2 DIVERGENT INTRAGENOMIC RDNA ITS2 TYPES WITHIN A MONOPHYLETIC LINEAGE OF THE FUNGUS FUSARIUM ARE NONORTHOLOGOUS

The evolutionary history of the phytopathogenic Gibberella fujikuroi c omplex of Fusarium and related species was investigated by cladistic a nalysis of DNA sequences obtained from multiple unlinked loci. Gene ph ylogenies inferred from the mitochondrial small subunit (mtSSU) rDNA, nuclear 28S rDNA, and beta-tubulin gene were generally concordant, pro viding strong support for a fully resolved phylogeny of all biological and most morphological species. Discordance of the nuclear rDNA inter nal transcribed spacer 2 (ITS2) gene tree is due to paralogous or xeno logous ITS2 sequences. PCR and sequence analysis demonstrated that eve ry strain of the ingroup species tested possesses two highly divergent nonorthologous ITS2 types designated type I and type II. Only the maj or ITS2 type, however, is discernable when PCR products are amplified and sequenced directly with conserved primers. The minor ITS2 type was recovered using ITS2 type-specific PCR primers, Distribution of the m ajor ITS2 type within the species lineages exhibits a homoplastic patt ern of evolution, thus obscuring true phylogenetic relationships. The results suggest that the ancestral ITS2 types may have arisen followin g an ancient interspecific hybridization or gene duplication which occ urred prior to the evolutionary radiation of the Gibberella fujikuroi complex and related species of Fusarium. The results also indicate tha t current morphological-based taxonomic schemes for these fungi are un natural and a new classification is required. (C) 1997 Academic Press.