Morphological and molecular characterization of the Tilletia fusca complex

Tilletia fusca (T. bromi) is a species complex comprised of several intergrading taxa infecting wild grass hosts, and is closely related to the wheat bunt species T. controversa, T. tritici and T. laevis. Three varieties were recognized within the complex based on host specificity: T. fusca vars. guyotiana and bromi-tectorum infecting Bromus spp., and T. fusca var. fusca infecting Vulpia spp. Random amplified polymorphic DNA analyses showed distinct separation of the Vulpia-infecting and brome-infecting varieties at less than 10% similarity. Tilletia controversa isolates are more similar to the brome-infecting varieties than to T. fusca var. fusca. Populations of T. fusca var. fusca infecting different species of Vulpia had a relatively low level of similarity (35%). Results of the RAPD analysis were supported by RFLP-ITS data. Examination of the nuclear condition of the primary sporidia showed up to 12% binucleate sporidia in T. fusca var. fusca, and less than 1% binucleate sporidia in the brome-infecting varieties. These findings, in addition to morphological differences in the ornamentation of the teliospores, support the recognition of T. bromi for the brome-infecting varieties as a separate species from T. fusca. Tilletia goloskokovii, a species closely related to the T. fusca complex, occurs on Apera interrupta, and was previously known only from the type specimen collected in the Republic of Kazakhstan. Based on RAPD and RFLP-ITS analyses of collections from central Washington we showed that T. goloskokovii is distinct from T. separata, the only other Tilletia species occurring on an Apera host, and more closely related to T. bromi than to T. fusca. Our results show that molecular and cytological data can supplement existing morphological characters. Additionally, specificity at the host species level may be as important as morphology in delimiting natural taxa among closely related species of Tilletia.