Linear Plasmids in the Phytopathogenic Fungus Claviceps Purpurea

Abstract

The ascomycete Claviceps purpurea is a common parasite of cereals and nonagricultural grasses. The life cycle of this fungus involves two morphologically and physiologically different mycelial forms. After infection of young ovaries (by ascospores or conidia), initially the so-called sphac-elial mycelium is formed, which produces masses of conidia. This mycelial form may be propagated in axenic culture without special requirements. In a later stage of infection the mycelium becomes dense and forms the well-known sclerotial stage. Since medieval times these sclerotia were used as drugs; they contain the so-called ergot alkaloids which still have much pharmaceutical importance (for review, see Ref. 2). Because of this bio-technological relevance, C. purpurea has become one of the best investigated fungi, but mainly with respect to the physiology of alkaloid synthesis (see Ref. 14); otherwise, only a few genetic data have been published for C . purpurea. This may be due to its long life cycle, which may be performed only partially in the laboratory: sclerotia are only formed on the host, never in axenic culture. Sclerotia are the starting point for the sexual cycle; they germinate to form perithecia-containing stroma heads. The needle-like ascospores are homokaryotic. Claviceps has been shown to be monoecious or homothallic (3), a characteristic which does not facilitate genetic analysis.