Scientists are taking action on fish! They are trying every possible way to make fish tastier, cheaper, and more productive...

Crucian carp, grass carp, carp, bighead carp, tilapia, sea bass, snakehead carp...Have you ever eaten these fish?

Thanks to the application of scientific research results such as freshwater fish habit research, breeding research, and aquaculture technology innovation, we can eat more and more delicious freshwater fish at cheap prices.

In fact, agricultural research has never been as popular as medical research. Even among agriculture, forestry, animal husbandry, sideline production and fishery, the status of "fish" is the most marginal. But fortunately, there are always people who move forward and constantly open up new paths.

Researcher Yin Zhan, former director of the Institute of Hydrobiology of the Chinese Academy of Sciences, is one of them. He said, " Fish are very important to the Chinese people, and I want to study them thoroughly ."

Every March and April, he and his research team would go to the breeding base in Guangdong to conduct scientific research. In that warmer place, carp can reach sexual maturity within a year, greatly saving research time.

Recently, Yin Zhan accepted our interview and told us in detail about "him and his 'fish'".

What does fishing mean in China?

China is the world's largest fishing country, and its aquaculture output accounts for more than 60% of the world's aquaculture output. In other words, the aquaculture output of all other countries combined is not as much as that of China.

In Western countries and Japan, the main supply of aquatic products comes from marine fishing, and freshwater fish farming is very small. Therefore, apart from experts in Norway and Denmark who specialize in freshwater fish farming, China is the only country in the world that studies freshwater fish farming most seriously and systematically.

In 2005, Yin Zhan, who had studied and worked abroad for more than ten years, returned to China and joined the Institute of Hydrobiology. With the help of the genetics and developmental biology training he obtained abroad, he began to conduct research on the endocrine regulation of fish growth metabolism and reproduction.

At that time, there was basically no research on fish endocrinology in China using genetic manipulation. He believed that hormones in the endocrine system could be produced in the endocrine glands of fish, circulate throughout the body, and cause the animal to have an overall response. If the mechanism could be figured out, active intervention could be made in the growth, metabolism, and reproduction of fish. Such research would have both basic theory and application value.

“I want to cultivate

Fish needed for China's fishery development"

"Grass, silver carp, bighead carp, carp, crucian carp and bream" (known as "commodity freshwater fish" in the industry), these seven fish account for 70% of China's freshwater fish farming output. In the past, "big fish and big meat" were delicacies that could only be enjoyed during the Chinese New Year. Thanks to the rapid expansion of China's aquaculture scale - from 1989 to 2020, China's aquaculture output increased 6.5 times - we can eat delicious freshwater fish at a cheap price.

However, behind the rapid growth of aquaculture is the rapid expansion of freshwater aquaculture area, the resulting eutrophication of water bodies, the sharp increase in ecological pressure, and the rapid consumption of resources, all of which will be difficult to sustain.

In addition, fish farming in China used to be mainly based on mixed pond farming, and so far, the proportion of factory farming production is still very low. my country's traditional farming species are not yet able to adapt to high-density farming, and there is no fish farming species whose annual production per cubic meter of pond can exceed 50 kilograms - far lower than the production of domesticated salmon, trout, tilapia, etc. under high-density farming models internationally.

"China's aquaculture is facing a new round of transformation and upgrading. In terms of aquaculture space, it is also advancing towards saline-alkali land and deep sea, and vigorously promoting facility-based, high-density intensive aquaculture. All these industrial issues require us to cultivate new varieties of fish that are adapted to these aquaculture models. "

Yin Zhan said: "This kind of fish must be 'well-behaved', able to adapt to high-density environments, not be stressed, and 'sleep' after eating, so that one pound of bait can grow more meat; it must be able to adapt to saline-alkali environments, not be afraid of cold, and be able to survive in saline-alkali lands in the north and deep seas; it would be best if it could control the production of only the favorable sex without causing genetic pollution..."

Source: Provided by the interviewee

"Cultivating fish that meet the needs of China's fishery development" is the idea that runs through Yin Zhan's entire scientific research career. Looking back on his decades of scientific research career, Yin Zhan said that the most exciting and impressive moment was when he learned that the laboratory had obtained all "male" fish.

Single-sex fish populations mean that scientists need to get fish of a specific sex. This is useful - fish of different sexes are often different sizes. For some fish, males grow faster and bigger; for some fish, females grow faster and bigger. In aquaculture, if you only keep fish of the larger sex, it will help increase production.

The experiment was conducted on zebrafish. There is a gene in zebrafish that controls the synthesis of androgens. In some zebrafish, androgens are converted to estrogen to a greater extent, which is how female fish are produced. If this gene is knocked out, zebrafish will not be able to synthesize androgens, and naturally there will be no estrogen.

Yin Zhan asked his students to do this work and found that without the ability to synthesize androgen, all the zebrafish grew into "male" fish. What's more interesting is that these "male" fish can also produce healthy sperm with activity. However, subsequent studies have found that these "male" fish cannot mate with female fish and produce their own offspring due to the lack of androgen in their bodies.

The sperm of these "male" fish can be squeezed out and collected, and then fertilized with eggs collected from female fish through artificial insemination to produce normal offspring.

However, when the research team submitted this result to various journals for publication, they were rejected one after another. The journal editors and reviewers questioned the research team for not completely knocking out the gene responsible for synthesizing androgen in zebrafish. Coincidentally, another research team at the Institute of Hydrobiology was doing research on removing androgen receptors. They found that if the androgen receptors were removed, even if androgen existed in the fish, the zebrafish testis would be seriously affected and no active sperm would be produced.

" I mated my fish with fish that lacked androgen receptors, and produced offspring that had neither androgen synthase nor androgen receptors. As a result, I once again obtained fully developed testes and fully male fish," said Yin Zhan. "So I had to speculate that perhaps other hormones were replacing androgens to maintain sperm production."

Could it be progesterone? Progesterone is also a hormone. It is the raw material for zebrafish to synthesize androgens. When the gene for synthesizing androgens in zebrafish is knocked out, progesterone in the body can no longer be converted into androgens. Therefore, a large amount of progesterone accumulates in these zebrafish, reaching 7 to 10 times the normal value in the testicular tissue.

Therefore, Yin Zhan asked his students to knock out the gene encoding the progesterone nuclear receptor. This time, the fish obtained only had testes but no sperm. This shows that it is progesterone that promotes the normal development of the testes and the production of sperm. This result was published in the famous academic journal eLife in the field of biology.

"But I didn't do the weight loss research.

Because I work in the aquatic industry.”

After the paper on fish progesterone research was published, a medical research institute contacted Yin Zhan because this discovery had the potential to bring new ideas to the treatment of human prostate cancer. Coincidentally, one month later, a team led by the Shanghai Institute of Biochemistry of the Chinese Academy of Sciences published a paper reporting that progesterone levels in late-stage prostate cancer patients increased, and proposed evidence that blocking progesterone signals could further treat prostate cancer.

Who would have thought that the study of a fish could also bring new inspiration and direction to biomedical research? However, this is not the first time that Yin Zhan has encountered such a situation.

Yin Zhan's team is one of the first teams in China to systematically edit genes of fish and endocrine-related molecules, including important protein hormones, and construct mutant models for research. Around 2014, Yin Zhan's team began to study the mechanism of action of steroid hormones.

As we all know, in the human body, vitamin D (VD) is a cholesterol derivative that helps calcium absorption. Yin Zhan's research team knocked out a gene encoding an activating enzyme of zebrafish VD, hindering its activation of VD in the body. They thought they would get fish with skeletal dysplasia. Unexpectedly, the skeletons of these zebrafish developed basically normally, but fat accumulated in the abdomen. After taking in activated VD, the abdominal fat of these zebrafish was consumed. It turns out that the main function of VD in fish is to oxidize and utilize abdominal fat to provide energy for themselves - which is different from the focus of promoting calcium absorption in the human body.

After the relevant paper was published, it was recommended by the chairman of the International Vitamin D Conference at the 2018 annual meeting as one of the 12 most watched vitamin D research papers in the world in 2017. Because the study used an animal model to prove for the first time the mechanism by which VD regulates fat metabolism in animals, it is also an important clue for using VD to regulate human fat metabolism.

Weight loss has long been a hot topic in the biomedical field. After Yin Zhan published his paper, research groups and biologists studying weight loss came one after another, hoping to cooperate with Yin Zhan in research.

" But I didn't do that because I was in the aquatic products business ," said Yin Zhan.

A ray of hope in scientific research

The final publication of each scientific research result is accompanied by countless failed explorations. Before an original scientific research experiment is carried out, the results are mostly unpredictable. Sometimes researchers need to be tenacious, because failure is not meaningless. They should persist in a field, or even a failed experimental result, and dare to challenge and draw inferences from one instance to another. Perhaps a discovery since the development of human science and technology can bring new inspiration.

Yin Zhan still remembers a study conducted more than 10 years ago. At that time, he was focusing on fish prolactin. Studies have shown that mice without prolactin cannot give birth, and the ovaries of female mice also develop abnormally. So, how would a similar situation manifest in fish? "Anyway, (zebrafish) embryos develop in vitro, so we should knock out the gene first."

However, zebrafish that cannot synthesize prolactin can still lay eggs successfully. Four days later, the baby zebrafish hatched normally - this is completely different from the result of mice that cannot give birth at all, which is beyond everyone's expectations.

However, what is even more dramatic is that these little zebrafish, which were once lively and energetic, began to die one by one from the 10th day. By the 16th day, not a single one was left, and all of them were "sacrificed".

Fish pond, source: provided by the interviewee

Yin Zhan and his students carefully dissected and observed all the dead fish, and the development of their tissues and organs seemed to be basically normal. So they cultivated a group of similar zebrafish, cut one of the fish into more than 200 tissue slices through continuous sectioning, stained the cells of each slice, and carefully examined its development, and there was no significant abnormality either.

The cause of death remained a mystery and the experiment came to a standstill.

A few days later, everyone experienced a "miracle turning point". A student actually saved these mutated zebrafish!

It turned out that the student noticed that a study had shown that salmon do not secrete prolactin in the ocean, but when they migrate to freshwater upstream to spawn, their prolactin secretion levels gradually increase and reach their highest level before giving birth. The scientists who published the article at the time believed that the prolactin secretion level of fish is related to the degree of sexual maturity.

This student tried to interpret the paper from a different angle. He wondered if salmon might not secrete prolactin in the ocean with high osmotic pressure, but only secrete prolactin in fresh water. The more sexually mature the salmon are, the closer they migrate to the upstream of fresh water, and the lower the osmotic pressure. So he added a certain amount of salt to the fish tank and let the mutant zebrafish that did not produce prolactin soak in brackish water. Unexpectedly, the zebrafish actually survived.

Therefore, Yin Zhan's research team concluded that prolactin secreted by fish helps absorb environmental ions and regulate osmotic pressure, and mutant zebrafish lacking prolactin can survive and reproduce in water with increased salinity. This is different from the production of prolactin by mammals such as humans for reproduction and feeding offspring.

Who would have thought that an accidental "flash of inspiration" would save the zebrafish and discover another scientific truth.

Source: Provided by the interviewee

Conclusion

The biomedical field is always bustling, and Yin Zhan is still doing his fish research in the 500 large fish tanks in the fish farm. The original zebrafish that could not synthesize androgens has already had tens of thousands of offspring, which are multiplying in the fish tanks, providing a steady supply of materials for the experiment.

Yin Zhan envisions that if the technology of controlling the sex and sterility of zebrafish is applied to all carp fish, it can better serve the development of fisheries. Now, Yin Zhan's team has successfully constructed a gene-edited all-female Yellow River carp population, which has increased production by more than 20% compared to mixed pond farming. They are studying how to make fish "female or male" and sterile, so as to effectively prevent the "contamination" of artificially edited gene sites in natural populations.

In addition, Yin Zhan's team has successfully made zebrafish "well-behaved", accelerated their growth, increased their feed conversion rate, and improved their cold-resistance and salt-alkali resistance. They are currently "transplanting" the results to other farmed carp fish.

In the process of studying fish, there are novel and interesting discoveries, puzzling phenomena, explorations that have guiding significance in treating human diseases, and results that are more conducive to the development of the fishery industry. The scientific research goals planned by Yin Zhan when he joined the Institute of Hydrobiology in 2005 are gradually being achieved.

Planning and production

Author: Feiwan Popular Science Creator

Reviewer: Yin Zhan, Researcher and former Director of the Institute of Hydrobiology, Chinese Academy of Sciences, Director of the State Key Laboratory of Freshwater Ecology and Biotechnology

Planning丨Lin Lin

Editors: Lin Lin, Xu Lai