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

Diversified photosynthetic eukaryotes have evolved through the acquisition of plastid by secondary endosymbiosis, and several steps are required. At the late stage just before plastid acquisition, the genome of symbiont dramatically reduces, but few studies have focused on a genomic change of symbiont at the early stage of secondary endosymbiosis. Therefore, to reveal the genetic basis from facultative to obligate endosymbionts, we compared the genomes of Chlorella variabilis, representative symbiotic algae with Paramecium bursaria, to closely related free-living species and transcriptomes between under symbiotic and non-symbiotic conditions. As a result, we find that the non-reduction genome of C. variabilis and genes playing a crucial role under endosymbiosis, which is involved in the cell wall biogenesis/degradation, metabolic exchange to the host. Furthermore, our results suggest that the genetic mechanism underlying the enhancement of photosynthesis under symbiosis is the increasing light absorption efficiency and carbon fixation capacity, resulting in the massive supply of maltose to the P. bursaria.