22 February 2020 | New Scientist | 47during their evolutionary history, each one
thought to coincide with global temperature
peaks. This gives grounds for optimism that
it can happen again.
The major difference this time, however,
is that along with rising temperature, the
animals face other challenges their ancestors
didn’t have to deal with. “Many species have
lost the connecting habitat that would have
allowed them to shift their distributions to
more suitable climates in the past,” says
Mitchell. Habitat destruction also reduces
population sizes, which can limit the ability to
adapt. “The likelihood of harbouring beneficial
genetic variants is significantly greater in
larger populations,” says Pezaro. “And many
of today’s populations are nowhere near the
sizes they were at their historic peaks.” That
means they have less of the raw material on
which evolution can get to work. And even
if they can adapt, they may not be able to
evolve fast enough if the rate of temperature
change outpaces the speed at which they
can reproduce.
Here is yet another reason why we need
to act now to limit climate change and
environmental degradation. Our efforts
won’t just help polar bears on thin ice, they
will also help turtles, tilapia and tuataras that
find themselves in increasingly hot water. ❚mutated genes would, de facto, become a
nascent sex chromosome, obliging embryos
that inherit it to become female. Only embryos
inheriting two copies of the chromosome
lacking the mutated genes would be capable
of developing into males.
This is precisely the system universally
found in birds, where females have a set of
ZW sex chromosomes – where W dictates
femaleness – and males have ZZ. According
to Pezaro and his colleagues, birds might be
a prime example of animals whose ancestors
originally had temperature-dependent sex
determination, but who evolved a pro-female
sex chromosome as a way out of a catastrophic
male bias. This, the researchers suggest,
could have been triggered by the evolution
of hot-bloodedness and egg-brooding
behaviour, which maintained incubating
eggs at consistent, warm temperatures. The
evolution of sex chromosomes might have
contributed to the rise of birds in the fossil
record some 150 million years ago. Meanwhile,
some researchers believe that continued
reliance on temperature-dependent sex
determination may have played a role in the
eventual demise of the dinosaurs. Birds are
their only descendants alive today.
This evolutionary innovation could also be
the saving strategy for today’s species that rely
on temperature to determine sex. Some of the
fish involved already have sex chromosomes,
so the solution for them could be to simply
find a way to prevent temperature overriding
these. And evidence suggests that turtles have
evolved sex chromosomes at least six timesSofia Deleniv is a doctoral
student in neuroscience at
the University of OxfordC^1 A1 P^1
C^1 O1 M
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IMAGESof temperature-dependent sex determination
include the tuatara, the last member of a
reptile-like lineage dating back to the Triassic
era, 250 million years ago. Today, most tuatara
populations are isolated on a few dozen
islands off New Zealand, and Nicki Mitchell
at the University of Western Australia thinks
we should be concerned for their future. Her
research has identified an escalating male
bias on one of these islands. “We suspect an
extinction vortex is in play,” she says. “By 2085,
there will be almost no nest microclimates on
the island that would produce females, even
with radical adjustments to nesting behaviour.”
Tuataras are by no means alone. Fish with
temperature-dependent sex determination
also produce more males when the going
gets hot – the number of species that do it
is unknown but it includes commercially
important ones such as sea bass, tilapia and
grayling. In both freshwater and marine
species, the sex ratio can shift from equal
numbers to three males for every female
if temperatures rise by just 1°C to 2°C, which
is lower than some projections of the rise
in global sea temperatures by the end of
this century as a result of global warming.
Many of these species have sex
chromosomes that would normally dictate
whether they become male or female, but
temperature routinely overrides these genetic
instructions. Some fish, such as Nile tilapia, may
be contributing to this effect and fast-tracking
the development of a sex imbalance in their
own populations. A recent study found that
they prefer spending time in warmer waters
during their critical period of development,
causing many genetically female tilapia to
effectively convert themselves into males.
A sexual revolution
So how will things pan out for these species
as global temperatures continue to rise? Will
populations become trapped in a runaway
male bias and go extinct? That is certainly a
possibility. However, a study by Pezaro and
others suggests there may be a way out, one
that appears to have been used in the past.
According to their model, a prolonged
shortage of females could eventually trigger
the evolution of a primitive sex chromosome
dictating female development. This might
begin with a chance event in which one or
more genes become mutated, causing them to
switch off the genes that regulate an embryo’s
thermal sensitivity. This would prevent it
from developing into a male, whatever the
temperature. The chromosome carrying these
You ng talapia
prefer warmer
waters, making
them more likely
to become male