Complex outcome of reciprocal exchange of nuclear DNA between two members of the basidiomycete genus Stereum

Abstract

Strains of the basidiomycete fungi Stereum hirsutum from the USSR and Stereum complicatum from the eastern USA were interfertile, yielding viable, variable, meiotic progeny. However, interactions between paired homokaryotic mycelia of these species were unstable, yielding a uniform mycelial mat on the S. complicatum side, but a macroscopically heterogeneous mycelium on the S. hirsutum side. Hyphal tips excised from the interactions gave rise to colonies identifiable as one or other of the progenitor strains or two non-progenitor types (C-types and H-types). C-types were all morphologically similar to one another and resembled the S. complicatum progenitor except in being inaccessible to nuclear migration and in exhibiting a light-induced phenotypic switch. They could be isolated from both sides of interactions and were somatically compatible with one another. H-types were only isolated from the S. hirsutum side of interactions and had a range of morphologies, generally similar to the S. hirsutum progenitor. Pairings between H-types, and between H-types and C-types sometimes resulted in pigmented interaction zones due to somatic incompatibility. Single, dual and minisatellite DNA probes were used to investigate restriction fragment length polymorphisms in progenitor and derivative strains. Nuclear, but not mitochondrial DNA from both species was detected in all non-progenitor strains obtained from beyond the interaction interface. However, whereas S. complicatum-specific nuclear sequences were consistently present, the distribution of S. hirsutum-specific sequences varied, and was partly correlated with possession of H-type morphologies.