Archaeplastida

The Archaeplastida (or Plantae sensu lato) are a major group of eukaryotes, comprising the red algae (Rhodophyta), the green algae and the land plants, together with a small group of freshwater unicellular algae called glaucophytes. The chloroplasts of all these organisms are surrounded by two membranes, suggesting that they developed directly from endosymbiotic cyanobacteria. In all other groups besides the amoeboid Paulinella chromatophora, the chloroplasts are surrounded by three or four membranes, suggesting that they were acquired secondarily from red or green algae.

Although many studies have suggested that the Archaeplastida form a monophyletic group, a 2009 paper argues that they are in fact paraphyletic. The enrichment of novel red algal genes in a recent study demonstrates a strong signal for Plantae (Archaeplastida) monophyly and an equally strong signal of gene sharing history between the red/green algae and other lineages. This study provides insight on how rich mesophilic red algal gene data is crucial for testing controversial issues in eukaryote evolution and for understanding the complex patterns of gene inheritance in protists.