Background: Mitochondrial genomic sequences are known to be variable. Comparative analyses of mitochondrial genomes can reveal the nature and extent of their variation.Results: Draft mitochondrial genomes of 16 Tremella fuciformis isolates (TF01-TF16) were assembled from Illumina and PacBio sequencing data. Mitochondrial DNA contigs were extracted and assembled into complete circular molecules, ranging from 35,104 bp to 49,044 bp in size. All mtDNAs contained the same set of 41 conserved genes with identical gene order. Comparative analyses revealed that introns and intergenic regions were variable, whereas genic regions (including coding sequences, tRNA, and rRNA genes) were conserved. Among 24 introns detected, 11 were in protein-coding genes, 3 in tRNA genes, and the other 10 in rRNA genes. In addition, two mobile fragments were found in intergenic regions. Interestingly, six introns containing N-terminal duplication of the host genes were found in five conserved protein-coding gene sequences. Comparison of genes with and without these introns gave rise to the following proposed model: gene fragment exchange with other species can occur via gain or loss of introns with N-terminal duplication of the host genes.Conclusions: Our findings suggest a novel mechanism of fungal mitochondrial gene evolution: partial foreign gene replacement though intron mobility.

BackgroundMitochondrial genomic sequences are known to be variable. Comparative analyses of mitochondrial genomes can reveal the nature and extent of their variation. ResultsDraft genomes of 16 Tremella fuciformis isolates (TF01-TF16) were assembled from Illumina and PacBio sequencing data. Mitochondrial DNA contigs were extracted and assembled into complete circular molecules, ranging from 35,104 bp to 49,044 bp in size. All mtDNAs contained the same set of 41 conserved genes with identical gene order. Comparative analyses revealed that introns and intergenic regions were variable, whereas genic regions (including coding sequences, tRNA, and rRNA genes) were conserved. Among 24 introns detected, 11 were in protein-coding genes, 3 in tRNA genes, and the other 10 in rRNA genes. In addition, two mobile fragments were found in intergenic regions. Interestingly, six introns containing N-terminal duplication of the host genes were found in five conserved protein-coding gene sequences. Comparison of genes with and without these introns gave rise to the following proposed model: gene fragment exchange with other species can occur via gain or loss of introns with N-terminal duplication of the host genes. ConclusionsOur findings suggest a novel mechanism of fungal mitochondrial gene evolution: partial foreign gene replacement though intron mobility. employed to recognize the target region through complementarity [ 22]. Group II introns are much less common in fungal mitochondrial genomes [ 5], where splicing occurs by two transesterification steps virtually identical to nuclear pre-mRNA splicing [ 23].Recent studies provide evidence that mobility of introns may affect their host genes, including gene structure and DNA composition. The Gigapora rosea cox1 gene is broken up into two fragments via group I intron-mediated trans-splicing. The two fragments are on the same strand in the mitochondrial genome, and are separated by a sequence of ~30 kbp, which includes 15 genes.Similar cases of group I intron-mediated trans-splicing have also been reported in the cox1 gene in Gigaspora margarita[ 24 ], Isoetes engelmannii [ 25], Selaginella moellendorffii [ 26], Helicosporidium sp. [ 27], andplacozoan animals [ 28], and in the rns gene in G. margarita [ 24]. A higher density of single nucleotide polymorphisms in exons near self-splicing introns was detected when analyzing the mitochondrial genomes of Saccharomyces cerevisiae,

Background Mitochondrial genomic sequences are known to be variable. Comparative analyses of mitochondrial genomes can reveal the nature and extent of their variation. Results Draft mitochondrial genomes of 16 Tremella fuciformis isolates (TF01-TF16) were assembled from Illumina and PacBio sequencing data. Mitochondrial DNA contigs were extracted and assembled into complete circular molecules, ranging from 35,104 bp to 49,044 bp in size. All mtDNAs contained the same set of 41 conserved genes with identical gene order. Comparative analyses revealed that introns and intergenic regions were variable, whereas genic regions (including coding sequences, tRNA, and rRNA genes) were conserved. Among 24 introns detected, 11 were in protein-coding genes, 3 in tRNA genes, and the other 10 in rRNA genes. In addition, two mobile fragments were found in intergenic regions. Interestingly, six introns containing N-terminal duplication of the host genes were found in five conserved protein-coding gene sequences. Comparison of genes with and without these introns gave rise to the following proposed model: gene fragment exchange with other species can occur via gain or loss of introns with N-terminal duplication of the host genes. Conclusions Our findings suggest a novel mechanism of fungal mitochondrial gene evolution: partial foreign gene replacement though intron mobility.

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