Structural characteristics and possible horizontal transfer of group I introns between closely related plant pathogenic fungi

We have characterized structural features and the distribution pattern of n uclear group I introns found in ribosomal DNA (rDNA) of closely related pla nt pathogenic fungi of the family Sclerotiniaceae. Sixteen introns, at two distinct positions in the small-subunit (SSU) and large-subunit (LSU) rDNA, were sequenced and analyzed among the 29 taxa included in the initial scre ening. Genera found to contain introns were Botrytis, Dumontinia, Encoelia, Grovesinia, Myriosclerotinia, and Sclerotinia. Secondary-structure analyse s of the group I introns concluded that all belong to the common IC1 subcla ss. Interestingly, the SSU rDNA intron from Myriosclerotinia caricisampulla cea contains an insertion-like sequence extension which may be a relic of a n open reading frame. Incongruent branching patterns of intron-based and rD NA-based (internal transcribed spacer) phylogenetic trees suggest that the fungal host genomes and the group I introns do not share a common evolution ary history. A model to explain how horizontal intron transfers may have oc curred among the closely related fungal taxa is proposed.