Phylogenetic analyses of the ITS region of ribosomal DNA reveal thatSeptoria passerinii from barley is closely related to the wheat pathogenMycosphaerella graminicola

摘要

Septoria passerinii, a mitosporic ascomycete, causes speckled leaf blotch of barley. Although this fungus has been known to science for more than 120 yr, no teleomorph has been found and its phylogenetic relationships are not known. On the basis of spore morphology and growth characteristics in culture, S. passerinii appears similar to Septoria tritici, the anamorph of the wheat pathogen Mycosphaerella graminicola. To test the hypothesis that S. passerinii and M. graminicola are closely related, the internal transcribed spacer (ITS) region (ITS1, ITS2 and the intervening 5.8S rRNA gene) of both species was sequenced and compared to those of many other fungi in the Dothideales and Pleosporales of the Loculoascomycetes. Two ITS sequences were identified among seven isolates of S. passerinii. Six isolates from cultivated barley (Hordeum vulgare) shared the same 569 base-pair sequence. One isolate from the wild barley H. jubatum had a 569 base-pair sequence that differed from the first sequence by seven nucleo tides. The ITS sequence of M. graminicola differed from those of both forms of S. passerinii by 10 nucleotides. These differences were large enough that all three ITS sequences could be distinguished by restriction enzyme digestion of polymerase chain reaction-amplified products. Cluster analyses revealed that both forms of S. passerinii were closely related to M. graminicola and were contained within a large, monophyletic group of Mycosphaerella species. This confirms that the teleomorph of S. passerinii, if it exists, must be a Mycosphaerella. Furthermore, the isolate from H. jubatum probably represents a new, undescribed species that is separated from S. passerinii by host specificity. Our analysis also revealed the phylogenetic relationships of the halophilic black yeasts Trimmatostroma salinum and Phaeotheca triangularis within Mycosphaerella and of Rhizopycnis vagum within the Pleosporales. The Pleosporalean genera Leptosphaeria and Phaeosphaeria were apparently polyphyletic, as were most of the anamorph genera in both the Pleosporales and Dothideales. Pathogenicity to the same host occurred in phylogenetically distinct species, indicating convergent evolution for host specificity. Large differences in size of the ITS1 region made alignment difficu within the Dothideales and Pleosporales, the best phylogenetic analyses were obtained with ITS2 sequences.