Taxonomy, species concept and systematics of the genus Fusarium: Progress, current status and problems to be solved

Taxonomy of the genus Fusarium, transition of its species concept, introduction and development of molecular phylogeny and their current conditions have been outlined by reviewing our own studies. Taxonomy of the genus has traditionally been based on the phenotypes represented by the species, mainly their morphological features. Microscopic phenotypes including those in culture are, however, not always stable characters for taxonomy and, therefore, severe arguments on the species definition or their circumscriptions have been conducted among taxonomic workers. Efforts to establish a worldwide reasonable taxonomic system have been facing a lot of difficulties. Molecular-phylogenetic analyses based on DNA sequences of their gene regions, introduced substantially in 1990s, have made a significant influence to the taxonomy of Fusarium. Species concepts of the genus have been converged into those recognizing narrowly splitted species. On the other hand, a lot of cryptic species have been discovered within the previously-known species that were defined briefly. Requirements are to describe and recognize splitted species accurately based on still limited phenotypic features. To obtain constant and stable data for such species delimitation, observation of fungal materials and examination of phenotypic features should become more precise. Consequently, a lot of novel species have been described, in addition to division of well-known species so far. Phenotypically-defined species (i.e., morphological species) are not always equivalent to phylogenetically-recognized species (i.e., molecular-phylogenetic species). To guarantee objectiveness of species definition, novel species are being described based on phenotypic features that are also supported by the phylogenetic analyses. However, limitation of available phenotypic features for the satisfactory definition of these species is becoming serious. A possible new procedure to satisfy requirements of the nomenclatural code is expected for describing taxa based on the direct application of molecular-phylogenetic data for the Fusarium taxonomy.