Comparative Evolutionary Analysis of Organellar Genomic Diversity in Green Plants

Endosymbiotic bacteria have been reported with extraordinary reduced genome in numerous cases. Many endosymbiotic green algae also show extreme genomic reduction of their nuclear genomes, but they may retain a fully functional plastid genome if they maintain photosynthetic ability or if they can survive outside of their host. In order to better understand how the endosymbiotic lifestyle has affected the organellar genomes of photosynthetic green algae, we generated the complete organellar genome sequences from three green algal endosymbionts (Chlorella heliozoae, Chlorella sp. ATCC30562 and Micractinium conductrix) isolated from a ciliate and heliozoan. Compared with other trebouxiophycean green algae, the three newly sequenced species have regular genome size and gene content in both the mitochondria and the plastid genome, providing no evidence for organellar genomic reduction in these endosymbionts. Instead, the organellar genomes of the three endosymbionts are generally larger and more intron rich than other species of Chlorella. Phylogenetic analysis of plastid and mitochondrial genes demonstrated that M. conductrix clusters together with Chlorella strains, suggesting that it should be considered a species of Chlorella. In addition, the three endosymbionts do not form a monophyletic group, indicating that the endosymbiotic lifestyle has evolved multiple times within Chlorella.