Outcrossing or inbreeding: DNA markers provide evidence for type of reproductive mode in Phellinus nigrolimitatus (Basidiomycota)

The aim of this study was to clarify the reproductive mode in the wood-rotting fungus Phellinus nigrolimitatus, using molecular markers and pairing experiments of cultural isolates. A 511 bp segment of the translation elongation factor 1a (EF1a) gene and the internal and intergenic transcribed spacer sequences (ITS and IGS) of the nuclear ribosomal DNA (nrDNA), were amplified by PCR from somatic cultures derived directly from basidiomes and single spore cultures of the fungus. Sequence analyses of the partial EF1a sequence (511 bp) and ITS (732 bp) of single spore cultures gave five and fourteen variable sites, respectively. Five somatic isolates yielded double ITS sequences, which suggested the existence of different ITS types within the same isolate. Restriction digestion of the partial EF1a and the complete ITS and IGS (3.4 ± 0.2 kbp) sequences with Hinfl and Haelll gave restriction fragment length polymorphisms (RFLPs) among the isolates. The sum of fragments exceeded the lengths of the initial PCR product in several somatic isolates, which indicated the existence of EF1a and nrDNA heterotypes within isolates. Segregation of ITS and IGS RFLPs in single spore isolates suggested heterokaryosis in somatic isolates of P. nigrolimitatus. The polymorphic EF1a and ITS restriction sites were used as codominant markers in a test for Hardy-Weinberg expectations. The molecular data suggests a predominant outcrossing reproductive mode in P. nigrolimitatus. Results from cultural pairing experiments with single spore isolates corresponded poorly with the molecular data, suggesting that laboratory pairing experiments do not necessarily provide reliable information of the mode of reproduction in natural populations of fungi.