Gene loss and genome rearrangement in the plastids of five Hemiparasites in the family Orobanchaceae

BACKGROUND: The chloroplast genomes (plastome) of most plants are highly conserved in structure, gene content, and gene order. Parasitic plants, including those that are fully photosynthetic, often contain plastome rearrangements. These most notably include gene deletions that result in a smaller plastome size. The nature of gene loss and genome structural rearrangement has been investigated in several parasitic plants, but their timing and contributions to the adaptation of these parasites requires further investigation, especially among the under-studied hemi-parasites. RESULTS: De novo sequencing, assembly and annotation of the chloroplast genomes of five photosynthetic parasites from the family Orobanchaceae were employed to investigate plastome dynamics. Four had major structural rearrangements, including gene duplications and gene losses, that differentiated the taxa. The facultative parasite Aureolaria virginica had the most similar genome content to its close non-parasitic relative, Lindenbergia philippensis, with similar genome size and organization, and no differences in gene content. In contrast, the facultative parasite Buchnera americana and three obligate parasites in the genus Striga all had enlargements of their plastomes, primarily caused by expansion within the large inverted repeats (IRs) that are a standard plastome feature. Some of these IR increases were shared by multiple investigated species, but others were unique to particular lineages. Gene deletions and pseudogenization were also both shared and lineage-specific, with particularly frequent and independent loss of the ndh genes involved in electron recycling. CONCLUSIONS: Five new plastid genomes were fully assembled and compared. The results indicate that plastome instability is common in parasitic plants, even those that retain the need to perform essential plastid functions like photosynthesis. Gene losses were slow and not identical across taxa, suggesting that different lineages had different uses or needs for some of their plastome gene content, including genes involved in some aspects of photosynthesis. Recent repeat region extensions, some unique to terminal species branches, were observed after the divergence of the Buchnera/Striga clade, suggesting that this otherwise rare event has some special value in this lineage.