Evidence that cryptomonad chloroplasts evolved from photosynthetic eukaryotic endosymbionts

Unicellular algae of the division Cryptophyta possess an unusual subcellular compartment of unknown derivation. This compartment, which is partitioned off from the main cytoplasm by two membranes, contains a chloroplast and a small nucleus-like organelle surrounded by ribosomelike particles. Electron-microscopic in situ hybridization has been used to show that the ribosomes in this subcellular compartment are eukaryotic. In addition, eukaryotic rRNA has been localised within the nucleus-like organelle, suggesting that the rRNAs may be transcribed from genes in this nucleus. This identification of a second, nucleuscontaining eukaryotic compartment within these cells supports the hypothesis that cryptomonads contain a reduced photosynthetic eukaryotic endosymbiont.