Abstract
ALTHOUGH it is widely accepted that the plastids of plants and algae originated as endosymbionts1, the details of this evolutionary process are unclear2'3. It has been proposed that in organisms whose plastids are surrounded by more than two membranes, the endosymbiont was a eukaryotic alga rather than a photosynthetic prokaryote4. The DNA-containing5 nucleomorph6 of cryptomonad algae appears to be the vestigial nucleus of such an algal endosymbiont7. Eukaryotic-type ribosomal RNA sequences have been localized to a nucleolus-like structure in the nucleomorph8. In support of the hypothesis that cryptomonads are evolutionary chimaeras of two distinct eukaryotic cells, we show here that Cryptomonas Φ contains two phylogenetically separate, nuclear-type small-subunit rRNA genes, both of which are transcriptionally active. We incorporate our rRNA sequence data into phylogenetic trees, from which we infer the evolutionary ancestry of the host and symbiont components of Cryptomonas Φ. Such trees do not support the thesis3 that chromophyte algae evolved directly from a cryp-tomonad-like ancestor.
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Douglas, S., Murphy, C., Spencer, D. et al. Cryptomonad algae are evolutionary chimaeras of two phylogenetically distinct unicellular eukaryotes. Nature 350, 148–151 (1991). https://doi.org/10.1038/350148a0
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DOI: https://doi.org/10.1038/350148a0
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