Molecular evidence for a single common origin of chromalveolate plastids

Abstract

Plastids (photosynthetic organelles of plants and algae) are known to have spread between eukaryotic lineages by secondary endosymbiosis, or the uptake of a eukaryotic alga by another eukaryote, but the number of times this has taken place is controversial. This is particularly so in the case of eukaryotes with plastids derived from red algae, which are numerous and diverse. Despite their diversity, it has been suggested that all these eukaryotes share a recent common ancestor and that their plastids originated in a single endosymbiosis, the so‐called “chromalveolate hypothesis”. Here, we describe a novel molecular character that supports the chromalveolate hypothesis. Fructose‐1,6‐bisphosphate aldolase (FBA) is a glycolytic/Calvin cycle enzyme that exists as one of two non‐homologous types, class I and class II. Red algal plastid‐targeted FBA is a class I enzyme related to homologues from plants and green algae, and it would be predicted that the plastid‐targeted FBA from algae with red algal secondary endosymbionts should be related to this class I enzyme. However, we show that heterokonts, cryptomonads, haptophytes and dinoflagellates (all photosynthetic chromalveolates) encode two paralogous class II enzymes, one of which strongly supports a specific relationship between chromalveolates. Exactly where chromalveolates got their plastid‐targeted FBA genes is presently uncertain, but their common possession of this unexpected plastid characteristic is compelling evidence for their close relationship and a common origin for their plastids.