42 Scientific American, February 2022

Douglas Fox writes about biology, geology and climate science

from California. He wrote the July 2021 article “The Carbon Rocks

of Oman,” about efforts to turn carbon dioxide into solid minerals.

View the animal up close, and another oddity be ­

comes apparent: its cells are up to 300 times larger than

those of a lizard, bird or mammal. You can see, with a

simple magnifying glass, individual blood cells zipping

through the capillaries in its transparent gills.

The Neuse River waterdog, Necturus lewisi, and other

salamanders represent a long­standing conundrum sci­

entists are only now starting to understand. The animal’s

strange traits stem from a hidden burden: Each of its cells

is bloated with 38 times more DNA than a human cell.

The waterdog has the largest genome of any four­footed

beast on Earth. The only comparable animals of any kind

are lungfish, which also have sluggardly tendencies.

Most mammal, bird, reptile and fish genomes fall

within a narrow range of half a billion to six billion chem­

ical building blocks, or base pairs, of DNA. The base pairs

form genes—links in a long chain that constitute an ani­

mal’s genome. But salamander genomes range wildly,

from 10  billion to 120 billion base pairs (10 to 120 giga­

bases). Salamanders don’t have more genes than other

animals; instead their genomes are cluttered with seg­

ments of parasitic DNA that have multiplied out of con­

trol. Everything about their lives is dominated by their

massive genome, which has pushed them into the

extreme slow lane of existence. They slog through life

with underdeveloped bodies, simplified brains and

hearts as flimsy as paper bags, sometimes for 100 years.

In exchange for this burden, salamanders may have

gained at least one amazing ability: regeneration. They

can regrow not only limbs but also up to a quarter of

their brain if it is cut out—handy for survival.

Salamanders owe their weird traits to their DNA, but

not in the way you’d expect. DNA is often called the blue­

print of life. It contains the precise information that

determines the structure and function of every cell in

every species. But the latest discoveries about salaman­

ders upend this long­held notion of a fine­tuned genome.

They reveal that DNA also shapes its owner in ways that

have nothing to do with its informational content. DNA

can distort bodies and organs like a funhouse mirror; a

species can tolerate only so much DNA before experienc­

ing those side effects. We humans, in fact, may be close

to our limit: make our genome any larger, and it could

compromise our species’ greatest asset, our intelligence.

As for salamanders, one has to wonder why their bur­

den hasn’t dragged them down to extinction. Their very

perseverance suggests that our idea of evolution, partic­

ularly “survival of the fittest,” has a serious moralistic

bias: Work hard, young species, hone your body and

brain for high performance, and someday you will suc-

ceed. But salamanders owe their success to lying around.

They have found a way to cheat the system.

INFLATED GENOMES

The mysTery of gigantic genomes began during a pivotal

moment decades ago, when biologists had just identi­

fied DNA as the hereditary molecule of life. A genome,

unique to every species, contains thousands of genes,

composed of DNA, that instruct cells to make proteins

and other molecules that make an organism what it is.

Researchers initially assumed that advanced species with

complex bodies, such as primates and humans, would

have more genes and therefore larger genomes.

But Alfred Mirsky and Hans Ris of the Rockefeller

Institute for Medical Research overturned this notion in

1951. They measured the amount of DNA in individual

T

he Neuse river waTerdog lives a sluggish exisTeNce, as if burdeNed by aN

invisible weight. This mottled brown salamander, about as long as a

human hand, rarely strays far from its concealed burrows beneath rocks

or logs in the rivers of North Carolina. It “hunts” by sitting still in the riv­

erbed, waiting for an insect to swim by, then lurches forward to swallow

the object—a mindless reflex. It spends its entire life confined to water,

an overgrown larva that never completes metamorphosis, with flaccid

legs too small for its body, toes that haven’t finished sprouting, a missing

upper jawbone and puffy larval gills bulging from its neck.