One of the first chemical reactions children learn is the recipe for photosynthesis, combining carbon dioxide, water and solar energy to produce organic compounds. Many of the world's most important photosynthetic eukaryotes such as plants did not develop the ability to combine these ingredients themselves. Rather, they got their light-harnessing organelles -- chloroplasts -- indirectly by stealing them from other organisms. In some instances, this has resulted in algae with multiple, distinct genomes, the evolutionary equivalent of a "turducken*."
Chloroplasts originally evolved from photosynthetic bacteria by primary endosymbiosis, in which a bacterium or other prokaryote is engulfed by a eukaryotic host. The chloroplasts of red and green algae have subsequently come to reside within other, previously non-photosynthetic eukaryotes by secondary endosymbiosis. Such events have contributed to the global diversity of photosynthetic organisms that play a crucial role in regulating and maintaining the global carbon cycle. In most organisms that acquired photosynthesis by this mechanism, the nucleus from the ingested algal cell has disappeared, but in some cases it persists as a residual organelle known as a nucleomorph. Such organisms have four distinct genomes.
To better understand the process of secondary endosymbiosis and why nucleomorphs persist in some organisms, an international team composed of 73 researchers at 27 institutions, including the U.S. Department of Energy Joint Genome Institute (DOE JGI), collaborated to sequence and analyze the genomes and transcriptomes (the expressed genes) of two tiny algae. The team led by John Archibald of Canada's Dalhousie University published their findings on the algae Bigelowellia natans and Guillardia theta online November 29, 2012 in Nature.
Archibald compared these algae to Russian nesting dolls with "sophisticated sub-cellular protein-targeting machinery" and four
|Contact: David Gilbert|
DOE/Joint Genome Institute