Scientists have sequenced the genome of a bizarre miniature nucleus, found in a complex algae with unusual evolutionary origins. The findings could provide insights into how genomes evolve and how small a eukaryotic genome can get.
| Algal efficiency. The nucleomorph of Guillardia theta, surrounded here by other algal species (all in false colors) has an extremely dense genome. |
CREDIT: D. HILL
The strange micronucleus is the result of an ancient acquisition. Hundreds of millions of years ago, the algae Guillardia theta came into being when a eukaryotic cell nabbed a red algae, a primitive plant cell which itself had formed when a eukaryotic cell incorporated a cyanobacterium. Because cyanobacteria can make food from sunlight, so did the red algae. In its new home, the cyanobacterium settled down to became a chloroplast, while the red algae's nucleus was whittled down to form a "nucleomorph"--something resembling a second cell nucleus.
A team led by biologist Thomas Cavalier-Smith at the University of British Columbia in Vancouver, Canada (who's now at the University of Oxford, United Kingdom), sequenced the genome of Guillardia theta's nucleomorph. They found all the trappings of a miniature nucleus, such as three linear chromosomes, but it was extremely dense, with some 500 genes packed into just 551 kilobases. Unlike the human genome, where only 1% of the genome codes for proteins, the nucleomorph has very little noncoding DNA, and many genes that overlap.
Only 30 of the nucleomorph's genes are actually needed by the algae, as they perform some function in the chloroplast; those genes' presence explains why the nucleomorph has persisted for so long, the researchers report in the 26 April issue of Nature. The rest of the genes merely serve to build and maintain the nucleomorph itself. Other algae have lost their nucleomorph and moved the genes needed by the chloroplast to the nucleus. Given that the nucleomorph has some cyanobacterial genes, it offers a nice peek into the past, says Cavalier-Smith, because it "tells you something about the nucleus of the cell that was engulfed and formed a cell within a cell."
The study is "really an exciting achievement," says evolutionary biologist Geoffrey McFadden of the University of Melbourne, Australia. His group has sequenced the genome of another nucleomorph that originated with a green algae and found that oddly enough, it stalled at exactly the same point of miniaturization as Guillardia theta's nucleomorph. McFadden suspects that the size of the nucleomorph's chromosomes may be the constraining factor. "Something profound is at work here," he says.
Evolutionary history of eukaryotic cells trapped inside other cells
McFadden Lab site