Somatic incompatibility and nuclear reassortment in Heterobasidion annosum

Abstract

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