A single-celled phytoplankton that forms enormous blooms in the ocean and plays a vital role in regulating the carbon cycle has an unusual defense against a virus: When the virus appears, the microbe switches into a different life phase, thereby avoiding an attack from the virus. Researchers call the clever defense a “Cheshire Cat escape strategy” after the cat in Alice in Wonderland that occasionally vanished.
“In this paper, we show how a species can escape from [environmental] pressure by switching to a life-cycle phase or form that’s not recognizable by a predator,” said Miguel Frada, a marine microbiologist [The Scientist]. The microbe, named Emiliania huxleyi, is so abundant in the ocean that its massive blooms can form turquoise patches visible from space, yet these blooms are often cut off abruptly in a boom-and-bust cycle. The new study suggests that the busts are caused when a virus causes the microbes to switch forms.
The single-celled microbe has two distinct life cycle stages: a “diploid” phase where it contains two copies of the genome (like a regular human cell), and a “haploid” phase where it contains just one copy (like a human sperm or egg). In the diploid stage, E. huxleyi dons a calcium carbonate coat called a coccolith, and forms extensive blooms that help regulate the chemical equilibrium in the ocean and atmosphere by cycling carbon dioxide. In the haploid stage, the phytoplankton sheds its calcium carbonate outer layer and becomes mobile, using its flagella to navigate [The Scientist]. In the haploid stage, the microbe is immune to all E. huxleyi viruses because it doesn’t have the surface receptors that the viruses need to latch onto.
Like many other single-celled eukaryotes (including yeast), E. hux switches between these two forms in a version of sex. Two haploid cells can fuse to make a diploid, which can continue to divide in that form or, under certain conditions, divide up its DNA and split into haploid cells again. This process exchanges DNA among pairs of chromosomes through recombination, helping improve the genetic health of the species [Ars Technica]. But researchers didn’t know what triggered the split into haploid cells. In the study, published in the Proceedings of the National Academy of Sciences [subscription required], researchers introduced E. huxleyi viruses into a cell culture, and saw that the population of diploid cells crashed, but the haploid cells multiplied. The findings suggest a new way to look at the microbes’ sex lives–they’re not just reproducing, they’re also protecting themselves from viruses.
For more on the strange evolution of sex, see the DISCOVER article “The Real Dirty Secret About Sex.”
Image: Glynn Gorick and Miguel Frada