The origin of plastids

Abstract

It is generally accepted that plastids first arose by acquisition of photosynthetic prokaryotic endosymbionts by non-photosynthetic eukaryotic hosts. It is also accepted that photosynthetic eukaryotes were acquired on several occasions as endosymbionts by non-photosynthetic eukaryote hosts to form secondary plastids. In some lineages, secondary plastids were lost and new symbionts were acquired, to form tertiary plastids. Most recent work has been interpreted to indicate that primary plastids arose only once, referred to as a 'monophyletic' origin. We critically assess the evidence for this. We argue that the combination of Ockham's razor and poor taxon sampling will bias studies in favour of monophyly. We discuss possible concerns in phylogenetic reconstruction from sequence data. We argue that improved understanding of lineage-specific substitution processes is needed to assess the reliability of sequence-based trees. Improved understanding of the timing of the radiation of presentday cyanobacteria is also needed. We suggest that acquisition of plastids is better described as the result of a process rather than something occurring at a discrete time, and describe the 'shopping bag' model of plastid origin. We argue that dinoflagellates and other lineages provide evidence in support of this.

Functional remodeling of RNA processing in replacement chloroplasts by pathways retained from their predecessors

“…Our observations suggest an important addition to conventional models of chloroplast evolution: that host lineages can retain biogenesis pathways from prior symbioses and apply them to replacement chloroplasts, in which they may confer new functions. This might enhance the stability of the replacement chloroplast in the host cell or adapt the metabolic or regulatory pathways of the chloroplast to the needs of the host (1,2,19). In the light of recent data indicating that serial endosymbiosis has occurred extensively across the eukaryotes, we propose that features of the chloroplast biology of many prominent photosynthetic eukaryotes have been optimized by functions derived from previous endosymbioses.…”

“…organelle | fucoxanthin | haptophyte | poly(U) | alveolate C hloroplasts originate through endosymbiosis, in which a freeliving photosynthetic symbiont is taken up by a eukaryotic host, with processes becoming established within the host to support the biogenesis and maintenance of the symbiont (1,2). It has long been understood that many such endosymbiotic events have occurred across the eukaryotes, giving rise to a diverse array of extant chloroplast lineages (2)(3)(4).…”

The evolutionary history of haptophytes and cryptophytes: phylogenomic evidence for separate origins

“…This crucial event in eukaryote evolution is generally seen as unique: primary plastids probably evolved only once in the common ancestor of glaucophytes, red algae and green plants (green algae þ land plants), together making the Plantae supergroup [4] (but see [5]). A much more recent case of cyanobacterium to eukaryote endosymbiosis has been reported in the rhizarian Paulinella chromatophora [6], but this event appears to have had less impact on the diversification of plastids.…”

Oxygenic photosynthesis and the distribution of chloroplasts

“…Resolving questions of gene reduction, and whether specific individual genes were retained after a plastid loss, has been difficult to substantiate partly due to an unequal weighting of selected gene comparisons and because of some presumed chloroplast genes which have broader cell functions. Cautions against oversimplifications from gene sampling and potential losses are valid and are growing (Archibald 2009;Bodyl et al 2009;Howe et al 2008;Inagaki et al 2009;Stiller 2007;Stiller et al 2009), though not always popular with the current multitude who continue to try to find the right place(s) for Cinderella's slipper.…”