Gene flow and intersterility between allopatric and sympatric populations of Heterobasidion abietinum and H. parviporum in Europe

The tree pathogenic fungi Heterobasidion parviporum and H. abietinum are recently described sibling species that show marked host preference for Picea abies and Abies alba, respectively. H. parviporum shows a higher reproductive character displacement (i.e. higher intersterility) towards H. abietinum in sympatry than in allopatry. We inspected whether this pattern was also present in H. abietinum by studying the inter-sterility and gene flow amongst sympatric central European populations of H. abietinum and H. parviporum, and two isolated allopatric populations, Pyrenean H. abietinum and Scandinavian H. parviporum. Inter- and intra-specific fixation index values were calculated from DNA sequence data of the GST-1 locus and four microsatellite loci. Present allopatric and sympatric populations of H. abietinum were equally inter-sterile towards H. parviporum, not suggestive of relaxed reinforcement. Several hypotheses explaining the observed pattern are presented. Genetic differentiation was observed between H. abietinum in the Pyrenees and in the Alps, suggesting geographical structure of H. abietinum in Europe.