Retention of an apparently functional plastome in an apparently mycoheterotrophic orchid, Dipodium roseum D.L.Jones & M.A.Clem. (Orchidaceae)

Abstract

Context Giving up photosynthetic function is a bold evolutionary step for a plant, yet the evolutionary transition from autotrophy to mycoheterotrophy has occurred dozens of times. Comparing plastome sequences of mycoheterotrophs and autotrophs has identified recurring patterns of gene loss throughout a range of land plants, though more comparisons are required to see how broadly these patterns apply across the diversity of terrestrial plants. Mycoheterotrophy is especially common in Orchidaceae, with more than 40 transitions from autotrophy to mycoheterotrophy in the family. Aims We sought to test generalised patterns of plastome degradation seen in other mycoheterotrophs by comparing two species in the genus Dipodium (Orchidaceae); one species is photosynthetic and the other appears to be a facultative mycoheterotroph species. Methods We sequenced and assembled the plastomes of two Dipodium species and compared the two genomes to identify gene degradation or loss. Results The two plastomes were nearly identical, with no degradation of photosynthesis genes in the putative mycoheterotroph, and both species have undergone loss or pseudogenisation of all plastid ndh (nicotinamide adenine dinucleotide + hydrogen specific dehydrogenase) genes. Conclusions These results contrast with most other comparisons between photosynthetic and likely mycoheterotrophic relatives, where rapid degradation in mycoheterotroph plastome genes is common, and may suggest the leafless Dipodium species are capable of photosynthesis and may be in the early stages of transitioning to a fully heterotrophic lifestyle. Implications Further investigation of trophic transitions in Dipodium, including sequencing more plastomes and measuring photosynthetic capability of the putative heterotrophs, will yield insights into the evolution of plant lineages that lose the ability to photosynthesise.