Ribosomal variation in six species of Fusarium

Eighteen isolates representing six Fusarium species from diverse hosts and geographical origins were evaluated to determine ribosomal DNA variation using polymerase chain reaction and restriction fragment length polymorphisms. No length variation was observed for amplified 18S and 28S regions. However, amplification of the ITS region showed one isolate, a F. oxysporum, to be about 120 bp larger than the remaining 17. Restriction digestions in the 18S region revealed polymorphisms within species of F. oxysporum and F. solani. An amplified variable stretch of the 28S gene showed restriction site differences between F. avenecum, F. sambucinum and F. sporotrichioides. A large degree of polymorphism was observed both between and within species in the ITS region. Therefore, entire sequences of the ITS and the 5.8S subunit were obtained for 17 of the 18 isolates. These sequences, along with those from eight additional isolates, were analysed using PAUP to assess the occurrence of DNA sequence divergence within the ITS region. The lack of correlation between molecular-based relationships and species affinities inferred from morphology for some isolates indicates that species designation can be unreliable using morphological data alone. Possible reasons for the discordance of the sequence and morphological data are discussed.