Association of intron loss with high mutation rate in Arabidopsis: implications for genome size evolution

Genome-wide identification of bHLH gene family and its response to cadmium stress in Populus × canescens

The basic helix-loop-helix (bHLH) gene family is integral to various aspects of plant development and the orchestration of stress response. This study focuses on the bHLH genes within Populus × canescens, a poplar species noted for its significant tolerance to cadmium (Cd) stress. Through our comprehensive genomic analysis, we have identified and characterized 170 bHLH genes within the P. canescens genome. These genes have been systematically classified into 22 distant subfamilies based on their evolutionary relationships. A notable conservation in gene structure and motif compositions were conserved across these subfamilies. Further analysis of the promoter regions of these genes revealed an abundance of essential cis-acting element, which are associated with plant hormonal regulation, development processes, and stress response pathway. Utilizing quantitative PCR (qPCR), we have documented the differential regulation of PcbHLHs in response to elevated Cd concentrations, with distinct expression patterns observed across various tissues. This study is poised to unravel the molecular mechanism underpinning Cd tolerance in P. canescens, offering valuable insights for the development of new cultivars with enhanced Cd accumulation capacity and tolerance. Such advancements are crucial for implementing effective phytoremediation strategies to mitigate soil pollution caused by Cd.

Toll like receptor 4 (TLR4) gene polymorphism and its association with somatic cell score and milk production traits in Indian dromedary camels

Our study aimed to explore the genetic variation in the Toll-like receptor 4 (TLR4) gene and establish its association with somatic cell score (SCS) and milk production traits in four Indian camel breeds namely Bikaneri, Kachchhi, Jaisalmeri and Mewari. TLR4 gene fragment of 573 bp spanning 5' UTR, exon-1 and partial intron-1 region was amplified and genotyped using the PCR-sequence based typing method. Only one SNP located at position C472T was identified. Genotyping revealed two alleles (C and T) and three genotypes: CC, CT and TT. The genotype frequencies for CC, CT and TT were 0.116, 0.326 and 0.558 and allele frequencies for C and T alleles were 0.279 and 0.721, respectively. Association study inferred that the effect of genotype on SCS, lactation yield (LY) and peak yield (PY) was non-significant however heterozygote (CT) genotypes recorded lower SCS and higher LY and PY. It can be concluded that the TLR4 gene possesses limited genetic variation, depicting polymorphism at a single locus in Indian camel breeds with a predominance of the TT genotype. The association study indicated that heterozygote animals possess better udder health and production performance, the statistical significance of which needs to be established using a large data set.

C and G are frequently mutated into T and A in coding regions of human genes

Nucleotide mutations in human genes have long been a hot subject for study because some of them may lead to severe human diseases. Understanding the general mutational process and evolutionary trend of human genes could help answer such questions as why certain diseases occur and what challenges we face in protecting human health. In this study, we conducted statistics on 89,895 single-nucleotide variations identified in coding regions of 18,339 human genes. The results show that C and G are frequently mutated into T and A in human genes. C/G (C or G)-to-T/A mutations lead to reduction of hydrogen bonds in double-stranded DNA because C-G and T-A base pairs are maintained by three and two hydrogen bonds respectively. C-to-T and G-to-A mutations occur predominantly in human genes because they not only reduce hydrogen bonds but also belong to transition mutation. Reduction of hydrogen bonds could reduce energy consumption not only in separating double strands of mutated DNA for transcription and replication but also in disrupting stem-loop structure of mutated mRNA for translation. It is thus considered that to reduce hydrogen bonds (and thus to reduce energy consumption in gene expression) is one of the driving forces for nucleotide mutation. Moreover, codon mutation is positively correlated to its content, suggesting that most mutations are not targeted on changing any specific codons (amino acids) but are merely for reducing hydrogen bonds. Our study provides an example of utilizing single-nucleotide variation data to infer evolutionary trend of human genes, which can be referenced to conduct similar studies in other organisms.

Genetic variants of CSN1S1, CSN2, CSN3, and BLG genes and their association with dairy production traits in Sahiwal cattle and Nili-Ravi buffaloes

Milk protein genes are associated with milk yield and composition in dairy animals. The present study aimed to identify milk protein genes (CSN1S1, CSN2, CSN3, and BLG) genetic variants and their association with milk yield in Sahiwal cattle and Nili-Ravi buffaloes. One hundred animals from each species were selected to collect blood samples and milk production records. Primers were designed for these milk protein genes for PCR amplification. Sequencing of resultant PCR products revealed a higher number of SNPs (13 vs. 7, 5 vs. 1, and 6 vs. 2) in Sahiwal as compared to Nili-Ravi animals in CSN1S1, CSN2, and CSN3 genes, respectively. However, a single SNP was observed in BLG gene of both species. Association analysis revealed that one SNP in BLG gene of Nili-Ravi was associated (p < 0.05) with 305-day milk yield. Two SNPs at CSN1S1 gene in Sahiwal were associated with dry-period. Similarly, one SNP at CSN1S1 and two SNPs at CSN3 gene showed significant association (p < 0.05) with average calving-interval in Sahiwal while two SNPs in CSN1S1 gene were associated (p < 0.05) with this trait in Nili-Ravi. These SNPs could be helpful as candidate variants for marker-assisted selection in cattle and buffaloes for improvement of lactation performance.

Gene Structural Specificity and Expression of MADS-Box Gene Family in Camellia chekiangoleosa

MADS-box genes encode transcription factors that affect plant growth and development. Camellia chekiangoleosa is an oil tree species with ornamental value, but there have been few molecular biological studies on the developmental regulation of this species. To explore their possible role in C. chekiangoleosa and lay a foundation for subsequent research, 89 MADS-box genes were identified across the whole genome of C. chekiangoleosa for the first time. These genes were present on all the chromosomes and were found to have expanded by tandem duplication and fragment duplication. Based on the results of a phylogenetic analysis, the 89 MADS-box genes could be divided into either type I (38) or type II (51). Both the number and proportion of the type II genes were significantly greater than those of Camellia sinensis and Arabidopsis thaliana, indicating that C. chekiangoleosa type II genes experienced a higher duplication rate or a lower loss rate. The results of both a sequence alignment and a conserved motif analysis suggest that the type II genes are more conserved, meaning that they may have originated and differentiated earlier than the type I genes did. At the same time, the presence of extra-long amino acid sequences may be an important feature of C. chekiangoleosa. Gene structure analysis revealed the number of introns of MADS-box genes: twenty-one type I genes had no introns, and 13 type I genes contained only 1~2 introns. The type II genes have far more introns and longer introns than the type I genes do. Some MIKC^C genes have super large introns (≥15 kb), which are rare in other species. The super large introns of these MIKC^C genes may indicate richer gene expression. Moreover, the results of a qPCR expression analysis of the roots, flowers, leaves and seeds of C. chekiangoleosa showed that the MADS-box genes were expressed in all those tissues. Overall, compared with that of the type I genes, the expression of the type II genes was significantly higher. The CchMADS31 and CchMADS58 genes (type II) were highly expressed specifically in the flowers, which may in turn regulate the size of the flower meristem and petals. CchMADS55 was expressed specifically in the seeds, which might affect seed development. This study provides additional information for the functional characterization of the MADS-box gene family and lays an important foundation for in-depth study of related genes, such as those involved in the development of the reproductive organs of C. chekiangoleosa.

Interplay between gene expression and gene architecture as a consequence of gene and genome duplications: evidence from metabolic genes of Arabidopsis thaliana

Gene and genome duplications have been widespread during the evolution of flowering plant which resulted in the increment of biological complexity as well as creation of plasticity of a genome helping the species to adapt to changing environments. Duplicated genes with higher evolutionary rates can act as a mechanism of generating novel functions in secondary metabolism. In this study, we explored duplication as a potential factor governing the expression heterogeneity and gene architecture of Primary Metabolic Genes (PMGs) and Secondary Metabolic Genes (SMGs) of Arabidopsis thaliana. It is remarkable that different types of duplication processes controlled gene expression and tissue specificity differently in PMGs and SMGs. A complex relationship exists between gene architecture and expression patterns of primary and secondary metabolic genes. Our study reflects, expression heterogeneity and gene structure variation of primary and secondary metabolism in Arabidopsis thaliana are partly results of duplication events of different origins. Our study suggests that duplication has differential effect on PMGs and SMGs regarding expression pattern by controlling gene structure, epigenetic modifications, multifunctionality and subcellular compartmentalization. This study provides an insight into the evolution of metabolism in plants in the light of gene and genome scale duplication.

Supplementary Information

The online version contains supplementary material available at 10.1007/s12298-022-01188-2.

Bone marrow stromal antigen 2 (BST-2) genetic variants influence expression levels and disease outcome in HIV-1 chronically infected patients

Background

Bone marrow stromal antigen 2 (BST-2) also known as Tetherin (CD317/HM1.24), is a host restriction factor that blocks the release of HIV-1 virions from infected cells. Previous studies reported that BST-2 genetic variants or single nucleotide polymorphims (SNPs) have a preventative role during HIV-1 infection. However, the influence of BST-2 SNPs on expression levels remains unknown. In this study, we investigated the influence of BST-2 SNPs on expression levels and disease outcome in HIV-1 subtype C chronically infected antiretroviral therapy naïve individuals.

Results

We quantified BST-2 mRNA levels in peripheral blood mononuclear cells (PBMCs), determined BST-2 protein expression on the surface of CD4⁺ T cells using flow cytometry and genotyped two intronic single nucleotide polymorphisms (SNPs) rs919267 and rs919266 together with one SNP rs9576 located in the 3’ untranslated region (UTR) of bst-2 gene using TaqMan assays from HIV-1 uninfected and infected participants. Subsequently, we determined the ability of plasma antibody levels to mediate antibody-dependent cellular phagocytosis (ADCP) using gp120 consensus C and p24 subtype B/C protein. Fc receptor-mediated NK cell degranulation was evaluated as a surrogate for ADCC activity using plasma from HIV-1 positive participants. BST-2 mRNA expression levels in PBMCs and protein levels on CD4⁺ T cells were lower in HIV-1 infected compared to uninfected participants (p = 0.075 and p < 0.001, respectively). rs919267CT (p = 0.042) and rs919267TT (p = 0.045) were associated with lower BST-2 mRNA expression levels compared to rs919267CC in HIV-1 uninfected participants. In HIV-1 infected participants, rs919267CT associated with lower CD4 counts, (p = 0.003), gp120-IgG1 (p = 0.040), gp120-IgG3 (p = 0.016) levels but higher viral loads (p = 0.001) while rs919267TT was associated with lower BST-2 mRNA levels (p = 0.046), CD4 counts (p = 0.001), gp120-IgG1 levels (p = 0.033) but higher plasma viral loads (p = 0.007). Conversely, rs9576CA was associated with higher BST-2 mRNA expression levels (p = 0.027), CD4 counts (p = 0.079), gp120-IgG1 (p = 0.009), gp120-IgG3 (p = 0.039) levels but with lower viral loads (p = 0.037).

Conclusion

Our findings show that bst-2 SNPs mediate BST-2 expression and disease outcome, correlate with gp120-IgG1, gp120-IgG3 levels but not p24-IgG levels, ADCC and ADCP activity.

Graphical Abstract

Supplementary Information

The online version contains supplementary material available at 10.1186/s12977-022-00588-2.

Costly circRNAs, Effective Population Size, and the Origins of Molecular Complexity

While much excitement has attended the discovery and study of circular RNAs, a new study in Cell Reports suggests that most mammalian circRNAs are not only functionless, but in fact costly. Comparison across three species is also consistent with the influential but rarely tested Drift-Barrier Hypothesis of molecular complexity. According to this hypothesis, nonessential genomic elements are slightly deleterious elements that fix by genetic drift and, thus, are generally more abundant in species with small effective population sizes. I discuss the implications of these new results for the Drift-Barrier hypothesis. In particular, I note the distinction between two classes of genomic elements, based on whether they are created by 'standard' small-scale mutations (basepair substitutions, indels, etc.) or larger, more idiosyncratic mutations (segmental duplications, transposable element propagation, etc.) I suggest that the Drift-Barrier Hypothesis is likely to apply to the former class, but perhaps not the latter class.

Deciphering the association of intronic single nucleotide polymorphisms of crystallin gene family with congenital cataract

Purpose

Introns play an important role in gene regulation and expression. Single nucleotide polymorphisms (SNPs) in introns have the potential to cause disease and alter the genotype-phenotype association. Hence, this study aimed to decipher the association of SNPs in the introns of the crystallin gene in congenital cataracts.

Methods

SNPs in the introns of crystallin gene family - CRYAA (rs3788059), CRYAB (rs2070894), CRYBA4 (rs2071861), and CRYBB2 (rs5752083, rs5996863) - were genotyped in 248 participants consisting of 141 congenital cataracts and 107 healthy controls by allele-specific oligonucleotide polymerase chain reaction method. Around 10% of samples for each SNPs were sequenced to confirm the genotypes. The allele, genotype, and haplotype frequency were evaluated by the SHEsis online tool.

Results

Using dominant model, the "A" allele of rs3788059 was found to have an increased risk toward congenital cataract development whereas the "G" allele was found to be protective (AA + AG vs. GG; odds ratio [OR] 95% confidence interval [CI] = 3.73 [1.71, 8.15], P = 0.0009). The "A" allele of both rs2070894 (AA + AG vs. GG; OR [95% CI] = 0.49 [0.29, 0.84], P = 0.012) and rs5752083 (AA + AC vs. CC; OR [95% CI] = 0.25 [0.08, 0.76], P = 0.016) were suggested to have a protective role by the dominant model. The A-C-T haplotype (rs2071861, rs5752083, and rs5996863) was found to be a significant risk factor for the development of congenital cataract.

Conclusion

Intronic SNPs in crystallin genes may play a role in the predisposition toward congenital cataract. However, the present findings need to be replicated in a large cohort with more number of samples.

An Intron of Invertebrate Microphthalmia Transcription Factor Gene Is Evolved from a Longer Ancestral Sequence

Introns are highly variable in number and size. Sequence simulation is an effective method to elucidate intron evolution patterns. Previously, we have reported that introns are more likely to evolve through mutation-and-deletion (MD) rather than through mutation-and-insertion (MI). In the present study, we further studied evolution models by allowing insertion in the MD model and by allowing deletion in the MI model at various frequencies. It was found that all deletion-biased models with proper parameter settings could generate sequences with attributes matchable to 16 invertebrate introns from the microphthalmia transcription factor gene, whereas all insertion-biased models with any parameter settings failed to generate such sequences. We conclude that the examined invertebrate introns may have evolved from a longer ancestral sequence in a deletion-biased pattern. The constructed models are useful for studying the evolution of introns from other genes and/or from other taxonomic groups. (C++ scripts of all deletion- and insertion-biased models are available upon request.)

Development of Molecular Marker Linked with Bacterial Fruit Blotch Resistance in Melon (Cucumis melo L.)

Bacterial fruit blotch (BFB) causes losses in melon marketable yield. However, until now, there has been no information about the genetic loci responsible for resistance to the disease or their pattern of inheritance. We determined the inheritance pattern of BFB resistance from a segregating population of 491 F₂ individuals raised by crossing BFB-resistant (PI 353814) and susceptible (PI 614596) parental accessions. All F₁ plants were resistant to Acidovorax citrulli strain KACC18782, and F₂ plants segregated with a 3:1 ratio for resistant and susceptible phenotypes, respectively, in a seedling bioassay experiment, indicating that BFB resistance is controlled by a monogenic dominant gene. In an investigation of 57 putative disease-resistance related genes across the melon genome, only the MELO3C022157 gene (encoding TIR-NBS-LRR domain), showing polymorphism between resistant and susceptible parents, revealed as a good candidate for further investigation. Cloning, sequencing and quantitative RT-PCR expression of the polymorphic gene MELO3C022157 located on chromosome 9 revealed multiple insertion/deletions (InDels) and single nucleotide polymorphisms (SNPs), of which the SNP A²⁰³⁵T in the second exon of the gene caused loss of the LRR domain and truncated protein in the susceptible accession. The InDel marker MB157-2, based on the large (504 bp) insertion in the first intron of the susceptible accession, was able to distinguish resistant and susceptible accessions among 491 F₂ and 22 landraces/inbred accessions with 98.17% and 100% detection accuracy, respectively. This novel PCR-based, co-dominant InDel marker represents a practical tool for marker-assisted breeding aimed at developing BFB-resistant melon accessions.

Simulation of Chordate Intron Evolution Using Randomly Generated and Mutated Base Sequences

Introns are well known for their high variation not only in length but also in base sequence. The evolution of intron sequences has aroused broad interest in the past decades. However, very little is known about the evolutionary pattern of introns due to the lack of efficient analytical method. In this study, we designed 2 evolutionary models, that is, mutation-and-deletion (MD) and mutation-and-insertion (MI), to simulate intron evolution using randomly generated and mutated bases by referencing to the phylogenetic tree constructed using 14 chordate introns from TF4 (transcription factor-like protein 4) gene. A comparison of attributes between model-generated sequences and chordate introns showed that the MD model with proper parameter settings could generate sequences that have attributes matchable to chordate introns, whereas the MI model with any parameter settings failed in doing so. These data suggest that the surveyed chordate introns have evolved from a long ancestral sequence through gradual reduction in length. The established methodology provides an effective measure to study the evolutionary pattern of intron sequences from organisms of various taxonomic groups. (C++ scripts of MD and MI models are available upon request.).

Inheritance Pattern and Molecular Markers for Resistance to Blackleg Disease in Cabbage

The inheritance and causal loci for resistance to blackleg, a devastating disease of Brassicaceous crops, are yet to be known in cabbage (Brassica oleracea L.). Here, we report the pattern of inheritance and linked molecular marker for this trait. A segregating BC₁ population consisting of 253 plants was raised from resistant and susceptible parents, L29 (♀) and L16 (♂), respectively. Cotyledon resistance bioassay of BC₁ population, measured based on a scale of 0-9 at 12 days after inoculation with Leptosphaeria maculans isolate 03-02 s, revealed the segregation of resistance and ratio, indicative of dominant monogenic control of the trait. Investigation of potential polymorphism in the previously identified differentially expressed genes within the collinear region of 'B. napus blackleg resistant loci Rlm1' in B. oleracea identified two insertion/deletion (InDel) mutations in the intron and numerous single nucleotide polymorphisms (SNPs) throughout the LRR-RLK gene Bol040029, of which six SNPs in the first exon caused the loss of two LRR domains in the susceptible line. An InDel marker, BLR-C-InDel based on the InDel mutations, and a high resolution melting (HRM) marker, BLR-C-2808 based on the SNP C²⁸⁰⁸T in the second exon were developed, which predicated the resistance status of the BC₁ population with 80.24%, and of 24 commercial inbred lines with 100% detection accuracy. This is the first report of inheritance and molecular markers linked with blackleg resistance in cabbage. This study will enhance our understanding of the trait, and will be helpful in marker assisted breeding aiming at developing resistant cabbage varieties.

Dates and rates in grape's plastomes: evolution in slow motion

The family Vitaceae includes the domesticated grapevine (Vitis vinifera), one of the most economically important crops in the world. Despite the importance of Vitaceae, there is still considerable controversy surrounding their phylogenetic relationships and evolutionary timescales. Moreover, variation in rates of molecular evolution among Vitaceae remains mostly unexplored. The present research aims to fill these knowledge gaps through the analysis of plastome sequences. Thirteen newly sequenced grape plastomes are presented and their phylogenetic relationships examined. Divergence times and absolute substitution rates are inferred under different molecular clocks by the analysis of 95 non-coding plastid regions and 43 representative accessions of the major lineages of Vitaceae. Furthermore, the phylogenetic informativeness of non-coding plastid regions is investigated. We find strong evidence in favor of the random local clock model and rate heterogeneity within Vitaceae. Substitution rates decelerate in Ampelocissus, Ampelopsis, Nekemias, Parthenocissus, Rhoicissus, and Vitis, with genus Vitis showing the lowest values up to a minimum of ~ 4.65 × 10^-11 s/s/y. We suggest that liana-like species of Vitaceae evolve slower than erect growth habit plants and we invoke the "rate of mitosis hypothesis" to explain the observed pattern of the substitution rates. We identify a reduced set of 20 non-coding regions able to accurately reconstruct the phylogeny of Vitaceae and we provide a detailed description of all 152 non-coding regions identified in the plastomes of subg. Vitis. These polymorphic regions will find their applications in phylogenetics, phylogeography, and population genetics as well in grapes identification through DNA barcoding techniques.

Small, but surprisingly repetitive genomes: transposon expansion and not polyploidy has driven a doubling in genome size in a metazoan species complex

BACKGROUND

The causes and consequences of genome size variation across Eukaryotes, which spans five orders of magnitude, have been hotly debated since before the advent of genome sequencing. Previous studies have mostly examined variation among larger taxonomic units (e.g., orders, or genera), while comparisons among closely related species are rare. Rotifers of the Brachionus plicatilis species complex exhibit a seven-fold variation in genome size and thus represent a unique opportunity to study such changes on a relatively short evolutionary timescale. Here, we sequenced and analysed the genomes of four species of this complex with nuclear DNA contents spanning 110-422 Mbp. To establish the likely mechanisms of genome size change, we analysed both sequencing read libraries and assemblies for signatures of polyploidy and repetitive element content. We also compared these genomes to that of B. calyciflorus, the closest relative with a sequenced genome (293 Mbp nuclear DNA content).

RESULTS

Despite the very large differences in genome size, we saw no evidence of ploidy level changes across the B. plicatilis complex. However, repetitive element content explained a large portion of genome size variation (at least 54%). The species with the largest genome, B. asplanchnoidis, has a strikingly high 44% repetitive element content, while the smaller B. plicatilis genomes contain between 14 and 25% repetitive elements. According to our analyses, the B. calyciflorus genome contains 39% repetitive elements, which is substantially higher than previously reported (21%), and suggests that high repetitive element load could be widespread in monogonont rotifers.

CONCLUSIONS

Even though the genome sizes of these species are at the low end of the metazoan spectrum, their genomes contain substantial amounts of repetitive elements. Polyploidy does not appear to play a role in genome size variations in these species, and these variations can be mostly explained by changes in repetitive element content. This contradicts the naïve expectation that small genomes are streamlined, or less complex, and that large variations in nuclear DNA content between closely related species are due to polyploidy.

Genome-wide identification and analysis of the EIN3/EIL gene family in allotetraploid Brassica napus reveal its potential advantages during polyploidization

BACKGROUND

Polyploidization is a common event in the evolutionary history of angiosperms, and there will be some changes in the genomes of plants other than a simple genomic doubling after polyploidization. Allotetraploid Brassica napus and its diploid progenitors (B. rapa and B. oleracea) are a good group for studying the problems associated with polyploidization. On the other hand, the EIN3/EIL gene family is an important gene family in plants, all members of which are key genes in the ethylene signaling pathway. Until now, the EIN3/EIL gene family in B. napus and its diploid progenitors have been largely unknown, so it is necessary to comprehensively identify and analyze this gene family.

RESULTS

In this study, 13, 7 and 7 EIN3/EIL genes were identified in B. napus (2n = 4x = 38, A_nC_n), B. rapa (2n = 2x = 20, A_r) and B. oleracea (2n = 2x = 18, C_o). All of the identified EIN3/EIL proteins were divided into 3 clades and further divided into 8 sub-clades. Ka/Ks analysis showed that all identified EIN3/EIL genes underwent purifying selection after the duplication events. Moreover, gene structure analysis showed that some EIN3/EIL genes in B. napus acquired introns during polyploidization, and homolog expression bias analysis showed that B. napus was biased towards its diploid progenitor B. rapa. The promoters of the EIN3/EIL genes in B. napus contained more cis-acting elements, which were mainly involved in endosperm gene expression and light responsiveness, than its diploid progenitors. Thus, B. napus might have potential advantages in some biological aspects.

CONCLUSIONS

The results indicated allotetraploid B. napus might have potential advantages in some biological aspects. Moreover, our results can increase the understanding of the evolution of the EIN3/EIL gene family in B. napus, and provided more reference for future research about polyploidization.

Multivariate analysis of genomic variables, effective population size, and mutation rate

Objective

The relationship between genomic variables (genome size, gene number, intron size, and intron number) and evolutionary forces has two implications. First, they help to unravel the mechanism underlying genome evolution. Second, they provide a solution to the debate over discrepancy between genome size variation and organismal complexity. Previously, a clear correlation between genomic variables and effective population size and mutation rate (Neu) led to an important hypothesis to consider random genetic drift as a major evolutionary force during evolution of genome size and complexity. But recent reports also support natural selection as the leading evolutionary force. As such, the debate remains unresolved.

Results

Here, we used a multivariate method to explore the relationship between genomic variables and Neu in order to understand the evolution of genome. Previously reported patterns between genomic variables and Neu were not observed in our multivariate study. We found only one association between intron number and Neu, but no relationships were observed between genome size, intron size, gene number, and Neu, suggesting that Neu of the organisms solely does not influence genome evolution. We, therefore, concluded that Neu influences intron evolution, while it may not be the only force that provides mechanistic insights into genome evolution and complexity.

Electronic supplementary material

The online version of this article (10.1186/s13104-019-4097-3) contains supplementary material, which is available to authorized users.

Microsporidian Introns Retained against a Background of Genome Reduction: Characterization of an Unusual Set of Introns

Spliceosomal introns are ubiquitous features of eukaryotic genomes, but the mechanisms responsible for their loss and gain are difficult to identify. Microsporidia are obligate intracellular parasites that have significantly reduced genomes and, as a result, have lost many if not all of their introns. In the microsporidian Encephalitozoon cuniculi, a relatively long intron was identified and was spliced at higher levels than the remaining introns. This long intron is part of a set of unique introns in two unrelated genes that show high levels of sequence conservation across diverse microsporidia. The introns possess a unique internal conserved region, which overlaps with a shared, predicted stem–loop structure. The unusual similarity and retention of these long introns in reduced microsporidian genomes could indicate that these introns function similarly, are homologous, or both. Regardless, the significant genome reduction in microsporidia provides a rare opportunity to understand intron evolution.

Higher Rates of Protein Evolution in the Self-Fertilizing Plant Arabidopsis thaliana than in the Out-Crossers Arabidopsis lyrata and Arabidopsis halleri

The common transition from out-crossing to self-fertilization in plants decreases effective population size. This is expected to result in a reduced efficacy of natural selection and in increased rates of protein evolution in selfing plants compared with their outcrossing congeners. Prior analyses, based on a very limited number of genes, detected no differences between the rates of protein evolution in the selfing Arabidopsis thaliana compared with the out-crosser Arabidopsis lyrata. Here, we reevaluate this trend using the complete genomes of A. thaliana, A. lyrata, Arabidopsis halleri, and the outgroups Capsella rubella and Thellungiella parvula. Our analyses indicate slightly but measurably higher nonsynonymous divergences (dN), synonymous divergences (dS) and dN/dS ratios in A. thaliana compared with the other Arabidopsis species, indicating that purifying selection is indeed less efficacious in A. thaliana.

Current Bacterial Gene Encoding Capsule Biosynthesis Protein CapI Contains Nucleotides Derived from Exonization

Since the proposition of introns-early hypothesis, although many studies have shown that most eukaryotic ancestors possessed intron-rich genomes, evidence of intron existence in genomes of ancestral bacteria has still been absent. While not a single intron has been found in all protein-coding genes of current bacteria, analyses on bacterial genes horizontally transferred into eukaryotes at ancient time may provide evidence of intron existence in bacterial ancestors. In this study, a bacterial gene encoding capsule biosynthesis protein CapI was found in the genome of sea anemone, Nematostella vectensis. This horizontally transferred gene contains a phase 1 intron of 40 base pairs. The nucleotides of this intron have high sequence identity with those encoding amino acids in current bacterial CapI gene, indicating that the intron and the amino acid-coding nucleotides are originated from the same ancestor sequence. Moreover, 5′-splice site of this intron is located in a GT-poor region associated with a closely following AG-rich region, suggesting that deletion mutation at 5′-splice site has been employed to remove this intron and the intron-like amino acid-coding nucleotides in current bacterial CapI gene are derived from exonization. These data suggest that bacterial CapI gene contained intron(s) at ancient time. This is the first report providing the result of sequence analysis to suggest possible existence of spliceosomal introns in ancestral bacterial genes. The methodology employed in this study may be used to identify more such evidence that would aid in settlement of the dispute between introns-early and introns-late theories.

Functional Promiscuity of Two Divergent Paralogs of Type III Plant Polyketide Synthases

Plants effectively defend themselves against biotic and abiotic stresses by synthesizing diverse secondary metabolites, including health-protective flavonoids. These display incredible chemical diversity and ubiquitous occurrence and confer impeccable biological and agricultural applications. Chalcone synthase (CHS), a type III plant polyketide synthase, is critical for flavonoid biosynthesis. It catalyzes acyl-coenzyme A thioesters to synthesize naringenin chalcone through a polyketidic intermediate. The functional divergence among the evolutionarily generated members of a gene family is pivotal in driving the chemical diversity. Against this backdrop, this study was aimed to functionally characterize members of the CHS gene family from Rheum emodi, an endangered and endemic high-altitude medicinal herb of northwestern Himalayas. Two full-length cDNAs (1,179 bp each), ReCHS1 and ReCHS2, encoding unique paralogs were isolated and characterized. Heterologous expression and purification in Escherichia coli, bottom-up proteomic characterization, high-performance liquid chromatography-electrospray ionization-tandem mass spectrometry analysis, and enzyme kinetic studies using five different substrates confirmed their catalytic potential. Phylogenetic analysis revealed the existence of higher synonymous mutations in the intronless divergents of ReCHS. ReCHS2 displayed significant enzymatic efficiency (Vmax/Km) with different substrates. There were significant spatial and altitudinal variations in messenger RNA transcript levels of ReCHSs correlating positively with metabolite accumulation. Furthermore, the elicitations in the form of methyl jasmonate, salicylic acid, ultraviolet B light, and wounding, chosen on the basis of identified cis-regulatory promoter elements, presented considerable differences in the transcript profiles of ReCHSs. Taken together, our results demonstrate differential propensities of CHS paralogs in terms of the accumulation of flavonoids and their relative substrate selectivities.

Genes of the de novo and Salvage Biosynthesis Pathways of Vitamin B6 are Regulated under Oxidative Stress in the Plant Pathogen Rhizoctonia solani

Vitamin B6 is recognized as an important cofactor required for numerous metabolic enzymes, and has been shown to act as an antioxidant and play a role in stress responses. It can be synthesized through two different routes: salvage and de novo pathways. However, little is known about the possible function of the vitamin B6 pathways in the fungal plant pathogen Rhizoctonia solani. Using genome walking, the de novo biosynthetic pathway genes; RsolPDX1 and RsolPDX2 and the salvage biosynthetic pathway gene, RsolPLR were sequenced. The predicted amino acid sequences of the three genes had high degrees of similarity to other fungal PDX1, PDX2, and PLR proteins and are closely related to other R. solani anastomosis groups. We also examined their regulation when subjected to reactive oxygen species (ROS) stress inducers, the superoxide generator paraquat, or H₂O₂, and compared it to the well-known antioxidant genes, catalase and glutathione-S-transferase (GST). The genes were differentially regulated with transcript levels as high as 33 fold depending on the gene and type of stress reflecting differences in the type of damage induced by ROS. Exogenous addition of the vitamers PN or PLP in culture medium significantly induced the transcription of the vitamin B6 de novo encoding genes as early as 0.5 hour post treatment (HPT). On the other hand, transcription of RsolPLR was vitamer-specific; a down regulation upon supplementation of PN and upregulation with PLP. Our results suggest that accumulation of ROS in R. solani mycelia is linked to transcriptional regulation of the three genes and implicate the vitamin B6 biosynthesis machinery in R. solani, similar to catalases and GST, as an antioxidant stress protector against oxidative stress.

Evaluation of the mechanisms of intron loss and gain in the social amoebae Dictyostelium

Background

Spliceosomal introns are a common feature of eukaryotic genomes. To approach a comprehensive understanding of intron evolution on Earth, studies should look beyond repeatedly studied groups such as animals, plants, and fungi. The slime mold Dictyostelium belongs to a supergroup of eukaryotes not covered in previous studies.

Results

We found 441 precise intron losses in Dictyostelium discoideum and 202 precise intron losses in Dictyostelium purpureum. Consistent with these observations, Dictyostelium discoideum was found to have significantly more copies of reverse transcriptase genes than Dictyostelium purpureum. We also found that the lost introns are significantly further from the 5′ end of genes than the conserved introns. Adjacent introns were prone to be lost simultaneously in Dictyostelium discoideum. In both Dictyostelium species, the exonic sequences flanking lost introns were found to have a significantly higher GC content than those flanking conserved introns. Together, these observations support a reverse-transcription model of intron loss in which intron losses were caused by gene conversion between genomic DNA and cDNA reverse transcribed from mature mRNA. We also identified two imprecise intron losses in Dictyostelium discoideum that may have resulted from genomic deletions. Ninety-eight putative intron gains were also observed. Consistent with previous studies of other lineages, the source sequences were found in only a small number of cases, with only two instances of intron gain identified in Dictyostelium discoideum.

Conclusions

Although they diverged very early from animals and fungi, Dictyostelium species have similar mechanisms of intron loss.

Electronic supplementary material

The online version of this article (doi:10.1186/s12862-015-0567-y) contains supplementary material, which is available to authorized users.

The dynamic loss and gain of introns during the evolution of the Brassicaceae

Sequence comparison allows the detailed analysis of evolution at the nucleotide and amino acid levels, but much less information is known about the structural evolution of genes, i.e. how the number, length and distribution of introns change over time. We constructed a parsimonious model for the evolutionary rate of intron loss (IL) and intron gain (IG) within the Brassicaceae and found that IL/IG has been highly dynamic, with substantial differences between and even within lineages. The divergence of the Brassicaceae lineages I and II marked a dramatic change in the IL rate, with the common ancestor of lineage I losing introns three times more rapidly than the common ancestor of lineage II. Our data also indicate a subsequent declining trend in the rate of IL, although in Arabidopsis thaliana introns continue to be lost at approximately the ancestral rate. Variations in the rate of IL/IG within lineage II have been even more remarkable. Brassica rapa appears to have lost introns approximately 15 times more rapidly than the common ancestor of B. rapa and Schenkiella parvula, and approximately 25 times more rapidly than its sister species Eutrema salsugineum. Microhomology was detected at the splice sites of several dynamic introns suggesting that the non-homologous end-joining and double-strand break repair is a common pathway underlying IL/IG in these species. We also detected molecular signatures typical of mRNA-mediated IL, but only in B. rapa.

Imprecise intron losses are less frequent than precise intron losses but are not rare in plants

Abstract

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.

Genome size evolution in macroparasites

Reduction in genome size has been associated not only with a parasitic lifestyle in intracellular microparasites but also in some macroparasitic insects and nematodes. We collected the available data on genome size for flatworms, annelids, nematodes and arthropods, compared those with available data for the phylogenetically closest free-living taxa and found evidence of smaller genome sizes for parasites in six of nine comparisons. Our results suggest that despite great differences in evolutionary history and life cycles, parasitism as a lifestyle promotes convergent genome size reduction in macroparasites. We discuss factors that could be associated with small genome size in parasites which require further exploration in the future.

Higher frequency of intron loss from the promoter proximally paused genes of Drosophila melanogaster

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.

Transposable elements and genome size variations in plants

Although the number of protein-coding genes is not highly variable between plant taxa, the DNA content in their genomes is highly variable, by as much as 2,056-fold from a 1C amount of 0.0648 pg to 132.5 pg. The mean 1C-value in plants is 2.4 pg, and genome size expansion/contraction is lineage-specific in plant taxonomy. Transposable element fractions in plant genomes are also variable, as low as ~3% in small genomes and as high as ~85% in large genomes, indicating that genome size is a linear function of transposable element content. Of the 2 classes of transposable elements, the dynamics of class 1 long terminal repeat (LTR) retrotransposons is a major contributor to the 1C value differences among plants. The activity of LTR retrotransposons is under the control of epigenetic suppressing mechanisms. Also, genome-purging mechanisms have been adopted to counter-balance the genome size amplification. With a wealth of information on whole-genome sequences in plant genomes, it was revealed that several genome-purging mechanisms have been employed, depending on plant taxa. Two genera, Lilium and Fritillaria, are known to have large genomes in angiosperms. There were twice times of concerted genome size evolutions in the family Liliaceae during the divergence of the current genera in Liliaceae. In addition to the LTR retrotransposons, non-LTR retrotransposons and satellite DNAs contributed to the huge genomes in the two genera by possible failure of genome counter-balancing mechanisms.

Antagonistic relationships between intron content and codon usage bias of genes in three mosquito species: functional and evolutionary implications

Genome biology of mosquitoes holds potential in developing knowledge-based control strategies against vectorborne diseases such as malaria, dengue, West Nile, and others. Although the genomes of three major vector mosquitoes have been sequenced, attempts to elucidate the relationship between intron and codon usage bias across species in phylogenetic contexts are limited. In this study, we investigated the relationship between intron content and codon bias of orthologous genes among three vector mosquito species. We found an antagonistic relationship between codon usage bias and the intron number of genes in each mosquito species. The pattern is further evident among the intronless and the intron-containing orthologous genes associated with either low or high codon bias among the three species. Furthermore, the covariance between codon bias and intron number has a directional component associated with the species phylogeny when compared with other nonmosquito insects. By applying a maximum likelihood-based continuous regression method, we show that codon bias and intron content of genes vary among the insects in a phylogeny-dependent manner, but with no evidence of adaptive radiation or species-specific adaptation. We discuss the functional and evolutionary significance of antagonistic relationships between intron content and codon bias.