Phylogenetic and differentiation analysis of the trunk rot pathogen F omitiporia tsugina in North America

Fomitiporia hartigii s . l . is an important trunk rot pathogen of conifers throughout the N orthern H emisphere. In N orth A merica, this pathogen primarily attacks T suga , but is also found on A bies , P icea and P seudotsuga . Previous research showed that isolates of F . hartigii from N orth A merica represent a distinct phylogenetic species, known as F . tsugina . However, that conclusion is based on limited data. To better understand the phylogenetic relationships of F . tsugina in N orth A merica, a phylogenetic analysis was performed using three loci (internal transcribed spacer, nuclear large subunit and tef1 ) with 23 isolates originating from the northern U nited S tates, C anada and central E urope. N orth A merican isolates formed a monophyletic group with significant statistical support, confirming previous reports that F . tsugina represents a unique phylogenetic species, distinct from European F . hartigii s.s . Population subdivision between isolates of F . tsugina from eastern ( M assachusetts, M ichigan, N ew H ampshire, N ew Y ork and W isconsin) and western ( B ritish C olumbia, C olorado, O regon and W ashington) N orth A merica was investigated. Tests of gene flow and genetic differentiation based on region of origin detected significant variation ( F ST = 0.761; K ST = 0.625, p < 0.01), suggesting gene flow between the two populations may be limited. Neutrality tests revealed significant, negative departures from the standard neutral model, which could indicate that a purifying or stabilizing selection has maintained low levels of polymorphisms in the population, perhaps favouring an advantageous phenotype. Further studies are required to better understand the occurrence of F . tsugina on A bies and P icea in boreal forests outside the natural range of T suga .