Paulinella: An Emerging Model in Photosynthesis Research and Plastid Evolution

Background

A critical event in the evolutionary history of eukaryotes was the establishment of plastids (e.g., chloroplast). Plastids containing chlorophyll are capable of carrying out photosynthesis, a process that converts light energy, CO2 and H2O into O2 and organic compounds. Plastids allowed the evolution of algae and the plants, which form the base of the food chain for life on earth.

The origin of the first plastid can be traced to a single primary endosymbiosis where a non−photosynthetic protist engulfed and enslaved a cyanobacterium as a cytoplasmic organelle. Descendants gave rise to the red, green, and glaucophyte algae. By secondary and tertiary endosymbiosis, photosynthesis spread into other eukaryotic groups (e.g., brown seaweeds, diatoms, and dinoflagellates). However, our knowledge of plastid evolution is still limited because most of those endosymbiotic events occurred more than a billion years ago.

Paulinella chromatophora is a member of the Cercozoa that is remarkable because it very recently acquired a plastid (termed cyanelle in this species) via a putative primary endosymbiosis involving a Prochlorococcus or Synechococcus − like cyanobacterium. The closely related Paulinella ovalis lacks a plastid but feeds actively on cyanobacteria.

Using this unique model for understanding organelle genesis, we propose to use a hybrid approach (454 Life Sciences™ and Sanger sequencing) to determine draft nuclear genome sequences from Paulinella chromatophora FK01 and Paulinella ovalis using DNA isolated and amplified from single (or a few) cells. We will also assemble the cyanelle genome from Paulinella chromatophora FK01 to determine which genes were lost and potentially transferred to the host nucleus.

endosymbiotic gene transfer

This process is termed endosymbiotic gene transfer (EGT) and is a foundational event in primary endosymbiosis. In addition, we will use "454 Life Sciences™" pyrosequencing to assemble a rich cDNA set from Paulinella chromatophora FK01 to aid gene identification and annotation. The Paulinella ovalis genome will be critical to generate a gene/genome catalogue before and after endosymbiosis and to discriminate between the origin of genes in these taxa through horizontal gene transfer (HGT) from cyanobacterial prey versus bona fide endosymbiotic gene transfer (EGT). Genome comparisons should also help us to identify the initial genetic inventions that allowed the critical transition from heterotrophy animal to autotrophy (plant). Such inventions are postulated to include the retargeting of existing transporters to the plastid to facilitate metabolite exchange and the transfer to the host cell of cyanobacterial genes that regulate organelle division (e.g., ftsZ).

Broader Impacts