Plastid Evolution

Abstract

The ancestors of modern cyanobacteria invented O(2)-generating photosynthesis some 3.6 billion years ago. The conversion of water and CO(2) into energy-rich sugars and O(2) slowly transformed the planet, eventually creating the biosphere as we know it today. Eukaryotes didn't invent photosynthesis; they co-opted it from prokaryotes by engulfing and stably integrating a photoautotrophic prokaryote in a process known as primary endosymbiosis. After approximately a billion of years of coevolution, the eukaryotic host and its endosymbiont have achieved an extraordinary level of integration and have spawned a bewildering array of primary producers that now underpin life on land and in the water. No partnership has been more important to life on earth. Secondary endosymbioses have created additional autotrophic eukaryotic lineages that include key organisms in the marine environment. Some of these organisms have subsequently reverted to heterotrophic lifestyles, becoming significant pathogens, microscopic predators, and consumers. We review the origins, integration, and functions of the different plastid types with special emphasis on their biochemical abilities, transfer of genes to the host, and the back supply of proteins to the endosymbiont.

Algal genomes reveal evolutionary mosaicism and the fate of nucleomorphs

“…1.9.4.1.2, together with a predicted set of .600 evolutionarily conserved endoplasmic reticulum and Golgi proteins (Supplementary Information 1.9). The limited overlap in proteins predicted to be targeted to different compartments suggests that the search strategies successfully differentiated among plastid-, PPC-and nucleomorph-, and host endoplasmicreticulum-and Golgi-targeted proteins, which is important because in both cryptophytes and chlorarachniophytes the signal peptide secretion system is the first step in trafficking proteins to each of these compartments 1 . We analysed these proteomes in order to compare and contrast the biology of the independently evolved plastid and periplastidial compartments in G. theta and B. natans.…”

“…On balance, the PPC of cryptophytes and chlorarachniophytes is highly reduced, but has retained an unexpectedly broad range of biochemical processes. These data provide the basis for addressing many fundamental questions about algal cell biology, including how many homologues of G. theta and B. natans PPC proteins are retained in the nucleomorphlacking PPC of algae such as diatoms and haptophytes 21 , and what exactly are the biochemical determinants of the protein trafficking pathways in cryptophytes, chlorarachniophytes and other secondary plastid-bearing algae 1,27 . Making sense of the hundreds of predicted PPC and nucleomorph proteins in G. theta and B. natans with unknown functions (Supplementary Table 1.9.4.1.1) will be a substantial challenge.…”

“…The photosynthetic organelles (plastids) of algae evolved from cyanobacteria by endosymbiosis 1,2 . The 'primary' plastids of red algae, glaucophyte algae and green algae, and their land-plant descendants, probably arose just once, more than a billion years ago 3,4 .…”

Endosymbiotic associations within protists

“…Additionally, and maybe more importantly, evolution of a protein import mechanism to target the former prokaryotic compartment, likely the main hurdle for genetic integration of a prokaryote into a eukaryotic cell, is already established in secondary and tertiary endosymbioses. The new eukaryotic endosymbiont can be accessed via the endomembrane system (Gould et al 2008).…”

“…In the context of plastid acquisition, endosymbioses involving a prokaryotic endosymbiont (a cyanobacterium) are referred to as 'primary endosymbioses' leading to 'primary plastids', as opposed to sequential endosymbioses involving photosynthetic eukaryotes that are termed 'secondary' and 'tertiary endosymbioses' leading to secondary and 'tertiary plastids' (Gould et al 2008;Archibald 2009;Keeling 2009). It is, however, important to note that the same terms are used differently by zoologists: 'primary endosymbionts' are here bacterial endosymbionts that live inside specialized animal host cells in mutually obligate associations, whereas 'secondary endosymbionts' are facultative bacterial endosymbionts that coexist with a primary endosymbiont and are not essential for the survival of the host (Moya et al 2008).…”