38 THENEWYORKER,JANUARY18, 2021
yet to be imagined. Mathematical mod-
elling suggests that an effective suppres-
sion drive would be extremely efficient;
a hundred gene-drive mice released on
an island could take a population of fifty
thousand ordinary mice down to zero
within a few years. “So that’s quite strik-
ing,” Thomas said. “That’s the best-case
scenario. It’s something to aim for.”
I
t’s often said that we live in the An-
thropocene, a new geological epoch
defined by human impacts on the planet.
One of the features of this new epoch
is a redistribution of the world’s rodents.
Everywhere that people have settled—
and even some places they’ve only vis-
ited—mice and rats have tagged along,
often with ugly consequences.
The Pacific rat (Rattus exulans) was
once confined to Southeast Asia. Start-
ing about three thousand years ago, sea-
faring Polynesians carried it to nearly
every island in the Pacific. Its arrival set
off wave after wave of destruction that
claimed an estimated thousand species
of birds. Later, European colonists
brought to those islands—and many
others—ship rats (Rattus rattus), set-
ting off further waves of extinctions that
are still ongoing. In the case of New
Zealand’s Big South Cape Island, ship
rats arrived in the nineteen-sixties, by
which point naturalists were on hand
to watch the carnage. Despite intensive
efforts to save them, three species en-
demic to the island—one bat and two
birds—disappeared.
The house mouse (Mus musculus) orig-
inated on the Indian subcontinent; it can
now be found from the tropics to very
near the poles. According to Lee Silver,
the author of “Mouse Genetics,” “Only
humans are as adaptable (some would
say less so).” Under the right circumstances,
mice can be just as fierce as rats, and every
bit as deadly. Gough Island, which lies
more or less midway between Africa and
South America, is home to the world’s last
two thousand breeding pairs of Tristan
albatross. Video cameras installed on the
island have recorded gangs of Mus mus-
culus attacking albatross chicks and eat-
ing them alive. “Working on Gough Is-
land is like working in an ornithological
trauma center,” Alex Bond, a Canadian
conservation biologist, has written.
For the past few decades, the weapon
of choice against invasive rodents has
been brodifacoum, an anticoagulant that
induces internal hemorrhaging. Brodi-
facoum can be incorporated into bait and
then dispensed from feeders, or it can be
spread by hand, or dropped from the air.
(First you ship a species around the world,
then you poison it from helicopters.)
Hundreds of uninhabited islands have
been demoused and deratted in this way,
and such campaigns have helped bring
scores of species back from the edge, in-
cluding New Zealand’s Campbell Island
teal, a small, flightless duck, and the An-
tiguan racer, a grayish lizard-eating snake.
The downside of brodifacoum, from
a rodent’s perspective, is pretty obvious:
internal bleeding is a slow and painful
way to go. From an ecologist’s perspec-
tive, too, there are drawbacks. Non-target
animals often take the bait or eat rodents
that have eaten it. In this way, poison
spreads up and down the food chain.
And if just one pregnant mouse survives
an application, she can readily repopu-
late an island.
Gene-drive mice would scuttle around
these problems. Impacts would be tar-
geted. There would be no more bleed-
ing to death. And, perhaps best of all,
gene-drive rodents could be released on
inhabited islands, where dropping anti-
coagulants from the air is, understand-
ably, frowned upon.
But as is so often the case, solving
one set of problems introduces new ones.
In this case, big ones. Humongous ones.
Gene-drive technology has been com-
pared to Kurt Vonnegut’s ice-nine, a sin-
gle shard of which is enough to freeze
all the water in the world. A single
X-shredder mouse on the loose could,
it’s feared, have a similarly chilling
effect—a sort of mice-nine.
To guard against a Vonnegutian ca-
tastrophe, various fail-safe schemes have
been proposed, with names like killer
rescue, multi-locus assortment, and daisy
chain. All of them share a basic, hope-
ful premise: it should be possible to en-
gineer a gene drive that’s effective but
not too effective. Such a drive might be
engineered so as to exhaust itself after
a few generations, or it might be yoked
to a gene variant that’s limited to a sin-
gle population on a single island. It has
also been suggested that if a gene drive
did somehow manage to go rogue it
might be possible to send out into the
world another gene drive, featuring a
“Cas9-triggered chain ablation”—or
CATCHA—sequence, to chase it down.
What could possibly go wrong?
W
hile I was in Australia, I wanted
to get out of the lab and into
the countryside. I thought it would be
fun to see some northern quolls. In the
photos I’d found online, they looked
“There had better not be any socialists under my bed.”
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