The Cryphonectria parasitica mitochondrial rns gene: plasmid-like elements, introns and homing endonucleases

Intraspecific comparison of mitochondrial genomes reveals the evolution in medicinal fungus Ganoderma lingzhi

Several mitogenomes of the genus Ganoderma have been assembled, but intraspecific comparisons of mitogenomes in Ganoderma lingzhi have not been reported. In this study, 19 G. lingzhi mitogenomes were assembled and analyzed combined with three mitogenomes of G. lingzhi from GenBank in term of the characteristics, evolution, and phylogeny. The results showed that the mitogenomes of the G. lingzhi strains are closed circular ranging from 49.23 kb to 68.37 kb. The genetic distance, selective pressure, and base variation indicate that the 14 common protein coding genes were highly conserved. The differences in introns, open reading frames, and repetitive sequences in the mitogenome were the main factors leaded to the variations in mitogenome. The introns were horizontally transferred in mitogenomes, and the differences between introns in the same insertion, which were primarily caused by the repetitive sequence, showed that the introns may be under degeneration. Besides, the frequent insertion and deletion of introns showed an evolutionary rate faster than protein coding genes. Phylogenetic analysis showed that the G. lingzhi strains gathered with high support, and those with the same intron distribution law had closer clustering relationships.

Comparative mitochondrial genome analyses reveal conserved gene arrangement but massive expansion/contraction in two closely related Exserohilum pathogens

Exserohilum turcicum and E. rostratum, two closely related fungal species, are both economically important pathogens but have quite different target hosts (specific to plants and cross-kingdom infection, respectively). In the present study, complete circular mitochondrial genomes of the two Exserohilum species were sequenced and de novo assembled, which mainly comprised the same set of 13 core protein-coding genes (PCGs), two rRNAs, and a certain number of tRNAs and unidentified open reading frames (ORFs). Comparative analyses indicated that these two fungi had significant mitogenomic collinearity and consistent mitochondrial gene arrangement, yet with vastly different mitogenome sizes, 264,948 bp and 64,620 bp, respectively. By contrast with the 17 introns containing 17 intronic ORFs (one-to-one) in the E. rostratum mitogenome, E. turcicum involved far more introns (70) and intronic ORFs (126), which was considered as the main contributing factors of their mitogenome expansion/contraction. Within the generally intron-rich gene cox1, a total of 18 and 10 intron position classes (Pcls) were identified separately in the two mitogenomes. Moreover, 16.16% and 10.85% ratios of intra-mitogenomic repetitive regions were detected in E. turcicum and E. rostratum, respectively. Based on the combined mitochondrial gene dataset, we established a well-supported topology of phylogeny tree of 98 ascomycetes, implying that mitogenomes may act as an effective molecular marker for fungal phylogenetic reconstruction. Our results served as the first report on mitogenomes in the genus Exserohilum, and would have significant implications in understanding the origin, evolution and pathogenic mechanisms of this fungal lineage.

The Mitochondrial Genome of the Phytopathogenic Fungus Bipolaris sorokiniana and the Utility of Mitochondrial Genome to Infer Phylogeny of Dothideomycetes

A number of species in Bipolaris are important plant pathogens. Due to a limited number of synapomorphic characters, it is difficult to perform species identification and to estimate phylogeny of Bipolaris based solely on morphology. In this study, we sequenced the complete mitochondrial genome of Bipolaris sorokiniana, and presented the detailed annotation of the genome. The B. sorokiniana mitochondrial genome is 137,775 bp long, and contains two ribosomal RNA genes, 12 core protein-coding genes, 38 tRNA genes. In addition, two ribosomal protein genes (rps3 gene and rps5 gene) and the fungal mitochondrial RNase P gene (rnpB) are identified. The large genome size is mostly determined by the presence of numerous intronic and intergenic regions. A total of 28 introns are inserted in eight core protein-coding genes. Together with the published mitochondrial genome sequences, we conducted a preliminary phylogenetic inference of Dothideomycetes under various datasets and substitution models. The monophyly of Capnodiales, Botryosphaeriales and Pleosporales are consistently supported in all analyses. The Venturiaceae forms an independent lineage, with a distant phylogenetic relationship to Pleosporales. At the family level, the Mycosphaerellaceae, Botryosphaeriaceae. Phaeosphaeriaceae, and Pleosporaceae are recognized in the majority of trees.

Mitochondrial genome and diverse inheritance patterns in Pleurotus pulmonarius

Pleurotus pulmonarius, a member of the Pleurotaceae family in Basidiomycota, is an edible, economically important mushroom in most Asian countries. In this study, the complete mitochondrial genomes (mtDNA) of three P. pulmonarius strains - two monokaryotic commercial (J1-13 and ZA3) and one wild (X1-15) - were sequenced and analyzed. In ZA3 and X1-15, the mtDNA molecule was found to be a single circle of 68,305 bp and 73,435 bp, respectively. Both strains contain 14 core protein-coding genes and two ribosomal RNA (rRNA) subunit genes. The ZA3 strain has 22 transfer RNA (tRNA) genes and nine introns: eight in cytochrome c oxidase subunit 1 (coxl), and one in the rRNA large subunit (rnl). Monokaryotic J1-13 and ZA3 mtDNAs were found to be similar in their structure. However, the wild strain X1-15 contains 25 tRNA genes and only seven introns in coxl. Open reading frames (ORFs) of ZA3/J1-13 and X1-15 encode LAGLIDADG, ribosomal protein S3, and DNA polymerase II. In addition, mtDNA inheritance in J1-13, ZA3, and X1-15 was also studied. Results showed that the mtDNA inheritance pattern was uniparental and closely related to dikaryotic hyphal location with respect to the parent. Results also show that mtDNA inheritance is influenced by both the parental nuclear genome and mitogenome in the zone of contact between two compatible parents. In summary, this analysis provides valuable information and a basis for further studies to improve our understanding of the inheritance of fungal mtDNA.

Mobile genetic elements explain size variation in the mitochondrial genomes of four closely-related Armillaria species

Background

Species in the genus Armillaria (fungi, basidiomycota) are well-known as saprophytes and pathogens on plants. Many of them cause white-rot root disease in diverse woody plants worldwide. Mitochondrial genomes (mitogenomes) are widely used in evolutionary and population studies, but despite the importance and wide distribution of Armillaria, the complete mitogenomes have not previously been reported for this genus. Meanwhile, the well-supported phylogeny of Armillaria species provides an excellent framework in which to study variation in mitogenomes and how they have evolved over time.

Results

Here we completely sequenced, assembled, and annotated the circular mitogenomes of four species: A. borealis, A. gallica, A. sinapina, and A. solidipes (116,443, 98,896, 103,563, and 122,167 bp, respectively). The variation in mitogenome size can be explained by variable numbers of mobile genetic elements, introns, and plasmid-related sequences. Most Armillaria introns contained open reading frames (ORFs) that are related to homing endonucleases of the LAGLIDADG and GIY-YIG families. Insertions of mobile elements were also evident as fragments of plasmid-related sequences in Armillaria mitogenomes. We also found several truncated gene duplications in all four mitogenomes.

Conclusions

Our study showed that fungal mitogenomes have a high degree of variation in size, gene content, and genomic organization even among closely related species of Armillara. We suggest that mobile genetic elements invading introns and intergenic sequences in the Armillaria mitogenomes have played a significant role in shaping their genome structure. The mitogenome changes we describe here are consistent with widely accepted phylogenetic relationships among the four species.

Electronic supplementary material

The online version of this article (10.1186/s12864-019-5732-z) contains supplementary material, which is available to authorized users.

The mitochondrial genome of Endoconidiophora resinifera is intron rich

Endoconidiophora resinifera (=Ceratocystis resinifera) is a blue-stain fungus that occurs on conifers. The data showed that the Endoconidiophora resinifera mitochondrial genome is one of the largest mitochondrial genomes (>220 kb) so far reported among members of the Ascomycota. An exceptional large number of introns (81) were noted and differences among the four strains were restricted to minor variations in intron numbers and a few indels and single nucleotide polymorphisms. The major differences among the four strains examined are due to size polymorphisms generated by the absence or presence of mitochondrial introns. Also, these mitochondrial genomes encode the largest cytochrome oxidase subunit 1 gene (47.5 kb) reported so far among the fungi. The large size for this gene again can be attributed to the large number of intron insertions. This study reports the first mitochondrial genome for the genus Endoconidiophora, previously members of this genus were assigned to Ceratocystis. The latter genus has recently undergone extensive taxonomic revisions and the mitochondrial genome might provide loci that could be applied as molecular markers assisting in the identification of taxa within this group of economically important fungi. The large mitochondrial genome also may provide some insight on mechanisms that can lead to mitochondrial genome expansion.

Comparison of the Mitochondrial Genome Sequences of Six Annulohypoxylon stygium Isolates Suggests Short Fragment Insertions as a Potential Factor Leading to Larger Genomic Size

Mitochondrial DNA (mtDNA) is a core non-nuclear genetic material found in all eukaryotic organisms, the size of which varies extensively in the eumycota, even within species. In this study, mitochondrial genomes of six isolates of Annulohypoxylon stygium (Lév.) were assembled from raw reads from PacBio and Illumina sequencing. The diversity of genomic structures, conserved genes, intergenic regions and introns were analyzed and compared. Genome sizes ranged from 132 to 147 kb and contained the same sets of conserved protein-coding, tRNA and rRNA genes and shared the same gene arrangements and orientation. In addition, most intergenic regions were homogeneous and had similar sizes except for the region between cytochrome b (cob) and cytochrome c oxidase I (cox1) genes which ranged from 2,998 to 8,039 bp among the six isolates. Sixty-five intron insertion sites and 99 different introns were detected in these genomes. Each genome contained 45 or more introns, which varied in distribution and content. Introns from homologous insertion sites also showed high diversity in size, type and content. Comparison of introns at the same loci showed some complex introns, such as twintrons and ORF-less introns. There were 44 short fragment insertions detected within introns, intergenic regions, or as introns, some of them located at conserved domain regions of homing endonuclease genes. Insertions of short fragments such as small inverted repeats might affect or hinder the movement of introns, and these allowed for intron accumulation in the mitochondrial genomes analyzed, and enlarged their size. This study showed that the evolution of fungal mitochondrial introns is complex, and the results suggest short fragment insertions as a potential factor leading to larger mitochondrial genomes in A. stygium.

Three new active members of the I-OnuI family of homing endonucleases

In vitro characterization of 3 LAGLIDADG-type homing endonucleases (HEs) (I-CcaI, I-CcaII, and I-AstI) that belong to the I-OnuI family showed that they are functional HEs that cleave their respective cognate target sites. These endonucleases are encoded within group ID introns and appear to be orthologues that have inserted into 3 different mitochondrial genes: rns, rnl, and cox3. The endonuclease activity of I-CcaI was tested using various substrates, and its minimum DNA recognition sequence was estimated to be 26 nt. This set of HEs may provide some insight into how these types of mobile elements can migrate into new locations. This study provides additional endonucleases that can be added to the catalog of currently available HEs that may have various biotechnology applications.

Mitochondrion-to-Chloroplast DNA Transfers and Intragenomic Proliferation of Chloroplast Group II Introns in Gloeotilopsis Green Algae (Ulotrichales, Ulvophyceae)

To probe organelle genome evolution in the Ulvales/Ulotrichales clade, the newly sequenced chloroplast and mitochondrial genomes of Gloeotilopsis planctonica and Gloeotilopsis sarcinoidea (Ulotrichales) were compared with those of Pseudendoclonium akinetum (Ulotrichales) and of the few other green algae previously sampled in the Ulvophyceae. At 105,236 bp, the G planctonica mitochondrial DNA (mtDNA) is the largest mitochondrial genome reported so far among chlorophytes, whereas the 221,431-bp G planctonica and 262,888-bp G sarcinoidea chloroplast DNAs (cpDNAs) are the largest chloroplast genomes analyzed among the Ulvophyceae. Gains of non-coding sequences largely account for the expansion of these genomes. Both Gloeotilopsis cpDNAs lack the inverted repeat (IR) typically found in green plants, indicating that two independent IR losses occurred in the Ulvales/Ulotrichales. Our comparison of the Pseudendoclonium and Gloeotilopsis cpDNAs offered clues regarding the mechanism of IR loss in the Ulotrichales, suggesting that internal sequences from the rDNA operon were differentially lost from the two original IR copies during this process. Our analyses also unveiled a number of genetic novelties. Short mtDNA fragments were discovered in two distinct regions of the G sarcinoidea cpDNA, providing the first evidence for intracellular inter-organelle gene migration in green algae. We identified for the first time in green algal organelles, group II introns with LAGLIDADG ORFs as well as group II introns inserted into untranslated gene regions. We discovered many group II introns occupying sites not previously documented for the chloroplast genome and demonstrated that a number of them arose by intragenomic proliferation, most likely through retrohoming.

Intron Derived Size Polymorphism in the Mitochondrial Genomes of Closely Related Chrysoporthe Species

In this study, the complete mitochondrial (mt) genomes of Chrysoporthe austroafricana (190,834 bp), C. cubensis (89,084 bp) and C. deuterocubensis (124,412 bp) were determined. Additionally, the mitochondrial genome of another member of the Cryphonectriaceae, namely Cryphonectria parasitica (158,902 bp), was retrieved and annotated for comparative purposes. These genomes showed high levels of synteny, especially in regions including genes involved in oxidative phosphorylation and electron transfer, unique open reading frames (uORFs), ribosomal RNAs (rRNAs) and transfer RNAs (tRNAs), as well as intron positions. Comparative analyses revealed signatures of duplication events, intron number and length variation, and varying intronic ORFs which highlighted the genetic diversity of mt genomes among the Cryphonectriaceae. These mt genomes showed remarkable size polymorphism. The size polymorphism in the mt genomes of these closely related Chrysoporthe species was attributed to the varying number and length of introns, coding sequences and to a lesser extent, intergenic sequences. Compared to publicly available fungal mt genomes, the C. austroafricana mt genome is the second largest in the Ascomycetes thus far.

Evolution of group II introns

Present in the genomes of bacteria and eukaryotic organelles, group II introns are an ancient class of ribozymes and retroelements that are believed to have been the ancestors of nuclear pre-mRNA introns. Despite long-standing speculation, there is limited understanding about the actual pathway by which group II introns evolved into eukaryotic introns. In this review, we focus on the evolution of group II introns themselves. We describe the different forms of group II introns known to exist in nature and then address how these forms may have evolved to give rise to spliceosomal introns and other genetic elements. Finally, we summarize the structural and biochemical parallels between group II introns and the spliceosome, including recent data that strongly support their hypothesized evolutionary relationship.

The 203 kbp mitochondrial genome of the phytopathogenic fungus Sclerotinia borealis reveals multiple invasions of introns and genomic duplications

Here we report the complete sequence of the mitochondrial (mt) genome of the necrotrophic phytopathogenic fungus Sclerotinia borealis, a member of the order Helotiales of Ascomycetes. The 203,051 bp long mtDNA of S. borealis represents one of the largest sequenced fungal mt genomes. The large size is mostly determined by the presence of mobile genetic elements, which include 61 introns. Introns contain a total of 125,394 bp, are scattered throughout the genome, and are found in 12 protein-coding genes and in the ribosomal RNA genes. Most introns contain complete or truncated ORFs that are related to homing endonucleases of the LAGLIDADG and GIY-YIG families. Integrations of mobile elements are also evidenced by the presence of two regions similar to fragments of inverton-like plasmids. Although duplications of some short genome regions, resulting in the appearance of truncated extra copies of genes, did occur, we found no evidences of extensive accumulation of repeat sequences accounting for mitochondrial genome size expansion in some other fungi. Comparisons of mtDNA of S. borealis with other members of the order Helotiales reveal considerable gene order conservation and a dynamic pattern of intron acquisition and loss during evolution. Our data are consistent with the hypothesis that horizontal DNA transfer has played a significant role in the evolution and size expansion of the S. borealis mt genome.

Intronic and plasmid-derived regions contribute to the large mitochondrial genome sizes of Agaricomycetes

Sizes of mitochondrial genomes vary extensively between fungal species although they typically contain a conserved set of core genes. We have characterised the mitochondrial genome of the conifer root rot pathogen Heterobasidion irregulare and compared the size, gene content and structure of 20 Basidiomycete mitochondrial genomes. The mitochondrial genome of H. irregulare was 114, 193 bp and contained a core set of 15 protein coding genes, two rRNA genes and 26 tRNA genes. In addition, we found six non-conserved open reading frames (ORFs) and four putative plasmid genes clustered in three separate regions together with 24 introns and 14 intronic homing endonuclease genes, unequally spread across seven of the core genes. The size differences among the 20 Basidiomycetes can largely be explained by length variation of intergenic regions and introns. The Agaricomycetes contained the nine largest mitochondrial genomes in the Basidiomycete group and Agaricomycete genomes are significantly (p < 0.001) larger than the other Basidiomycetes. A feature of the Agaricomycete mitochondrial genomes in this study was the simultaneous occurrence of putative plasmid genes and non-conserved ORFs, with Cantharellus cibarius as only exception, where no non-conserved ORF was identified. This indicates a mitochondrial plasmid origin of the non-conserved ORFs or increased mitochondrial genome dynamics of species harbouring mitochondrial plasmids. We hypothesise that two independent factors are the driving forces for large mitochondrial genomes: the homing endonuclease genes in introns and integration of plasmid DNA.

In vivo conformation and replication intermediates of circular mitochondrial plasmids in Neurospora and Cryphonectria parasitica

The in vivo conformation and replication intermediates of fungal circular mitochondrial plasmids and plasmid-like mitochondrial element (plMEs) were analyzed by two-dimensional gel electrophoresis and electron microscopy. Plasmids with circular restriction maps exist predominantly as circular molecules and were found to replicate by rolling circle mechanisms. However, the reverse transcriptase-encoding Mauriceville plasmid of Neurospora crassa was observed to replicate by two possible mechanisms: one that is consistent with a reverse transcriptase-mediated process and a second one might involve rolling circle DNA replication. Like the mtDNA-derived plasmid-like elements of N. crassa (Hausner et al. 2006a, b), a plasmid-like element of Cryphonectria parasitica (plME-C9), which consists predominantly of a 1.4 kb nucleotide sequence different from mitochondrial DNA, also was found to replicate by a rolling circle mechanism. Although the techniques used in this study were not suited for the establishment of the in vivo conformation and mode of replication of the mtDNAs of Neurospora or Cryphonectria, we surmise that the rolling circle mechanism might be the predominant mode of DNA replication in fungal mitochondria.

Mitochondrial genome evolution in species belonging to the Phialocephala fortinii s.l. - Acephala applanata species complex

Background

Mitochondrial (mt) markers are successfully applied in evolutionary biology and systematics because mt genomes often evolve faster than the nuclear genomes. In addition, they allow robust phylogenetic analysis based on conserved proteins of the oxidative phosphorylation system. In the present study we sequenced and annotated the complete mt genome of P. subalpina, a member of the Phialocephala fortinii s.l. – Acephala applanata species complex (PAC). PAC belongs to the Helotiales, which is one of the most diverse groups of ascomycetes including more than 2,000 species. The gene order was compared to deduce the mt genome evolution in the Pezizomycotina. Genetic variation in coding and intergenic regions of the mtDNA was studied for PAC to assess the usefulness of mt DNA for species diagnosis.

Results

The mt genome of P. subalpina is 43,742 bp long and codes for 14 mt genes associated with the oxidative phosphorylation. In addition, a GIY-YIG endonuclease, the ribosomal protein S3 (Rps3) and a putative N-acetyl-transferase were recognized. A complete set of tRNA genes as well as the large and small rRNA genes but no introns were found. All protein-coding genes were confirmed by EST sequences. The gene order in P. subalpina deviated from the gene order in Sclerotinia sclerotiorum, the only other helotialean species with a fully sequenced and annotated mt genome. Gene order analysis within Pezizomycotina suggests that the evolution of gene orders is mostly driven by transpositions. Furthermore, sequence diversity in coding and non-coding mtDNA regions in seven additional PAC species was pronounced and allowed for unequivocal species diagnosis in PAC.

Conclusions

The combination of non-interrupted ORFs and EST sequences resulted in a high quality annotation of the mt genome of P. subalpina, which can be used as a reference for the annotation of other mt genomes in the Helotiales. In addition, our analyses show that mtDNA loci will be the marker of choice for future analysis of PAC communities.

Recurrent insertion of 5'-terminal nucleotides and loss of the branchpoint motif in lineages of group II introns inserted in mitochondrial preribosomal RNAs

A survey of sequence databases revealed 10 instances of subgroup IIB1 mitochondrial ribosomal introns with 1 to 33 additional nucleotides inserted between the 5' exon and the consensus sequence at the intron 5' end. These 10 introns depart further from the IIB1 consensus in their predicted domain VI structure: In contrast to its basal helix and distal GNRA terminal loop, the middle part of domain VI is highly variable and lacks the bulging A that serves as the branchpoint in lariat formation. In vitro experiments using two closely related IIB1 members inserted at the same ribosomal RNA site in the basidiomycete fungi Grifola frondosa and Pycnoporellus fulgens revealed that both ribozymes are capable of efficient self-splicing. However, whereas the Grifola intron was excised predominantly as a lariat, the Pycnoporellus intron, which possesses six additional nucleotides at the 5' end, yielded only linear products, consistent with its predicted domain VI structure. Strikingly, all of the introns with 5' terminal insertions lack the EBS2 exon-binding site. Moreover, several of them are part of the small subset of group II introns that encode potentially functional homing endonucleases of the LAGLIDADG family rather than reverse transcriptases. Such coincidences suggest causal relationships between the shift to DNA-based mobility, the loss of one of the two ribozyme sites for binding the 5' exon, and the exclusive use of hydrolysis to initiate splicing.

Evolutionary dynamics of the mS952 intron: a novel mitochondrial group II intron encoding a LAGLIDADG homing endonuclease gene

Examination of the mitochondrial small subunit ribosomal RNA (rns) gene of five species of the fungal genus Leptographium revealed that the gene has been invaded at least once at position 952 by a group II intron encoding a LAGLIDADG homing endonuclease gene. Phylogenetic analyses of the intron and homing endonuclease sequences indicated that each element in Leptographium species forms a single clade and is closely related to the group II intron/homing endonuclease gene composite element previously reported at position 952 of the mitochondrial rns gene of Cordyceps species and of Cryphonectria parasitica. The results of an intron survey of the mt rns gene of Leptographium species superimposed onto the phylogenetic analysis of the host organisms suggest that the composite element was transmitted vertically in Leptographium lundbergii. However, its stochastic distribution among strains of L. wingfieldii, L. terebrantis, and L. truncatum suggests that it has been horizontally transmitted by lateral gene transfer among these species, although the random presence of the intron may reflect multiple random loss events. A model is proposed describing the initial invasion of the group II intron in the rns gene of L. lundbergii by a LAGLIDADG homing endonuclease gene and subsequent evolution of this gene to recognize a novel DNA target site, which may now promote the mobility of the intron and homing endonuclease gene as a composite element.

Mitochondrial inheritance in fungi

Faithful inheritance of mitochondria is essential for growth and development. Uniparental inheritance of mitochondria is a common phenomenon in sexual eukaryotes and has been reported for numerous fungal species. Uniparental inheritance is a genetically regulated process, aimed to gain a homoplasmic state within cells, and this is often associated with selective elimination of one parental mitochondria population. This review will focus on recent developments in our understanding of common and specified regulatory circuits of selective mitochondrial inheritance during sexual development. It further refers to the influence of mitochondrial fusion on generation of recombinant mitochondrial DNA molecules. The latter aspect appears rather exciting in the context of intron homing and could bring a new twist to the debate on the significance of uniparental inheritance. The emergence of genome-wide studies offers new perspectives to address potential relationships between uniparental inheritance, vegetative inheritance and last but not least cellular scavenging systems to dispose of disintegrated organelles.