Molecular systematics and phylogeography of the Gibberella fujikuroi species complex

Abstract

Phylogenetic relationships of the phytopathogenic Gibberella fujikuroi species complex were investigated by maximum parsimony analysis of DNA sequences from multiple loci. Gene trees inferred from the β-tubulin 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 phylogeny consistent with species radiations in South America, Africa, and Asia. These analyses place the American clade as a monophyletic sister-group of an African-Asian clade. Africa is the most phylogenetically diverse area examined with 16 species, followed by America (12 species) and Asia (8 species). The biogeographic hypothesis proposed from the phylogenetic evidence is based primarily on the formation of natural barriers associated with the fragmentation of the ancient super-continent Gondwana. Discordance of the nuclear ribosoreal internal transcribed spacer (ITS) based tree with gene trees from the other loci sequenced is due to nonorthologous ITS2 sequences. The molecular evidence suggests the divergent ITS2 types were combined by an ancient interspecific hybridization (xenologous origin) or gene duplication (paralogous origin) that predates the evolutionary radiation of the G. fujikuroi complex. Two highly divergent nonorthologous ITS2 types designated type I and type II were identified and characterized with conserved ITS and ITS2 type-specific polymerase chain reaction (PCR) primers and DNA sequence analysis. Only the major ITS2 type is discernible when conserved ITS primers are used; however, a minor ITS2 type was amplified from every strain tested with type-specific PCR primers. The evolutionary pattern exhibited by the major ITS2 type is homoplastic when mapped onto the species lineages inferred from the combined nuclear 28S rDNA, mtSSU rDNA, and β-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. Twenty-six of the 45 species included in this study represent either new species (23 species), new combinations (F. bulbicola and F. phyllophilum), or a rediscovered species (F. lactis). The results further indicate that traditional sectional and species-level taxonomic schemes for this lineage are artificial and a more natural classification is proposed.