February 2022, ScientificAmerican.com 49correct, then a larger human genome might leave us with
fewer brain cells—and less smarts.
Frogs and toads, close relatives to salamanders, often
have relatively large genomes, ranging up to 13.1 giga
bases of DNA. But the ornate burrowing frog, whose
genome was published in 2021, has only 1.06 gigabases,
similar to a hummingbird’s. It inhabits the Australian
desert, laying eggs in puddles that form after infrequent
rains. The tadpoles have only a few days to sprout legs
before their aquatic nurseries evaporate. They simply
can’t afford to hoard genomic junk. Even among plants,
the fastspreading weeds that take over vacant lots—this
tle, dandelions, and the like—often have smaller genomes
than the slowergrowing species that they smother.
In contrast to leanandmean species, the salaman
der probably evolved its bloated genome gradually. Greg
ory and Mueller think that 200 million years ago, the
ancestor of all salamanders probably occupied the slow
lane of life, with low energy needs and sluggish develop
ment. As a result, it suffered no immediate harm as
transposons accumulated in its genome. As salamander
genomes expanded, they pushed the critters further into
niches where the slowandfrugal strategy paid off.
In a 2020 paper, Gregory suggests that this process
eventually hit a tipping point: transposons changed from
being mere inhabitants of the genomic landscape to
being fullblown ecosystem engineers. When a transpo
son inserts a new copy of itself, there is always a risk that
it will disrupt a gene and harm the host—which is bad if
you’re a parasite because you depend on the host for sur
vival. A newly inserted transposon won’t be passed on to
the next generation if it causes its host to be sterile, for
example. But as transposons multiplied, their very pres
ence provided more “habitat” in the genome where new
transposons could insert themselves without hitting
genes. “There is a feedback loop,” Gregory says. “The
more transposons you have, the more safe places you
have to insert them.”
And so it was that the Neuse River waterdog eventu
ally found itself lugging around 118 gigabases of DNA.
Its sister species, the dwarf waterdog ( Necturus puncta-
tus ), is right behind, with 117 gigabases.
It’s easy to look at the Neuse River waterdog and feel
a pang of pity. Its slow development not only leaves it
unable to metamorphose but may also prevent adult sal
amanders from regenerating limbs, a cruel irony. Unable
to traverse dry land, the waterdog remains isolated in
two small river systems in North Carolina. Agriculture
and development have led to worsening water quality.
In June 2021 the U.S. government listed the waterdog,
whose population is falling, as “threatened.” Although
salamanders have survived as a group for 200 million
years, it is tempting to think that this one species’ ginor
mous ge nome has pushed it toward extinction.
Sessions isn’t so sure. These bloated beasts have dem
onstrated, time and again, that when it comes to sur
vival of the fittest, our notion of “fitness” is biased to
ward strength and agility. Genomic parasites have
slowed the waterdog’s development, swelled its cells and
distorted its anatomy. This odd circumstance has
pushed the animal onto a bizarre evolutionary side track
that redefines fitness in such a way that hearts and com
plex brains are reduced to an afterthought. Yet some
how the animal’s lineage persists, even as fires, floods
and asteroids obliterate other species—furry, feathered
and scaled—that seem more fit.
“Salamanders,” Sessions says, “are tough survivors.”FROM OUR ARCHIVES
Hacking the Genome. Deborah Erickson; April 1992.
scientificamerican.com/magazine/sa