Evolutionary Extremes in the Organelle Genomes of the Ancient Parasitic Plant Hydnora visseri (Hydnoraceae, Piperales)

Plastid genomes of photosynthetic flowering plants are usually highly conserved in both structure and gene content. However, the plastomes of parasitic and mycoheterotrophic plants may be released from selective constraint due to the reduction or loss of photosynthetic ability. The nonphotosynthetic plant Hydnora visseri (Hydnoraceae, Piperales) is particularily interesting for several reasons, one is being among the oldest extant parasitic lineages.Genomic sequence data of Hydnora allowed us to identify and characterize its organellar genomes. The plastome is greatly reduced in size (only 27 kb in length), encoding 24 highly divergent genes. Several complementary approaches using the highest possible sensitivity were required for identification and annotation of this plastome. Despite the extreme reduction of the genome and high sequence divergence, the presence of syntenic, long transcriptionally-active open reading frames with distant similarity to other plastid genomes and a high plastome stoichiometry relative to the mitochondrial and nuclear genomes suggests that the plastome remains functional in Hydnora.In contrast to the plastome, the mitochondrial genome of Hydnora visseri is among the larger ones across the angiosperms (1.63 MB in length). The gene set is surprisingly conservative, representing a fairly complete and functional set of common plant mitochondrial genes.We demonstrate that divergent and reduced plastomes can now be detected in genomic NGS sequence data, where “traditional” methods required extensive effort. This greatly completes (and may change) our understanding of organellar genome evolution in nonphotosynthetic plants.