Researchers studying the single-cell organism Oxytricha trifallax have discovered that the algae-muncher breaks its own DNA into a quarter-million pieces and rapidly reassembles those bits every time it’s about to have sex. Their findings were published in Cell earlier this month.
"It's one of nature's early attempts to become more complex despite staying small in the sense of being unicellular," says Laura Landweber from Princeton. "There are other examples of genomic jigsaw puzzles, but this one is a leader in terms of complexity. People might think that pond-dwelling organisms would be simple, but this shows how complex life can be, that it can reassemble all the building blocks of chromosomes."
Oxytricha is about 10 times the size of a typical human cell, and it contains about 16,000 chromosomes (humans have 46). And unlike most other single-celled organisms, it has two nuclei: It keeps active DNA in one (called the working nucleus) and uses the second one (the germ-line nucleus) to store an archive of genetic material that it’ll pass along to the next generation. The encrypted genome of the germ-line nucleus -- which contains over 3,500 scrambled genes -- is the one that gets dismantled and reconstructed to produce a new working nucleus for the offspring.
When it’s time to mate, Oxytricha rummages through thousands of jumbled, stored gene bits and pieces together 225,000 or so tiny DNA segments. This all happens in about 60 hours.
But all new Oxytricha populations spawn from a single organism (like with a plant cutting), so sex is used solely to exchange DNA -- and not to reproduce. During sex (pictured above), two cells fuse together and share half of their genetic information. Each organism replaces its aging genes with new genes and DNA parts from its partner. That way, both get to construct new working nuclei using fresh chromosomes to diversify their genetic material. "It's kind of like science fiction -- they stop aging by trading in their old parts," Landweber says in a news release.
Each partner uses those quarter-million DNA segments to reconstruct its rejuvenated chromosomes, which helps Oxytricha protect its DNA by only transmitting healthy, robust material when it reproduces. This novel method of encryption and rapidly reassembly could explain why they've successfully spread around the world.
Furthermore, its orderly, step-by-step chromosome reconstruction abilities could make it a template for understanding what happens during the chaotic and unpredictable onset of cancer. "It's basically bad when human chromosomes break apart and reassemble in a different order," Landweber explains. "The process in Oxytricha recruits some of the same biological mechanisms that normally protect chromosomes from falling apart and uses them to do something creative and constructive instead."