Cochliobolus phylogenetics and the origin of known, highly virulent pathogens, inferred from ITS and glyceraldehyde-3-phosphate dehydrogenase gene sequences

We evaluate the phylogenetic distribution of known, highly virulent plant p athogens in the genus Cochliobolus (sexually reproducing species in the Asc omycota, Pleosporaceae) and assess the relationship between Cochliobolus sp ecies and species of Curvularia and Bipolaris (asexual states of fungi in t he Ascomycota, Pleosporaceae). To infer a phylogeny, we have used two seque nce regions: (i) the complete ITS 1, ITS 2, and 5.88 rDNA sequences for 65 fungal isolates and (ii) a 600 bp fragment of the housekeeping gene gpd, co ding for glyceraldehyde-3-phosphate dehydrogenase, for 54 isolates. We comb ined ITS, 5.8S and gpd sequence data from 41 species. Ln the Cochliobolus c lade, 31 out of 32 species fit clearly into one of two groups. One species, Cochliobolus homomorphus, did not fit clearly into either group. The 13 sp ecies in Cochliobolus Group I grouped together with 100% bootstrap support from the combined ITS/gpd data. This group included Cochliobolus and Bipola ris species that cause serious crop losses, such as Co. sativus, Co. miyabe anus, Co. carbonum and Co. heterostrophus. However; within Group 1, the kno wn, highly virulent pathogens did not form a monophyletic group of species. Average substitution levels between pairs of species in the Group 1 were l ow, about 1.7% in the ITS region, suggesting that these species had radiate d rapidly and recently. The 18 species in Cochliobolus Group 2 formed a mon ophyletic group in 96% of parsimony bootstrap replicates of the combined IT S and gpd data. The Cochliobolus species that were transferred into the seg regate genus Pseudocochliobolus were in this second group. This study inclu ded 9 Curvularia and Bipolaris species without known sexual states and they all appear to be recently derived from among sexual species of Cochliobolu s. Both Curvularia and Bipolaris were polyphyletic, but only Bipolaris stat es were associated with Group 1 Cochliobolus species. Both Curvularia and B ipolaris states were associated with species in Cochliobolus Group 2.