Genetic variation in Cephalosporium gramineum, cause of Cephalosporium stripe

Juliane Carol Griffith Evans

Cephalosporium stripe is a disease of winter wheat caused by the fungus Cephalosporium gramineum that causes significant yield losses. Eastern Washington has a long history of the disease, likely due to its favorable climate. So far, only asexual reproduction of C. gramineum has been observed; it is not known whether this fungus undergoes sexual reproduction. The objective of this study was to assess genetic variation and infer reproductive strategies by studying population genetic structure of C. gramineum. Pathogen isolates were collected from symptomatic winter wheat in WA, other states, and other countries. Two-hundred forty-two isolates were compared quantitatively and qualitatively based on growth rate, color, and texture. Amplified Fragment Length Polymorphism (AFLP) was used to evaluate genetic diversity of isolates. Significant differences in growth rate existed among the C. gramineum isolates tested, ranging from 0.5-2.9 mm/day. Isolates were divided into five textural classes and six color classes, indicating that visual phenotypic differences existed among isolates. AFLP data from 255 isolates collected from the Pacific Northwest (PNW), other regions in the U.S., and Scotland were used to determine that the Index of Association was significantly greater than 0, resulting in rejection of the null hypothesis of random mating, suggesting that C. gramineum is reproducing asexually. Additional analysis revealed that a large amount of genetic variation exists within the species and that 234 of the 255 isolates studied had unique multilocus genotypes based on AFLP data for 40 loci. Nei’s genetic distances were calculated and used in principal coordinate analysis, which showed isolates clustered based on collection date and geography. Analysis of molecular variation (AMOVA) supported the clustering. Resulting significant variation allowed for the inference that C. gramineum isolates contain more variation within than between populations as determined by collection date or location. Overall, asexual reproduction appears to be consistently occurring within C. gramineum among populations tested, and morphological and genetic variation was observed in a large number of isolates over time and space. A large amount of genetic variation was observed among C. gramineum isolates, but reasons for this are not yet understood and require further investigation.

Genetic variation in Cephalosporium gramineum, cause of Cephalosporium stripe

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