Chromalveolates and the Evolution of Plastids by Secondary Endosymbiosis

The establishment of a new plastid organelle by secondary endosymbiosis represents a series of events of massive complexity, and yet we know it has taken place multiple times because both green and red algae have been taken up by other eukaryotic lineages. Exactly how many times these events have succeeded, however, has been a matter of debate that significantly impacts how we view plastid evolution, protein targeting, and eukaryotic relationships. On the green side it is now largely accepted that two independent events led to plastids of euglenids and chlorarachniophytes. How many times red algae have been taken up is less clear, because there are many more lineages with red alga-derived plastids (cryptomonads, haptophytes, heterokonts, dinoflagellates and apicomplexa) and the relationships between these lineages are less clear. Ten years ago, Cavalier-Smith proposed that these plastids were all derived from a single endosymbiosis, an idea that was dubbed the chromalveolate hypothesis. No one observation has yet supported the chromalveolate hypothesis as a whole, but molecular data from plastid-encoded and plastid-targeted proteins have provided strong support for several components of the overall hypothesis, and evidence for cryptic plastids and new photosynthetic lineages (e.g. Chromera) have transformed our view of plastid distribution within the group. Collectively, these data are most easily reconciled with a single origin of the chromalveolate plastids, although the phylogeny of chromalveolate host lineages (and potentially Rhizaria) remain to be reconciled with this plastid data.