The mitochondrial apocytochrome b genes of two Agrocybe species suggest lateral transfers of group I homing introns among phylogenetically distant fungi

The Agrocybe chaxingu and Agrocybe aegerita mitochondrial apocytochrome b coding sequences are highly similar (97% of nt identity), but have highly different sizes (2312 and 4867 nt, respectively), due to the presence of three large group IB introns: two (iAae1 and iAae2) in A. aegerita, one (iAch1) in A. chaxingu. All these introns encode a homing endonuclease (HE) similar to those described in introns of mitochondrial genes (cob, cox1, and nad5) from various organisms. Phylogenetic trees were built with these HE sequences. From these trees, the Agrocybe coding introns argue for recent lateral transfers, i.e., occurring after the separation of the two Agrocybe species, involving phylogenetically distant fungi such as members of the Ascomycota phylum (for iAch1 and iAae2) and, for the first time to our knowledge, a member of the Chytridiomycota phylum (for iAae1). The grouping of the HE gene (HEG) sequences according to the mitochondrial gene (cob, cox1, and nad5) where they are inserted, suggests modifications of the interactions between the HE and the recognized sequences, leading to new target genes. The largest distribution of the iAch1 HE, shared by several cob and cox1 mitochondrial genes from Ascomycota, Basidiomycota, and Chytridiomycota phyla, suggests a higher target flexibility of this HE, perhaps related to the presence of two different LAGLIDADG motifs in the catalytic site of the enzyme.