Source | discover magazine
Author | connor lynch
Translate | Xu Chuchu
Edit | Wei Xiao
There may be no more "ambitious" cell in the human body than a sperm, which has enough patience to wait to become a complete organism, and the only cell to perform such a task: to leave its "home" and then try to live to the moment when it can reproduce.
To achieve this, sperm have evolved into the most distinctive of all cells. Most of us think of sperm as a small swimmer with a head and a long tail, but the appearance of sperm cells varies depending on the species to which they belong. For example, the sperm of the crustacean mesquite (ostracods) is made up almost entirely of the head; some mollusks produce a huge sperm "assistant", the spermatozeugma, which allows small "swimmers" to stick to it, like the armrests that commuters cling to on the bus.
What's even more striking than the difference in appearance of sperm is their difference in size. The most representative example is a North American fruit fly called drosophila bifurca, which is only a few millimeters long, but can produce sperm that can exceed 7 centimeters in length, reaching 20 times the length of the body.
Different forms of sperm in nature. a. Sperm of sea urchin; b. reptile sperm of pig roundworm; c. The longest known sperm - sperm of bifida flies; d-f. sperm from different plants, each with more than 2-1000 flagellar; g, h. two bird sperm. Image source: https://link.springer.com/article/10.1007%2fs10265-017-0936-2
Even considering the body proportions, the sperm of the bifida flies is the longest in the world, which dwarfs even the sperm of humans, chimpanzees, fish, sharks and even whales, far more than those that can produce giant sperm. Of course, there are many other invertebrates that can also produce sperm of considerable size, such as centipedes of the genus Centipede and Sysphore of the genus Macrophyllum.
The origin of these giant sperm has always been a mystery. But a June 2021 study published in Nature Ecology & Evolution revealed the true drivers of giant sperm and what contributes to their overall diversity: the fertilization patterns of females.
Approaching the "Giant Sperm"
Ariel Kahrl is an evolutionary biologist at Stockholm University in Sweden and one of the paper's co-authors. She explained that she and her colleagues would love to learn about how and why sperm evolve.
Their research was able to answer many questions, such as why flies are 20 times the size of their sperm. Kahrl said: "It's like comparing a grain of rice to a train. Previously, many people have tried to understand the origin of giant sperm by looking at specific groups, such as birds, fish and insects, but no one has ever tried to take a step back and interpret it in the broader natural system with the laws of evolution. ”
The researchers hypothesized that the organisms undergoing in vivo fertilization would produce larger sperm than those of the IVF organisms. In vivo-fertilized males and sperm casters can release sperm into environments that can be absorbed by females, while those ivally fertilized animals release both sperm and eggs into their surroundings. The sperm of the former two may evolve faster than the latter, and their sperm "interact" more frequently with females.
Scientists have also speculated that males with in vivo insemination and sprinkling insemination have a greater diversity of sperm lengths. Because their sperm have adapted to the moment and the female reproductive tract to maintain a consistent pace of development.
All told, the researchers analyzed more than 3,000 species in 21 phylums, from sponges to arthropods to chordates (such as humans). They distinguished these study subjects into vertebrates and invertebrates and divided them by different fertilization methods, including in vitro fertilization, in vivo fertilization and ejection. Subsequently, they constructed a tree of life that included each species and plotted the connections between them to compensate for interspecific relationships and avoid bias in results.
Kahrl said: "We found that our previous assumption was correct. "The results showed that in vivo fertilization produced sperm about 6 times longer than other forms of fertilization; in addition, animals that were fertilized and sprinkled in vivo evolved more quickly and tended to exhibit more diverse size variations."
Kahrl adds that in in vivo fertilized animals, almost all giant sperm are produced by invertebrates. Their bodies are small enough that sperm can grow enough to replace each other. Imagine a group of runners racing on a field where the first person to run to the finish line will win for the whole team. When you're on a small, crowded field, having plenty of room to grow is key, and when you're in a big space, the more team members you have, the more you can ensure victory.
The "second sex" at the top of the list
According to the study, the fastest-evolving sperm tend to have the most contact with the female reproductive system. Kahrl said that the size of the female reproductive system is closely related to the size of the sperm it interacts with, and together, these factors can indicate that the female drives the evolution of the sperm.
Scott Pitnick, a sperm scientist at Syracuse University in the United States, explains: "It's a miracle. What this research team is doing is actually very difficult. He thinks that perhaps the problems kahrl wants to solve are very simple, but they may also be the most difficult to answer.
Combing through a large amount of scientific literature and obtaining sufficient computer computing power to carry out comparative biology research has only recently been technically realized. In addition, the team has made public the resulting vast database, allowing other researchers to take advantage of their work.
Pitnick said the paper also highlights the most important, but least asked, question in the field: How do sperm interact with females? "The reality, however, is that there is a huge male bias in biology because we don't know anything about the role that the female reproductive tract plays in this process," he said. ”
After all, the mission of sperm cells is to effectively find the right direction in the female reproductive tract to fertilize the egg. Studying sperm only under a microscope and ignoring the anatomy of females is like pitnick describing "doing comparative biology studies only in zoos." Pitnick points out that because scientists ignore the role that females play in this process, the traditional paradigm of sperm quality and quantity research tends to be just to look at the microscope.
What's more, sperm cells don't usually develop until they enter the female's body, a process known as post-ejaculatory modifications to sperm (pems). Pitnick adds that many of the sperm's core proteins, such as metabolic proteins, come from females. By observing sperm changes across the animal kingdom, scientists have found many strange phenomena, such as a ten-fold increase in sperm length, or an internal and external reversal that occurs when it enters the female's body.
Pitnick concludes: "We are still in the dark ages of female biology research, and this paper makes a good point about this. If we want to understand the diversity of sperm, we must abandon the prejudices of the past and turn our attention to female biology. ”
Original link:
https://www.discovermagazine.com/planet-earth/why-fruit-flies-won-natures-prize-for-longest-sperm
Thesis Link:
https://www.nature.com/articles/s41559-021-01488-y
2020 Hot Articles Selection
1. The cup is up! A cup full of hot coffee, full of microplastics...
2. American, British and Australian scientists "Natural Medicine" adds another strong proof: the new crown virus is a natural evolutionary product, or has two origins...
3. NEJM: Effects of intermittent fasting on health, aging and disease
4. Cure insomnia in one year! Studies have found that to improve sleep, you may only need a heavy blanket
5. New Harvard study: Just 12 minutes of strenuous exercise can bring huge metabolic benefits to health
6. The first human intervention trial: 28 days of "crawling" in nature, enough to improve immunity
7. Junk food is "true. Garbage"! It takes away telomere length and makes people age faster!
8. Cell Puzzle: If you don't sleep, you will really die! But the lethal changes do not occur in the brain, but in the intestines...
9. Nature Communications' Mega Scale Study: Iron Levels in the Blood Are Key to Health and Aging!
10. Incredible! Scientists reversed the animal's "permanent" brain damage overnight, and also restored the old brain to a youthful state...