Evolution of algal plastids from eukaryotic endosymbionts

Summary

Introduction

Endosymbiosis has been a key process in the evolution of eukaryotes. Molecular phylogenetic data prove ‘beyond reasonable doubt’ that mitochondria and green chloroplasts are of eubacterial origin (Pace et al., 1986), probably derived from endosymbionts (Margulis, 1981). Hence, the eukaryotic cell is a polyphyletic chimera of two or more organisms created by cross-bridges between different streams of evolution. Accordingly, the plant cell comprises three subcellular compartments (cytoplasm, chloroplast and mitochondrion) derived from three distantly related organisms now amalgamated into a single cell (Margulis, 1981). However, the morphology of certain algal cells (e.g. the Divisions (or Phyla) Cryptoophyta and Chlorarachniophyta) suggests that they contain four different subcellular compartments; possibly derived from a union of two eukaryotes (Whatley & Whatley, 1981, Cavalier-Smith 1988, Cattolicco, 1986, Cavalier-Smith & Lee, 1985). In order to determine the affinity of the sub-cellular compartments in these algae, high resolution in situ hybridisation using lineage specific probes for rRNA has been employed. This cytochemical approach demonstrates that cryptomonads and Chlorarachnion not only have two prokaryotic subcelluar compartments (plastids and mitochondria) but also contain two eukaryotic subcellular compartments, each with 80S-like eukaryotic ribosomes and a nucleus. These results support the hypothesis that certain algae attained their plastids from eukaryotic endosymbionts, and that vestigial remnants of the endosymbionts nucleus and cytoplasm remain in the algal cells (Whatley & Whatley, 1981; Hibberd & Norris, 1984; Ludwig and Gibbs, 1985, 1987, 1989; Hansmann, 1988).