Polymorphism at the ribosomal DNA spacers and its relation to breeding structure of the widespread mushroom Schizophyllum commune

The common split-gilled mushroom Schizophyllum commune is found throughout the world on woody substrates. This study. addresses the dispersal and popu lation structure of this fungal species by, studying the phylogeny and evol utionary dynamics of ribosomal DNA (rDNA) spacer regions. Extensive samplin g (N = 195) of sequences of the intergenic spacer region (IGS1) revealed a large number of unique haplotypes (n = 143). The phylogeny of these IGS1 se quences revealed strong geographic patter ns and supported three evolutiona rily distinct lineages within the global population. The same three geograp hic lineages were found in phylogenetic analysis of both other I DNA spacer regions (IGS2 and ITS). However, nested clade analysis of the IGS1 phyloge ny suggested the population structure of S. commune has undergone recent ch anges, such as a long distance colonization of western North America fr-om Europe as well as a recent range expansion in the Caribbean. Among all spac er regions, variation in length and nucleotide sequence was observed betwee n but not within the tandem rDNA repeats (arrays). This pattern is consiste nt with strong within-array and weak among-array homogenizing forces. We pr esent evidence for the suppression of recombination between rDNA arrays on homologous chromosomes that may account for this pattern of concerted evolu tion.