Shulou(Shulou.com)11/24 Report--

Flying fish, a strange way of moving, seems to be a strategy to avoid predators under the water. (photo Source: John Cobb) suddenly, a clever figure pierced the tranquil water and glided rapidly above the water-the vigorous posture of a flying fish. What makes these fish that should live in the water have an incredible ability to "fly"?

Flying fish (Exocoetidae) is a kind of fish widely distributed in the global ocean. Compared with ordinary fish, they are slightly strange in shape, with long, hard pectoral and ventral fins, just like majestic wings. But it only needs these few changes in the body, these creatures that can only swim in the water will have the ability to "fly". Strictly speaking, of course, they can only glide: using acceleration under water to jump out of the water, using aerodynamics to extend gliding time in the air, and hitting the surface continuously with their tails when they are about to enter the water again. Flying fish can glide as far as 180 meters in a single glide and 400 meters several times, which can help them escape predators.

For a long time, however, scientists do not know what genetic mechanism makes flying fish have this trait. Recently, led by Matthew Harris of Harvard Medical School, a group of researchers revealed the genetic mechanism by which flying fish evolved strange fins. Combined with a variety of technologies, they found that it takes only two genetic changes to make flying fish have this strange body structure, and the mechanism is surprisingly simple. The study was published last November in the journal Current Biology.

The secret of fin growth Harris's team sequenced and compared the genomes of 35 species, including flying fish and their relatives, in order to search for the genetic mechanism that gives flying fish a strange body structure. They looked at DNA regions where there were significant differences between different species and found some genes that seemed to have evolved under selection pressure.

Joost Woltering, an evolutionary biologist at the University of Constance in Germany, believes that this comparative study could help uncover the drivers of new body shapes. "but how do you prove that this gene is responsible for the difference? it's hard to shorten the fins of flying fish back," he said. "We have to find another animal that can carry out experiments."

So Harris' team experimented with zebrafish (Danio rerio). Using chemicals and gamma rays, they induced random mutations in more than 10,000 zebrafish embryos and then looked for traits of interest to individuals who survived to adults. This practice is not common because genetic studies in zebrafish usually focus on their development.

Zebrafish is widely used in genetics and development research, and is also a popular pet. (photo: Michael Goderre / Boston Children's Hospital; In Laos) Jacob Daane was a postdoctoral researcher at Harris Lab at the time. He and his colleagues also screened a number of previous zebrafish long-fin mutants to look more accurately for genetic variants that may regulate zebrafish fin growth. They focused on two mutant genes, kcnh2a, which causes overexpression of potassium channel proteins on the outer membrane, and lat4a, which leads to loss of leucine transporter function.

The researchers found that in zebrafish, mutations in leucine transporters caused all fins to become shorter, while mutations in potassium channels caused all fins to become longer. If only one of these two mutations occurs, the motor ability of zebrafish will be affected. But when these two mutations occur at the same time, the zebrafish's pectoral fins get longer, while median fin, the asymmetrical fins that fish grow along the midline of the body, such as dorsal, anal and caudal fins, get shorter and look like flying fish. In response, Dan said: "as far as I know, it is not common in animals that such a very simple mechanism can have such a huge effect on the size of organs."

The victory of evolution looks at the vast expanse of nature, different animal body shapes can be described as varied, this morphological diversity is largely due to the role of natural selection. Even a slight change in the time and speed at which a piece of tissue grows may affect the length and size of the body structure somewhere, or even make the animal grow more or less a bone, thus creating new adaptive traits and winning a new niche (niche) for the species. The unique trait of flying fish can be said to be an evolutionary victory.

In different flying fish lineages, their body structure has undergone several independent evolution, but all of them are related to the mutations of leucine transporter and potassium channel genes. The leucine transporter mutations in different lineages are not exactly the same, but they involve the same amino acid changes, indicating that different lineages independently adopt the same genetic mechanism, thus evolving the shape of the fin. "in different backgrounds, nature targets the same specific gene." Says Sarah McMenamien (Sarah McMenamin), an evolutionary developmental biologist at Boston College.

The leucine transporter gene (intermediate column) is highly conserved in most fish species (upper). However, in flying fish (lower) and some of their related species (middle), there is an amino acid residue (right column) in the transporter protein that is different from other fish. So the researchers suspect that the gene affects the proportion of fin length. (photo: Samuel Velasco / Quanta Magazine, cropped; source: https://doi.org/ 10.1016 / j.cub.2021.08.054) scientists still don't know how mutations in potassium channels lead to overgrowth of fins. When potassium channels are overexpressed, the resting membrane potential of the cell membrane and the pH value of the cytoplasm will change, enhancing the cell activity and response to stimulation. As a result, fin cells begin to send signals similar to those emitted by neurons and stem cells. Harris said that perhaps changes in cellular signals will affect the growth of fins, but this is just a guess. "these are all new problems in biology, and few people have studied them, and people know very little about the mechanism." He said.

By inhibiting the expression of potassium channel mutant genes, the researchers inhibited the flow of potassium ions and found that the growth of fish fins was blocked. They suspect that at some stage of development, the cells in the fins become similar to syncytial bodies (syncytium, a single cytoplasmic mass with multiple nuclei). If this is the case, the electric field generated by potassium ion current can be transmitted to the whole fin to achieve long-range signal regulation, which may be stronger than the chemical signals that affect morphogenesis (morphogenesis), such as morphogenetic factors or secretory factors. In other words, bioelectric signals may also regulate the growth and shape of fish fins, and may even affect other body structures.

The study of the development and evolution of fins to the left and limbs to the right may also help us to answer the question of quadruped limb evolution. Zebrafish's pectoral fin and body are connected by only one layer of bone structure, the proximal radiofin bone (proximal radial), which forms a joint with the "shoulder" of the fish. Last February, BrentHawkins, KatrinHenke and Harris published a study in the journal Cell. They found that a mutation in one expression pathway could affect the pattern of zebrafish fin development, showing the potential to develop into limbs.

The fins (upper) of bony fish lack such fine structures as the limbs (lower) of quadrupeds. However, only a mutation in one expression pathway can affect the pattern of zebrafish fin development and make it show the potential to develop into limbs (middle). (photo source: https://doi.org/ 10.1016 / j.cell.2021.01.003) mutated zebrafish form two "middle fin bones" (intermediate radial), which form joints with the proximal radius fin. These newly formed bones even have muscles attached to them. Such a structure has revealed the embryonic form of more elaborate limbs. However, the ancestors of zebrafish and quadrupeds parted ways as early as about 450 million years ago. This atavism (atavism) restores the evolutionary picture of hundreds of millions of years ago and reveals that the genetic mechanisms that encode fins and limbs are very ancient and may be common in vertebrates.

For the study of the evolution of appendages, their findings are gratifying. "the work of the Harris team actually shows that living radiofin fish genomes still retain information that can produce fine endoskeleton structures, and they have the potential to develop more elaborate structures." McMenemin said.

Two other studies, published at the same time in the journal Cell, analyzed the early branches of radial fin fish and the genomes of African lungfish, respectively, suggesting that the common ancestor of all bony fish had the potential to develop limbs.

Follow-up studies have also continuously shown that the genetic mechanisms related to fin and limb development are quite conservative. For example, a study published in the Proceedings of the National Academy of Sciences (PNAS) last November found a gene that regulates the formation of both the toe structure and the outer edge of the fin in quadrupeds. In the same month, a study published in current Biology showed that jerboas have long hind feet because a gene causes the bones of their limbs to grow disproportionately. The phenomenon is similar to the allometric growth of flying fish fins (the growth rate or pattern of a part is out of proportion to the growth of the individual).

Mystery of Origin Marcus Davis, an evolutionary developmental biologist at James Madison University, believes that one question that academics should think about now is where did this set of procedures for the development of fins and limbs come from in the first place? "such a program must have a source and cannot be created out of thin air." Davis said.

This procedure may come from an older set of developmental procedures in other parts of the animal's body. Tetsuya Nakamura, a developmental biologist at Rutgers University, speculates that the genetic mechanism that encodes even fins (paired fin, the symmetrically grown fins of fish, such as pectoral and ventral fins) and limbs is derived from the mechanism that codes for the development of dorsal and anal fins, which are older than even fins. For example, lampreys (lampreys) are a class of jawless fish that evolved about 500 million years ago. They have dorsal and anal fins, but no even fins.

Flying fish's paired pectoral fins (located in the front of the body) and ventral fins (in front of the tail) are longer and harder than most other bony fish and can act like wings. The odd fins on the upper and lower sides of the body are relatively small and can reduce air resistance. (photo source: Valerii Evlakhov) although these very different appendages and body shapes share a common and ancient genetic origin, Waltling points out that the transformation from one form to another is a very important evolutionary event: "I believe that the birth of quadruped limbs is definitely a pioneering work in the history of evolution."

How these evolutionary events occur remains to be studied, and the novel research methods of Harris team provide new ideas for the study of evolutionary developmental biology. When looking for genes that regulate developmental processes, scientists often focus on some common and closely related mechanisms, such as insulin signaling pathways that regulate allometric growth, and Hox genes that regulate limb and fin development patterns. But Harris' team used comparative genomics and large-scale genetic screening techniques to identify individual zebrafish with target phenotypes. This approach is more uncertain and it is difficult to predict the results of the experiment.

But Harris believes that their efforts are worth it: "once we are thinking in the right direction, we will get unexpected results that cannot be obtained by classic group-level studies."

Original text link:

Https://www.quantamagazine.org/flying-fish-and-aquarium-pets-yield-secrets-of-evolution-20220105/

Research papers:

Https://doi.org/10.1016/j.cub.2021.08.054

Reference link:

Https://doi.org/10.1016/j.cell.2021.01.003

Https://www.britannica.com/animal/flying-fish

Https://en.wikipedia.org/wiki/Flying_fish

This article comes from the official account of Wechat: global Science (ID:huanqiukexue), translated by Li Shiyuan, revised by Erqi.

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