MOLECULAR SYSTEMATICS AND PHYLOGEOGRAPHY OF THE GIBBERELLA-FUJIKUROI SPECIES COMPLEX

Phylogenetic relationships of the phytopathogenic Gibberella fujikuroi species complex were investigated by maximum parsimony analysis of DN A sequences from multiple loci. Gene trees inferred from the beta-tubu lin gene exons and introns, mitochondrial small subunit (mtSSU) rDNA, and 5' portion of the nuclear 28S rDNA were largely concordant, and in a combined analysis, provide strong statistical support for a phyloge ny consistent with species radiations in South America, Africa, and As ia. These analyses place the American clads as a monophyletic sister-g roup of an African-Asian clade. Africa is the most phylogenetically di verse area examined with 16 species, followed by America (12 species) and Asia (8 species). The biogeographic hypothesis proposed from the p hylogenetic evidence is based primarily on the formation of natural ba rriers associated with the fragmentation of the ancient super-continen t Gondwana. Discordance of the nuclear ribosomal internal transcribed spacer (ITS) based tree with gene trees from the other loci sequenced is due to nonorthologous ITS2 sequences. The molecular evidence sugges ts the divergent ITS2 types were combined by an ancient interspecific hybridization (xenologous origin) or gene duplication (paralogous orig in) that predates the evolutionary radiation of the G. fujikuroi compl ex. Two highly divergent nonorthologous ITS2 types designated type I a nd type II were identified and characterized with conserved ITS and IT S2 type-specific polymerase chain reaction (PCR) primers and DNA seque nce analysis. Only the major ITS2 type is discernible when conserved I TS primers are used; however, a minor ITS2 type was amplified from eve ry strain tested with type-specific PCR primers. The evolutionary patt ern exhibited by the major ITS2 type is homoplastic when mapped onto t he species lineages inferred from the combined nuclear 28S rDNA, mtSSU rDNA, and beta-tubulin gene sequences. Remarkably, the data indicate the major ITS2 type has switched between a type I and type II sequence at least three times during the evolution of the G. fujikuroi complex , but neither type has been fixed in any of the 45 species examined. T wenty-six of the 45 species included in this study represent either ne w species (23 species), new combinations (F. bulbicola and F. phylloph ilum), or a rediscovered species (F. lactis). The results further indi cate that traditional sectional and species-level taxonomic schemes fo r this lineage are artificial and a more natural classification is pro posed.