However, several variations in the life cycle occur among Ustilaginaceae

species. For instance, their systemic growth ability differs according to the plant organ infected (root, leaf or flower) (see Vánky, 1994). Among these variations, the role of solopathogenic strains was poorly investigated although such strains were considered as useful genetic tools. In the literature, solopathogenic strains of Ustilaginaceae were isolated from the progeny of in vitro mated haploid and compatible yeast strains, either wild types (Ehrlich, 1958; Puhalla, 1968) or auxotrophic mutants (Holliday, 1974; MEK inhibitor Harrison & Sherwood, 1994). Our strategy was to isolate solopathogenic strains from germinating teliospores to evaluate and compare the production of such spores under control conditions by three different smut fungi. In Ustilaginaceae,

dikaryotic cells are unstable and revert to haploid yeasts (Trueheart & Herskowitz, 1992), whereas solopathogenic strains are stable in axenic culture. RG7422 purchase This characteristic was used to eliminate fuzzy-dikaryotic strains from successive subcultures. Using this protocol, here we report the first isolation of a naturally occurring solopathogenic strain of S. reilianum, SRZS1. Nucleus staining revealed that SRZS1 is monokaryotic. The amplification and restriction enzyme digestion of mating type genes showed that SRZS1 has the two MATb alleles provided by the parental strains SRZM (MATb2) and SRZN (MATb1). This result is in agreement with the hypothesis that this monokaryotic strain is diploid. However, the strategy used did not allow us to exclude that the presence of the compatible

allele could also be the result of a parasexual transfer leading to the formation of a merodiploid strain (Zeigler et al., 1997), although such a mechanism has not been observed as yet in solopathogenic strains of U. maydis. Using specific primers of S. reilianum, PCR detection of SRZS1 in caulinar apices after crown infection showed that the strain is infectious. Its pathogenicity is weak as colonization did not lead to the formation of a sorus. We obtained similar conclusions with two other solopathogenic strains isolated from a poly-teliosporal sample (Table 1): inoculated plants present symptoms (dwarf Erythromycin plants and/or chlorotic spots on leaves), but did not develop smutted ears. Although they are infectious, the solopathogenic strains of S. reilianum seem unable to perform the entire life cycle of the fungus. We compared the ability of teliospores from M. penicillariae, S. reilianum and U. maydis to form solopathogenic cells. Surprisingly, all strains formed by the M. penicillariae were solopathogenic. It has already been described that monoisolates of this species can be infectious (Wilson & Bondari, 1990). Under our assay conditions, the solopathogenicity of monoisolates formed after teliospore germination is the usual cell status.