Abstract
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.
Highlights
The phytopathogenic fungus Sclerotinia borealis Bubak & Vleugel, which is an ascomycete belonging to the family Sclerotiniaceae of the order Helotiales, has a broad host range and causes diseases in at least 17 plant genera from the families Alliaceae, Asteraceae, Brassicaceae, Campanulaceae, Fabaceae, Iridaceae, Pinaceae, and Poaceae
Our comparative analysis indicates that the 203,051 bp long S. borealis mt genome is the second largest fungal mt genome sequenced to date, after the 235,849 bp long mtDNA of the basidiomycete R. solani Rhs1AP [7]
Expansion of the mitochondrial genome size in R. solani Rhs1AP was driven by accumulation of introns, HEG-like genes and hypothetical genes, but the most peculiar feature was the presence of various interspersed repeats, which occupy about one third of the genome [7]
Summary
The phytopathogenic fungus Sclerotinia borealis Bubak & Vleugel, which is an ascomycete belonging to the family Sclerotiniaceae of the order Helotiales, has a broad host range and causes diseases in at least 17 plant genera from the families Alliaceae, Asteraceae, Brassicaceae, Campanulaceae, Fabaceae, Iridaceae, Pinaceae, and Poaceae (notably wheat and corn). Fungal mt genomes range in size from about 19 kbp (Hanseniaspora uvarum) to 235,849 bp for Rhizoctonia solani AG-3 strain Rhs1AP [7], and usually contain 14 genes that encode oxidative phosphorylation system proteins, the large (rnl) and small (rns) ribosomal RNA subunits, and a fairly constant set of tRNAs genes [8], [9]. Besides this core set of genes, a varying number of introns, often including
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