Cryptomonad algae are evolutionary chimaeras of two phylogenetically distinct unicellular eukaryotes

Abstract

ALTHOUGH it is widely accepted that the plastids of plants and algae originated as endosymbionts¹, the details of this evolutionary process are unclear^2'3. 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.