Red and Green Algal Monophyly and Extensive Gene Sharing Found in a Rich Repertoire of Red Algal Genes

The Plantae comprising red, green (including land plants), and glaucophyte algae are postulated to have a single common ancestor that is the founding lineage of photosynthetic eukaryotes. However, recent multiprotein phylogenies provide little or no support for this hypothesis. This may reflect limited complete genome data available for red algae, currently only the highly reduced genome of Cyanidioschyzon merolae, a reticulate gene ancestry, or variable gene divergence rates that mislead phylogenetic inference. Here, using novel genome data from the mesophilic Porphyridium cruentum and Calliarthron tuberculosum, we analyze 60,000 novel red algal genes to test the monophyly of red + green (RG) algae and their extent of gene sharing with other lineages. Using a gene-by-gene approach, we find an emerging signal of RG monophyly (supported by ∼50% of the examined protein phylogenies) that increases with the number of distinct phyla and terminal taxa in the analysis. A total of 1,808 phylogenies show evidence of gene sharing between Plantae and other lineages. We demonstrate that a rich mesophilic red algal gene repertoire is crucial for testing controversial issues in eukaryote evolution and for understanding the complex patterns of gene inheritance in protists.