogenic bacteria strains with whole genome sequenced, and obtained specific orthologous gene groups for phylum, class, order, family and genus respectively, by using bioinformatic and genomic methods. These orthologous genes were then annotated by similarity search against COG database. Further analyses on the differences of functions of consered genes of each taxonomic rank showed that a total of 19563 gene groups are fitted for the identification and typing in hierarchical taxonomic ranks (2891 for phylum, 1016 for class, 3601 for order, 10130 for family and 1925 for genus). It is also showed that the functions of these gene groups are different in each taxonomic rank, as well as in gram positive/negative bacteria. These findings will help us to design primers and microarrays for fast detection of pathogen bacteria in China. Meanwhile, it is also the first time to combine the whole genome sequences of bacteria and the taxonomy of pathogenic bacteria in China, which can provide a new and effective method to select candidate genes/loci for the pathogen detection. These results may also shed light on the research of bacteria metagenomics.

Exploring the symbiotic pangenome of the nitrogen-fixing bacterium Sinorhizobium meliloti

BackgroundSinorhizobium meliloti is a model system for the studies of symbiotic nitrogen fixation. An extensive polymorphism at the genetic and phenotypic level is present in natural populations of this species, especially in relation with symbiotic promotion of plant growth. AK83 and BL225C are two nodule-isolated strains with diverse symbiotic phenotypes; BL225C is more efficient in promoting growth of the Medicago sativa plants than strain AK83. In order to investigate the genetic determinants of the phenotypic diversification of S. meliloti strains AK83 and BL225C, we sequenced the complete genomes for these two strains.ResultsWith sizes of 7.14 Mbp and 6.97 Mbp, respectively, the genomes of AK83 and BL225C are larger than the laboratory strain Rm1021. The core genome of Rm1021, AK83, BL225C strains included 5124 orthologous groups, while the accessory genome was composed by 2700 orthologous groups. While Rm1021 and BL225C have only three replicons (Chromosome, pSymA and pSymB), AK83 has also two plasmids, 260 and 70 Kbp long. We found 65 interesting orthologous groups of genes that were present only in the accessory genome, consequently responsible for phenotypic diversity and putatively involved in plant-bacterium interaction. Notably, the symbiosis inefficient AK83 lacked several genes required for microaerophilic growth inside nodules, while several genes for accessory functions related to competition, plant invasion and bacteroid tropism were identified only in AK83 and BL225C strains. Presence and extent of polymorphism in regulons of transcription factors involved in symbiotic interaction were also analyzed. Our results indicate that regulons are flexible, with a large number of accessory genes, suggesting that regulons polymorphism could also be a key determinant in the variability of symbiotic performances among the analyzed strains.ConclusionsIn conclusions, the extended comparative genomics approach revealed a variable subset of genes and regulons that may contribute to the symbiotic diversity.

A comparison of the low temperature transcriptomes and CBF regulons of three plant species that differ in freezing tolerance: Solanum commersonii, Solanum tuberosum, and Arabidopsis thaliana

Solanum commersonii and Solanum tuberosum are closely related plant species that differ in their abilities to cold acclimate; whereas S. commersonii increases in freezing tolerance in response to low temperature, S. tuberosum does not. In Arabidopsis thaliana, cold-regulated genes have been shown to contribute to freezing tolerance, including those that comprise the CBF regulon, genes that are controlled by the CBF transcription factors. The low temperature transcriptomes and CBF regulons of S. commersonii and S. tuberosum were therefore compared to determine whether there might be differences that contribute to their differences in ability to cold acclimate. The results indicated that both plants alter gene expression in response to low temperature to similar degrees with similar kinetics and that both plants have CBF regulons composed of hundreds of genes. However, there were considerable differences in the sets of genes that comprised the low temperature transcriptomes and CBF regulons of the two species. Thus differences in cold regulatory programmes may contribute to the differences in freezing tolerance of these two species. However, 53 groups of putative orthologous genes that are cold-regulated in S. commersonii, S. tuberosum, and A. thaliana were identified. Given that the evolutionary distance between the two Solanum species and A. thaliana is 112–156 million years, it seems likely that these conserved cold-regulated genes—many of which encode transcription factors and proteins of unknown function—have fundamental roles in plant growth and development at low temperature.