Characterization of the organelle genomes of mycoheterotrophic Thismia angustimitra and evidence suggesting horizontal gene transfer

Background Our knowledge about mitochondrial genome evolution in plants is lacking far behind our knowledge of plastome evolution. In heterotrophic plants, it is well-known that plastome degradation and reduced or lost photosynthesis go hand in hand, and there is little if any difference between plastome evolution in mycoheterotrophic and haustorial parasites. The mitochondrial genome appears less affected by a heterotrophic lifestyle, although exceptions occur. In contrast to plastome evolution, mitochondrial evolution may differ between mycoheterotrophic and haustorial parasites, in particular with regards to the occurrence of horizontal gene transfer. However, data from mycoheterotrophs are scarce, and complete mitogenomes are only known from three of ten clades. Results Here we present data from the complete plastome and mitogenome from Thismia angustimitra , representing a fourth clade of mycoheterotrophs. The 18,215 bp plastome is much reduced and includes only few genes. The mitogenome has a multicircular structure consisting of 22 chromosomes with a total length of 422,400 bp. The gene content and the pattern of RNA editing are normal. The mitogenome includes only few repeats and recombinational activity is limited. A search for foreign DNA did not reveal any fungal sequences, and the content of plastid DNA is low. Just a few inserts appear to have originated through intracellular transfer, whereas others most likely stem from horizontal gene transfer. Conclusion The highly reduced plastome of T. angustimitra fits completely the general picture of plastome degradation in heterotrophic plants. Whereas the mitogenome in many aspects appears unaffected by the lifestyle, the recombinational activity is unusually low. Surprisingly, we find evidence of horizontal gene transfer, suggesting that this phenomenon may be more common among mycoheterotrophic plants than previously thought.