Widespread distribution of low-copy-number variants of mitochondrial plasmid pEM in the genus Agaricus

Fungal mitochondrial genomes and genetic polymorphisms

Mitochondria are the powerhouses of eukaryotic cells, responsible for ATP generation and playing a role in a diversity of cellular and organismal functions. Different from the majority of other intracellular membrane structures, mitochondria contain their own genetic materials that are capable of independent replication and inheritance. In this mini-review, we provide brief summaries of fungal mitochondrial genome structure, size, gene content, inheritance, and genetic variation. We pay special attention to the relative genetic polymorphisms of the mitochondrial vs nuclear genomes at the population level within individual fungal species. Among the 20 species/groups of species reviewed here, there is a range of variation among genes and species in the relative nuclear and mitochondrial genetic polymorphisms. Interestingly, most (15/20) showed a greater genetic diversity for nuclear genes and genomes than for mitochondrial genes and genomes, with the remaining five showing similar or slower nuclear genome genetic variations. This fungal pattern is different from the dominant pattern in animals, but more similar to that in plants. At present, the mechanisms for the variations among fungal species and the overall low level of mitochondrial sequence polymorphisms are not known. The increasing availability of population genomic data should help us reveal the potential genetic and ecological factors responsible for the observed variations.

The mitochondrial genome of a cytoplasmic male sterile line of perennial ryegrass (Lolium perenne L.) contains an integrated linear plasmid-like element

The mitochondrial genome of a cytoplasmic male sterile line of perennial ryegrass (Lolium perenne L.) was shown to contain a 9.6 kb element, LpCMSi, that is absent in the mitochondrial genome of fertile lines. LpCMSi contains the previously described chimeric gene orfC9, and three additional open reading frames (orfs) encoding a unique 45 kDa predicted protein of unknown function, a family B-like DNA polymerase (LpDpo), and a phage-type single subunit RNA polymerase (LpRpo). The latter two proteins shared significant similarity with DNA and RNA polymerases encoded by extrachromosomal linear mitochondrial plasmids of plants and fungi, and also to integrated plasmid-like sequences found in various plant and fungal mitochondrial genomes. Transcripts for both LpDpo and LpRpo were detected by RT-PCR in mitochondria of the CMS line. PCR-based investigations further revealed the presence of LpCMSi-like sequences in fertile L. perenne lines that are likely maintained as low-copy number extrachromosomal replicons. The absence of integrated forms of LpCMSi in the mitochondrial genome of fertile lines suggests that LpCMSi integration adjacent to the atp9 gene may be responsible, directly or indirectly, for the sterility phenotype of the CMS line.

Molecular evolution of a mitochondrial polB gene, encoding a family B DNA polymerase, towards the elimination from Agrocybe mitochondrial genomes

Two genes (Ac-polB O1and Ac-polB O2), each encoding a family B DNA polymerase, were characterized from the mitochondrial genome of the basidiomycete Agrocybe chaxingu. These two polB genes constitute orthologs of the potentially functional Aa-polB gene and its disrupted paralog Aa-polB P1, previously described in the closely related species A. aegerita. Unlike the case in Aa-polB, both gene copies in A. chaxingu are constituted by large but disrupted ORFs, which very probably encode nonfunctional enzymes: Ac-polB O1 has a deletion of 126 bp between the segments encoding the Exo II and Pol I domains and a 78-bp insertion between the Exo II and Exo III domains, whereas Ac-polB O2 has a large deletion of 1208 bp between the Exo II and Pol III domains and a deletion of 54-bp involving the 3' end of the gene. Hence, rearrangements in the Ac-polB ORFs appear to have led to their functional erosion in the mitochondrial genome in this species. Phylogenetic analysis has shown a close relationship between the mitochondrial polB genes and homologous genes carried by fungal linear plasmids, suggesting that they may have been acquired by the integration of linear plasmids into the mitochondrial genome.

Evolution in action: molecular evidence for recent emergence of secotioid genera Endxoptychum, Gyrophragmium and Longula from Agaricus ancestors

Phylogenetic analyses of 29 Agaricus species and one representative from genera Endoptychum, Gyrophragmium and Longula each were conducted based on sequence data of the entire internal transcribed spacers and partial large subunit of ribosomal DNA. The Agaricus species formed several distinct clades both confirming and challenging previous morphological sections in several cases. Endoptychum depressum, Gyrophragmium dunalii and Longula texensis were nested among species of the genus Agaricus. This study provides evidence for independent emergence of these secotioid fungi from Agaricus ancestors, most likely from species placed in the section Arvenses as inferred from molecular data.