Plastid Genomes of the Hemiparasitic GenusKrameria(Zygophyllales) Are Intact and Exhibit Little Relaxation in Selection

Abstract

Premise of research. Parasitic plants are characterized by a reduced or absent ability to conduct photosynthesis and accompanying morphological, physiological, and genomic changes. The plastid genome (or plastome) houses many key photosynthetic genes and is consequently highly conserved in autotrophic plants. This molecule is thus a useful model for documenting the genomic effects of a loss of autotrophy, which is typically associated with some reduction in plastome size and coding content. Twelve lineages of angiosperms have seen independently evolved haustorial parasitism. One of these lineages, Krameria, is a genus of obligate hemiparasites that appears to subvert the expectation of plastome reduction and instead has a substantially longer plastid genome than its nearest photosynthetic relatives.Methodology. Two plastid genomes have been reported from this genus but have not yet been analyzed in depth. This study adds a third assembled Krameria plastome and then investigates their structure and sequence composition in comparison with that of the autotrophic Tribulus terrestris from the group’s sister clade.Pivotal results. We find that Krameria plastomes have essentially intact coding sequences and that the unexpected increase in their sizes is due to the accumulation of elevated numbers of tandem repeats in the intergenic spaces of the large and small single-copy regions. Photosynthetic genes are maintained under purifying selection with dN/dS values commensurate with those observed in lineages of autotrophic plants.Conclusions. Krameria contains both the largest and the most intact plastid genomes reported to date from parasitic angiosperms. Our results suggest that these plants are still reliant on photosynthesis as an important part of their nutrient acquisition strategy and that plastid genomes of Krameria remain evolutionarily stable.