The origin of red algae and cryptomonad nucleomorphs: A comparative phylogeny based on small and large subunit rRNA sequences of Palmaria palmata, Gracilaria verrucosa, and the Guillardia theta nucleomorph.

The complete large subunit rRNA sequences from the red algae Palmaria palmata and Gracilaria verrucosa, and from the nucleomorph of the cryptomonad Guillardia theta, were determined in order to assess their phylogenetic relationships relative to each other and to other eukaryotes. Neighbor-joining, maximum-parsimony, and maximum-likelihood trees were constructed on the basis of small subunit rRNA, large subunit rRNA, and a combination of both molecules. Our results support the hypothesis that the cryptomonad plastid is derived from a primitive red alga, in that an ancient common ancestor of rhodophytes and cryptomonad nucleomorphs is indicated. This cluster shows some affinity with chlorobionts, which could point to a monophyletic origin of green and red plastids. However, the exact branching order of the crown eukaryotes remains uncertain and further research is required.