Plastomes on the edge: the evolutionary breakdown of mycoheterotroph plastid genomes

Summary We examine recent evidence for ratchet‐like genome degradation in mycoheterotrophs, plants that obtain nutrition from fungi. Initial loss of the NADH dehydrogenase‐like ( NDH ) complex may often set off an irreversible evolutionary cascade of photosynthetic gene losses. Genes for plastid‐encoded subunits of RNA polymerase and photosynthetic enzymes with secondary functions (Rubisco and ATP synthase) can persist initially, with nonsynchronous and quite broad windows in the relative timing of their loss. Delayed losses of five core nonbioenergetic genes (especially trn E and acc D, which respectively code for glutamyl tRNA and a subunit of acetyl‐CoA carboxylase) probably explain long‐term persistence of heterotrophic plastomes. The observed range of changes of mycoheterotroph plastomes is similar to that of holoparasites, although greater diversity of both probably remains to be discovered. These patterns of gene loss/retention can inform research programs on plastome function.ContentsSummary 48 I. Introduction 48 II. Modelling plastid genome degradation in mycoheterotrophs 50 III. Conclusions: a modified model for plastome degradation, and future research directions 53Acknowledgements 54References 54