Figure 1. The albino species cultivated in the laboratory of the Mexican Salamander are cute internet celebrity animals. Image source: sciencefocus.com
- Introduction -
Unlike the tadpole that is bound to turn into a frog, the grown baby salamander will come to the intersection of salamanders: perverted, or impervious?
Written by 丨 莘莘
Responsible editor 丨 Dai Wei
In the ancient Aztec culture, there is such a legend.
One day, in order to create a new sun, the wind god decided to kill all the gods and sacrifice themselves, but Zoroa, the god of change, was unwilling to sacrifice himself. In order to escape the wind god, Zoratra first turned into corn, then into tequila, and finally into the Mexican Salamander in the water.
In the end, the wind god still found Zoratra, and in order to punish him for cowardice and selfishness, Zorado was cursed, and he would live in the water forever and never grow up.
Figure 2. The wild species of mexican salamander looks bad. Source: Wikimedia Commons
A pending case in the Botanical Garden of Paris
The Mexican axolotl (Ambystoma mexicanum), also known as the Mexican salamander, is an amphibian found only in Mexico because of the six pigtails on its head, also known as the Hexagonal Dinosaur.
The earliest entry of the Salamander into the academic field can be traced back to more than 200 years ago. At the beginning of the 19th century, the famous Alexander von Humboldt presented two salamander specimens obtained during expeditions to the Americas to the famous French scholar George Cuvier.
Cuvier was the founder of comparative anatomy, and at the time he was working to refine the amphibian and reptile parts of the Linnaean classification system by analyzing the structure of animals, and was the absolute authority of the two reptiles. Cuvier, with the assistance of his assistant André Duméril, dissected the specimens of the two Salamanders and came to the conclusion that the two Salamanders should be juveniles of some sort of animal – that is, in the pre-metamorphosis state.
Metamorphosis refers to the developmental process of animals after birth, the significant changes in physical structure and living habits, the transformation of caterpillars into butterflies, tadpoles into frogs, these are all familiar to us. As for what the adult body looks like after the metamorphosis of the Salamander, you have to go to the Americas to catch another adult body.
Since then, some specimens of Salamander have visited Europe, but no new conclusions have emerged in the academic community.
It wasn't until January 1864, on a winter day in Paris, that six Salamanders came quietly from Mexico, 9,190 kilometers away, to the Jardin des plantes on the banks of the Seine. This was the first time that the Salamander had set foot on the european continent alive, and it was none other than Auguste Duméril, the son of André Dumery, Cuvier's assistant.
By observing these six Salamanders, Du believed that Cuvier's conclusion was not wrong, so he published a paper in 1864 stating that the Salamander was indeed the juvenile of some kind of reptile.
However, xiao Du had just published a paper on the front foot, and the reversal came: his Mesai salamander was born.
Since these Salamanders can breed offspring, they are of course sexually mature adults, how can they be juvenile? Xiao Du had to publish a paper in the same year: Sorry, I (and Cuvier) got it wrong, these Mexican salamanders are adults.
However, only a year later, when Xiao Du went to check on his Salamander again, he found that he had made a mistake again.
"Wait, how did I get one in my Mexican Salamander... What animal is that? Who put it in? ”
Figure 3. Dumeli depicts two appearances of the Salamander. Source: Duméril [2]
However, the salamander that appeared out of thin air was no other salamander, but the Mexican salamander had undergone metamorphosis.
Now Xiao Du is completely confused by his Mexican Salamander, are those American Salamanders native to Mexico an adult? If not, how come they gave birth to the little Salamander again? If so, how did the second-generation Salamander they gave birth to become a completely different form?
Xiao Du then found that among these second-generation Mexican salamanders, some would be perverted and some would not be perverted. But what exactly determines the perversion or inversion of the Mexican Salamander? Unfortunately, until Du Xiao died in 1870, there was no perfect answer to this question.
Figure 4. The mystery of the mermaid's adult body. Image source: Hand-drawn by the author
Perverted or unchanging? The crossroads of life
Tadpoles turn frogs are the most familiar metamorphosis process. Similar to tadpoles, juvenile salamanders also live aquatic lives breathing on their gills – the six small braids on their faces are their cheeks.
However, unlike the tadpole that inevitably becomes a frog, as the juvenile salamander grows up, it will face a salamander crossroads: if it has been living in the water, then its lifelong form will be frozen in its juvenile state, breeding offspring in a juvenile form; if its living environment is biased towards dry land, then the salamander may undergo a perverted process like a tadpole to a frog, from a "fat-headed fish" with six small braids to a "four-legged snake" suitable for land life.
Figure 5. Reducing degree 100% of metamorphosis and non-perverted Salamander. Image source: Hand-drawn by the author
In the development of animals, environmental factors at a certain point in time can play a decisive role in the morphology of animals. One scientist who learned the mystery of human ancestors from the life course of the Salamander was the British scientist Walter Garstang.
Gaston loved two things in his life, animal evolution and poetry. We will not list the poems for the time being, but let's first introduce two creatures that fascinated Gastang.
In the vast expanse of the sea, there is an inconspicuous animal, the Branchiostoma lanceolatum. The Wenchang fish is not actually a fish (Figure 6), nor is it a vertebrate, it has no vertebrae, only a nerve cord that descends along the back, and this nerve cord indicates that the Wenchang fish is the closest living animal to the ancestor of the vertebrates.
Figure 6. Wenchang fish Image source: embrc-france.fr
At the same time, another ancient animal, the sea squirt (Tunicata), is widely distributed in the sea. The sea squirt looks like a hollow sponge tube (Figure 7), and for most of its life it is inserted into the seabed and does not move. However, the sea squirt is not born this way, and the juvenile sea squirt is shaped like a tadpole and can swim freely in the sea, and there is a ridge at the tail end that is similar to the structure of the Wenchang fish. However, after metamorphosis, the cord of the sea squirt disappears and becomes a tube on the seabed.
Figure 7. Sea Squirt Image source: Wikimedia Commons
In Gastang's mind, the Salamander connected the Wenchang fish to the sea squirt: since it is possible for the Salamander to remain in its juvenile state due to environmental factors, something similar could have happened to some ancient creature that resembled a sea squirt. At some ancient point in time, a creature resembling a sea squirt was frozen in its juvenile state and multiplied in a form similar to a Wenchang fish, becoming the ancestor of vertebrates, including humans.
Figure 8. Juvenile continuation theory: A sea-like organism was frozen in the juvenile state and multiplied in a form similar to the Wenchang fish, becoming the ancestor of vertebrates. Image source: Hand-drawn by the author
This theory was called the theory of juvenile continuation by Gasten, and Gastang, who was keen on poetry, did not write these studies as ordinary academic papers, but all of them were written into poems, which were included in his poetry collection "Larval Morphology and Other Zoological Verses". While modern DNA research has confirmed some shortcomings in introducing vertebrate origins in juvenile continuations, Gastang's work certainly provides us with a groundbreaking perspective on animal evolution (and a unique interest in writing poems about scientific research).
At the right time, in the right place, become an experimental star
Although until his death, Xiao Du did not find the answer to the metamorphosis of the Salamander, but he contributed to the American Salamander to become an enduring experimental star.
In the scientific spirit of selfless sharing, the generous Xiao Du has donated his breeding Salamander to many universities, research institutions, public and private zoos and aquariums. At the same time, with the development of air pump technology, there was a wave of aquarium fever in Europe in the mid-to-late 19th century, and water tanks became almost a must-have item for home viewing or scientific research. With this shareholder wind, the easier-to-breed Salamander soon spread across Europe, becoming the first laboratory-bred laboratory animal in history to breed autonomously.
In the process of reviewing the history of Salamander research, I wrote an e-mail asking Christian Reiss, a professor of the history of science at universität Regensburg in Germany: What makes Salamander so special?
Dr. Lacey's answer was: at the right time, in the right place, the right animal was encountered.
In the 19th century, when zoology began to take shape, from embryonic to comparative anatomy to biological evolution, the main reason scientists initially chose salamander as a test subject was that it was easy to reproduce and well supplied. With more and more understanding of the Salamander, the magical qualities of the Salamander have fascinated scientists more and more, and have sent countless new topics to the scientific community.
At the beginning of the 20th century, the German scientists Vilem Laufberger and Julian Huxley confirmed with experiments that thyroxine is the key to regulating the metamorphosis of salamander, and given thyroxine can artificially induce the metamorphosis of juvenile salamander. This research not only put a perfect end to the doubts that Panhuan Xiaodu had for many years, but also greatly promoted the study of thyroid function and metabolism in other animals. To this day, the scientific community still calls the test of thyroid hormone distribution in animals "salamander test".
Today, the most fascinating thing about the scientific community is its powerful regenerative ability, from organ regeneration to anti-aging, the key to solving these mysteries is still on the body of the salamander.
Artificial breeding is thriving, and wild Salamanders are endangered
In Aztec legend, the Salamander was the embodiment of Zorotta, the god of change in the underworld, but this aura did not allow them to live in the wild without worry.
Wild Salamanders are found only in the central part of Lake Xochimilco and Lake Chalco. With the urbanization of Mexico City, the continuous loss of habitat suitable for the Salamander, coupled with the tradition of Mexicans preying on the Salamander, the number of Mexican Salamander has decreased sharply in recent years and was endangered in 2010, although the Mexican government has made some efforts to restore the population of the Wild Salamander, but the situation is not optimistic, the Salamander in the Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES) is still critically Endangered, This means that it is likely to become extinct in the wild.
A secret contained in a small Salamander that we humans have studied for 200 years and have not yet fully unraveled. If we allow such a species to disappear, we will never know what we have missed, which will be an eternal regret for all mankind.
Author's Statement
Special thanks to Professor Christian Reiss, author of references[1], for his enthusiastic response to my questions.
The author wants to thank Prof Reiss for his help and support.
Resources
[1] Reiss, C., Olsson, L., Hoßfeld, U., ”The history of the oldest self‐sustaining laboratory animal: 150 years of axolotl research.” Journal of Experimental Zoology Part B: Molecular and Developmental Evolution. 324, 5, (2015):393–404.
[2] Dumeril, A., 1866. "Observations on the reproductions in the menagerie of reptiles of the Museum of Natural History, axolotls, amphibian urodeles with external gills of Mexico, on their development and metamorphoses." New Archives of the Museum of Natural History. 2, (1866):265–292.
[3] Shubin, N. Some Assembly Required: Decoding Four Billion Years of Life, from Ancient Fossils to DNA. New York: Pantheon, 2020.
[4] Richard Dawkins, Wang Xiuqiang (translation), "The Story of ancestors: A Pilgrimage to the Origin of Life", Jiangsu Science and Technology Press, 2010.