Species gain and shed startling amounts of DNA as they evolve, and even genomes that look stable churn furiously. What does it mean?
Quanta Magazine said:Take an onion. Slice it very thin. Thinner than paper thin: single-cell thin. Then dip a slice in a succession of chemical baths cooked up to stain DNA. The dyed strands should appear in radiant magenta — *the fingerprints of life’s instructions as vivid as rose petals on a marital bed. Now you can count how much DNA there is in each cell. It’s simply a matter of volume and density. A computer can flash the answer in seconds: 17 picograms. That’s about 16 billion base pairs — the molecular links of a DNA chain.
Maybe that number doesn’t mean much to you. Or maybe you’re scratching your head, recalling that your own hereditary blueprint weighs in at only 3 billion base pairs. “Huh?” joked Ilia Leitch, a biologist at the Royal Botanic Gardens, Kew, in England. Her reaction mimicked the befuddlement of countless anthropocentric minds who have puzzled over this discrepancy since scientists began comparing species’ genomes more than 70 years ago. “Why would an onion have five times more DNA than we have? Are they five times more clever?”
Of course, it wasn’t just the onion that upended assumptions about a link between an organism’s complexity and the heft of its genetic code. In the first broad survey of animal genome sizes, published in 1951, Arthur Mirsky and Hans Ris —pioneers in molecular biology and electron microscopy, respectively — reported with disbelief that the snakelike salamander Amphiuma contains 70 times as much DNA as a chicken, “a far more highly developed animal.” The decades that followed brought more surprises: flying birds with smaller genomes than grasshoppers; primitive lungfish with bigger genomes than mammals; flowering plants with 50 times less DNA than humans, and flowering plants with 50 times more; single-celled protozoans with some of the largest known genomes of all.
Even setting aside the genetic miniatures of viruses, cellular genome sizes measured to date vary more than a millionfold. Think pebbles versus Mount Everest. “It’s just crazy,” Leitch said. “Why would that be?”
By the 1980s, biologists had a partial answer: Most DNA does not consist of genes — those functional lines of code that translate into the molecules carrying out the business of a cell. “Large genomes have vast amounts of noncoding DNA,” Leitch said. “That’s what’s driving the difference.”
But although this explanation solved the paradox of the clever onion, it wasn’t particularly satisfying. “It just opened up more cans of worms,” said Ryan Gregory, a biologist at the University of Guelph who runs the online Animal Genome Size Database. Why, for instance, do some genomes contain very little noncoding DNA — also, controversially, often called “junk DNA” — while others hoard it? Does all this clutter — or lack of it — serve a purpose?
This past February, a tantalizing clue arose from research led by Aurélie Kapusta while she was a postdoctoral fellow working with Cedric Feschotte, a geneticist then at the University of Utah, along with Alexander Suh, an evolutionary biologist at Uppsala University in Sweden. The study, one of the first of its kind, compared genome sequences across diverse lineages of mammals and birds. It showed that as species evolved, they gained and shed astonishing amounts of DNA, although the average size of their genomes stayed relatively constant. “We see the genome is very dynamic, very elastic,” said Feschotte, who is now at Cornell University.
To explain this tremendous DNA turnover, Feschotte proposes an “accordion model” of evolution, whereby genomes expand and contract, forever gathering up new base pairs and dumping old ones. These molecular gymnastics represent more than a curiosity. They hint at hidden forces shaping the genome — and the organisms that genomes beget.
...