Massive and Widespread Organelle Genomic Expansion in the Green Algal Genus Dunaliella

The mitochondrial genomes of chlamydomonadalean green algae are renowned for their highly reduced and conserved gene repertoires, which are almost fixed at 12 genes across the entire lineage. The sizes of these genomes, however, are much more variable, with some species having small, compact mitochondrial DNAs (mtDNAs) and others having expanded ones. Earlier work demonstrated that the halophilic genus Dunaliella contains extremely inflated organelle genomes, but to date the mtDNA of only one isolate has been explored. Here, by surveying mtDNA architecture across the Chlamydomonadales, we show that various Dunaliella species have undergone massive levels of mitochondrial genomic expansion, harboring the most inflated, intron-dense mtDNAs available from chlorophyte green algae. The same also appears to be true for their plastid genomes, which are potentially among the largest of all plastid-containing eukaryotes. Genetic divergence data are used to investigate the underlying causes of such extreme organelle genomic architectures, and ultimately reveal order-of-magnitude differences in mitochondrial versus plastid mutation rates within Dunaliella.