Mycorrhizal fungi are essential for the growth and survival of orchids, which obtain nutrients, including carbon, from the interaction. To obtain the necessary resources for growth, especially at early life history stages, which are non-photosynthetic, orchids interact with mycorrhizal fungi in several fungal families, particularly the Tulasnellaceae and Ceratobasidiaceae. Little is known about the mechanisms that are associated with orchid acquisition of resources from mycorrhizal fungi. One of the first steps in understanding the dynamics of orchid-fungal interactions is to characterize the genomics of the different groups of fungi that are known to form orchid mycorrhizae. None of the mitochondrial genomes of the Tulasnellaceae and Ceratobasidium have been investigated. The genomic DNA of Tulasnella sp. (Tulasnellaceae) and Ceratobasidium sp. (Ceratobasidiaceae) were extracted from pure cultures of single pelotons isolated from two native orchids; Goodyera pubescens and Platanthera lacera (Orchidaceae), respectively. The raw data obtained by the Illumina Hi-Seq system were filtered by BLAST searches and reference mapping with the complete mitogenome sequences of Agaricomycetes. The filtered reads were de novo assembled using Velvet 1.2.10 and Geneious Assembler in Geneious version 10, and ambiguous regions were reassembled under adjusted settings.
Results/Conclusions
The assembled mitogenome of Tulasnella sp. was smaller (37,993 bp) than previously-published mitogenomes of members of the Cantharellales. It nevertheless contained all 15 core protein coding genes (PCGs), 2 rRNA genes, and 46 tRNA genes. The minimal number of genome rearrangements under the model of Double-Cut-and-Join (DCJ) rearrangements was eight between the two mitogenomes of Tulasnella sp. and the other species in Cantharellales. Fifteen scaffolds of Ceratobasidium sp. mitogenome were assembled and the complete mitogenome size was predicted to be >117 kb, which was three times bigger than the Tulasnella sp. mitogenome. Despite having only draft scaffolds, all 15 PCGs and two rRNA genes were detected in them, suggesting again the independence of fungal mitochondrial function from the mycorrhizal association. The similarities and differences between the two mitogenomes for Tulasnellaceae and Ceratobasidiaceae families that were assembled for this study will provide insights into features and evolution of mitogenomes in orchid mycorrhizal fungi.