Multiple cryptic species with divergent substrate affinities in the Serpula himantioides species complex.

Serpula himantioides is a widespread saprotrophic morphospecies mainly colonising coniferous wood in nature, but it appears frequently in buildings as well. From an earlier study, it is known that at least three divergent lineages occur within the S. himantioides species complex. In this study, a broader sample of S. himantioides isolates has been analysed by multi-locus sequencing, including new isolates from Asia, North and South America. Altogether five phylogenetical species (PS1-5) were detected, all recognised across independent gene phylogenies. A new southern South American phylogenetic species (PS1) was found, representing an early diverging lineage within the S. himantioides species complex. The two closely related PS2 and PS3 lineages included isolates from North America only, and PS4 was also dominated by North American isolates. Most of the investigated isolates (76%) clustered into PS5, a lineage that has been found on most continents, including North America. Overall, little phylogeographical structure was found in PS5, indicating frequent and recent long-distance dispersal events within this widespread lineage. Our analyses indicate that South and North America are the centres of divergence for the S. himantioides species complex. Some of the lineages seem adapted to various substrates, but PS5 is able to decay a wide array of angiosperms and gymnosperms, which may have facilitated the spread of this lineage throughout the world.

High variability in a mating type linked region in the dry rot fungus Serpula lacrymans caused by frequency-dependent selection?

BACKGROUND: The mating type loci that govern the mating process in fungi are thought to be influenced by negative frequency-dependent selection due to rare allele advantage. In this study we used a mating type linked DNA marker as a proxy to indirectly study the allelic richness and geographic distribution of mating types of one mating type locus (MAT A) in worldwide populations of the dry rot fungus Serpula lacrymans. This fungus, which causes serious destruction to wooden constructions in temperate regions worldwide, has recently expanded its geographic range with a concomitant genetic bottleneck. RESULTS: High allelic richness and molecular variation was detected in the mating type linked marker as compared to other presumably neutral markers. Comparable amounts of genetic variation appeared in the mating type linked marker in populations from nature and buildings, which contrast the pattern observed with neutral genetic markers where natural populations were far more variable. Some geographic structuring of the allelic variation in the mating type linked marker appeared, but far less than that observed with neutral markers. In founder populations of S. lacrymans, alleles co-occurring in heterokaryotic individuals were more divergent than expected by chance, which agrees with the expectation for populations where few mating alleles exists. The analyzed DNA marker displays trans-species polymorphism wherein some alleles from the closely related species S. himantoides are more similar to those of S. lacrymans than other alleles from S. himantoides. CONCLUSIONS: Our results support the idea that strong negative frequency-dependent selection maintains high levels of genetic variation in MAT-linked genomic regions, even in recently bottlenecked populations of S. lacrymans.

Two invasive populations of the dry rot fungus Serpula lacrymans show divergent population genetic structures.

The dry rot fungus Serpula lacrymans is a devastating basidiomycete occurring in wooden constructions in temperate regions worldwide. In this study, we compare the genetic structures of two invasive populations from Europe and Japan. Microsatellite data from 14 loci and DNA sequences from four loci demonstrated that the two populations were highly differentiated. Significant isolation by distance effect was observed in Europe and Japan. Higher genetic variation was observed within the Japanese population than within the European population, corresponding with the observed higher richness of vegetative compatibility types in Japan, indicating that there has been a higher level of gene flow from the Asian source populations to Japan than to Europe. The European population is genetically more homogenous with only six detected vegetative compatibility types. Various tests indicate that both the European and the Japanese populations have gone through population bottlenecks prior to population expansion. No identical multi-locus genotypes were observed within Japan and very few within Europe, indicating limited clonal dispersal. Deviations from Hardy Weinberg expectations were observed both in Europe and Japan and heterozygote excess were observed at several loci, especially in Europe. Possible explanations for this pattern are discussed.