Scientific American - USA (2022-02)

February 2022, ScientificAmerican.com 43

cells from several dozen animal species. To their surprise,
the African lungfish and a giant salamander from the
southeastern U.S., called amphiuma, had dozens of times
more DNA per cell than humans, rats, birds or reptiles
did. As scientists measured more species, it became clear
that salamanders and lungfish were outliers.
Over the next two decades researchers got a closer
view of gigantic genomes. Shigeki Mizuno and Herbert
MacGregor of the University of Leicester in England stud­
ied a handful of North American salamander species
called plethodontids. The species looked nearly identi­
cal, yet their genomes ranged from 18 to 55 gigabases—
about five to 16 times the human genome, which has
3.06 gigabases.
In all the species, the DNA chain was wound into
sausage­shaped structures called chromosomes. But in
species with bigger genomes, the chromosome shape
looked enlarged, like an overinflated sausage balloon.
Extra DNA seemed to be sprinkled throughout the chro­
mosomes’ length.
Mizuno and MacGregor had no idea what that extra
material was. But during the 1980s scientists found that
cells in other species, from flies to humans, harbored
“parasitic” DNA—short DNA segments, called transpo­
sons, which vaguely resemble viruses. Transposons con­
tain several genes that allow the parasite to make copies
of itself, which then insert themselves, sometimes ran­
domly, into other parts of a cell’s genome.

The exploration of gigantic genomes went slowly for

several decades. Scientists labored to fully sequence the

genomes of fruit flies, worms and humans, but most

avoided salamanders, whose sheer volume of DNA would

have been a nightmare to handle. Then in 2011 Rachel

Mueller, an evolutionary biologist at Colorado State Uni­

versity, took a major step forward.

Mueller and her colleagues used high­through put

sequencing to analyze hundreds of thousands of random

DNA snippets from six species of plethodontid salaman­

ders, as well as another species called hellbender. The

results confirmed what people had suspected: salaman­

der genomes were overinflated with transposons. Many

of the same transposons were present in both the

plethodontids and hellbender, which suggested that the

parasites had first multiplied out of control in the

an cestor of all living salamanders, more than 200 mil­

lion years ago.

The mystery of why that explosion happened has

intrigued Mueller ever since. “It wasn’t like one [trans­

poson] went bananas,” she says. “It was a global change

in how those [transposon] sequences were permitted to

in habit the genome,” allowing dozens of them to multi­

ply simultaneously.

Although Mueller hasn’t determined why, she has

solved another puzzle: Even when transposons do prolif­

erate in a host’s genome, they are usually deleted over

time through random mutations. This pruning happens

SALAMANDERS

have giant

genomes that

en cumber the

critters with

infantile bodies.

But they can

provide the

power to regen­

erate limbs and

even parts

of the brain.