The algae Cryptomonas gyropyrenoidosa has seven genomes within its cells
Emma George and others
Single-celled algae collected more than 50 years ago and grown in the lab turned out to be a strange assemblage of once-independent organisms, containing at least seven different genomes. I was.
“To my knowledge, a single cell has a record seven different genomes,” says Emma George, who conducted the study at the University of British Columbia in Canada.
Algae called cryptomonads were collected by naturalist Ernst Georg Pringsheim before 1970 and are now part of the collection of the University of Göttingen, Germany. In 1988, microscopic studies revealed bacteria within algae cells and viruses within some bacteria.
After reading the study, George asked for algae samples so that her team could sequence all the DNA in the cells and identify the viruses and bacteria in them.
It is not uncommon for cells to host symbiotic bacteria. Complex cells are thought to have arisen about 3 billion years ago when bacteria began to live inside other simple cells and form partnerships. This phenomenon is known as endosymbiosis. That bacterium evolved into the energy-producing mitochondria found in nearly every complex cell.
Although the main genome of complex cells resides in the cell nucleus, mitochondria hold their own small genome. This means that most animal cells have two different genomes of her, with up to thousands of mitochondrial copies of her genome per cell.
About 1 billion years ago, plant cells acquired photosynthetic ability by acquiring cyanobacteria. This evolved into the chloroplast, which also retains part of its genome, so the plant cell has her three different genomes.
However, Cryptomonas algae are not plant cells. They began as free-swimming predatory cells and gained the ability to photosynthesize by engulfing red algae, a complex plant cell rather than cyanobacteria.
The nucleus of this red alga is held in a contracted form because it contains several genes essential for photosynthesis. The major genome of the cell nucleus, the remnant nucleus of red algae, mitochondria, and the chloroplast of red algae.
The Göttingen strain also has three different genomes. George’s team discovers that instead of just one he has acquired two additional bacterial endosymbionts, one of which he is infected with a bacteriophage his virus.
“It’s amazing to have two different cells, one of which is infected with a phage, and all within one cell,” says George.
Her team has transformed host cells into Cryptomonas gylopyrenoidosatwo bacteria many grelia and Megaira polyxenophilaand viral infection M. polyxenophila as a mankiphage.
George believes that this assemblage was present in the algae collected by Pringsheim and has since been passed down to all descendants for about 4400 generations.
Surprisingly, phage-infected bacteria are more abundant in host Cryptomonas than in uninfected bacteria. It’s not clear how the phage has survived without wiping out the host bacteria, but the phage has genes that may help the bacterium get along with Cryptomonas, says George. “There needs to be balance in that system,” she says.
Dave Speijer, from the University of Amsterdam in the Netherlands, who studies the evolution of complex cells, says the work has been thoroughly studied and shows the surprisingly complex relationship between the host and the bacteria and viruses within it. says that But he wonders whether these relationships survived in real-world conditions, or if they persisted only because of the stable laboratory environment in which the cells have been maintained.
It was already known that there are single-celled organisms called dinoflagellates, within which there are single-celled algae called diatoms, with at least six different genomes per cell. Her one of these “dinosaurs”, discovered by Noriko Yamada of the University of Konstanz in Germany, may have acquired diatoms on four separate occasions and has nine different genomes.
But Yamada says her unpublished results suggest that the same diatom species was acquired each time, and could still have only six different genomes, depending on what you consider different. It means that there is
“Either way, both systems are very complex, and these ‘records’ could be broken by another system that has yet to be discovered,” says George.
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