Evidence that cryptomonad chloroplasts evolved from photosynthetic eukaryotic endosymbionts

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Abstract
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.