Multi-omics analysis of the symbiotic green algae, Chlorella variabilis, revealing the genetic basis of the obligate endosymbiotic lifestyle

Abstract

Background: Photosynthetic eukaryotes have evolved through the acquisition of plastids by secondary endosymbiosis, a process that requires several steps. Immediately before plastid acquisition, the genome of the symbiont is known to be dramatically reduced, but few studies have focused on the genomic changes in the symbiont at the early stages of secondary endosymbiosis. Methods: To investigate the genetic basis of the transition from facultative to obligate endosymbiosis, we compared the genomes of Chlorella variabilis, a representative symbiotic alga, with that of Paramecium bursaria, to compare closely related free-living species and transcriptomes between organisms in symbiotic and non-symbiotic conditions. Results: We found that the non-reduced genome of C. variabilis and its genes play a crucial role in endosymbiosis, being involved in cell wall biogenesis and degradation, and metabolic exchanges with the host. Our results suggest that the genetic mechanism underlying the enhancement of photosynthesis under symbiosis is the increasing light absorption efficiency and carbon fixation capacity of the endosymbiont, resulting in an increase in the supply of maltose to P. bursaria.