Relative rates of evolution among the three genetic compartments of the red alga Porphyra differ from those of green plants and do not correlate with genome architecture.

In photosynthetic eukaryotes, relative silent-site nucleotide substitution rates (which can be used to approximate relative mutation rates) among mitochondrial, plastid, and nuclear genomes (mtDNAs, ptDNAs, and nucDNAs) are estimated to be 1:3:10 respectively for seed plants and roughly equal for green algae. These estimates correlate with certain genome characteristics, such as size and coding density, and have therefore been taken to support a relationship between mutation rate and genome architecture. Plants and green algae, however, represent a small fraction of the major eukaryotic plastid-bearing lineages. Here, we investigate relative rates of mutation within the model red algal genus Porphyra. In contrast to plants, we find that the levels of silent-site divergence between the Porphyra purpurea and Porphyra umbilicalis mtDNAs are three times that of their ptDNAs and five times that of their nucDNAs. Moreover, relative mutation rates do not correlate with genome architecture: despite an estimated three-fold difference in their mutation rate, the mitochondrial and plastid genome coding densities are equivalent - an observation that extends to organisms with secondary red algal plastids. These findings are supported by within-species silent-site polymorphism data from P. purpurea.