Habitat preference and the evolution of sympatric intersterility groups in the Heterbasidion annosum species complex
Populations of the basidiomycete Heterobasidion annosum display varying degrees, of intersterility and differential host specialization. At least three intersterility groups have been formally described, each characterized by a range of "preferred" hosts. It has been hypothesized that processes of host-pathogen compatibility may have been driving the evolution of sympatric populations of this fungus. Host specialization may also determined a selective disadvantage on inter-ISG hybrids, and keep the geene [gene] pools of these otherwise partially interfertile populations (intersterility in fact is only partial) separate. Molecular data generated in the past five years is indicating that in fact there are more than three genetically distinct groups worldwide. The geographic distribution of these groups suggests that allopatric processes are involved in the evolution of distinct fungal populations.
The authors analyzed the relationships among European and North American populations of H. annosum with a range of nuclear and mitochondrial genetic markers. Results indicate that (a) each population (intended here as all the individuals belonging to the same ISG and from the same world region) is genetically well distinct from the others; and (b) the European S and F ISGs as the two closest groups and have probably diverged more recently than the others.Because the European S and F ISGs are found preferentially on different hosts (Picea and Abies spp.respectively), it is plausible that host specificity may be driving the evolution of this organism. Furthermore, because (1) both ISGs are present (sometimes even on the same stump) in European mixed conifer forests; and (2) paleobotanical data indicate Picea and Abies spp. shared in the past a largely overlapping geographic range, these results support the hypothesis of sympatric speciation in the H. annosum complex. To provide evidence for possible mechanisms of sympatric speciation in H. annosum, the authors have used North American S, P, and field SP hybrid isolates in greenhouse inoculation experiments. SP isolates were significantly less virulent than P isolates on P-hosts (Pinus spp.) and significantly less virulent than S isolates on S-hosts (Abies, Tsuga), but were as virulent as S or P isolates on the greenhouse ?universal? host Sitka spruce.These results support the hypothesis that inter-ISG hybrids may be at a selective disadvantage in nature and provide evidence that mechanisms of host-pathogen interaction act as a driving force or as a reinforcement of the genetic isolation of the two North American ISG, which in the laboratory show moderate levels of infertility.