Transmission of antibiotic-resistance markers by hyphal fusion suggests partial presence of parasexuality in the root endophytic fungus Glutinomyces brunneus

Abstract

The genetic-exchange process of some endophytic root fungi whose teleomorphs have not been discovered is unknown. To assess parasexuality in the root-colonizing fungus Glutinomyces brunneus, we investigated the transmission of genetic markers in this fungus. We generated strains resistant to benomyl and hygromycin B and performed co-cultivation; dual resistance was used as a selection marker for genetic exchange between hyphae. Co-cultivation of benomyl- and hygromycin B-resistant strains yielded significantly larger numbers of dual-resistant colonies than did co-cultivation of strains resistant to the same agent, which suggests genetic exchange between the two vegetative hyphae. This was not simply due to heterokaryosis because all dual-resistant strains were monokaryotic. Also, half of the analyzed dual-resistant strains were homozygous in β-tubulin gene which is associated with benomyl resistance; thus, their simple diploidization was rejected for these strains. During selective cultivation, temporal hyphal growth occurred from the point at which two hyphae touched, which stopped after 2 weeks; this was followed by growth of true dual-resistant hyphae. Transmission of resistance markers was more frequent during co-cultivation on 2% malt extract agar than on other media, which suggests a role of medium composition. Genetic exchange between resistant strains that did not share an ancestor was limited, which implies vegetative incompatibility in this fungus. Our findings suggest the partial presence of parasexual potential in G. brunneus.