SOMATIC INCOMPATIBILITY AND NUCLEAR REASSORTMENT IN HETEROBASIDION-ANNOSUM

Somatic incompatibility in Heterobasidion annosum is expressed as a na rrow gap between interacting secondary mycelia. The gap has sparse aer ial hyphae, and few conidiophores. Hyphae on or in agar, by contrast m ingle freely. Anastomoses were observed between incompatible hyphae in micro-culture chambers, and no evidence of cell death or disruption w as noted, although there was evidence of changed metabolic activity in interaction zones. Secondary mycelia of H. annosum contain both clamp ed and non-clamped hyphae. The latter usually produce conidia of a sin gle mating type, and are apparently homokaryotic. It is hypothesized t hat anastomoses between homokaryotic hyphae in the gap between incompa tible heterokaryotic mycelia result in the localized establishment of new heterokaryotic combinations. Some heterokaryotic hyphae isolated f rom gaps as single hyphal tips were somatically incompatible with eith er progenitor, and contained one mating-type allele from each progenit or. This novel mechanism of nuclear reassortment may allow new outcome s when individuals meet and compete in roots and stumps, resulting in an enhanced capability to maintain fitness in the population.