NATURE NEWS FEATURE by Erik
Vance, 11/15/17
Axolotls inhabit thousands of
labs and home aquariums around the world, but are vanishing from their natural
habitat.Credit: Brett Gundlock for Nature
When biologist Luis Zambrano
began his career in the late 1990s, he pictured himself working miles from
civilization, maybe discovering new species in some hidden corner of Mexico’s
Yucatán Peninsula. Instead, in 2003, he found himself counting amphibians in
the polluted, murky canals of Mexico City’s Xochimilco district. The job had
its advantages: he was working minutes from his home and studying the axolotl
(Ambystoma mexicanum), a national icon in Mexico and arguably the world’s most
recognizable salamander. But in that first year, Zambrano couldn’t wait for it
to be over.
“Let me tell you, I hated the
project at the beginning,” he says. For one thing, “I couldn’t catch anything”.
Over time, however, he did catch
some axolotls. What he found surprised him — and changed the course of his
career. In 1998, the first robust study to count axolotls estimated that there
were about 6,000 of them per square kilometre in Xochimilco1. Zambrano — who
now is a professor at the National Autonomous University of Mexico (UNAM) in
Mexico City — discovered in 2000 that the number had dropped to about
1,000 animals per square kilometre. By 2008, it was down to 100; today,
thanks to pollution and invasive predators, there are fewer than 35 animals per
square kilometre1.
The axolotl is on the brink of
annihilation in the canals of Mexico City, its only natural habitat. But
although there might be just a few hundred individuals left in the wild, tens
of thousands can be found in home aquariums and research laboratories around
the world. They are bred so widely in captivity that certain restaurants in
Japan even serve them up deep-fried
“The axolotl is a complete
conservation paradox,” says Richard Griffiths, an ecologist at the University
of Kent in Canterbury, UK, who recruited Zambrano to the project. “Because it’s
probably the most widely distributed amphibian around the world in pet shops
and labs, and yet it’s almost extinct in the wild.”
This creates a problem for
biologists. Thanks to its unique physiology and remarkable
ability to regenerate severed limbs, the axolotl has become an
important lab model for everything from tissue repair to development and
cancer. But after centuries of inbreeding, captive populations are vulnerable
to disease. And the loss of genetic diversity in wild axolotls — owing to their
diminishing population — means that scientists lose out on learning all they
can about the animal’s biology.
As lab scientists continue to
study the captive animal and its large and complex genome, Zambrano and a
handful of other researchers are doing their best to preserve the wild version.
They are breeding and releasing axolotls into control ponds and canals in and
around Xochimilco to see how they fare, and hopefully to retain some of their
natural genetic diversity. The task of saving them is difficult, but should be
doable given the animal’s hardiness — if the Mexican government would only
engage with the process.
“I’ve seen that in other places
in the world, these kinds of huge tasks are possible,” Zambrano says. “If they
can do it, why can’t we?”
Axolotls evolved relatively
recently compared to other salamander species in the region, and they thrived
along the banks of Lake Texcoco in the mountains of central Mexico. They are
neotenic, meaning that the adults retain traits seen only in juveniles of
similar species. Although other salamanders metamorphose into terrestrial
creatures, axolotls hold on to their feathery gills and stay in the water for
their entire lives. It’s as if they never grow up.
The species might have perished
entirely under colonial rule, except that its odd inability to grow up caught
the attention of European scientists, who puzzled over it in the late
nineteenth century.
Visitors to Mexico brought the
creatures back and began breeding them. The animal turned out to be ideal for
research: it reproduces readily in the lab, is a hardy survivor and is easy to
care for. Axolotls have large cells that simplify investigations into
development. Their eggs are almost 30 times larger than a human’s. And in
an axolotl embryo, the neural plate cells — a precursor to the brain and spinal
cord — are almost 600 times larger by volume.
Also, the pigmentation of
axolotls varies greatly from one cell to the next, unlike in humans or other
animals, in which cell traits tend to be uniform. This can help researchers to
track which tissues in an embryo become which organs. Yet it has a large genome
— roughly ten times the size of a human’s — which can make it challenging to
study in some respects.
“It is not a good genetic model
organism, but it does regenerate — and that makes it an awesome biological
model,” says David Gardiner, a developmental biologist at the University of
California, Irvine, who has studied axolotl regeneration for decades.
In the early twentieth century,
axolotls were central to understanding how organs develop and function in
vertebrates. They helped scientists to unpick the causes of spina bifida in
humans — a birth defect in which the spine doesn’t form properly. And they
played a part in the discovery of thyroid hormones: in the 1920s, scientists
fed thyroid tissue from livestock to axolotls. If the tissue had been secreting
hormone, the axolotls would metamorphose, losing their gills and shedding their
larval skin.
In the 1980s, axolotls helped
scientists to develop a model explaining how cells take on different forms in
embryos. The ‘cell state splitter’ model proposes that many stem cells turn
into specific tissues in the body through
waves of pulling and stretching as embryos. Scientists found that
they could watch the axolotl’s cells squeeze and stretch before they formed
tissues. More recently, in 2011, extract from axolotl oocytes has been used to
stop breast-cancer cells multiplying by switching on a tumour-suppressor gene2.
But perhaps the most fascinating
contribution of the axolotl to science has been in regenerative medicine. The
animals can grow back missing limbs, tails, organs, parts of the eye and even
portions of the brain. Many scientists have presumed this is because, being
neotenic, they retain some trait from their embryonic stages, although other
salamanders seem to regenerate even as adults.
Biologists have been trying to
identify the mechanisms behind their regenerative abilities for decades, says
Tatiana Sandoval Guzmán, a regeneration researcher at the Technical University
of Dresden, Germany. “How do they do it? What is it that they have that we
don’t? Or maybe the opposite — what in mammals is stopping that?”
Sandoval Guzmán is interested in
bone and muscle regeneration and has taken over a long-standing axolotl
laboratory in Dresden. A Mexican national who went to school not far from
Xochimilco, she never thought much about the animal and certainly never
considered studying it until she came to Germany. Today she is fascinated by
the creature, and has shown3 that many of the mechanisms in axolotl
regeneration — such as those involving muscle-tissue stem cells — are not so
different from those found in humans.
Most regeneration research
focuses on the stub — or blastema — that forms over the wound of a severed
limb. Whereas such a wound in humans gets covered with skin tissue, axolotls
transform nearby cells into stem cells and recruit others from farther away to
gather near the injury. There, the cells begin forming bones, skin and veins in
almost the same way as when the animal was developing inside the egg. Each
tissue contributes its own stem cells to the effort.
Researchers showed that a protein
called transforming growth factor-β is key both in axolotl regeneration and in
preventing scar tissue in injured human embryos during the first trimester. Adult
mice and humans can regenerate digit tips, although humans lose
this ability with age, suggesting that regenerative abilities could be
reawakened in mammals.
“There will be a day when we as
humans can regenerate,” says Gardiner. His studies are not focused on
rebuilding limbs, but on curing paralysis, growing healthy organs and even
reversing ageing by repairing damaged and worn-out tissues. “And when they
write that story, it will go back to these model organisms,” he says.
By the time that day comes,
however, the wild axolotl may be gone. That worries Gardiner and Sandoval
Guzmán because the animals that they study, like many lab animals, are highly
inbred. Scientists use an ‘inbreeding coefficient’ to measure how small a gene
pool is. Identical twins have a coefficient of 100%; totally unrelated
individuals would score close to zero. A score above 12% indicates a population
in which individuals are mostly breeding with their first cousins, and is
considered a serious concern by ecologists and geneticists. The
famously inbred and unhealthy Spanish Habsburg kings of the seventeenth century
often had a coefficient somewhere above 20%. The average axolotl inbreeding
coefficient is 35%.
“These animals that we have, they
still work just fine, they regenerate just fine. But they are so inbred. It’s a
bottleneck,” Gardiner says. “Populations are very vulnerable to disease when
inbred.”
Their high level of inbreeding is
partly a result of the bizarre historical path captive axolotls have taken.
Most laboratory specimens trace their heritage back to a single group of
34 animals that were taken out of Xochimilco by a French-funded expedition
in 1863. They sparked an axolotl-breeding craze across Europe by museums and
naturalists.
In 1935, some of the animals
travelled from a Polish laboratory back to North America, where they eventually
became a breeding stock at the University of Buffalo, New York. Here,
scientists brought in a series of wild axolotls to mix up the gene pool and at one
point even added in tiger salamanders (Ambystoma tigrinum). The Buffalo
population thrived and eventually moved to the University of Kentucky in
Lexington, which is now the centre of global axolotl breeding. This means that,
in addition to being inbred, almost all of the axolotls in labs and aquariums
today are actually part tiger salamander.
“They got bottlenecked in Europe
for sure and then they got bottlenecked again,” says Randal Voss, head of the
programme in Kentucky, which holds some 2,000 adults and 3,000–5,000
larvae.
Voss says that axolotl research
today is expanding throughout the world, thanks to modern genetics and
stem-cell research. In 2015, he and his group published an initial assembly of
the axolotl genome4, a Herculean task given its large size, estimated to be
about 32 billion bases. But it is incomplete — the size and complexity of
the genome proved too much for the computational power Voss’s group could throw
at it. Scientists in several centres continue to work on completing the picture.
But as they work on that, the
creature’s vulnerability to disease has already caused mysterious massive
die-offs in Voss’ facility. Scientists worry that if a new infectious disease
were to race around labs worldwide, it might force them to abandon the axolotl,
potentially setting research back by years.
What’s more, no one can be sure
that lab axolotls haven’t already diverged so much from their wild counterparts
that they have lost key elements of regeneration. “Going back to study the wild
population can give you a different mechanism or different genes,” says
Sandoval Guzmán. “Losing the genetic diversity — of course it’s a loss for
science.”
“I can’t always know for sure,
but the axolotls from Kentucky do have some differences,” says Arturo Vergara
Iglesias, staring into a tank of axolotls lazily crawling about. “They have a
lot of malformations. For example, they often have too many fingers.”
Vergara Iglesias is a biologist
at the Centre for Biological and Aquaculture Research (CIBAC), an axolotl
breeding facility near Xochimilco that is hoping to preserve a few wild lines.
On the side, he breeds his own wild axolotls to sell to labs and pet
distributors. He is standing over a salamander tank on a traditional Xochimilco
farm plot, or chinampa, that is used as an educational facility for tourists.
These animals, and the others he sells, were bred from a group of 32 pulled out
of the water not far from the plot. In Mexico, the axolotl is a prized pet and
a source of national pride. It’s the subject of countless Mexican memes and
souvenirs, and is even the official emoji for Mexico City.
It’s hard to know exactly how
many axolotls are left in the wild there. Zambrano guesses that during his last
survey, in 2014, there were fewer than 1,000 in total, and perhaps fewer than
500. But he can’t be more specific — in the past two years, he’s been unable to
raise the money to do any follow-up studies. That he can’t obtain funding for a
simple census does not bode well for conservation efforts.
Students supervised by biologist Luis Zambrano release an
axolotl into a protected pond near the National Autonomous University of
Mexico in Mexico City. Credit: Brett Gundlock for Nature
Zambrano says that to save the
wild axolotl, policymakers must address its two primary threats. The first is
non-native fish such as the common carp (Cyprinus carpio) and tilapia
(Oreochromis niloticus). Ironically, these were introduced to Xochimilco in the
1970s and 1980s through programmes run by the Food and Agriculture Organization
of the United Nations, with the aim of getting more protein into local diets.
Zambrano says he has mapped the areas where axolotls still remain; he envisions
a team of local fishers being paid to sweep them of fish on an ongoing basis.
Although this wouldn’t remove all the fish, for a few hundred thousand dollars
it might give the salamanders a window in which to re-establish themselves. His
work has shown that axolotls are most vulnerable to carp when they are at the
egg stage, and to tilapia when they are juveniles, but reveals that if they can
grow beyond a certain size, they might still thrive5.
The second threat is trickier.
Every time a powerful storm fills the city’s ageing sewer system, treatment
facilities release human waste into Xochimilco, carrying with it ammonia, heavy
metals and untold other toxic chemicals. Amphibians, which breathe in part
through their highly permeable skin, are vulnerable
to these regular pollution dumps. It’s a testament to the
animal’s resilience that it exists in the wild at all.
These are complex issues, but
they are not unsolvable. So far, however, there have been no efforts to save
the wild axolotl beyond a few halfhearted outreach programmes and some photo
opportunities. In 2013, CIBAC released a few thousand axolotls for a
behavioural study; some of them survived and even seemed to breed the following
year. This suggests that lab-bred salamanders might be able to thrive in the
wild if they are raised in captivity to a certain size. But biologists caution
that this doesn’t mean Mexico should start releasing them into canals.
“There’s probably not much point
in doing releases into the wild until you can neutralize the threats,” says
Griffiths. “You just might be increasing the fish population by just chucking
out more fish food.”
When Griffiths first started
working in Xochimilco in 2000, his plan was to create a breeding programme
aimed at releasing axolotls into the wild. But he and his Mexican partners
quickly abandoned the idea once they saw the condition of the ecosystem, which
was polluted and teeming with predators. It seemed pointless to send axolotls
off to their deaths. Successful reintroductions, such as those of the pool frog
(Pelophylax lessonae) in Britain or
the hellbender salamander (Cryptobranchus
alleganiensis) in the United States, require managing the ecosystem as a
whole and working with the community.
“If we had a million dollars per
year for ten years, we would save Xochimilco. Which is nothing compared with
the amount of money that is spent in this city,” says Zambrano.
One afternoon in October,
Zambrano and a group of volunteers gather by the ponds near the UNAM campus to
release ten lab-raised wild axolotls into a protected pond. If the animals
survive and breed, they might someday act as a sort of genetic bank for the
organism. Zambrano has been sporadically releasing and tracking animals here
over two years to understand their behaviour and habitat preferences. His work
so far suggests that the salamanders prefer fairly dirty ponds over the most
pristine ones — another sign that axolotls might still thrive in Xochimilco if
other pressures are removed. Similarly, CIBAC is breeding wild-type animals in an
effort to preserve the axolotl’s genetic diversity. But if axolotls do not have
a suitable home, most researchers say that their extinction in the wild might
be inevitable, no matter what they do.
“I would be frustrated if I saw
it in that way,” says Zambrano. “I see it with another view — that I am
doing my best to keep that from happening.”
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