Evolution of a Photosynthetic Organelle

The amoeba Paulinella chromatophora (Figure 1) contains photosynthetic organelles (chromatophores) derived from a primary endosymbiosis (with an α-cyanobacterium) that is independent of the origin of plastids in algae and plants. The nascent organelle has lost many genes, some of which have been transferred to the host genome by endosymbiotic gene transfer (EGT). Some of those transferred genes are light regulated, particularly by blue light.  Furthermore, a number of the proteins synthesized in the cytoplasm of the cell are targeted to the chromatophore where they assemble into macromolecular complexes. 

Recently, to deduce the rules that govern the integration of organelle metabolism into cellular metabolism of the host, we analyzed a high quality transcriptome database and draft genome assembly from P. chromatophora. Reconstruction of metabolic pathways revealed that nuclear and chromatophore gene inventories provide highly complementary functions. Interestingly, at least 229 nuclear genes were acquired via horizontal gene transfers (HGTs) from various bacteria, of which only 25% putatively arose through endosymbiotic gene transfer (EGT) from the chromatophore. Many of the bacterial genes fill gaps in pathways crucial to chromatophore maintenance such as peptidoglycan biosynthesis and DNA replication. 

Figure 1. Image of Paulinella chromatophora using light microscopy. Note  size of the scale bar at  left bottom of the figure.

Our results demonstrate the dominant role of HGT in compensating for organelle genome reduction (loss of genes on the chromatophore genome) and suggest that phagotrophy may be a major driver of HGT (and the evolution of organelles).