Populations of Armillaria mellea (Basidiomycota, Agaricales) across much of its range
are heterothallic; homothallic populations occur only in Africa (A. mellea ssp. africana),
China (China Biological Species CBS G), and Japan (A. mellea ssp. nipponica). Monosporous
isolates of heterothallic A. mellea are haploid and their mating behaviour is consistent
with the requirement of two different alleles at two mating-type loci (tetrapolar
mating system) to create a diploid individual. In contrast, monosporous isolates of
homothallic A. mellea are putatively diploid; they bypass the haploid phase by undergoing
karyogamy in the basidium (a unique type of secondary homothallism/pseudohomothallism).
In order to determine the genetic origin of this homothallism, we analyzed genetic
variation of 47 heterothallic isolates from China, Europe, and North America, and
14 homothallic isolates from Africa, China, and Japan. Gene trees and mutational networks
were constructed for partial mitochondrial gene ATP synthase subunit 6 (ATP6) and
for the following nuclear genes: actin (ACTIN), elongation factor subunit 1-alpha
(EFA), glyceraldehyde 3-phosphate dehydrogenase (GPD), and the RNA polymerase subunit
II (RPB2). Homothallic isolates from Africa and Japan shared a common mitochondrial
ATP6 haplotype with homothallic isolates from China, and are likely introductions.
Homothallic isolates 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 homothallic isolates
from China is indicative of hybridization between lineages originating from China
and Europe.