Evidence of natural hybridization among homothallic members of the basidiomycete Armillaria mellea sensu stricto

Abstract Populations of Armillaria mellea( Basidiomycota, Agaricales) across much of its range are heterothallic; homothallicpopulations occur only in Africa ( A. mellea ssp. africana ), China (China Biological Species CBS G), and Japan ( A. mellea ssp. nipponica ). Monosporous isolates of heterothallicA. mellea are haploid and their mating behaviour is consistent with the requirement of two different alleles at two mating-typeloci (tetrapolar mating system) to create a diploid individual. In contrast, monosporous isolates of homothallicA. mellea are putatively diploid; they bypass the haploid phase by undergoing karyogamyin the basidium(a unique typeof secondary homothallism/pseudohomothallism). In order to determine the genetic origin of this homothallism, we analyzed genetic variation of 47 heterothallicisolates from China, Europe, and North America, and 14 homothallicisolates from Africa, China, and Japan. Gene trees and mutational networks were constructed for partial mitochondrial gene ATP synthasesubunit 6 (ATP6) and for the following nuclear genes: actin (ACTIN), elongation factorsubunit 1-alpha (EFA), glyceraldehyde 3-phosphate dehydrogenase(GPD), and the RNA polymerase subunit II (RPB2). Homothallicisolates from Africa and Japan shared a common mitochondrial ATP6 haplotype with homothallicisolates from China, and are likely introductions. Homothallicisolates from China that shared a common mitochondrial haplotype with all European isolates did not share European nuclear haplotypes, as revealed by median-joining networks, but instead clustered with haplotypes from China or were intermediate between those of China and Europe. Such mitochondrial–nuclear discordance in homothallicisolates from China is indicative of hybridization between lineages originating from China and Europe.