Ancient Horizontal Gene Transfers from Plastome to Mitogenome of a Nonphotosynthetic Orchid, Gastrodia pubilabiata (Epidendroideae, Orchidaceae)

Gastrodia pubilabiata is a nonphotosynthetic and mycoheterotrophic orchid belonging to subfamily Epidendroideae. Compared to other typical angiosperm species, the plastome of G. pubilabiata is dramatically reduced in size to only 30,698 base pairs (bp). This reduction has led to the loss of most photosynthesis-related genes and some housekeeping genes in the plastome, which now only contains 19 protein coding genes, three tRNAs, and three rRNAs. In contrast, the typical orchid species contains 79 protein coding genes, 30 tRNAs, and four rRNAs. This study decoded the entire mitogenome of G. pubilabiata, which consisted of 44 contigs with a total length of 867,349 bp. Its mitogenome contained 38 protein coding genes, nine tRNAs, and three rRNAs. The gene content of G. pubilabiata mitogenome is similar to the typical plant mitogenomes even though the mitogenome size is twice as large as the typical ones. To determine possible gene transfer events between the plastome and the mitogenome individual BLASTN searches were conducted, using all available orchid plastome sequences and flowering plant mitogenome sequences. Plastid rRNA fragments were found at a high frequency in the mitogenome. Seven plastid protein coding gene fragments (ndhC, ndhJ, ndhK, psaA, psbF, rpoB, and rps4) were also identified in the mitogenome of G. pubilabiata. Phylogenetic trees using these seven plastid protein coding gene fragments suggested that horizontal gene transfer (HGT) from plastome to mitogenome occurred before losses of photosynthesis related genes, leading to the lineage of G. pubilabiata. Compared to species phylogeny of the lineage of orchid, it was estimated that HGT might have occurred approximately 30 million years ago.