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Phylogenetic generalized least squares (PGLS) regression is widely used to analyze evolutionary correlations between two traits. In contrast to the equal treatment of analyzed traits in conventional correlation methods such as Pearson and Spearman's rank tests, PGLS regression accounts for phylogenetic non-independence unbalanced between independent and dependent variables when estimating some evolutionary parameters. We investigated the correlations of CRISPR-Cas and prophage contents with optimal growth temperature and minimal doubling time using Pagel's λ model and found that switching independent and dependent variables resulted in conflicting results in 26.3% of cases. To better understand this phenomenon, we conducted 12,000 simulations of the evolution of two traits (X1 and X2) along a binary tree with 100 terminal nodes, using different models and variances. We found that switching dependent and independent variables in PGLS regression analysis using Pagel's λ Model resulted in conflicting outcomes in 17.2% of cases. To assess correlations in each simulation, we established a gold standard by analyzing changes in traits along phylogenetic branches. Next, we tested seven potential criteria for dependent variable selection: log-likelihood, Akaike information criterion, R2, p-value, Pagel's λ, Blomberg et al.'s K, and the estimated λ in Pagel's λ model. We determined that the last three criteria performed equally well in selecting the dependent variable and were superior to the other four. As Pagel's λ and Blomberg et al.'s K values are indicators of phylogenetic signals and are commonly calculated at the beginning of phylogenetic comparative studies, we suggest using the trait with a higher λ or K value as the dependent variable for practicality in future PGLS regressions. Logical cause and effect analysis should be conducted after establishing a significant correlation by PGLS regression, rather than providing an indicator for the dependent variable.

BackgroundMany environmental factors are associated with GC content evolution. However, the relationship between GC content and halophilicity has been poorly explored. Higher GC contents have been linked to halophiles because of a few solitary archaeal cases. The only previous statistical study on the relationship ignored the phylogenetic non-independence of species. The habitats of halophilic archaea typically have high-intensity ultraviolet radiation. Avoiding the DNA photoproduct is widely believed as a selective force to increase GC content in archaea living in high saline/ultraviolet-radiation environments.ResultsUsing phylogenetic generalized least squares (PGLS), we examined the relationship between GC content and halophilicity in 1226 bacteria and 181 archaea. Significant positive correlations have been found in bacteria but not in archaea. We suspect that the crowding of most archaeal halophiles in a few phylogenetic lineages dramatically reduces the effective sample size and probably makes the PGLS method less efficient in detecting correlations. The GC contents of different genomic components were explored separately. Bacterial intergenic sequences have GC contents more strongly correlated with halophilicity than protein-coding genes, and 5S rRNA and tRNA genes have GC contents more strongly correlated with halophilicity than 16S and 23S rRNA genes. Using PGLS, we measured the relationship between GC content and photoreactivity, a measure of the vulnerability of DNA sequences under ultraviolet radiation. No significant correlation was observed in the 1226 bacteria or the 181 archaea.ConclusionsWe proposed that the correlation between bacterial GC content and environmental salinity should be attributed to the GC-biased DNA repairing process and the insertion of GC-rich sequences.

CRISPR-Cas systems provide adaptive immunity for prokaryotic cells by recognizing and eliminating the recurrent genetic invaders whose sequences had been captured in a prior infection and stored in the CRISPR arrays as spacers. However, the biological/environmental factors determining the efficiency of this immune system have yet to be fully characterized. Recent studies in cultured bacteria showed that slowing the growth rate of bacterial cells could promote their acquisition of novel spacers. This study examined the relationship between the CRISPR-Cas content and the minimal doubling time across the bacteria and the archaea domains. Every completely sequenced genome could be used to predict a minimal doubling time. With a large dataset of 4142 bacterial samples, we found that the predicted minimal doubling times are positively correlated with spacer number and other parameters of the CRISPR-Cas systems, like array number,Casgene cluster number, andCasgene number. Different datasets gave different results. Weak results were obtained in analyzing bacterial empirical minimal doubling times and the archaea domain. Still, the conclusion of more spacers in slowly-grown prokaryotes was supported. In addition, we found that the minimal doubling times are negatively correlated with the occurrence of prophages, and the spacer numbers per array are negatively associated with the number of prophages. These observations support the existence of an evolutionary trade-off between bacterial growth and adaptive defense against virulent phages.

Background GC pairs are generally more stable than AT pairs; GC-rich genomes were proposed to be more adapted to high temperatures than AT-rich genomes. Previous studies consistently showed positive correlations between growth temperature and the GC contents of structural RNA genes. However, for the whole genome sequences and the silent sites of the codons in protein-coding genes, the relationship between GC content and growth temperature is in a long-lasting debate. Results With a dataset much larger than previous studies (681 bacteria and 155 archaea with completely assembled genomes), our phylogenetic comparative analyses showed positive correlations between optimal growth temperature (Topt) and GC content both in bacterial and archaeal structural RNA genes and in bacterial whole genome sequences, chromosomal sequences, plasmid sequences, core genes, and accessory genes. However, in the 155 archaea, we did not observe a significant positive correlation of Topt with whole-genome GC content (GCw) or GC content at four-fold degenerate sites. We randomly drew 155 samples from the 681 bacteria for 1000 rounds. In most cases (> 95%), the positive correlations between Topt and genomic GC contents became statistically nonsignificant (P > 0.05). This result suggested that the small sample sizes might account for the lack of positive correlations between growth temperature and genomic GC content in the 155 archaea and the bacterial samples of previous studies. Comparing the GC content among four categories (psychrophiles/psychrotrophiles, mesophiles, thermophiles, and hyperthermophiles) also revealed a positive correlation between GCw and growth temperature in bacteria. By including the GCw of incompletely assembled genomes, we expanded the sample size of archaea to 303. Positive correlations between GCw and Topt appear especially after excluding the halophilic archaea whose GC contents might be strongly shaped by intense UV radiation. Conclusions This study explains the previous contradictory observations and ends a long debate. Prokaryotes growing in high temperatures have higher GC contents. Thermal adaptation is one possible explanation for the positive association. Meanwhile, we propose that the elevated efficiency of DNA repair in response to heat mutagenesis might have the by-product of increasing GC content like that happens in intracellular symbionts and marine bacterioplankton.

Although performing adaptive immunity, CRISPR-Cas systems are present in only 40% of bacterial genomes. Here, we observed an abrupt transition of bacterial CRISPR-Cas abundance at around 45°C. Phylogenetic comparative analyses confirmed that the abundance correlates with growth temperature only at the temperature range around 45°C. Meanwhile, we noticed that the diversities of cellular predators have a precipitous decline at this temperature range. The grazing risk faced by bacteria reduces substantially at around 45°C and almost disappears above 60°C. So viral lysis would become the dominating factor of bacterial mortality, and antivirus immunity has a higher priority. In temperature ranges where the abundance of cellular predators does not change with temperature, temperatures would have negligible effects on CRISPR-Cas abundance. The hypothesis predicts that bacteria should also be rich in CRISPR-Cas systems if they live in other extreme conditions inaccessible to grazing predators.

Background Although horizontal gene transfer (HGT) is a widely accepted force in the evolution of prokaryotic genomes, its role in the evolution of eukaryotic genomes remains hotly debated. Some bdelloid rotifers that are resistant to extreme desiccation and radiation undergo a very high level of HGT, whereas in another desiccation-resistant invertebrate, the tardigrade, the pattern does not exist. Overall, the DNA double-strand breaks (DSBs) induced by prolonged desiccation have been postulated to open a gateway to the nuclear genome for exogenous DNA integration and thus to facilitate the HGT process, thereby enhancing the rate of endosymbiotic DNA transfer (EDT). Results We first surveyed the abundance of nuclear mitochondrial DNAs (NUMTs) and nuclear plastid DNAs (NUPTs) in five eukaryotes that are highly resistant to desiccation: the bdelloid rotifers Adineta vaga and Adineta ricciae, the tardigrade Ramazzottius varieornatus, and the resurrection plants Dorcoceras hygrometricum and Selaginella tamariscina. Excessive NUMTs or NUPTs were not detected. Furthermore, we compared 24 groups of desiccation-tolerant organisms with their relatively less desiccation-tolerant relatives but did not find a significant difference in NUMT/NUPT contents. Conclusions Desiccation may induce DSBs, but it is unlikely to dramatically increase the frequency of exogenous sequence integration in most eukaryotes. The capture of exogenous DNA sequences is possible only when DSBs are repaired through a subtype of non-homologous end joining, named alternative end joining (alt-EJ). Due to the deleterious effects of the resulting insertion mutations, alt-EJ is less frequently initiated than other mechanisms.

Changes in the copy number of protein-coding genes would lead to detrimental effects if the consequent changes in protein concentration disrupt essential cellular functions. Large-scale genomic studies have identified thousands of dosage-sensitive genes in human genome. We are interested in the dosage-sensitivity of transcription factor (TF) genes whose products are essential for the growth, division and differentiation of cells by regulating the expression of the genetic information encoded in the genome. We first surveyed the enrichment of human TF genes in four recently curated datasets of dosage-sensitive genes, including the haploinsufficient genes identified by a large-scale genomic study, the haploinsufficient genes predicted by a machine learning approach, the genes with conserved copy number across mammals, and the ohnologs. Then we selected the dosage-sensitive genes that are present in all the four dataset and regarded them as the most reliable dosage-sensitive genes, and the genes that are absent from any one of the four datasets as the most reliable dosage-insensitive genes, and surveyed the enrichments of TFs genes in these two datasets. A large number of TF genes were found to be dosage-insensitive, which is beyond the expectation based on the role of TFs. In spite of this, the likeness of TF genes to be dosage-sensitive were supported by five datasets, with the conserved-copy-number genes as the exception. The nuclear receptors are the only one family of TFs whose dosage-sensitivity was consistently supported by all the six datasets. In addition, we found that TF families with very few members are also more likely to be dosage-sensitive while the largest TF family, C2H2-ZF, are most likely dosage-insensitive. The most extensively studied TFs, p53, are not special in dosage-sensitivity. They are significantly enriched in only three datasets. We also confirmed that dosage-sensitive genes generally have long coding sequences, high expression levels and experienced stronger selective pressure. Our results indicate some TFs function in a dose-dependent manner while some other not. Gene dosage changes in some TF families like nuclear receptor would result in disease phenotypes while the effects of such changes in some TFs like C2H2-ZF would be mild.

Horizontal gene transfer (HGT) is a widely accepted force in the evolution of prokaryotic genomes. However, in eukaryotes, it is still in hot debate. Some bdelloid rotifers that are resistant to extreme desiccation and radiation were reported to have a very high level of HGTs. However, a similar report in another resistant invertebrate, tardigrades, has been mired in controversy. The DNA double-strand breaks (DSBs) induced by prolonged desiccation have been postulated to open the gateway of nuclear genome for foreign DNA integration and thus facilitate the HGT process. If so, the rate of endosymbiotic DNA transfer should also be enhanced. We first surveyed the abundance of nuclear mitochondrial DNAs (NUMTs) and nuclear plastid DNAs (NUPTs) in three groups of eukaryotes that are extremely resistant to desiccation, bdelloid rotifers, A. vaga and A. ricciae, tardigrades, H. dujardini and R. varieornatus, and the resurrection plants, D. hygrometricum and S. tamariscina. Excessive NUMTs or NUPTs have not been detected. Furthermore, we compared nine groups of desiccation-tolerant organisms with their desiccation-sensitive relatives but did not find significant difference in the NUMT/NUPT contents. Desiccation could induce DSBs, but it unlikely dramatically increase the frequency of foreign sequence integration in most eukaryotes. Only in the nuclear genomes enriched in repetitive sequences, the DSBs are predominantly repaired by non-homologous end joining (NHEJ) and desiccation-induced DSBs is possible to enhance the integration of foreign sequences into nuclear genome for some degree.

Background Among the four bases, guanine is the most susceptible to damage from oxidative stress. Replication of DNA containing damaged guanines results in G to T mutations. Therefore, the mutations resulting from oxidative DNA damage are generally expected to predominantly consist of G to T (and C to A when the damaged guanine is not in the reference strand) and result in decreased GC content. However, the opposite pattern was reported 16 years ago in a study of prokaryotic genomes. Although that result has been widely cited and confirmed by nine later studies with similar methods, the omission of the effect of shared ancestry requires a re-examination of the reliability of the results. Results When aerobic and obligate aerobic prokaryotes were mixed together and anaerobic and obligate anaerobic prokaryotes were mixed together, phylogenetic controlled analyses did not detect significant difference in GC content between aerobic and anaerobic prokaryotes. This result is consistent with two generally neglected studied that had accounted for the phylogenetic relationship. However, when obligate aerobic prokaryotes were compared with aerobic prokaryotes, anaerobic prokaryotes, and obligate anaerobic prokaryotes separately using phylogenetic regression analysis, a significant positive association was observed between aerobiosis and GC content, no matter it was calculated from whole genome sequences or the 4-fold degenerate sites of protein-coding genes. Obligate aerobes have significantly higher GC content than aerobes, anaerobes, and obligate anaerobes. Conclusions The positive association between aerobiosis and GC content could be attributed to a mutational force resulting from incorporation of damaged deoxyguanosine during DNA replication rather than oxidation of the guanine nucleotides within DNA sequences. Our results indicate a grade in the aerobiosis-associated mutational force, strong in obligate aerobes, moderate in aerobes, weak in anaerobes and obligate anaerobes. Electronic supplementary material The online version of this article (10.1186/s12862-019-1365-8) contains supplementary material, which is available to authorized users.

Staphylococcus aureus small colony variants (SCVs) can cause persistent infections. However, the genomes and transcriptomes of S. aureus SCVs remain poorly understood. A pair of isogenic wild-type and SCV methicillin-resistant S. aureus (MRSA) strains (IE1 and IE2, respectively) were isolated from a patient with prosthetic valve infectious endocarditis. The SCV strain IE2 grew more slowly than the wild-type strain, and serum killing and mouse lethality assays revealed that the virulence of SCV strain IE2 was decreased. Whole-genome sequencing of the SCV and wild-type strains revealed 15 mutations in nine genes associated with metabolism, virulence and DNA repair, including serine/threonine-protein kinase PrkC (prkC), glycerol-3-phosphate acyltransferase (plsY), 2-deoxyribose-5-phosphate aldolase (deoC), extracellular adherence protein (eap), iron compound ABC uptake transporter substrate-binding protein (sstD), RecU Holliday junction resolvase (recU), excinuclease ABC subunit B (uvrB), type I restriction-modification system, M subunit (hsdM) and smooth muscle caldesmon. Sequencing of RNA transcripts revealed that expression levels of 321 genes were upregulated and 582 genes were downregulated in SCV strain IE2 compared with IE1. Most of the differentially expressed genes were involved in metabolism. Expression levels of several genes involved in the pathways to which plsY, deoC, eap and sstD belonged were changed, associated with the metabolism and virulence of S. aureus. In conclusion, the reduced growth rate and decreased virulence of MRSA SCV strains may be related to mutations in and downregulation of genes associated with metabolism and virulence, especially plsY, deoC, eap and sstD.

Background:Among the four bases, guanine is the most susceptible to damage from oxidative stress. Replication of DNA containing damaged guanines result in G to T mutations. Therefore, the mutations resulting from oxidative DNA damage are generally expected to predominantly consist of G to T (and C to A when the damaged guanine is not in the reference strand) and result in decreased GC content. However, the opposite pattern was reported 16 years ago in a study of prokaryotic genomes. Although that result has been widely cited and confirmed by nine later studies with similar methods, the omission of the effect of shared ancestry requires a re-examination of the reliability of the results.Results:We retrieved 70 aerobe-anaerobe pairs of prokaryotes, and members of each pair were adjacent on the phylogenetic tree. Pairwise comparisons of either whole-genome GC content or the GC content at 4-fold degenerate sites of orthologous genes among these 70 pairs did not show significant differences between aerobes and anaerobes. The signature of guanine oxidation on GC content evolution has not been detected even after extensive controlling of other influencing factors. Furthermore, the anaerobes were not different from the aerobes in the rate of either G to T, C to A, or other directions of substitutions. The presence of the enzymes responsible for guanine oxidation in anaerobic prokaryotes provided additional evidence that guanine oxidation might be prevalent in anaerobic prokaryotes. In either aerobes or anaerobes, the rates of G:C to T:A mutations were not significantly higher than the reverse mutations.Conclusions:The previous counterintuitive results on the relationship between oxygen requirement and GC content should be attributed to the methodological artefact resulting from phylogenetically non-independence among the analysed samples. Our results showed that aerobiosis does not increase or decrease GC content in evolution. Furthermore, our study challenged the widespread belief that abundant G:C to T:A transversions are the signature of oxidative stress in prokaryotic evolution.

Although intron losses have been widely reported, it is not clear whether they are neutral and therefore random or driven by positive selection. Intron transcription and splicing are time-consuming and can delay the expression of its host gene. For genes that must be activated quickly to respond to physiological or stress signals, intron delay may be deleterious. Promoter proximally paused (PPP) genes are a group of rapidly expressed genes. To respond quickly to activation signals, they generally initiate transcription competently but stall after synthesizing a short RNA. In this study, performed in Drosophila melanogaster, the PPP genes were found to have a significantly higher rate of intron loss than control genes. However, further analysis did not find more significant shrinkage of intron size in PPP genes. Referring to previous studies on the rates of transcription and splicing and to the time saved by deletion of the introns from mouse gene Hes7, it is here suggested that transcription delay is comparable to splicing delay only when the intron is 28.5 kb or larger, which is greater in size than 95% of vertebrate introns, 99.5% of Drosophila introns, and all the annotated introns of Saccharomyces cerevisiae and Arabidopsis thaliana. Delays in intron splicing are probably a selective force, promoting intron loss from quickly expressed genes. In other genes, it may have been an exaptation during the emergency of developmental clocks.

Despite the prevalence of intron losses during eukaryotic evolution, the selective forces acting on them have not been extensively explored. Arabidopsis thaliana lost half of its genome and experienced an elevated rate of intron loss after diverging from A. lyrata. The selective force for genome reduction was suggested to have driven the intron loss. However, the evolutionary mechanism of genome reduction is still a matter of debate. In this study, we found that intron-lost genes have high synonymous substitution rates. Assuming that differences in mutability among different introns are conserved among closely related species, we used the nucleotide substitution rate between orthologous introns in other species as the proxy of the mutation rate of Arabidopsis introns, either lost or extant. The lost introns were found to have higher mutation rates than extant introns. At the genome-wide level, A. thaliana has a higher mutation rate than A. lyrata, which correlates with the higher rate of intron loss and rapid genome reduction of A. thaliana. Our results indicate that selection to minimize mutational hazards might be the selective force for intron loss, and possibly also for genome reduction, in the evolution of A. thaliana. Small genome size and lower genome-wide intron density were widely reported to be correlated with phenotypic features, such as high metabolic rates and rapid growth. We argue that the mutational-hazard hypothesis is compatible with these correlations, by suggesting that selection for rapid growth might indirectly increase mutational hazards.

Proteins that assimilate particular elements were found to avoid using amino acids containing the element, which indicates that the metabolic constraints of amino acids may influence the evolution of proteins. We suspected that low contents of carbon, nitrogen, and sulfur may also be selected for economy in highly abundant proteins that consume large amounts of the resources of cells. By analyzing recently available proteomic data in Escherichia coli, Saccharomyces cerevisiae, and Schizosaccharomyces pombe, we found that at least the carbon and nitrogen contents in amino acid side chains are negatively correlated with protein abundance. An amino acid with a high number of carbon atoms in its side chain generally requires relatively more energy for its synthesis. Thus, it may be selected against in highly abundant proteins either because of economy in building blocks or because of economy in energy. Previous studies showed that highly abundant proteins preferentially use cheap (in terms of energy) amino acids. We found that the carbon content is still negatively correlated with protein abundance after controlling for the energetic cost of the amino acids. However, the negative correlation between protein abundance and energetic cost disappeared after controlling for carbon content. Building blocks seem to be more restricted than energy. It seems that the amino acid sequences of highly abundant proteins have to compromise between optimization for their biological functions and reducing the consumption of limiting resources. By contrast, the amino acid sequences of weakly expressed proteins are more likely to be optimized for their biological functions.

Origin and subsequent accumulation of spliceosomal introns are prominent events in the evolution of eukaryotic gene structure. Recently gained introns would be especially useful for the study of the mechanisms of intron gain because randomly accumulated mutations might erase the evolutionary traces. The mechanisms of intron gain remain unclear due to the presence of very few solid cases. A widely cited model of intron gain is tandem genomic duplication, in which the duplication of an AGGT-containing exonic segment provides the GT and AG splicing sites for the new intron. We found that the second intron of the potato RNA-dependent RNA polymerase gene PGSC0003DMG402000361 originated mainly from a direct duplication of the 3′ side of the upstream intron. The 5' splicing site of this new intron was recruited from the upstream exonic sequence. In addition to the new intron, a downstream exonic segment of 178 bp also arose from duplication. Most of the splicing signals were inherited directly from the parental intron/exon structure, including a putative branch site, the polypyrimidine tract, the 3′ splicing site, two putative exonic splicing enhancers and the GC contents differentiated between the intron and exon. We propose a new version of the tandem genomic duplication model, termed as the partial duplication of the preexisting intron/exon structure. This new version and the widely cited version are not mutually exclusive.

Introns were found to enhance almost every steps of gene expression except increasing mRNA stability. By analyzing the genome-wide data of mRNA stability published by someone previously, we found that human intron-containing genes have more stable mRNAs than intronless genes, and the Arabidopsis thaliana genes with the most unstable mRNAs have fewer introns than other genes in the genome. After controlling for mRNA length, we found mRNA stability is still positively correlated with intron number in human intron-containing genes. But in yeast Saccharomyces cerevisiae, two different datasets on mRNA half-life gave conflicting results. The components of messenger ribonucleoprotein particles recruited during intron splicing may be retained in cytoplasmic mRNPs and act as signals of mRNA stability or simply insulators to avoid mRNA degradation.

The half-life of individual mRNA plays a central role in controlling the level of gene expression. However, the determinants of mRNA stability have not yet been well defined. Most previous studies suggest that mRNA length does not affect its stability. Here, we show significant negative correlations between mRNA length and stability in human and Escherichia coli, but not in Saccharomyces cerevisiae or Bacillus subtilis. This finding suggests the possibility that endonucleolytic attacks by RNA endonuclease and/or mechanical damage may strongly influence mRNA stability in both prokaryotes and eukaryotes.

The origin and subsequent accumulation of spliceosomal introns are prominent events in the evolution of eukaryotic gene structure. However, the mechanisms underlying intron gain remain unclear because there are few proven cases of recently gained introns. In anRNA-dependent RNA polymerase(RdRp) gene, we found that a tandem duplication occurred after the divergence of potato and its wild relatives among otherSolanumplants. The duplicated sequence crosses the intron-exon boundary of the first intron and the second exon. A new intron was detected at this duplicated region, and it includes a small previously exonic segment of the upstream copy of the duplicated sequence and the intronic segment of the downstream copy of the duplicated sequence. The donor site of this new intron was directly obtained from the small previously exonic segment. Most of the splicing signals were inherited directly from the parental intron/exon structure, including a putative branch site, the polypyrimidine tract, the 3′ splicing site, two putative exonic splicing enhancers, and the GC contents differed between the intron and exon. In the widely cited model of intron gain by tandem genomic duplication, the duplication of an AGGT-containing exonic segment provides the GT and AG splicing sites for the new intron. Our results illustrate that the tandem duplication model of intron gain should be diverse in terms of obtaining the proper splicing signals.

Strand compositional asymmetry has been observed in prokaryotes and used in predicting prokaryotic DNA replication origins and termini. However, it was not found in eukaryotic genomes by the same methods. We propose that transcription-associated strand asymmetries mask the replication-associated ones. By analyzing the nucleotide composition of intergenic sequences larger than 50 kb by cumulative skew diagrams (CSD), we found replication-associated strand asymmetry in vertebrate genomes. Furthermore, we found that the most common replicon sizes in vertebrates are 50-100 kb, and show evidence that the replication origin and termination regions of vertebrate genomes range from a discrete site to a broad zone.

In this study, we identified 19 intron losses, including 11 precise intron losses (PILs), six imprecise intron losses (IILs), one de-exonization, and one exon deletion in tomato and potato, and 17 IILs in Arabidopsis thaliana. Comparative analysis of related genomes confirmed that all of the IILs have been fixed during evolution. Consistent with previous studies, our results indicate that PILs are a major type of intron loss. However, at least in plants, IILs are unlikely to be as rare as previously reported. Reviewers This article was reviewed by Jun Yu and Zhang Zhang. For complete reviews, see the Reviewers’ Reports section. Electronic supplementary material The online version of this article (doi:10.1186/s13062-015-0056-7) contains supplementary material, which is available to authorized users.

Although intron losses have been widely reported, it is not clear whether they are neutral and therefore random or driven by positive selection. Intron transcription and splicing are time-consuming and can delay the expression of its host gene. For genes that must be activated quickly to respond to physiological or stress signals, intron delay may be deleterious. Promoter proximally paused (PPP) genes are a group of rapidly expressed genes. To respond quickly to activation signals, they generally initiate transcription competently but stall after synthesizing a short RNA. In this study, performed in Drosophila melanogaster, the PPP genes were found to have a significantly higher rate of intron loss than control genes. However, further analysis did not find more significant shrinkage of intron size in PPP genes. Referring to previous studies on the rates of transcription and splicing and to the time saved by deletion of the introns from mouse gene Hes7, it is here suggested that transcription delay is comparable to splicing delay only when the intron is 28.5 kb or larger, which is greater in size than 95% of vertebrate introns, 99.5% of Drosophila introns, and all the annotated introns of Saccharomyces cerevisiae and Arabidopsis thaliana. Delays in intron splicing are probably a selective force, promoting intron loss from quickly expressed genes. In other genes, it may have been an exaptation during the emergency of developmental clocks.

One of the large, unsolved problems in human genetics is the proportion of functional sequences in genomes. Recently, the encyclopedia of DNA elements consortium revealed that the majority of the genome is biochemically active, which were described as biochemical functions. This has been used as evidence to pronounce the death of the junk DNA concept. In evolutionary biology, junk DNAs are sequences whose gain or loss does not seriously affect fitness of the host organism. In the human genome, a large amount of biochemical activity should be to repress the sequences so as to avoid their harmful expression. The biochemical activity is very different from functionality in the light of evolution. The single nucleotide polymorphism sites associated with disease and other phenotypes may be functional, but their abundance in the active genome regions is not reliable evidence of functionality. Because of sequence-independent functions, the proportion of functional regions would be underestimated when sequence constraints are used alone. Knockout may be the most effective means of distinguishing functional sequences from junk DNA.

In non-mammalian eukaryotes, an abnormally long 3′ untranslated region (UTR) is generally thought to be the definitive signal in the recognition of a premature termination codon (PTC) in nonsense-mediated mRNA decay (NMD). However, because the lengths of 3′ UTRs in normal mRNAs are widely distributed, “abnormally long” is hard to define. Distinct peaks of nucleosome deposition and DNA methylation have recently been found at coding region boundaries. We propose that nucleosomes and DNA methylation just upstream of a normal stop codon are ideal indicators for the position of a normal stop codon and may thus serve as signals in PTC recognition.

Although intron loss and gain have been widely observed, their mechanisms are still to be determined. In four Aspergillus genomes, we found 204 cases of intron loss and 84 cases of intron gain. Using this data, we tested common hypotheses of intron loss or gain. Statistical analysis showed that adjacent introns tend to be lost simultaneously and small introns were preferentially lost, supporting the model of mRNA-mediated intron loss. The lost introns reside in internal regions of genes, which is inconsistent with the traditional version of the model (partial length cDNAs are reverse transcribed from 3′ ends of mRNAs), but consistent with an alternate version (partial length cDNAs are produced by self-primed reverse transcription). The latter version was not supported by examination of the abundance of T-rich segments in mRNAs. Preferential loss of internal introns might be explained by highly efficient recombination at internal regions of genes. Among the 84 cases of intron gain, we found a significantly higher frequency of short direct repeats near exon–intron boundary than in conserved introns, supporting the double-strand break repair model. We also found possible source sequences for two cases of intron gain, one by gene conversion and one by insertion of a mitochondrial sequence during double-strand break repair. Source sequences for most gained introns could not be identified and the possible reasons were discussed. In the four Aspergillus genomes studied, we did not find evidence of frequent parallel intron gains.

The cyanobacteria Prochlorococcus is a cyanbacterial genus, with some strains adapted to sea surface environments, which are poor in nutrients and have high-light intensity, and some strains adapted to deep sea conditions, which have relatively higher concentrations of nitrogen and phosphorus and lower light intensity. Here, we report pairwise comparisons between strains isolated from different depths of the same sea, which reveal a close association between atomic composition of the proteome and the availability nitrogen and phosphorus in the environment. The atomic composition of proteomes differs significantly among Prochlorococcus strains with different supplies of nitrogen in vivo; these different supplies result from different capacities for nitrogen assimilation. We repeated our whole-proteome analysis with the core proteomes of Prochlorococcus and obtained similar results. Our findings indicate that the elemental composition of proteomes is shaped by the availability of resources in the environment.

Background In animals, the moss Physcomitrella patens and the pollen of Arabidopsis thaliana, highly expressed genes have shorter introns than weakly expressed genes. A popular explanation for this is selection for transcription efficiency, which includes two sub-hypotheses: to minimize the energetic cost or to minimize the time cost. Results In an individual human, different organs may differ up to hundreds of times in cell number (for example, a liver versus a hypothalamus). Considered at the individual level, a gene specifically expressed in a large organ is actually transcribed tens or hundreds of times more than a gene with a similar expression level (a measure of mRNA abundance per cell) specifically expressed in a small organ. According to the energetic cost hypothesis, the former should have shorter introns than the latter. However, in humans and mice we have not found significant differences in intron length between large-tissue/organ-specific genes and small-tissue/organ-specific genes with similar expression levels. Qualitative estimation shows that the deleterious effect (that is, the energetic burden) of long introns in highly expressed genes is too negligible to be efficiently selected against in mammals. Conclusion The short introns in highly expressed genes should not be attributed to energy constraint. We evaluated evidence for the time cost hypothesis and other alternatives.

Background Accumulating evidence indicates that the nascent RNA can invade and pair with one strand of DNA, forming an R-loop structure that threatens the stability of the genome. In addition, the cost and benefit of introns are still in debate. Results At least three factors are likely required for the R-loop formation: 1) sequence complementarity between the nascent RNA and the target DNA, 2) spatial juxtaposition between the nascent RNA and the template DNA, and 3) accessibility of the template DNA and the nascent RNA. The removal of introns from pre-mRNA reduces the complementarity between RNA and the template DNA and avoids the spatial juxtaposition between the nascent RNA and the template DNA. In addition, the secondary structures of group I and group II introns may act as spatial obstacles for the formation of R-loops between nearby exons and the genomic DNA. Conclusion Organisms may benefit from introns by avoiding deleterious R-loops. The potential contribution of this benefit in driving intron evolution is discussed. I propose that additional RNA polymerases may inhibit R-loop formation between preceding nascent RNA and the template DNA. This idea leads to a testable prediction: intermittently transcribed genes and genes with frequently prolonged transcription should have higher intron density. Reviewers This article was reviewed by Dr. Eugene V. Koonin, Dr. Alexei Fedorov (nominated by Dr. Laura F Landweber), and Dr. Scott W. Roy (nominated by Dr. Arcady Mushegian).

Both transcription-associated and replication-associated strand compositional asymmetries have recently been shown in vertebrate genomes. In this paper, we illustrate that transcription-associated strand compositional asymmetries and replication-associated ones coexist in most vertebrate large genes, although in most case the former conceals the latter. Furthermore, we found that the transcription-associated strand compositional asymmetries of housekeeping genes are stronger than those of somatic cell expressed genes. Together with other evidence, we suggest that germline transcription-associated strand asymmetric mutations may be the main cause of the transcription-associated strand compositional asymmetries.

Multicellular eukaryotes that have high intron density have their introns almost evenly distributed within genes, but unicellular eukaryotes that are generally intron poor have their introns asymmetrically distributed toward the 5' ends of genes. This was explained by homologous recombination of genomic DNA with the cDNA reverse transcribed from the 3' polyadenylated tail of spliced mRNA. This paper is to study whether mRNA-mediated intron losses have ever occurred in multicellular eukaryotes. If intron losses were mRNA-mediated, adjacent introns should be commonly lost together. A direct result is fusion of several previously adjacent exons and producing a large exon. We found that extraordinarily large exons (ELEs) are common not only in unicellular eukaryotes but also in multicellular eukaryotes. The percentage of genes having ELEs is negatively correlated with intron abundance. In addition, the number of lost introns estimated from the relative lengths of ELEs is negatively correlated with the number of extant introns. These results support mRNA-mediated intron losses in all eukaryotes. Moreover, we found that the ELEs of intron-common eukaryotes (with more than 0.5 intron per gene on average) are not only located at 3' ends but also at 5' ends and the middle of genes. This is contrary to what would be expected if the involved cDNAs were reverse transcribed from the 3' polyadenosine ends. A remarkable difference in intron distribution was revealed between intron-rare eukaryotes and intron-common eukaryotes. The intron-rare eukaryotes show very strong 5'-biased intron distribution, whereas the intron-common eukaryotes display even intron distribution or only weak 5'-biased distribution. We suspected that intron losses from 3' end of genes may be limited in intron-rare eukaryotes. The intron losses from intron-common eukaryotes should have other priming mechanism, like self-primed reverse transcription.

Background The costs and benefits of spliceosomal introns in eukaryotes have not been established. One recognized effect of intron splicing is its known enhancement of gene expression. However, the mechanism regulating such splicing-mediated expression enhancement has not been defined. Previous studies have shown that intron splicing is a time-consuming process, indicating that splicing may not reduce the time required for transcription and processing of spliced pre-mRNA molecules; rather, it might facilitate the later rounds of transcription. Because the densities of active RNA polymerase II on most genes are less than one molecule per gene, direct interactions between the splicing apparatus and transcriptional complexes (from the later rounds of transcription) are infrequent, and thus unlikely to account for splicing-mediated gene expression enhancement. Presentation of the hypothesis The serine/arginine-rich protein SF2/ASF can inhibit the DNA topoisomerase I activity that removes negative supercoiling of DNA generated by transcription. Consequently, splicing could make genes more receptive to RNA polymerase II during the later rounds of transcription, and thus affect the frequency of gene transcription. Compared with the transcriptional enhancement mediated by strong promoters, intron-containing genes experience a lower frequency of cut-and-paste processes. The cleavage and religation activity of DNA strands by DNA topoisomerase I was recently shown to account for transcription-associated mutagenesis. Therefore, intron-mediated enhancement of gene expression could reduce transcription-associated genome instability. Testing the hypothesis Experimentally test whether transcription-associated mutagenesis is lower in intron-containing genes than in intronless genes. Use bioinformatic analysis to check whether exons flanking lost introns have higher frequencies of short deletions. Implications of the hypothesis The mechanism of intron-mediated enhancement proposed here may also explain the positive correlation observed between intron size and gene expression levels in unicellular organisms, and the greater number of intron containing genes in higher organisms. Reviewers This article was reviewed by Dr Arcady Mushegian, Dr Igor B Rogozin (nominated by Dr I King Jordan) and Dr Alexey S Kondrashov. For the full reviews, please go to the Reviewer's Reports section.

Background Previous studies have indicated that the wide variation in intron density (the number of introns per gene) among different eukaryotes largely reflects varying degrees of intron loss during evolution. The most popular model, which suggests that organisms lose introns through a mechanism in which reverse-transcribed cDNA recombines with the genomic DNA, concerns only one mutational force. Hypothesis Using exons as the units of splicing-site recognition, exon definition constrains the length of exons. An intron-loss event results in fusion of flanking exons and thus a larger exon. The large size of the newborn exon may cause splicing errors, i.e., exon skipping, if the splicing of pre-mRNAs is initiated by exon definition. By contrast, if the splicing of pre-mRNAs is initiated by intron definition, intron loss does not matter. Exon definition may thus be a selective force against intron loss. An organism with a high frequency of exon definition is expected to experience a low rate of intron loss throughout evolution and have a high density of spliceosomal introns. Conclusion The majority of spliceosomal introns in vertebrates may be maintained during evolution not because of potential functions, but because of their splicing mechanism (i.e., exon definition). Further research is required to determine whether exon definition is a negative force in maintaining the high intron density of vertebrates. Reviewers This article was reviewed by Dr. Scott W. Roy (nominated by Dr. John Logsdon), Dr. Eugene V. Koonin, and Dr. Igor B. Rogozin (nominated by Dr. Mikhail Gelfand). For the full reviews, please go to the Reviewers' comments section.