Genome nucleotide composition shapes variation in simple sequence repeats

Comparative genome analysis reveals driving forces behind Monkeypox virus evolution and sheds light on the role of ATC trinucleotide motif

Monkeypox (MPOX), a zoonotic disease originating in Western and Central Africa in 1970, has seen a recent surge in outbreaks across 100+ countries. A comparative analysis of 404 Monkeypox virus (MPXV) genomes revealed notable changes in microsatellite abundance and density, especially within Clades I, IIa, and IIb. Each clade exhibited unique microsatellite motifs, with twenty-six conserved loci specific to MPXV, suggesting their potential as molecular markers in diagnostics. Additionally, nine genes in the MPXV genome featured ten variable hotspot microsatellite regions associated with surface protein synthesis and host control. Notably, gene OPG153, especially at the SSR locus '(ATC)n', exhibited the most pronounced variations among lineages over time and plays a role in virus pathogenesis within the host cell. These findings not only enhance our understanding of MPXV unique molecular profile but also offer valuable insights into potential pathogenic and evolutionary implications.

Parallel evolution of the G protein-coupled receptor GrlG and the loss of fruiting body formation in the social amoeba Dictyostelium discoideum evolved under low relatedness

Aggregative multicellularity relies on cooperation among formerly independent cells to form a multicellular body. Previous work with Dictyostelium discoideum showed that experimental evolution under low relatedness profoundly decreased cooperation, as indicated by the loss of fruiting body formation in many clones and an increase of cheaters that contribute proportionally more to spores than to the dead stalk. Using whole-genome sequencing and variant analysis of these lines, we identified 38 single nucleotide polymorphisms in 29 genes. Each gene had 1 variant except for grlG (encoding a G protein-coupled receptor), which had 10 unique SNPs and 5 structural variants. Variants in the 5' half of grlG-the region encoding the signal peptide and the extracellular binding domain-were significantly associated with the loss of fruiting body formation; the association was not significant in the 3' half of the gene. These results suggest that the loss of grlG was adaptive under low relatedness and that at least the 5' half of the gene is important for cooperation and multicellular development. This is surprising given some previous evidence that grlG encodes a folate receptor involved in predation, which occurs only during the single-celled stage. However, non-fruiting mutants showed little increase in a parallel evolution experiment where the multicellular stage was prevented from happening. This shows that non-fruiting mutants are not generally selected by any predation advantage but rather by something-likely cheating-during the multicellular stage.

Insights on Microsatellite Characteristics, Evolution, and Function From the Social Amoeba Dictyostelium discoideum

Microsatellites are repetitive sequences commonly found in the genomes of higher organisms. These repetitive sequences are prone to expansion or contraction, and when microsatellite expansion occurs in the regulatory or coding regions of genes this can result in a number of diseases including many neurodegenerative diseases. Unlike in humans and other organisms, the social amoeba Dictyostelium discoideum contains an unusually high number of microsatellites. Intriguingly, many of these microsatellites fall within the coding region of genes, resulting in nearly 10,000 homopolymeric repeat proteins within the Dictyostelium proteome. Surprisingly, among the most common of these repeats are polyglutamine repeats, a type of repeat that causes a class of nine neurodegenerative diseases in humans. In this minireview, we summarize what is currently known about homopolymeric repeats and microsatellites in Dictyostelium discoideum and discuss the potential utility of Dictyostelium for identifying novel mechanisms that utilize and regulate regions of repetitive DNA.

Simple sequence repeat insertion induced stability and potential 'gain of function' in the proteins of extremophilic bacteria

Here, we analysed the genomic evolution in extremophilic bacteria using long simple sequence repeats (SSRs). Frequencies of occurrence, relative abundance (RA) and relative density (RD) of long SSRs were analysed in the genomes of extremophilic bacteria. Thermus aquaticus had the most RA and RD of long SSRs in its coding sequences (110.6 and 1408.3), followed by Rhodoferax antarcticus (77.0 and 1187.4). A positive correlation was observed between G + C content and the RA-RD of long SSRs. Geobacillus kaustophilus, Geobacillus thermoleovorans, Halothermothrix orenii, R. antarcticus, and T. aquaticus preferred trinucleotide repeats within their genomes, whereas others preferred a higher number of tetranucleotide repeats. Gene enrichment showed the presence of these long SSRs in metabolic enzyme encoding genes related to stress tolerance. To analyse the functional implications of SSR insertions, three-dimensional protein structure modelling of SSR containing diguanylate cyclase (DGC) gene encoding protein was carried out. Removal of SSR sequence led to an inappropriate folding and instability of the modelled protein structure.

An empirical analysis of mtSSRs: could microsatellite distribution patterns explain the evolution of mitogenomes in plants?

Microsatellites (SSRs) are tandem repeat sequences in eukaryote genomes, including plant cytoplasmic genomes. The mitochondrial genome (mtDNA) has been shown to vary in size, number, and distribution of SSRs among different plant groups. Thus, SSRs contribute with genomic diversity in mtDNAs. However, the abundance, distribution, and evolutionary significance of SSRs in mtDNA from a wide range of algae and plants have not been explored. In this study, the mtDNAs of 204 plant and algal species were investigated related to the presence of SSRs. The number of SSRs was positively correlated with genome size. Its distribution is dependent on plant and algal groups analyzed, although the cluster analysis indicates the conservation of some common motifs in algal and terrestrial plants that reflect common ancestry of groups. Many SSRs in coding and non-coding regions can be useful for molecular markers. Moreover, mitochondrial SSRs are highly abundant, representing an important source for natural or induced genetic variation, i.e., for biotechnological approaches that can modulate mtDNA gene regulation. Thus, this comparative study increases the understanding of the plant and algal SSR evolution and brings perspectives for further studies.

Comparison of the Microsatellite Distribution Patterns in the Genomes of Euarchontoglires at the Taxonomic Level

Microsatellite or simple sequence repeat (SSR) instability within genes can induce genetic variation. The SSR signatures remain largely unknown in different clades within Euarchontoglires, one of the most successful mammalian radiations. Here, we conducted a genome-wide characterization of microsatellite distribution patterns at different taxonomic levels in 153 Euarchontoglires genomes. Our results showed that the abundance and density of the SSRs were significantly positively correlated with primate genome size, but no significant relationship with the genome size of rodents was found. Furthermore, a higher level of complexity for perfect SSR (P-SSR) attributes was observed in rodents than in primates. The most frequent type of P-SSR was the mononucleotide P-SSR in the genomes of primates, tree shrews, and colugos, while mononucleotide or dinucleotide motif types were dominant in the genomes of rodents and lagomorphs. Furthermore, (A)n was the most abundant motif in primate genomes, but (A)n, (AC)n, or (AG)n was the most abundant motif in rodent genomes which even varied within the same genus. The GC content and the repeat copy numbers of P-SSRs varied in different species when compared at different taxonomic levels, reflecting underlying differences in SSR mutation processes. Notably, the CDSs containing P-SSRs were categorized by functions and pathways using Gene Ontology and Kyoto Encyclopedia of Genes and Genomes annotations, highlighting their roles in transcription regulation. Generally, this work will aid future studies of the functional roles of the taxonomic features of microsatellites during the evolution of mammals in Euarchontoglires.

Epigenomic differences in the human and chimpanzee genomes are associated with structural variation

Structural variation (SV), including insertions and deletions (indels), is a primary mechanism of genome evolution. However, the mechanism by which SV contributes to epigenome evolution is poorly understood. In this study, we characterized the association between lineage-specific indels and epigenome differences between human and chimpanzee to investigate how SVs might have shaped the epigenetic landscape. By intersecting medium-to-large human-chimpanzee indels (20 bp-50 kb) with putative promoters and enhancers in cranial neural crest cells (CNCCs) and repressed regions in induced pluripotent cells (iPSCs), we found that 12% of indels overlap putative regulatory and repressed regions (RRRs), and 15% of these indels are associated with lineage-biased RRRs. Indel-associated putative enhancer and repressive regions are approximately 1.3 times and approximately three times as likely to be lineage-biased, respectively, as those not associated with indels. We found a twofold enrichment of medium-sized indels (20-50 bp) in CpG island (CGI)-containing promoters than expected by chance. Lastly, from human-specific transposable element insertions, we identified putative regulatory elements, including NR2F1-bound putative CNCC enhancers derived from SVAs and putative iPSC promoters derived from LTR5s. Our results show that different types of indels are associated with specific epigenomic diversity between human and chimpanzee.

Identification of three new microsatellites and their effects on body measurement traits in pigs using time of flight-mass spectrometry (TOF-MS)

The body status of livestock affects their physiological function and productive performances. Microsatellites, one of the most used DNA markers, have been found to be associated with pig productive traits. However, their identifications and effects on body measurement traits of the Chinese Qinghai Bamei pig still uncovered. According to our previous sequencing data, in this study, three novel microsatellites were found in this breed. Using time of flight-mass spectrometry (TOF-MS) method, these microsatellites were further identified in a large Bamei pig population. TOF-MS spectra showed that there are three microsatellites loci, named P1, P2 and P3. These microsatellites were linkage equilibrium based on the values of D' and r² tests. Association results demonstrated that P1 locus was associated with the body length, body height and chest width and the beneficial genotype was 150-/150-bp (p < 0.05); and P2 locus was associated with the body height (p < 0.05), and the 145-/145-bp, 145-/147-bp and 145-/149-bp were claimed as favorable genotypes and 145-bp allele was considered as the favorable allele. These findings suggested that P1 and P2 microsatellites might be considered as the candidate genetic markers to select pigs with superior body sizes, especially in local breed.

Low Base-Substitution Mutation Rate but High Rate of Slippage Mutations in the Sequence Repeat-Rich Genome of Dictyostelium discoideum

We describe the rate and spectrum of spontaneous mutations for the social amoeba Dictyostelium discoideum, a key model organism in molecular, cellular, evolutionary and developmental biology. Whole-genome sequencing of 37 mutation accumulation lines of D. discoideum after an average of 1,500 cell divisions yields a base-substitution mutation rate of 2.47 × 10⁻¹¹ per site per generation, substantially lower than that of most eukaryotic and prokaryotic organisms, and of the same order of magnitude as in the ciliates Paramecium tetraurelia and Tetrahymena thermophila. Known for its high genomic AT content and abundance of simple sequence repeats, we observe that base-substitution mutations in D. discoideum are highly A/T biased. This bias likely contributes both to the high genomic AT content and to the formation of simple sequence repeats in the AT-rich genome of Dictyostelium discoideum. In contrast to the situation in other surveyed unicellular eukaryotes, indel rates far exceed the base-substitution mutation rate in this organism with a high proportion of 3n indels, particularly in regions without simple sequence repeats. Like ciliates, D. discoideum has a large effective population size, reducing the power of random genetic drift, magnifying the effect of selection on replication fidelity, in principle allowing D. discoideum to evolve an extremely low base-substitution mutation rate.

Machine learning for composition analysis of ssDNA using chemical enhancement in SERS

Surface-enhanced Raman spectroscopy (SERS) is an attractive method for bio-chemical sensing due to its potential for single molecule sensitivity and the prospect of DNA composition analysis. In this manuscript we leverage metal specific chemical enhancement effect to detect differences in SERS spectra of 200-base length single-stranded DNA (ssDNA) molecules adsorbed on gold or silver nanorod substrates, and then develop and train a linear regression as well as neural network models to predict the composition of ssDNA. Our results indicate that employing substrates of different metals that host a given adsorbed molecule leads to distinct SERS spectra, allowing to probe metal-molecule interactions under distinct chemical enhancement regimes. Leveraging this difference and combining spectra from different metals as an input for PCA (Principal Component Analysis) and NN (Neural Network) models, allows to significantly lower the detection errors compared to manual feature-choosing analysis as well as compared to the case where data from single metal is used. Furthermore, we show that NN model provides superior performance in the presence of complex noise and data dispersion factors that affect SERS signals collected from metal substrates fabricated on different days.

Relatively semi-conservative replication and a folded slippage model for short tandem repeats

Background

The ubiquitous presence of short tandem repeats (STRs) in virtually all genomes implicates their functional relevance, while a widely-accepted definition of STR is yet to be established. Previous studies majorly focus on relatively longer STRs, while shorter repeats were generally excluded. Herein, we have adopted a more generous criteria to define shorter repeats, which has led to the definition of a much larger number of STRs that lack prior analysis. Using this definition, we analyzed the short repeats in 55 randomly selected segments in 55 randomly selected genomic sequences from a fairly wide range of species covering animals, plants, fungi, protozoa, bacteria, archaea and viruses.

Results

Our analysis reveals a high percentage of short repeats in all 55 randomly selected segments, indicating that the universal presence of high-content short repeats could be a common characteristic of genomes across all biological kingdoms. Therefore, it is reasonable to assume a mechanism for continuous production of repeats that can make the replicating process relatively semi-conservative. We have proposed a folded replication slippage model that considers the geometric space of nucleotides and hydrogen bond stability to explain the mechanism more explicitly, with improving the existing straight-line slippage model. The folded slippage model can explain the expansion and contraction of mono- to hexa- nucleotide repeats with proper folding angles. Analysis of external forces in the folding template strands also suggests that expansion exists more commonly than contraction in the short tandem repeats.

Conclusion

The folded replication slippage model provides a reasonable explanation for the continuous occurrences of simple sequence repeats in genomes. This model also contributes to the explanation of STR-to-genome evolution and is an alternative model that complements semi-conservative replication.

An Evolutionary Insertion in the Mxra8 Receptor-Binding Site Confers Resistance to Alphavirus Infection and Pathogenesis

Alphaviruses are emerging, mosquito-transmitted RNA viruses with poorly understood cellular tropism and species selectivity. Mxra8 is a receptor for multiple alphaviruses including chikungunya virus (CHIKV). We discovered that while expression of mouse, rat, chimpanzee, dog, horse, goat, sheep, and human Mxra8 enables alphavirus infection in cell culture, cattle Mxra8 does not. Cattle Mxra8 encodes a 15-amino acid insertion in its ectodomain that prevents Mxra8 binding to CHIKV. Identical insertions are present in zebu, yak, and the extinct auroch. As other Bovinae lineages contain related Mxra8 sequences, this insertion likely occurred at least 5 million years ago. Removing the Mxra8 insertion in Bovinae enhances alphavirus binding and infection, while introducing the insertion into mouse Mxra8 blocks CHIKV binding, prevents infection by multiple alphaviruses in cells, and mitigates CHIKV-induced pathogenesis in mice. Our studies on how this insertion provides resistance to CHIKV infection could facilitate countermeasures that disrupt Mxra8 interactions with alphaviruses.

Comparative genomics in phytopathogenic prokaryotes reveals the higher relative abundance and density of long-SSRs in the smallest prokaryotic genome

Frequency and distribution of long-SSRs were studied in 18 phytopathogenic prokaryotes. Higher relative abundance of the long-SSRs was observed in phytopathogenic prokaryotes when compared to non-pathogenic control. The frequency of these SSRs was positively correlated with size and GC content of the genomes of phytopathogenic prokaryotes. Interestingly, phytopathogens with higher GC content in the genome were found to posses longer repeat motifs of SSRs, whereas those having lesser GC content were harbouring shorter repeat motifs. Higher abundance of tri- and hexa-nucleotide repeat motifs were the characteristic of actinomycetes, where as higher abundance of mono- and tetra-nucleotide repeats were the characteristic of the mollicutes. The maximum relative abundance and relative density of SSR were found in the smallest genome of host-adapted pathogen Aster yellow, however, length of microsatellite repeat units was the least. On the basis of presence of SSRs in the housekeeping genes, a phylogenetic relationship between these phytopathogenic prokaryotes was deduced and compared with the phylogeny developed based on 16S ribosomal RNA gene.

A comparative survey of microsatellites among wild and domestic cat provides valuable resources for marker development

Information on the level and distribution of genetic variation is important for conservation plan of captive population of an endangered species such as tiger and cheetah. We assayed the frequency of microsatellites in the genomic and genic sequences of wild cats (Panthera tigris, Acinonyx jubatus) and compared it with the domestic cat (Felis catus). Frequency, relative abundance and density of microsatellites were highest in the domestic cat when compared with wild cats. The frequency of microsatellites was positively correlated with the G+C content of genomic and genic sequences. The maximum frequency of microsatellites among all three sequence sets was of di-nucleotide repeats (genomic-88.1%; genic-70.4%), whereas the hexa-nucleotide repeat represents < 0.5%. Motif conservation study among the genomic and genic sequences revealed conservation of 81.3% and 51.0% motif within the members of family Felidae. A total 40,233 primers from genic sequences were designed in order to enrich the members of family Felidae with genomic resources. The designed primers could be useful in determining the molecular genetics of population structure and individualization of a particular cat.

Comparative analysis of low complexity regions in Plasmodia

Low complexity regions (LCRs) are a common feature shared by many genomes, but their evolutionary and functional significance remains mostly unknown. At the core of the uncertainty is a poor understanding of the mechanisms that regulate their retention in genomes, whether driven by natural selection or neutral evolution. Applying a comparative approach of LCRs to multiple strains and species is a powerful approach to identify patterns of conservation in these regions. Using this method, we investigate the evolutionary history of LCRs in the genus Plasmodium based on orthologous protein coding genes shared by 11 species and strains from primate and rodent-infecting pathogens. We find multiple lines of evidence in support of natural selection as a major evolutionary force shaping the composition and conservation of LCRs through time and signatures that their evolutionary paths are species specific. Our findings add a comparative analysis perspective to the debate on the evolution of LCRs and harness the power of sequence comparisons to identify potential functionally important LCR candidates.

Large-scale analysis reveals that the genome features of simple sequence repeats are generally conserved at the family level in insects

BACKGROUND

Simple sequence repeats (SSR), also called microsatellites, have been widely used as genetic markers, and have been extensively studied in some model insects. At present, the genomes of more than 100 insect species are available. However, the features of SSRs in most insect genomes remain largely unknown.

RESULTS

We identified 15.01 million SSRs across 136 insect genomes. The number of identified SSRs was positively associated with genome size in insects, but the frequency and density per megabase of genomes were not. Most insect SSRs (56.2-93.1%) were perfect (no mismatch). Imperfect (at least one mismatch) SSRs (average length 22-73 bp) were longer than perfect SSRs (16-30 bp). The most abundant insect SSRs were the di- and trinucleotide types, which accounted for 27.2% and 22.0% of all SSRs, respectively. On average, 59.1%, 36.8%, and 3.7% of insect SSRs were located in intergenic, intronic, and exonic regions, respectively. The percentages of various types of SSRs were similar among insects from the same family. However, they were dissimilar among insects from different families within orders. We carried out a phylogenetic analysis using the SSR frequencies. Species from the same family were generally clustered together in the evolutionary tree. However, insects from the same order but not in the same family did not cluster together. These results indicated that although SSRs undergo rapid expansions and contractions in different populations of the same species, the general genomic features of insect SSRs remain conserved at the family level.

CONCLUSION

Millions of insect SSRs were identified and their genome features were analyzed. Most insect SSRs were perfect and were located in intergenic regions. We presented evidence that the variance of insect SSRs accumulated after the differentiation of insect families.

A Comparison of Microsatellites in Phytopathogenic Aspergillus Species in Order to Develop Markers for the Assessment of Genetic Diversity among Its Isolates

The occurrence of Microsatellites (SSRs) has been witnessed in most of the fungal genomes however its abundance varies across species. In the present study, we analyzed the frequency of SSRs in the whole genome and transcripts of two phyto-pathogenic (Aspergillus niger and Aspergillus terreus) and compared them with two non-pathogenic (Aspergillus nidulans and Aspergillus oryzae) Aspergillus. Higher relative abundance and relative density of SSRs were observed in the whole genome and transcript sequences of the pathogenic Aspergillus when compared to the non-pathogenic. The relative abundance and density of SSRs were positively correlated with the G+C content of transcripts. Among the different classes of SSR, the percentage of tetra-nucleotide SSRs were maximum in A. niger (36.7%) and A. oryzae (35.9%) whereas A. nidulans and A. terreus preferred tri-nucleotide SSRs (38.2 and 42.1%) in whole genome sequences. In transcripts, tri-nucleotide SSRs were the most abundant whereas di-nucleotide SSRs were the least favored. Motif conservation study among the transcripts revealed conservation of only 27% motif within Aspergillus species. Furthermore, a similar relationship among the Ascomycetes was obtained on the basis of motif conservation and conserved genes (rDNA). To analyze the diversity present within the Indian isolates of Aspergillus, primers were successfully designed for 692 motifs in A. niger and A. terreus of which 20 were selected for diversity analysis. Among all the markers amplified, 10 markers (83.3%) were polymorphic, whereas remaining two markers (16.6%) were monomorphic. Ten polymorphic markers acquired in this investigation showed the utility of recently created SSR markers in the assessment of genetic diversity among various isolates of Aspergillus.

Genome-wide mapping and characterization of microsatellites in the swamp eel genome

We described genome-wide screening and characterization of microsatellites in the swamp eel genome. A total of 99,293 microsatellite loci were identified in the genome with an overall density of 179 microsatellites per megabase of genomic sequences. The dinucleotide microsatellites were the most abundant type representing 71% of the total microsatellite loci and the AC-rich motifs were the most recurrent in all repeat types. Microsatellite frequency decreased as numbers of repeat units increased, which was more obvious in long than short microsatellite motifs. Most of microsatellites were located in non-coding regions, whereas only approximately 1% of the microsatellites were detected in coding regions. Trinucleotide repeats were most abundant microsatellites in the coding regions, which represented amino acid repeats in proteins. There was a chromosome-biased distribution of microsatellites in non-coding regions, with the highest density of 203.95/Mb on chromosome 8 and the least on chromosome 7 (164.06/Mb). The most abundant dinucleotides (AC)n was mainly located on chromosome 8. Notably, genomic mapping showed that there was a chromosome-biased association of genomic distributions between microsatellites and transposon elements. Thus, the novel dataset of microsatellites in swamp eel provides a valuable resource for further studies on QTL-based selection breeding, genetic resource conservation and evolutionary genetics.

A Comprehensive Characterization of Simple Sequence Repeats in the Sequenced Trichoderma Genomes Provides Valuable Resources for Marker Development

Members of genus Trichoderma are known worldwide for mycoparasitism. To gain a better insight into the organization and evolution of their genomes, we used an in silico approach to compare the occurrence, relative abundance and density of SSRs in Trichoderma atroviride, T. harzianum, T. reesei, and T. virens. Our analysis revealed that in all the four genome sequences studied, the occurrence, relative abundance, and density of microsatellites varied and was not influenced by genome sizes. The relative abundance and density of SSRs positively correlated with the G + C content of their genomes. The maximum frequency of SSRs was observed in the smallest genome of T. reesei whereas it was least in second smallest genome of T. atroviride. Among different classes of repeats, the tri-nucleotide repeats were abundant in all the genomes and accounts for ∼38%, whereas hexa-nuceotide repeats were the least (∼10.2%). Further evaluation of the conservation of motifs in the transcript sequences shows a 49.5% conservation among all the motifs. In order to study polymorphism in Trichoderma isolates, 12 polymorphic SSR markers were developed. Of the 12 markers, 6 markers are from T. atroviride and remaining 6 belong to T. harzianum. SSR markers were found to be more polymorphic from T. atroviride with an average polymorphism information content value of 0.745 in comparison with T. harzianum (0.615). Twelve polymorphic markers obtained in this study clearly demonstrate the utility of newly developed SSR markers in establishing genetic relationships among different isolates of Trichoderma.

Patterns of microsatellite evolution inferred from the Helianthus annuus (Asteraceae) transcriptome

The distribution of microsatellites in exons, and their association with gene ontology (GO) terms is explored to elucidate patterns of microsatellite evolution in the common sunflower, Helianthus annuus. The relative position, motif, size and level of impurity were estimated for each microsatellite in the unigene database available from the Compositae Genome Project (CGP), and statistical analyses were performed to determine if differences in microsatellite distributions and enrichment within certain GO terms were significant. There are more translated than untranslated microsatellites, implying that many bring about structural changes in proteins. However, the greatest density is observed within the UTRs, particularly 5'UTRs. Further, UTR microsatellites are purer and longer than coding region microsatellites. This suggests that UTR microsatellites are either younger and under more relaxed constraints, or that purifying selection limits impurities, and directional selection favours their expansion. GOs associated with response to various environmental stimuli including water deprivation and salt stress were significantly enriched with microsatellites. This may suggest that these GOs are more labile in plant genomes, or that selection has favoured the maintenance of microsatellites in these genes over others. This study shows that the distribution of transcribed microsatellites in H. annuus is nonrandom, the coding region microsatellites are under greater constraint compared to the UTR microsatellites, and that these sequences are enriched within genes that regulate plant responses to environmental stress and stimuli.

Are mutagenic non D-loop direct repeat motifs in mitochondrial DNA under a negative selection pressure?

Non D-loop direct repeats (DRs) in mitochondrial DNA (mtDNA) have been commonly implicated in the mutagenesis of mtDNA deletions associated with neuromuscular disease and ageing. Further, these DRs have been hypothesized to put a constraint on the lifespan of mammals and are under a negative selection pressure. Using a compendium of 294 mammalian mtDNA, we re-examined the relationship between species lifespan and the mutagenicity of such DRs. Contradicting the prevailing hypotheses, we found no significant evidence that long-lived mammals possess fewer mutagenic DRs than short-lived mammals. By comparing DR counts in human mtDNA with those in selectively randomized sequences, we also showed that the number of DRs in human mtDNA is primarily determined by global mtDNA properties, such as the bias in synonymous codon usage (SCU) and nucleotide composition. We found that SCU bias in mtDNA positively correlates with DR counts, where repeated usage of a subset of codons leads to more frequent DR occurrences. While bias in SCU and nucleotide composition has been attributed to nucleotide mutational bias, mammalian mtDNA still exhibit higher SCU bias and DR counts than expected from such mutational bias, suggesting a lack of negative selection against non D-loop DRs.

Implications of HIV RNA structure for recombination, speciation, and the neutralism-selectionism controversy

The conflict between the needs to encode both a protein (impaired by non-synonymous mutation), and nucleic acid structure (impaired by synonymous or non-synonymous mutation), can sometimes be resolved in favour of the nucleic acid because its structure is critical for a selectively advantageous genome-wide activity--recombination. However, above a sequence difference threshold, recombination is impaired. It may then be advantageous for new species to arise. Building on the work of Grantham and others critical of the neutralist viewpoint, heuristic support for this hypothesis emerged from studies of the base composition and structure of retroviral genomes. The extreme enrichment in the purine A of the RNA of human immunodeficiency virus (HIV-1), parallels the mild purine-loading of the RNAs of most organisms, for which there is an adaptive explanation--immune evasion. However, human T cell leukaemia virus (HTLV-1), with the potential to invade the same host cell, shows extreme enrichment in the pyrimidine C. Assuming the low GC% HIV and the high GC% HTLV-1 to share a common ancestor, it was postulated that differences in GC% had arisen to prevent homologous recombination between these emerging lentiviral species. Sympatrically isolated by this intracellular reproductive barrier, prototypic HIV-1 seized the AU-rich (low GC%) high ground (thus committing to purine A rather than purine G). Prototypic HTLV-1 forwent this advantage and evolved an independent evolutionary strategy--similar to that of the GC%-rich Epstein-Barr virus--profound latency maintained by transcription of one purine-rich mRNA. The evidence supporting these interpretations is reviewed.

A conserved extraordinarily long serine homopolymer in Dictyostelid amoebae

Eukaryotic protein sequences often contain amino-acid homopolymers that consist of a single amino acid repeated from several to dozens of times. Some of these are functional but others may persist largely because of high expansion rates due to DNA slippage. However, very long homopolymers with over a hundred repeats are very rare. We report an extraordinarily long homopolymer consisting of 306 tandem serine repeats from the single-celled eukaryote Dictyostelium discoideum, which also has a multicellular stage. The gene has a paralog with 132 repeats and orthologs, also with high serine repeat numbers, in various other Dictyostelid species. The conserved gene structure and protein sequences suggest that the homopolymer is functional. The high codon diversity and very poor alignment of serine codons in this gene between species similarly indicate functionality. This is because the serine homopolymer is conserved despite much DNA sequence change. A survey of other very long amino-acid homopolymers in eukaryotes shows that high codon diversity is the rule, suggesting that these too may be functional.

Revisiting an important component of plant genomes: microsatellites

Microsatellites are some of the most highly variable repetitive DNA tracts in genomes. Few studies focus on whether the characteristic instability of microsatellites is linked to phenotypic effects in plants. We summarise recent data to investigate how microsatellite variations affect gene expression and hence phenotype. We discuss how the basic characteristics of microsatellites may contribute to phenotypic effects. In summary, microsatellites in plants are universal and highly mutable, they coexist and coevolve with transposable elements, and are under selective pressure. The number of motif nucleotides, the type of motif and transposon activity all contribute to the nonrandom generation and decay of microsatellites, and to conservation and distribution biases. Although microsatellites are generated by accident, they mature through responses to environmental change before final decay. This process is mediated by organism adjustment mechanisms, which maintain a balance between birth versus death and growth versus decay in microsatellites. Close relationships also exist between the physical structure, variation and functionality of microsatellites: in most plant species, sequences containing microsatellites are associated with catalytic activity and binding functions, are expressed in the membrane and organelles, and participate in the developmental and metabolic processes. Microsatellites contribute to genome structure and functional plasticity, and may be considered to promote species evolution in plants in response to environmental changes. In conclusion, the generation, loss, functionality and evolution of microsatellites can be related to plant gene expression and functional alterations. The effect of microsatellites on phenotypic variation may be as significant in plants as it is in animals.

Development and characterization of simple sequence repeat markers providing genome-wide coverage and high resolution in maize

Simple sequence repeats (SSRs) have been widely used in maize genetics and breeding, because they are co-dominant, easy to score, and highly abundant. In this study, we used whole-genome sequences from 16 maize inbreds and 1 wild relative to determine SSR abundance and to develop a set of high-density polymorphic SSR markers. A total of 264 658 SSRs were identified across the 17 genomes, with an average of 135 693 SSRs per genome. Marker density was one SSR every of 15.48 kb. (C/G)n, (AT)n, (CAG/CTG)n, and (AAAT/ATTT)n were the most frequent motifs for mono, di-, tri-, and tetra-nucleotide SSRs, respectively. SSRs were most abundant in intergenic region and least frequent in untranslated regions, as revealed by comparing SSR distributions of three representative resequenced genomes. Comparing SSR sequences and e-polymerase chain reaction analysis among the 17 tested genomes created a new database, including 111 887 SSRs, that could be develop as polymorphic markers in silico. Among these markers, 58.00, 26.09, 7.20, 3.00, 3.93, and 1.78% of them had mono, di-, tri-, tetra-, penta-, and hexa-nucleotide motifs, respectively. Polymorphic information content for 35 573 polymorphic SSRs out of 111 887 loci varied from 0.05 to 0.83, with an average of 0.31 in the 17 tested genomes. Experimental validation of polymorphic SSR markers showed that over 70% of the primer pairs could generate the target bands with length polymorphism, and these markers would be very powerful when they are used for genetic populations derived from various types of maize germplasms that were sampled for this study.

Microsatellite repeat dynamics in mitochondrial genomes of phytopathogenic fungi: frequency and distribution in the genic and intergenic regions

The frequency and distribution of microsatellites were analyzed in the 19 mitogenomes of phytopathogenic fungi covering five phyla. Our analysis revealed that in all the mitogenomes studied, the frequency and relative abundance varied, and it was neither influenced by genome size nor by GC content. SSRs were found to be differential distributed in genic and intergenic regions. An average of 5.14 (23.6%) SSRs were present in genic sequences and 21.7 (76.4%) SSRs were located in the intergenic sequences. Relative abundance of SSRs in mitogenomes was the highest in Aspergillus tubigensis, whereas, it was the least in Phaeosphaeria nodurum, the average being 0.45. Trinucleotide repeats were the most abundant motifs in the genic and intergenic regions of the mitogenomes of the phytopathogenic fungi. Among the genes, cox1 harbors the maximum SSRs, whereas cox3 and nad 7 contain the least. Based on the presence of SSRs in a particular gene, genetic relationships among individual organisms were also established.

Amino acid repeats cause extraordinary coding sequence variation in the social amoeba Dictyostelium discoideum

Protein sequences are normally the most conserved elements of genomes owing to purifying selection to maintain their functions. We document an extraordinary amount of within-species protein sequence variation in the model eukaryote Dictyostelium discoideum stemming from triplet DNA repeats coding for long strings of single amino acids. D. discoideum has a very large number of such strings, many of which are polyglutamine repeats, the same sequence that causes various human neurological disorders in humans, like Huntington’s disease. We show here that D. discoideum coding repeat loci are highly variable among individuals, making D. discoideum a candidate for the most variable proteome. The coding repeat loci are not significantly less variable than similar non-coding triplet repeats. This pattern is consistent with these amino-acid repeats being largely non-functional sequences evolving primarily by mutation and drift.

A second generation framework for the analysis of microsatellites in expressed sequence tags and the development of EST-SSR markers for a conifer, Cryptomeria japonica

Background

Microsatellites or simple sequence repeats (SSRs) in expressed sequence tags (ESTs) are useful resources for genome analysis because of their abundance, functionality and polymorphism. The advent of commercial second generation sequencing machines has lead to new strategies for developing EST-SSR markers, necessitating the development of bioinformatic framework that can keep pace with the increasing quality and quantity of sequence data produced. We describe an open scheme for analyzing ESTs and developing EST-SSR markers from reads collected by Sanger sequencing and pyrosequencing of sugi (Cryptomeria japonica).

Results

We collected 141,097 sequence reads by Sanger sequencing and 1,333,444 by pyrosequencing. After trimming contaminant and low quality sequences, 118,319 Sanger and 1,201,150 pyrosequencing reads were passed to the MIRA assembler, generating 81,284 contigs that were analysed for SSRs. 4,059 SSRs were found in 3,694 (4.54%) contigs, giving an SSR frequency lower than that in seven other plant species with gene indices (5.4–21.9%). The average GC content of the SSR-containing contigs was 41.55%, compared to 40.23% for all contigs. Tri-SSRs were the most common SSRs; the most common motif was AT, which was found in 655 (46.3%) di-SSRs, followed by the AAG motif, found in 342 (25.9%) tri-SSRs. Most (72.8%) tri-SSRs were in coding regions, but 55.6% of the di-SSRs were in non-coding regions; the AT motif was most abundant in 3′ untranslated regions. Gene ontology (GO) annotations showed that six GO terms were significantly overrepresented within SSR-containing contigs. Forty–four EST-SSR markers were developed from 192 primer pairs using two pipelines: read2Marker and the newly-developed CMiB, which combines several open tools. Markers resulting from both pipelines showed no differences in PCR success rate and polymorphisms, but PCR success and polymorphism were significantly affected by the expected PCR product size and number of SSR repeats, respectively. EST-SSR markers exhibited less polymorphism than genomic SSRs.

Conclusions

We have created a new open pipeline for developing EST-SSR markers and applied it in a comprehensive analysis of EST-SSRs and EST-SSR markers in C. japonica. The results will be useful in genomic analyses of conifers and other non-model species.

A matter of life or death: how microsatellites emerge in and vanish from the human genome

Microsatellites--tandem repeats of short DNA motifs--are abundant in the human genome and have high mutation rates. While microsatellite instability is implicated in numerous genetic diseases, the molecular processes involved in their emergence and disappearance are still not well understood. Microsatellites are hypothesized to follow a life cycle, wherein they are born and expand into adulthood, until their degradation and death. Here we identified microsatellite births/deaths in human, chimpanzee, and orangutan genomes, using macaque and marmoset as outgroups. We inferred mutations causing births/deaths based on parsimony, and investigated local genomic environments affecting them. We also studied birth/death patterns within transposable elements (Alus and L1s), coding regions, and disease-associated loci. We observed that substitutions were the predominant cause for births of short microsatellites, while insertions and deletions were important for births of longer microsatellites. Substitutions were the cause for deaths of microsatellites of virtually all lengths. AT-rich L1 sequences exhibited elevated frequency of births/deaths over their entire length, while GC-rich Alus only in their 3' poly(A) tails and middle A-stretches, with differences depending on transposable element integration timing. Births/deaths were strongly selected against in coding regions. Births/deaths occurred in genomic regions with high substitution rates, protomicrosatellite content, and L1 density, but low GC content and Alu density. The majority of the 17 disease-associated microsatellites examined are evolutionarily ancient (were acquired by the common ancestor of simians). Our genome-wide investigation of microsatellite life cycle has fundamental applications for predicting the susceptibility of birth/death of microsatellites, including many disease-causing loci.

Comparative genomics of the social amoebae Dictyostelium discoideum and Dictyostelium purpureum

Background

The social amoebae (Dictyostelia) are a diverse group of Amoebozoa that achieve multicellularity by aggregation and undergo morphogenesis into fruiting bodies with terminally differentiated spores and stalk cells. There are four groups of dictyostelids, with the most derived being a group that contains the model species Dictyostelium discoideum.

Results

We have produced a draft genome sequence of another group dictyostelid, Dictyostelium purpureum, and compare it to the D. discoideum genome. The assembly (8.41 × coverage) comprises 799 scaffolds totaling 33.0 Mb, comparable to the D. discoideum genome size. Sequence comparisons suggest that these two dictyostelids shared a common ancestor approximately 400 million years ago. In spite of this divergence, most orthologs reside in small clusters of conserved synteny. Comparative analyses revealed a core set of orthologous genes that illuminate dictyostelid physiology, as well as differences in gene family content. Interesting patterns of gene conservation and divergence are also evident, suggesting function differences; some protein families, such as the histidine kinases, have undergone little functional change, whereas others, such as the polyketide synthases, have undergone extensive diversification. The abundant amino acid homopolymers encoded in both genomes are generally not found in homologous positions within proteins, so they are unlikely to derive from ancestral DNA triplet repeats. Genes involved in the social stage evolved more rapidly than others, consistent with either relaxed selection or accelerated evolution due to social conflict.

Conclusions

The findings from this new genome sequence and comparative analysis shed light on the biology and evolution of the Dictyostelia.