The European S and F intersterility groups of Heterobasidion annosum may represent sympatric protospecies

In those regions of Europe where they coexist, the F and S intersterility groups (ISGs) of Heterobasidion annosum (Fr.) Bref. are primarily found on Abies spp. and Picea abies (L.) Karst., respectively. Eighty-three isolates of H. annosum were collected from Abies alba Mill. from 19 sites in Italy, including 10 Abies–Picea mixed conifer stands in the eastern Alps. The ISGs of a subsample of 34 isolates were determined by ISG-diagnostic arbitrary-primed (AP) PCR primers. For a subsample of 16 isolates, including two S isolates from Norway and one S isolate from California, nuclear markers generated by AP-PCR analysis, and mitochondrial markers generated by restriction fragment length polymorphisms and sequencing of the ML5–ML6 region of the mitochondrial large ribosomal RNA gene indicated that, in Europe, (i) the F and S ISGs can be found in the same forest stand but they are two genetically distinct units with restricted gene flow between them; (ii) each of the two ISGs is monophyletic and may lack strong genetic substructuring in subpopulations; and (iii) the two ISGs are closely related to each other and their nearest common close relative is the allopatric S ISG from North America. By combining these results with paleobotanical information and results from previous studies, we postulate a recent sympatric divergence of these two groups driven by differential host specificity and mating barriers.