Cyanobacterial Contribution to Algal Nuclear Genomes Is Primarily Limited to Plastid Functions

A single cyanobacterial primary endosymbiosis that occurred approximately 1.5 billion years ago [1-3] is believed to have given rise to the plastid in the common ancestor of the Plantae or Archaeplastida-the eukaryotic supergroup comprising red, green (including land plants), and glaucophyte algae [4-8]. Critical to plastid establishment was the transfer of endosymbiont genes to the host nucleus (i.e., endosymbiotic gene transfer [EGT]) [9, 10]. It has been postulated that plastid-derived EGT played a significant role in plant nuclear-genome evolution, with 18% (or 4,500) of all nuclear genes in Arabidopsis thaliana having a cyanobacterial origin with about one-half of these recruited for nonplastid functions [11]. Here, we determine whether the level of cyanobacterial gene recruitment proposed for Arabidopsis is of the same magnitude in the algal sisters of plants by analyzing expressed-sequence tag (EST) data from the glaucophyte alga Cyanophora paradoxa. Bioinformatic analysis of 3,576 Cyanophora nuclear genes shows that 10.8% of these with significant database hits are of cyanobacterial origin and one-ninth of these have nonplastid functions. Our data indicate that unlike plants, early-diverging algal groups appear to retain a smaller number of endosymbiont genes in their nucleus, with only a minor proportion of these recruited for nonplastid functions.