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Sci.Fun | 科普 | 鸭嘴兽基因组图谱：地球上最古怪的哺乳动物怎么变得如此奇异

时间: 2021-06-25 15:27:30 | 作者：SciFun | 来源: 喜蛋文章网 | 编辑: admin | 阅读: 99次

Sci.Fun | 科普 | 鸭嘴兽基因组图谱：地球上最古怪的哺乳动物怎么变得如此奇异

撰文 | Fice责编 | Lee

Often considered the world's oddest mammal, Australia's beaver-like, duck-billed platypus exhibits an array of bizarre characteristics: it lays eggs instead of giving birth to live babies, sweats milk, has venomous spurs and is even equipped with 10 sex chromosomes. Now, an international team of researchers led by University of Copenhagen has conducted a unique mapping of the platypus genome and found answers regarding the origins of a few of its stranger features.

澳大利亚既像海狸又像鸭子的鸭嘴兽常被视为世界上最古怪的哺乳动物，它具有一系列奇异的特征：它们是卵生而非胎生、分泌乳汁、有寒毒的毒刺并且有十条性染色体。最近，一个由哥本哈根大学领导的网上研究团队已分析、绘制出鸭嘴兽的基因，并发现了许多他们奇特特征起源的答案。

It lays eggs, but nurses, it is toothless, has a venomous spur, has webbed feet, fur that glows and has 10 sex chromosomes. Ever since Europeans discovered the platypus in Australia during the late 1700's, the quirky, duck-billed, semiaquatic creature has baffled scientific researchers.

它生蛋而不是胎生，它无牙，有毒刺，有脚蹼，有光滑的皮毛并有十条性染色体。自从欧洲人18世纪在澳大利亚发现了鸭嘴兽，这一古怪、鸭嘴型、半水生的生物就阻碍了科学研究的进步。

Modern day researchers are still trying to understand how the platypus -- often considered to be the world's oddest mammal -- got to be so unique. Their understandings have now advanced, to a great degree. For the first time, an international team of researchers, led by University of Copenhagen biologists, has mapped a complete platypus genome. The study is published in the scientific journal, Nature.

现代研究仍在尝试探究理解鸭嘴兽——常被视为地球上最古怪的哺乳动物——是怎样变得如此奇特的。他们的理解如今有了新进展。由哥本哈根大学领导的网上研究团队首次完整描绘出鸭嘴兽的基因。这项研究被发表在科学杂志《Nature》上。

"The complete genome has provided us with the answers to how a few of the platypus' bizarre features emerged. At the same time, decoding the genome for platypus is important for improving our understanding of how other mammals evolved -- including us humans. It holds the key as to why we and other eutheria mammals evolved to become animals that give birth to live young instead of egg-laying animals," explains Professor Guojie Zhang of the Department of Biology.

“复杂的基因向我们提供了鸭嘴兽一些特征是出现的原因。同时，破译鸭嘴兽的基因对于我们进一步理解其他哺乳动物是怎样进化十分重要——包括我们人类。它包含着为什么我们与其他真兽亚纲哺乳动物生育婴儿而不是卵生动物的关键，”生物部门教授Guojie Zhang解释到。

The platypus belongs to an ancient group of mammals -- monotremes -- which existed millions of years prior to the emergence of any modern-day mammal.

鸭嘴兽属于一种远古哺乳动物——单孔目动物——在任何现代哺乳动物出现前几百万年。

"Indeed, the platypus belongs to the Mammalia class. But genetically, it is a mixture of mammals, birds and reptiles. It has preserved many of its ancestors' original features -- which probably contribute to its success in adapting to the environment they live in," says Professor Zhang.

“实际上，鸭嘴兽属于哺乳分类。但从遗传学角度来说，它是哺乳动物、鸟类和爬行类的混合。它包含许多它祖先具有的特征——可能对他们适应生活环境有贡献，”张教授说到。

Lays eggs, sweats milk and has no teeth

One of the platypus' most unusual characteristics is that, while it lays eggs, it also has mammary glands used to feed its babies, not through nipples, but by milk -- which is sweat from its body.

鸭嘴兽最不寻常的特征之一是它生蛋，同时具备乳腺以哺育幼崽，不通过乳头而是挤出来——直接从身体分泌。

During our own evolution, we humans lost all three so-called vitellogenin genes, each of which is important for the production of egg yolks. Chickens on the other hand, continue to have all three. The study demonstrates that platypuses still carry one of these three vitellogenin genes, despite having lost the other two roughly 130 million years ago. The platypus continues to lay eggs by virtue of this one remaining gene. This is probably because it is not as dependent on creating yolk proteins as birds and reptiles are, as platypuses produce milk for their young.

在我们自己的进化中，我们人类失去了全部三种所谓卵黄蛋白原基因，每一种对于生产蛋黄都十分重要。另一方面，鸡一直具有这三种基因。研究表明鸭嘴兽仍带有这三种卵黄蛋白原基因中的一种，即使在大约一亿三千万年前失去了另外两种基因。鸭嘴兽由于这一遗留的基因坚持生蛋。可能是由于不像鸟类和爬行类那样以来产生蛋黄营养，鸭嘴兽才会产奶喂养幼崽。

In all other mammals, vitellogenin genes have been replaced with casein genes, which are responsible for our ability to produce casein protein, a major component in mammalian milk. The new research demonstrates that the platypus carries casein genes as well, and that the composition of their milk is thereby quite similar to that of cows, humans and other mammals.

在其他所有哺乳动物中，卵黄蛋白原基因都被负责合成酪蛋白能力酪蛋白基因取代，酪蛋白是哺乳动物奶中的主要成分。最新研究表明，鸭嘴兽同样含有酪蛋白基因，而它们的奶也因此与牛、人类和其他哺乳动物的奶有十分相似的成分

"It informs us that milk production in all extant mammal species has been developed through the same set of genes derived from a common ancestor which lived more than 170 million years ago -- alongside the early dinosaurs in the Jurassic period," says Guojie Zhang.

“这提醒我们，所有现存哺乳动物物种的乳汁生产由一亿七千万年前相同的祖先的同一基因发展而来——差不多在恐龙时代早期侏罗纪时代，” Guojie Zhang说。

Another trait that makes the platypus so unique is that, unlike the vast majority of mammals, it is toothless. Although this monotremes' nearest ancestors were toothed, the modern platypus is equipped with two horn plates that are used to mash food. The study reveals that the platypus lost its teeth roughly 120 million years ago, when four of the eight genes responsible for tooth development disappeared.

另一项使鸭嘴兽独特的特征是，不像绝大多数哺乳动物，鸭嘴兽无牙。虽然单孔目动物最近的祖先有牙，但是现代鸭嘴兽有两片用于磨碎食物的扁平嘴。研究证明鸭嘴兽在大约一亿两千万年前失去了牙齿，当时八种决定牙齿发育的基因中的四种基因消失了。

Only animal with 10 sex chromosomes

Yet another platypus oddity investigated by the researchers was how their sex is determined. Both humans and every other mammal on Earth have two sex chromosomes that determine sex -- the X and Y chromosome system in which XX is female and XY is male. The monotremes, however, including our duck-billed friends from Down Under, have 10 sex chromosomes, with five Y and five X chromosomes.

另一项研究者们进行的有关鸭嘴兽的奇怪研究是，它们的性如何运作。人类和任何其他地球上的哺乳动物都有两条性染色体——X和Y性染色体系统，XX的雌性而XY是雄性。然而，包括我们的澳洲鸭嘴型朋友在内的单孔目动物有十条性染色体，其中五条Y染色体和五条X染色体。

Thanks to the near-complete chromosomal level genomes, researchers can now suggest that these 10 sex chromosomes in the ancestors of the monotremes were organized in a ring form which was later broken away into many small pieces of X and Y chromosomes. At the same time, the genome mapping reveals that the majority of monotreme sex chromosomes have more in common with chickens than with humans. But what it shows, is an evolutionary link between mammals and birds.

多亏接近完全的染色体等级基因，研究者们可以推测出这10条性染色体在单孔目动物祖先中是以环状存在，并在后来破碎成了许多小的X和Y染色体。同时，基因组作图证明单孔目动物的性染色体相比人类，与鸡更加类似。但这表明了哺乳动物与鸟类间的进化关联。

Platypus Facts

The platypus is endemic to eastern Australia and Tasmania. It is a protected species and classified by the IUCN as near-threatened.Among the reasons why platypuses are considered mammals: they have mammary glands, grow hair and have three bones in their middle ears. Each trait helps to define a mammal.The platypus belongs to the mammalian order monotreme, so named because monotremes use a singular opening for urination, defecation and sexual reproduction.The animal is an excellent swimmer and spends much of its time hunting for insects and shellfish in rivers.Its distinctive beak is filled with electrical sensors which are used to locate prey in muddy river beds.The male platypus has a venomous spur behind each of its hind legs. The venom is poisonous enough to kill a dog and is deployed when males fight for territory.Another 2021 study demonstrated that platypus fur is fluorescent. The animal's brown fur reflects a blue-green color when placed under UV light. (source: https://doi.org/10.1515/mammalia-2021-0027)

About the Study

Advanced gene sequencing technology that combines numerous cutting-edge methods has allowed the research team to map a near-complete genome at the chromosomal level from both the platypus and its cousin, the echidna -- the only two currently living types of monotreme animals. The gene data fills in 90 percent of the gaps in previous genetic mappings. Over 96% of the genome sequences are placed in the chromosomes now.The researchers have compared the monotreme genes and genomes from chickens, humans, rats, Tasmanian devils and lizards.In addition to Yang Zhou (lead author) and Guojie Zhang of the University of Copenhagen, the research was carried out by, among others: Linda Shearwin-Whyatt of The University of Adelaide (Australia) and Jing Li of Zhejiang University (China). A complete list of the authors can be found in the research article.
文章标题: Sci.Fun | 科普 | 鸭嘴兽基因组图谱：地球上最古怪的哺乳动物怎么变得如此奇异
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