Occurrence of unilateral genetic transfer and genomic replacement between strains of Stereum hirsutum from non‐outcrossing and outcrossing populations

summary Populations of the wood‐decaying basidiomycete, Stereum hirsutum (Willd. ex Fr.) S. F. Gray, from different geographical regions, reproduce either by outcrossing or non‐outcrossing. Outcrossing generates variable progeny whilst non‐outcrossing results in clonal sub‐populations. Pairings between single‐basidiospore‐derived strains from non‐outcrossing and outcrossing populations commonly result in mutual rejection or mycelial invasion of the outcrosser by the non‐outcrosser. However, on the basis of both morphological evidence and studies of DNA restriction fragment length polymorphisms, some pairings result in unilateral transfer of nuclear DNA from the non‐outcrosser into the outcrosser. In some combinations the resultant putative heterokaryon may be stable and give rise to variable basidiospore progeny with unusual development and non‐self recognition responses and at least one mating‐type specificity evidently transferred from the non‐outcrosser. In other combinations, genomic takeover by the non‐outcrosser has been demonstrated, leading respectively to the emergence of mycelial sectors or production of basidiospore progeny in which solely or predominantly non‐outcrossing nuclear DNA is associated with outcrossing mitochondrial DNA. These observations add a new perspective to the selective pressures influencing sexual outcrossing and non‐outcrossing and the role and interplay of these breeding strategies in reproductive isolation and speciation.