A cyanobacterial gene in nonphotosynthetic protists - An early chloroplast acquisition in eukaryotes?

Since the incorporation of mitochondria and chloroplasts (plastids) into the eukaryotic cell by endosymbiosis [1], genes have been transferred from the organellar genomes to the nucleus of the host [2 - 4], via an ongoing process known as endosymbiotic gene transfer [5]. Accordingly, in photosynthetic eukaryotes, nuclear genes with cyanobacterial affinity are believed to have originated from endosymbiotic gene transfer from chloroplasts. Analysis of the Arabidopsis thaliana genome has shown that a significant fraction (2% - 9%) of the nuclear genes have such an endosymbiotic origin [3]. Recently, it was argued that 6-phosphogluconate dehydrogenase (gnd) -the second enzyme in the oxidative pentose phosphate pathway -was one such example [6]. Here we show that gnd genes with cyanobacterial affinity also are present in several nonphotosynthetic protistan lineages, such as Heterolobosea, Apicomplexa, and parasitic Heterokonta. Current data cannot definitively resolve whether these groups acquired the grid gene by primary and/or secondary endosymbiosis or via an independent lateral gene transfer event. Nevertheless, our data suggest that chloroplasts were introduced into eukaryotes much earlier than previously thought and that several major groups of heterotrophic eukaryotes have secondarily lost photosynthetic plastids.