Chloroplast Origins and Evolution

Abstract

SummaryPlastids from extant plants exhibit considerable diversity in morphological and biochemical characters. Although most authors have agreed on xenogenous (endosymbiotic) rather than autogenous origins of plastids (discussed by Doolittle (1982) in ‘The Biology of Cyanobacteria’), details concerning the endosymbiotic events remain unresolved.In the eleven years since ‘The Biology of Cyanobacteria’, many data have accumulated that, while supporting the xenogenous origin of plastids, have revived the controversy over single (monophyletic) versus multiple (polyphyletic) origins. These arguments revolve around the number and nature of the primary endosymbiont(s) that gave rise to the first plastid-bearing eucaryotes. The question of whether secondary endosymbiotic events, originally hypothesized on the basis of electron microscopic evidence, were responsible for the formation of ‘complex’ plastids (those surrounded by more than two membranes) has now been investigated by molecular methods.The purpose of this chapter is to present recent evidence bearing on the probable nature of the procaryotic ancestor(s) involved in the primary endosymbiotic event(s), and on the secondary endosymbiotic events that gave rise to eucaryotes bearing complex plastids. Comparisons of gene content, gene arrangement, gene expression and gene sequences between extant cyanobacteria and plastids give important clues about the possible ancestors of plastids and of the subsequent transformation of a eubacterial genome into a plastid genome. In the last several years, four complete land plant plastid genomes have been sequenced, contributing vastly to our knowledge of plastid architecture and expression. In addition, a great deal of molecular data has been acquired on cyanelle and cyanobacterial genes and genomes. Increased emphasis has now been placed on the study of non-land plant plastid genomes and a number of rhodophyte, chromophyte, cryptophyte and euglenophyte plastid genomes have been extensively mapped and sequenced. These data are presented and the phylogenetic implications evaluated.