Are red algae plants? A critical evaluation of three key molecular data sets

Whether red algae are related to green plants has been debated for over a century. Features present due to their shared photosynthetic habit have been interpreted as support for an evolutionary sisterhood of the two groups but, until very recently, characters endogenous to the host cell have provided no reliable indication of such a relationship. In this investigation, we examine three molecular data sets that have provided key evidence of a possible relationship between green plants and red algae. Analyses of an expanded alignment of DNA-dependent RNA polymerase II largest subunit sequences indicate that their support for independent origins of rhodophytes and chlorophytes is not the result of long-branch attraction, as has been proposed elsewhere. Differences in the pol II C-terminal domain, an essential component of plant mRNA transcription, also suggest different host cell ancestors for the two groups. In contrast, concatenated sequences of two groups of mitochondrial genes, those encoding subunits of NADH-dehydrogenase as well as cytochrome c oxidase subunits plus apocytochrome B, appear to cluster red algal and green plant sequences together because both groups have evolved relatively slowly and share a super-abundance of ancestral positions. Finally, analyses of elongation factor 2 sequences demonstrate a strong phylogenetic signal favoring a rhodophyte/chlorophyte sister relationship, but that signal is restricted to a contiguous segment comprising approximately half of the EF2 gene. These results argue for great caution in the interpretation of phylogenetic analyses of ancient evolutionary events but, in combination, indicate that there is no emerging consensus from molecular data supporting a sister relationship between red algae and green plants.