Red Algal Mitochondrial Genomes Are More Complete than Previously Reported

The enslavement of an alpha-proteobacterial endosymbiont by the last common eukaryotic ancestor resulted in large-scale gene transfer of endosymbiont genes to the host nucleus as the endosymbiont transitioned into the mitochondrion. Mitochondrial genomes have experienced widespread gene loss and genome reduction within eukaryotes and DNA sequencing has revealed that most of these gene losses occurred early in eukaryotic lineage diversification. On a broad scale, more recent modifications to organelle genomes appear to be conserved and phylogenetically informative. The first red algal mitochondrial genome was sequenced more than 20 years ago, and an additional 29 Florideophyceae mitochondria have been added over the past decade. A total of 32 genes have been described to have been missing or considered non-functional pseudogenes from these Florideophyceae mitochondria. These losses have been attributed to endosymbiotic gene transfer or the evolution of a parasitic life strategy. Here we sequenced the mitochondrial genomes from the red algal parasite Choreocolax polysiphoniae and its host Vertebrata lanosa and found them to be complete and conserved in structure with other Florideophyceae mitochondria. This result led us to resequence the previously published parasite Gracilariophila oryzoides and its host Gracilariopsis andersonii, as well as reevaluate reported gene losses from published Florideophyceae mitochondria. Multiple independent losses of rpl20 and a single loss of rps11 can be verified. However by reannotating published data and resequencing specimens when possible, we were able to identify the majority of genes that have been reported as lost or pseudogenes from Florideophyceae mitochondria.