高質量英語演講&專訪影片哪裡看？
請您點選上方“精彩英語演講”，並“設為星標”
全網最新的英語演講&專訪第一時間為您奉上
2021年，義大利撒丁島一家水族館的工作人員驚奇地發現一條星鯊出生了，並給它起名為Ispera。
令人震驚的是，在過去十年的裡，Ispera的母親一直只和其它雌性鯊魚生活在一起。那麼，它的出生是怎麼回事呢？它爹又是誰呢？其它物種可以這樣繁殖嗎？
大家知道，絕大多數胚胎形成的條件是精子與卵細胞結合。而在這些孤雌繁殖的案例中，雌性個體產生的卵細胞可以不經受精作用直接發育成新的個體，同時因為孩子的所有DNA均來自母親，所以這個孩子也就相當於是媽媽的克隆體。
孤雌生殖的鯊魚雖然很神奇，但這也側面說明了鯊魚種群已經非常危險，許多雌性鯊魚為了繁殖已經開始進化出了各種方法。
↓↓↓ 上下滑動，檢視演講稿 ↓↓↓
In 2021, workers at a Sardinian aquarium were stunned by the birth of a smoothhound shark, who they called Ispera.
What shocked them was that, for the last decade, Ispera’s mother had been living only with other females.
But it’s actually entirely possible that Ispera had no father— and the reason why that is also explains other biological curiosities, like the existence of an all-female lizard species.
Usually sexual species have sex cells that contain half the number of chromosomes required to create a viable embryo.
So an egg cell must be fertilized by a sperm cell to form two full sets of chromosomes.
But some species that have sex cells can undergo a type of asexual reproduction called parthenogenesis— meaning “virgin origin” in Greek.
In parthenogenesis, an embryo develops from an unfertilized egg cell that doubles its own chromosome count.
In fact, some animals only ever undergo parthenogenesis, while others can reproduce both sexually and parthenogenetically.
It's actually more common than previously thought.
More than 80 different sexual vertebrate species— including Komodo dragons and certain kinds of turkeys, pythons, and sharks— have surprised us by occasionally reproducing this way.
These discoveries were usually made when females unexpectedly gave birth in captivity.
Ispera’s birth, for one, may have been the first account of parthenogenesis in smoothhound sharks.
Scientists also confirmed that parthenogenesis was taking place in some wild snake populations.
But just how many fatherless creatures are running, slithering, and swimming around out there is unknown: it’s a tough thing to track without population-wide genetic analyses.
So, why is it happening at all? Scientists think parthenogenesis could be evolutionarily beneficial in some contexts because, well, sex can be a drag.
Mating and its associated demands and rituals can be time- and energy-intensive, leave individuals vulnerable to predators, and even be fatal.
Parthenogenesis, meanwhile, requires only one parent.
Mayflies can sometimes default to parthenogenesis if there are no males available, which is especially handy because they’ve only got a day or so to reproduce before dying.
It can also help rapidly expand a population.
In the summer, when food is abundant, pea aphids can rely on parthenogenesis, allowing their population to explode under favorable conditions.
And in the autumn, they switch back to sex.
But some aphids, katydids, lizards, geckos, and snakes only ever reproduce via parthenogenesis.
So, why do other animals bother with sex? Scientists hypothesize that sex makes up for its shortcomings with long-term gains.
It allows individuals to mix their genes, leading to greater genetic diversity.
That way, when the going gets tough, beneficial mutations can be selected and harmful ones can be removed without ending the entire population.
In a parthenogenetic population, on the other hand, individuals can only reproduce using their own genetic material.
According to a theory called Muller’s ratchet, that’s not good.
The theory predicts that parthenogenetic lineages will accumulate harmful mutations over time and eventually, after thousands of generations, will reach a point of so-called mutational meltdown.
At this stage, individuals will be so compromised that they can't reproduce, so the population will nosedive, leading to extinction.
We haven’t yet seen this entire process unfold in nature.
But scientists have observed an accumulation of harmful mutations in parthenogenetic stick insects that are absent in their sexual relatives.
Only time will tell whether this will cause their extinction.
Otherwise, some parthenogenetic species appear to have ways of circumventing a mutational meltdown.
New Mexico whiptail lizards came about when two different lizard species hybridized, creating this new all-female species.
As hybrids, their genome is a combination of the different sets of chromosomes from their two parent species.
This gives them a high level of genetic diversity, which may allow them to survive long into the future.
Bdelloid rotifers, meanwhile, have been reproducing parthenogenetically for 60 million years.
They might have managed this by taking in foreign genetic material.
Indeed, about 10% of their genes comes from other organisms, like fungi, bacteria, and algae.
How exactly they do this is unclear, but whatever the trick is, it seems to be working.
To totally untangle the mysteries of reproduction, we’ll need more research— and probably a few more surprises like Ispera.
2021 年，撒丁島一家水族館的工作人員驚奇地發現一條星鯊出生了，並給它起名為 Ispera。
令人震驚的是，過去十年裡，Ispera 的母親一直只和其它雌性鯊魚生活在一起。
但 Ispera 沒有父親是完全有可能的，而其背後的原因也可以解釋其它奇特的生物，比如全雌性的蜥蜴物種的存在。
有性繁殖的物種通常有性細胞，包含形成胚胎所需的一半染色體。
因此，卵細胞必須由精子受精才能形成兩套完整的染色體。
不過，一些具有性細胞的物種可以進行一種無性繁殖，即“孤雌生殖” (parthenogenesis)，在希臘語中是“處女起源”的意思。
在孤雌生殖中，胚胎由未受精的卵細胞在染色體數目翻倍後發育而來。
實際上，有些動物只進行孤雌生殖，而另些則既可以進行有性繁殖，也能進行孤雌生殖。
這種現象比先前認為的更常見。
超過80種不同的有性脊椎動物，包括科摩多巨蜥和一些火雞、蟒蛇、鯊魚等，偶爾會以這種令人驚訝的方式繁殖。
這些現象往往是透過被捕的雌性動物意外分娩發現的。
Ispera 的誕生則是記錄中的第一例星鯊孤雌生殖。
科學家們也發現，孤雌生殖還會在某些野生蛇類種群中發生。
但是，沒有人知道世界上到底有多少無父親的生物在四處遊走：由於缺乏種群範圍內的遺傳分析，追蹤變得非常困難。
那麼， 這到底為什麼發生呢？科學家們認為，孤雌生殖有時候在進化上更有利，因為交配可以成為一種負擔。
交配和其相關的需求規範需要大量的時間和能量，使個體易受到捕食者的傷害，甚至會致命。
而孤雌生殖只需要一位父母。
蜉蝣在沒有雄性存在的條件下可以轉為孤雌生殖，這非常方便，因為它們在死亡前只有一天左右的時間進行繁殖。
這也能幫助迅速擴大種群。
在夏季食物充足的時候，豌豆蚜可以孤雌生殖，讓種群數量在有利環境中迅速膨脹。
到了秋季，它們就會迴歸有性生殖。
而有些蚜蟲、蟈蟈、蜥蜴、壁虎、蛇等只進行孤雌生殖。
那麼，為什麼還需要有性生殖呢？科學家們提出假說，認為有性生殖的長期收益能彌補不足。
有性繁殖能混合不同個體的基因，從而提高遺傳多樣性。
這樣，當環境變得不利於生存時，自然選擇可以保留有益突變、剔除有害突變，而不至於讓整個種群滅絕。
另一方面，在孤雌生殖的群體中，個體只能利用自身的遺傳物質進行繁殖。
根據穆勒棘輪效應，這不是好事。
此理論推測，孤雌生殖的血族會逐漸積累有害突變，最終在幾千代以後，達到所謂的“變異熔斷”。
到這個節點，個體已經非常受損，無法再繼續繁殖，種群數量將猛跌造成滅絕。
我們還沒在自然界中觀察到整個過程發生，但科學家們已經發現，一種孤雌生殖的粘蟲在缺乏性伴侶時積累起有害的變異。
只有時間才能證明，這是否會導致它們的滅絕。
不過，一些孤雌生殖的物種似乎有策略規避變異熔斷。
新墨西哥鞭尾蜥是透過兩種不同蜥蜴的雜交形成，是一種新的全雌性物種。
作為雜交種，它們的基因組分別是由兩個親代物種的染色體組合而成。
這使它們具有高度的遺傳多樣性，也許有助於未來的長期生存。
與此同時，蛭形輪蟲已經孤雌生殖了近 6000 萬年。
它們可能是透過吸收外來的遺傳物質以避免變異熔斷。
實際上，它們大概 10% 的基因來自其它生物，像真菌、細菌、藻類等。
這其中的過程無人瞭解，但不論如何，這種方法似乎有作用。
為了完全揭開繁殖的奧秘，我們需要更多研究——以及像 Ispera 這樣的意外。
↓↓↓ 上下滑動，檢視演講稿 ↓↓↓
You have a mum and a dad and maybe a few siblings.
You are different – maybe very different – from everyone else in your family.
What would you and your siblings look like if you came from only a mother with no father? Would you look just like your mum? Just like your siblings? Would you all be a family of identical daughters? Or would you and your siblings all be different from your mum AND different from one another? Could you even be… a boy?? To answer questions like these, we have to know more about how eggs are formed by the process of oogenesis, which occurs in the ovaries of a female.
In the ovaries of a female, diploid oogonia divide by mitosis to produce more oogonia and specialized primary oocytes – still diploid – that are committed to producing eggs.
The egg may then be fertilized by a sperm to produce an offspring.
This is how offspring are usually produced.
Parthenogenesis is a form of reproduction in which an egg develops into a new individual without being fertilized by a sperm.
Let's examine normal oogenesis more closely before considering reproduction through parthenogenesis.
In normal oogenesis, the diploid primary oocyte divides by meiosis to produce haploid daughter cells.
Let's follow this process for a cell with 2n = 4 chromosomes.
Remember that each homologous pair of chromosomes consists of one paternal and one maternal chromosome.
These chromosomes carry the same genes in the same sequence, but may carry different alleles.
Each chromosome has been replicated in the S phase of interphase and now consists of two identical sister chromatids, joined at the centromere.
During Prophase I of Meiosis I, the chromosomes condense, become visible, and homologous chromosomes are paired up in synapsis.
Crossing over occurs as non-sister chromatids on homologous chromosomes exchange genetic information.
With crossing over, a chromosome is no longer fully maternal or fully paternal but rather a mixture of maternal and paternal alleles In Metaphase I, the paired chromosomes line up along the cell's equatorial plane.
Pairs of homologous chromosomes line up randomly, with either member of the pair oriented to one pole or the other.
The pairs also orient randomly relative to other pairs, in the process of independent assortment.
In Anaphase I, the chromosomes in each homologous pair are separated from each other and pulled to opposite poles.
In Telophase I, one haploid set of chromosomes is present at each pole.
The cell divides by cytokinesis to produce two cells: a large haploid daughter cell called a secondary oocyte and a tiny polar body, with a haploid nucleus and very little cytoplasm.
Meiosis II is very similar to mitosis, the type of cell division that occurs in somatic cells.
The Meiosis I products divide in Meiosis II to produce two daughter cells, one of which becomes the egg.
Remember, the Meiosis I products are haploid and so are its daughter cells.
Meiosis II starts with Prophase II, as the chromosomes condense.
In Metaphase II, the chromosomes move to the center of the cell and line up along the equatorial plane.
In Anaphase II, the chromatids making up a chromosome are separated from each other and pulled to opposite poles.
During Telophase II and cytokinesis, the cell divides to produce two haploid daughter cells: a large egg and a tiny polar body.
We will now discuss four ways that this process can be altered to produce parthenogenetic offspring.
Mitotic division of the primary oocyte; Combination of the primary polar body with the secondary oocyte; Combination of the egg with a secondary polar body; And the haploid ovum divides by mitosis instead of fusing with sperm.
Imagine that a female reproduces by parthogenesis without undergoing meiosis, as primary oocytes develop directly into offspring by mitosis, instead.
Will the parthenogenetic offspring be genetically identical to their mum or genetically different? Will the offspring be genetically identical to one another or genetically different? Will the offspring be male or female? Imagine that the two haploid cells produced by Meiosis I (the secondary oocyte and the polar body) fuse back together to yield a diploid cell and that this new cell develops into an offspring by parthenogenesis.
Will the parthenogenetic offspring be genetically identical to their mum or genetically different? Will the offspring be genetically identical to one another or genetically different? Will the offspring be male or female? Imagine that the egg and the polar body produced at the end of Meiosis II fuse to yield a diploid cell and that this new cell develops into an offspring by parthenogenesis.
Will the parthenogenetic offspring be genetically identical to their mum or genetically different? Will the offspring be genetically identical to one another or genetically different? Will the offspring be male or female? Usually, a haploid sperm fertilizes the haploid egg to produce a diploid zygote, which develops into an offspring.
What if, instead of fusing with a sperm, the haploid egg divides by mitosis? For this to occur, chromosomes have to be replicated.
During the S phase of interphase, the DNA of the single chromatid making up each chromosome in the developing egg is replicated.
The chromosomes now consist of two identical sister chromatids, joined at the centromere.
Remember that each chromosome is a product of crossing over and comprises a combination of maternal and paternal genes.
During mitosis, the replicated chromosomes line up along the equatorial plane at the center of the cell.
The sister chromatids are pulled apart from each other to opposite poles as individual chromosomes.
But instead of completing mitosis and cytokinesis, the new chromosomes stay in a single cell.
Each chromatid now becomes an individual chromosome as they separate from each other, so the cell is now diploid.
Imagine that this diploid cell develops into an offspring.
This is another form of parthenogenesis, because again, an unfertilized egg is developing into an offspring.
Will the parthenogenetic offspring be genetically identical to their mum or genetically different? Will the offspring be genetically identical to one another or genetically different? Will the offspring be male or female? So there are four different ways for parthenogenesis to take place to produce an offspring from a mum, with no dad.
Many animals develop this way, and some of them may surprise you.
防止再次失聯，請立即關注備用號
想第一時間觀看高質量英語演講&採訪影片？把“精彩英語演講”設定為星標就對了！操作辦法就是：進入公眾號——點選右上角的●●●——找到“設為星標”點選即可。
