Unusual ascospore germination in Hypoxylon fragiforme: first seps in the establishment of an endophytic symbiosis

An unusual germination mechanism in ascospores of Hypoxylon fragiforme is described and illustrated. In this xylariaceous, endophytic fungus, germination always involved the emergence of a bivalved, flexible structure from an outer rigid shell, formed by a differentiated transparent wall layer, and resulted in the exposure of the cell body. The series of fast movements leading to the emergence of activated ascospores from their shells was termed spore eclosion. Eclosion was a necessary initial step of germination, but eclosion without germ-tube production could be obtained by cycloheximide treatment. Major changes involved in eclosion occurred within a few seconds, some minutes after discharge of ascospores onto the host material (Fagus sylvatica). We postulate the existence of a host-derived, diffusible, specific factor eliciting those changes. This sophisticated recognition mechanism indicates a high degree of specialization of H. fragiforme to its endophytic symbiosis with beech trees. Key words: eclosion, tree, Fagus, Hypoxylon, host–fungus recognition.