Evolutionary dynamics of introns and homing endonuclease ORFs in a region of the large subunit of the mitochondrial rRNA in Glomus species (arbuscular mycorrhizal fungi, Glomeromycota)

The large subunit of the mitochondrial ribosomal RNA genes (mtLSU) has previously been identified as a highly sensitive molecular marker for intraspecies diversity in the arbuscular mycorrhizal fungus Glomus intraradices. In this study, the respective region was analyzed in five species of Glomus (G. mosseae, G. geosporum, G. caledonium, G. clarum, G. coronatum) from the same major clade (Glomus group A), Glomus sp. ISCB 34 from the related Glomus group B and two species of Scutellospora. Results show low level of genetic polymorphism between related morphospecies. Introns homologous to those found in G. intraradices were detected as well as new ones, some of them containing putative ORFs for homing endonucleases (HEs). Introns without ORFs for HEs seem to have been inherited strictly vertically from the ancestors of Glomus groups A and B while other introns indicate occasional horizontal transfer and possibly maintenance, degeneration and loss together with their associated HE ORFs. Overall, we provide first insights into the evolutionary dynamics of introns and HEs in this ecologically important group of fungi, which was previously not analyzed in this respect.