Multiple gene phylogenies support the monophyly of cryptomonad and haptophyte host lineages.

Cryptomonad algae acquired their plastids by the secondary endosymbiotic uptake of a eukaryotic red alga. Several other algal lineages acquired plastids through such an event [1], but cryptomonads are distinguished by the retention of a relic red algal nucleus, the nucleomorph [2]. The nucleomorph (and its absence in other lineages) can reveal a great deal about the process and history of endosymbiosis, but only if we know the relationship between cryptomonads and other algae, and this has been controversial. Several recent analyses have suggested a relationship between plastids of cryptomonads and some or all other red alga-containing lineages [3-6], but we must also know whether host nuclear genes mirror this relationship to determine the number of endosymbiotic events, and this has not been demonstrated. We have carried out an expressed sequence tag (EST) survey of the cryptomonad Guillardia theta. Phylogenetic analyses of 102 orthologous nucleus-encoded proteins (18,425 amino acid alignment positions) show a robust sister-group relationship between cryptomonads and the haptophyte algae, which also have a red secondary plastid. This relationship demonstrates that loss of nucleomorphs must have taken place in haptophytes independently of any other red alga-containing lineages and that the ancestor of both already contained a red algal endosymbiont.