The chloroplasts of Euglena may have evolved from symbiotic green algae

Abstract

It is proposed that the chloroplasts of Euglena have arisen from the progressive reduction of endosymbiotic green algae. The theory is supported by the presence of a third membrane around the chloroplasts of Euglena which is not endoplasmic reticulum (ER) in origin and may be derived from the plasmalemma of the original symbiont. In addition, Euglena is the only organism in which chloroplast loss can be induced experimentally. Dinoflagellate chloroplasts are also surrounded by three membranes, and it is proposed that they too evolved from symbiotic eucaryotic algae.

The excavate protozoan phyla Metamonada Grasse emend. (Anaeromonadea, Parabasalia, Carpediemonas, Eopharyngia) and Loukozoa emend. (Jakobea, Malawimonas): their evolutionary affinities and new higher taxa

“…The latter is much more likely, as tetraphyte green algae are the very group that donated the green plastid to the ancestor of the euglenoids, the only excavate group currently with photosynthetic members (Cavalier-Smith, 1993a). According to the cabozoan theory, this lateral transfer of green algal plastids took place not within the euglenoids, as commonly assumed (Gibbs, 1978;Delwiche, 1999), but in the common ancestor of Euglenozoa and Cercozoa, the phylum containing chlorarachnean algae, which also have green chloroplasts of secondary symbiogenetic origin (probably also from a tetraphyte source: Cavalier-Smith, 1999, 2003. On this hypothesis, for which there are strong arguments based on considerations of parsimony in the evolution of protein targeting (Cavalier-Smith, 1999, 2003, all excavates must be derived from a photosynthetic common ancestor by plastid losses.…”

Extrachromosomal DNA in the Apicomplexa

“…Apart from the presence of the 35-kb DNA, the sole outwardly visible sign of this endosymbiotic history would ultimately be the possession of an organelle with triple or quadruple membranes. It is this feature, characteristic of secondary endosymbiosis (11,43), which led us to the idea that the multimembraned "spherical body" in the malaria parasite and its analogs in other apicomplexans might be the organellar home of the circular genome (Fig. 1b).…”

Phylogenetic analysis of chloroplast small-subunit rRNA genes of the genus Euglena Ehrenberg.

“…The origin of the euglenoid chloroplast is not entirely certain. Most likely, it is monophyletic in nature and obtained by secondary symbiosis from green algae (Gibbs, 1978(Gibbs, , 1981Morden et al, 1992 ; Delwiche & Palmer, 1997 Abbreviations : ML, maximum-likelihood ; MP, maximum-parsimony ; 16S rDNA, chloroplast SSU rDNA.…”

“…The origin of the euglenoid chloroplast is not entirely certain. Most likely, it is monophyletic in nature and obtained by secondary symbiosis from green algae (Gibbs, 1978(Gibbs, , 1981Morden et al, 1992 ; Delwiche & Palmer, 1997 Abbreviations : ML, maximum-likelihood ; MP, maximum-parsimony ; 16S rDNA, chloroplast SSU rDNA.The GenBank accession numbers for chloroplast SSU rDNA (16S rDNA) sequences reported in this papers are given in Methods.The genus Euglena consists of organisms highly diversified with respect to cell architecture. Species included within the genus represent almost entire morphological monad diversity encountered within green euglenoids.…”