MOLECULAR-BIOLOGY OF RHODOPHYTE AND CHROMOPHYTE PLASTIDS

Recent information on the plastid genes and genomes of rhodophyte (red algae) and chromophyte (yellow and brown algae) plastids are summariz ed. The plastid genomes of these algae contain many more genes than th ose of metaphyte (land plant) plastids, and the encoded proteins are i nvolved in a much wider range of metabolic activities. Rhodophyte and chromophyte plastid genomes contain very few introns and maintain many ancestral gene operons, which suggests that they are more primitive t han those of metaphytes. Hypotheses of plastid evolution (monophyletic or polyphyletic primary origins) are discussed and assessed. The data available at present support the idea that all plastids arose from a single endosymbiotic event that involved a cyanobacterium and a eukary ote host (monophyletic origin). Information from rhodophyte and chromo phyte plastid genomes also provides insights into the evolution of oth er organisms. Finally, the implications for plastid metabolism that ar ise from the presence of several genes on rhodophyte and chromophyte p lastid genomes are discussed.