Cytonuclear coevolution in a holoparasitic plant with highly disparate organellar genomes.

KEY MESSAGE: Contrasting substitution rates in the organellar genomes of Lophophytum agree with the DNA repair, replication, and recombination gene content. Plastid and nuclear genes whose products form multisubunit complexes co-evolve. The organellar genomes of the holoparasitic plant Lophophytum (Balanophoraceae) show disparate evolution. In the plastid, the genome has been severely reduced and presents a > 85% AT content, while in the mitochondria most protein-coding genes have been replaced by homologs acquired by horizontal gene transfer (HGT) from their hosts (Fabaceae). Both genomes carry genes whose products form multisubunit complexes with those of nuclear genes, creating a possible hotspot of cytonuclear coevolution. In this study, we assessed the evolutionary rates of plastid, mitochondrial and nuclear genes, and their impact on cytonuclear evolution of genes involved in multisubunit complexes related to lipid biosynthesis and proteolysis in the plastid and those in charge of the oxidative phosphorylation in the mitochondria. Genes from the plastid and the mitochondria (both native and foreign) of Lophophytum showed extremely high and ordinary substitution rates, respectively. These results agree with the biased loss of plastid-targeted proteins involved in angiosperm organellar repair, replication, and recombination machinery. Consistent with the high rate of evolution of plastid genes, nuclear-encoded subunits of plastid complexes showed disproportionate increases in non-synonymous substitution rates, while those of the mitochondrial complexes did not show different rates than the control (i.e. non-organellar nuclear genes). Moreover, the increases in the nuclear-encoded subunits of plastid complexes were positively correlated with the level of physical interaction they possess with the plastid-encoded ones. Overall, these results suggest that a structurally-mediated compensatory factor may be driving plastid-nuclear coevolution in Lophophytum, and that mito-nuclear coevolution was not altered by HGT.