Want to sell fish at the market? You must first learn some basic knowledge about fish breeding

Produced by: Science Popularization China

Author: Zhang Ru (Institute of Hydrobiology, Chinese Academy of Sciences)

Producer: China Science Expo

China is a country that loves fish. Fish is an indispensable dish on the Chinese table. Recently, I heard that many people want to sell fish in the market after watching "The Storm". If you can master the secret of making fish reproduce stably and productively, you will never have to worry about not having fish to sell.

Recently, a research paper jointly conducted by Sun Yonghua's team from the Institute of Hydrobiology, Chinese Academy of Sciences and Chen Zhenxia's team from Huazhong Agricultural University explored the mystery of fish reproduction from the perspective of mitochondria.

(Photo source: Veer Gallery)

The water mice are on the battlefield!

Chinese people eat fish, not by taking unlimited fish from nature, but by raising fish by themselves. China is the only major fishing country in the world where the output of aquaculture exceeds that of fishing, and aquaculture has become one of the important industries related to the national economy and people's livelihood.

From ancient times to the present, the Chinese have made various efforts and attempts to obtain a large and stable supply of fishery resources. As early as the Shang Dynasty more than 3,000 years ago, our ancestors began to try artificial fish farming. Fan Li, the first generation of wealthy Chinese merchants, wrote the world's first book on fish farming, "Fish Farming Classics". In modern times, Chinese scientists have also made many contributions to exploring the secrets of fish breeding and achieving sustainable aquaculture.

When it comes to the study of fish reproductive development in modern biology, we have to thank a small tropical ornamental fish that is active in aquariums - zebrafish. This small fish from tropical streams is named because its body is covered with multiple dark blue stripes, which resembles a zebra.

Zebrafish is an important animal model for studying the development of fish and vertebrates, and is therefore known as the "little white mouse of the water." This small fish contains the secrets of fish maintenance and reproduction.

Zebrafish from the National Zebrafish Resource Center

(Image source: National Zebrafish Resource Center)

Where do fish come from? - Germline stem cells

Research using zebrafish as a model has identified many germ plasm factors (a special cytoplasm with a certain morphological structure composed of proteins and RNA in animal oocytes, called "germ plasm"), which determine the specialization of primordial germ cells and the maintenance of the fate of germ stem cells.

Wait, what are primordial germ cells and what are germline stem cells?

There is a unique group of cell types in the gonads of adult animals, called germline stem cells (GSCs). The starting cells of sperm and egg are called spermatogonial stem cells (SSC) and oogonial stem cells (OSC), respectively, which are derived from primordial germ cells (PGC) in the embryonic period.

Primordial germ cells are the first group of germline stem cells established during early embryonic development, and are therefore also called germline progenitor cells. Germline cells with stem cell characteristics in the embryonic stage and in adults are collectively referred to as germline stem and progenitor cells (GSPC).

Fish germ cell development

(Image source: author)

How do fish come from? ——Maintenance and differentiation of the fate of germline stem cells

As the only carrier and transmitter of genetic information to offspring within an individual, reproductive stem cells have the important mission of continuing life in the biological world. The specialization, migration and fate maintenance of reproductive stem cells are comprehensively regulated by multiple genes and signaling pathways. The directed differentiation of reproductive stem cells includes two decisions:

The first is to decide whether to self-renew or differentiate. If the reproductive stem cells undergo self-renewal, they will enter mitosis and continuously produce new stem cells; if they choose to differentiate, the reproductive stem cells will enter meiosis and differentiate into their ultimate form (that is, reproductive cells that can be passed down from generation to generation, such as sperm and eggs).

The second decision is whether a germline progenitor cell entering meiosis will develop into an egg or a sperm.

The self-renewal and differentiation of reproductive stem progenitor cells must achieve a dynamic balance in order to continuously produce sperm and eggs, allowing the species to reproduce and continue.

Self-renewal and differentiation of germline stem progenitor cells

When the self-renewal of germline stem cells is enhanced but the differentiation is weakened, it will lead to germ cell tumors (SEMINOMA); when the self-renewal is reduced but the differentiation is enhanced, it will lead to spermatogonial stem cell exhaustion (SPERMATOGENESIS DEPLETION)

(Image source: Xie, et al., Biomolecules, 2020)

Active mitochondrial network provides the "force" for stem cell fate decisions

Mitochondria are known as the "power plants" of cells. They produce ATP through aerobic respiration and oxidative phosphorylation to provide energy for cell activities. At the same time, mitochondria are highly dynamic organelles. Mitochondria are constantly undergoing dynamic changes in fusion and fission, which is called mitochondrial dynamics.

In simple terms, mitochondria in cells can either split into multiple mitochondria or fuse into one mitochondrial cell. The dynamic balance of mitochondrial fusion and fission is one of the important ways to control mitochondrial quality, which is essential for its self-renewal and function.

At different developmental stages of germ cells, the number, size and morphological distribution of mitochondria are different, and the regulation of reproductive function by mitochondrial dynamics has received increasing attention.

Mitochondrial fission (left) and fusion (right). The green part is the mitochondria and the purple part is the endoplasmic reticulum.

(Image source: https://www.cell.com/cell/fulltext/S0092-8674(18)31308-4)

(GIF)

Pld6: a stabilizer of mitochondrial dynamics in fish germline progenitor cells

The "force" that mitochondria provide for cell development needs to be inexhaustible, but not excessive. So what controls the function of mitochondria?

Researchers from the Institute of Hydrobiology, Chinese Academy of Sciences, used zebrafish as a model for research and discovered a peculiar gene in fish reproductive stem progenitor cells: Pld6, which can control mitochondrial fusion and mitochondrial dynamics.

So how does Pld6 work?

Simply put, in fish reproductive stem progenitor cells, Pld6 acts like a "stabilizer", controlling the orderly output of the highly dynamic mitochondria, the "energy pump" in the cells, thereby regulating the balance and stability of self-renewal and differentiation of reproductive stem progenitor cells.

The mitochondrial fusion regulator pld6 is specifically expressed in the germ cell lineage

Ovary: ovary; testis: testis

(Image source: provided by the author)

If you still feel that Pld6 is unfamiliar, you can take a look at the picture below:

Back cover of Advanced Science Vol. 9 No. 36: Yu the Great Controlling Floods

(Image source: Designed by Dr. Zhang Ru and Researcher Sun Yonghua, created by Bangtu)

Just as the "Yu the Great Controlling Floods" picture drawn by the researchers is very Chinese in nature, it vividly presents the scientific story of Pld6-mediated mitochondrial fusion and mitochondrial dynamic regulation of fish germ cell line development homeostasis and species reproduction.

The endless cycle of life is like the endless flow of the mother river, and the renewal and differentiation of germ cells is like "the water of the Yellow River comes from the sky, rushes to the sea and never returns"; the germ cells "continue from generation to generation" but do not proliferate, just like a flood rushing along the river to the sea.

This picture uses Dayu holding a plow to control the flood to symbolize that the mitochondrial fusion factor Pld6 controls and ensures the balance between the self-renewal and differentiation of reproductive stem progenitor cells, thus creating the mother river that stretches for thousands of miles, rushes endlessly, and gives birth to countless lives.

Conclusion

Fish breeding is closely related to our lives and is also a key issue of concern to the fishery industry. Although there are many species of fish, there is a lack of high-quality species that can be cultivated on a large scale. A deeper understanding of the reproductive regulation mechanism of fish can not only help promote the reproduction of existing fish, but also be conducive to the cultivation of new breeding species.

I believe that in the near future, more abundant, higher quality and more affordable fish will appear on the tables of thousands of Chinese households.

Editor: Guo Yaxin

Note 1: The relevant content of this article has been officially published in Advanced Science as a back cover paper, titled: "A Germline-Specific Regulator of Mitochondrial Fusion is Required for Maintenance and Differentiation of Germline Stem and Progenitor Cells".

Note 2: Latin text should be in italics.