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EVOLUTIONARY GENOMICS
The nuclear genomes of two of nature's most complex cells have been sequenced. The data will help to determine the evolutionary path from symbioses between species to a multi-compartmental unicellular organism.
The orchestration of cellular function from genetic information is a marvel, even in a typical cell that has a single genome. But a few unusual members of a group of microorganisms called protists pack four genomes, each of unique origin, into a single cell. In this issue, Curtis et al.1 report the nuclear-genome sequences for two of these organisms: the unicellular algae Guillardia theta and Bigelowiella natans. These species are each the derivatives of symbiotic relationships between multiple cells, and their endowment of genetic and cellular relicts from these symbioses makes them the most complex cells known. The genome sequences are the final pieces in a long-standing puzzle, and they will enrich our understanding of how this arrangement evolved and how the cells manage their intricate biochemistry*.
The unusual biology of these algae arose from endosymbiosis, which is when a cell lives in irreversible symbiosis within another cell. Endosymbiosis is an important concept - the photosynthetic organelles (plastids) of all photosynthetic eukaryotes ultimately trace back to a cyanobacterium that took up residence within a eukaryotic host more than 1.2 billion years ago2. (Eukaryotes are organisms with nucleated cells, such as animals, plants and fungi, whereas cyanobacteria are prokaryotes, whose cells do not have a nucleus or other membrane-bound organelles.) This fateful encounter is called the primary endosymbiosis (Fig. 1a), because it led to all plastids known today.
Plastids are usually surrounded by two membranes, which are derived from the inner and outer cell membranes of that ancestral cyanobacterium3. But the plastids of some algae have additional membranes - a further two in the case of the algal groups studied by Curtis et al. - and the origin of these extra membranes is the story of secondary endosymbiosis4. Secondary endosymbiosis occurs when a eukaryotic host acquires a eukaryotic alga as its...