Unraveling the evolutionary trajectory of LHCI in red-lineage algae: Conservation, diversification, and neolocalization

Red algae and the secondary symbiotic algae that engulfed a red alga as an endosymbiont are called red-lineage algae. Several photosystem (PS) I-light-harvesting complex I (LHCI) structures have been reported from red-lineage algae-two red algae <i>Cyanidioschyzon merolae</i> (Cyanidiophyceae) and <i>Porphyridium purpureum</i> (Rhodophytina), a diatom, and a Cryptophyte. Here, we clarified the orthologous relation of LHCIs by combining a detailed phylogenetic analysis and the structural information of PSI-LHCI. We found that the seven Lhcr groups in LHCI are conserved in Rhodophytina; furthermore, during both genome reduction in Cyanidioschyzonales and endosymbiosis leading to Cryptophyta, some LHCIs were lost and replaced by existing or differentiated LHCIs. We denominate "neolocalization" to these examples of flexible reorganization of LHCIs. This study provides insights into the evolutionary process of LHCIs in red-lineage algae and clarifies the need for both molecular phylogeny and structural information to elucidate the plausible evolutionary history of LHCI.