|Goodwin, Stephen - Steve|
Submitted to: Phytopathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/26/2001
Publication Date: N/A
Interpretive Summary: Fungi in the genera Cercospora and Mycosphaerella cause devastating diseases on virtually every major family of plants. Although both genera are very large - Cercospora has over 3,000 species and Mycosphaerella over 1,800 - the evolutionary relationships among them are not known and species identification can be difficult. This can complicate development of effective disease control strategies. To simplify species identificatio and to test hypotheses about the evolution of these pathogens, a large database of ribosomal DNA spacer sequences was constructed by generating new data for some species and mining existing databases. A large-scale analysis showed that all species of Mycosphaerella evolved from a single common ancestor, and that asexual spore stages usually are not good indicators of evolutionary relationships. The genus Cercospora was derived from a common ancestor from within Mycosphaerella. Therefore, if Cercospora species have sexual stages, they must be in Mycosphaerella. Only species of Cercospora produced the plant toxin cercosporin. Thus, ability to produce cercosporin must have been acquired by the common ancestor of all Cercospora species. The analysis also confirmed previous suspicions that gray leaf spot of corn is caused by two distinct species of Cercospora, and that C. sorghi and C. sorghi var. maydis are two different species. Numerous other species with no known sexual stage were shown to be associated with Mycosphaerella. This information will be of great use to plant pathologists and mycologists for species identification. The information also may help fungicide companies and plant pathologists target disease management strategies, because methods and products that work on one fungus may also be effective against close relatives.
Technical Abstract: Most of the 3000 species in the genus Cercospora have no known teleomorph, although one in Mycosphaerella has been identified for a few. Mycosphaerella also is an extremely large genus of plant pathogens, with more than 1800 species and at least 43 associated anamorphs. The goal of this research was to perform a large-scale analysis to test hypotheses about the past evolutionary history of Cercospora and Mycosphaerella. Phylogenetic analysis of internal transcribed spacer (ITS) sequence data revealed that the genus Mycosphaerella almost certainly is monophyletic. In contrast, many anamorph genera within Mycosphaerella were polyphyletic and were not useful for grouping species. One exception was Cercospora, which formed a highly supported monophyletic group. Most Cercospora species from cereal crops formed a subgroup within the main Cercospora cluster. Only species within the Cercospora cluster produced the toxin cercosporin, suggesting that the ability to produce this compound evolved only once. Intraspecific variation for 25 taxa in the Mycosphaerella clade averaged 1.7 nucleotides in the ITS region. Thus, isolates with ITS sequences that differ by two or more nucleotides may be distinct species. ITS sequences of Groups I and II of the gray leaf spot pathogen C. zeae-maydis differed by seven nucleotides and clearly represent different species. There were 6.5 nucleotide differences on average between the ITS sequences of C. sorghi and C. sorghi var. maydis, indicating that the latter is a separate species and not simply a variety of C. sorghi. The large Mycosphaerella cluster contained a number of anamorph genera with no known teleomorph associations. Therefore, the number of anamorph genera related to Mycosphaerella may be much larger that suspected previously.