In this study, we examined the genetic structures of the ambrosia fungus isolated from mycangia of the scolytine beetle, Xylosandrus germanus to understand their co-evolutionary relationships. We analyzed datasets of three ambrosia fungus loci (18S rDNA, 28S rDNA, and the β-tubulin gene) and a X. germanus locus dataset (cytochrome c oxidase subunit 1 (COI) mitochondrial DNA). The ambrosia fungi were separated into three cultural morphptypes, and their haplotypes were distinguished by phylogenetic analysis on the basis of the three loci. The COI phylogenetic analysis revealed three distinct genetic lineages (clades A, B, and C) within X. germanus, each of which corresponded to specific ambrosia fungus cultural morphptypes. The fungal symbiont phylogeny was not concordant with that of the beetle. Our results suggest that X. germanus may be unable to exchange its mycangial fungi, but extraordinary horizontal transmission of symbiotic fungi between the beetle's lineages occurred at least once during the evolutionary history of this symbiosis.
Keywords: Ambrosiella hartigii; Xylosandrus germanus; coevolution; mycangium; phylogeography.