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2021年，意大利撒丁岛一家水族馆的工作人员惊奇地发现一条星鲨出生了，并给它起名为Ispera。
令人震惊的是，在过去十年的里，Ispera的母亲一直只和其它雌性鲨鱼生活在一起。那么，它的出生是怎么回事呢？它爹又是谁呢？其它物种可以这样繁殖吗？
大家知道，绝大多数胚胎形成的条件是精子与卵细胞结合。而在这些孤雌繁殖的案例中，雌性个体产生的卵细胞可以不经受精作用直接发育成新的个体，同时因为孩子的所有DNA均来自母亲，所以这个孩子也就相当于是妈妈的克隆体。
孤雌生殖的鲨鱼虽然很神奇，但这也侧面说明了鲨鱼种群已经非常危险，许多雌性鲨鱼为了繁殖已经开始进化出了各种方法。
这种全雌性的物种是如何繁殖的？
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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，撒丁岛一家水族馆的工作人员惊奇地发现一条星鲨出生了，并给它起名为 Ispera。
令人震惊的是，过去十年里，Ispera 的母亲一直只和其它雌性鲨鱼生活在一起。
但 Ispera 没有父亲是完全有可能的，而其背后的原因也可以解释其它奇特的生物，比如全雌性的蜥蜴物种的存在。
有性繁殖的物种通常有性细胞，包含形成胚胎所需的一半染色体。
因此，卵细胞必须由精子受精才能形成两套完整的染色体。
不过，一些具有性细胞的物种可以进行一种无性繁殖，即“孤雌生殖” (parthenogenesis)，在希腊语中是“处女起源”的意思。
在孤雌生殖中，胚胎由未受精的卵细胞在染色体数目翻倍后发育而来。
实际上，有些动物只进行孤雌生殖，而另些则既可以进行有性繁殖，也能进行孤雌生殖。
这种现象比先前认为的更常见。
超过80种不同的有性脊椎动物，包括科摩多巨蜥和一些火鸡、蟒蛇、鲨鱼等，偶尔会以这种令人惊讶的方式繁殖。
这些现象往往是通过被捕的雌性动物意外分娩发现的。
Ispera 的诞生则是记录中的第一例星鲨孤雌生殖。
科学家们也发现，孤雌生殖还会在某些野生蛇类种群中发生。
但是，没有人知道世界上到底有多少无父亲的生物在四处游走：由于缺乏种群范围内的遗传分析，追踪变得非常困难。
那么， 这到底为什么发生呢？科学家们认为，孤雌生殖有时候在进化上更有利，因为交配可以成为一种负担。
交配和其相关的需求规范需要大量的时间和能量，使个体易受到捕食者的伤害，甚至会致命。
而孤雌生殖只需要一位父母。
蜉蝣在没有雄性存在的条件下可以转为孤雌生殖，这非常方便，因为它们在死亡前只有一天左右的时间进行繁殖。
这也能帮助迅速扩大种群。
在夏季食物充足的时候，豌豆蚜可以孤雌生殖，让种群数量在有利环境中迅速膨胀。
到了秋季，它们就会回归有性生殖。
而有些蚜虫、蝈蝈、蜥蜴、壁虎、蛇等只进行孤雌生殖。
那么，为什么还需要有性生殖呢？科学家们提出假说，认为有性生殖的长期收益能弥补不足。
有性繁殖能混合不同个体的基因，从而提高遗传多样性。
这样，当环境变得不利于生存时，自然选择可以保留有益突变、剔除有害突变，而不至于让整个种群灭绝。
另一方面，在孤雌生殖的群体中，个体只能利用自身的遗传物质进行繁殖。
根据穆勒棘轮效应，这不是好事。
此理论推测，孤雌生殖的血族会逐渐积累有害突变，最终在几千代以后，达到所谓的“变异熔断”。
到这个节点，个体已经非常受损，无法再继续繁殖，种群数量将猛跌造成灭绝。
我们还没在自然界中观察到整个过程发生，但科学家们已经发现，一种孤雌生殖的粘虫在缺乏性伴侣时积累起有害的变异。
只有时间才能证明，这是否会导致它们的灭绝。
不过，一些孤雌生殖的物种似乎有策略规避变异熔断。
新墨西哥鞭尾蜥是通过两种不同蜥蜴的杂交形成，是一种新的全雌性物种。
作为杂交种，它们的基因组分别是由两个亲代物种的染色体组合而成。
这使它们具有高度的遗传多样性，也许有助于未来的长期生存。
与此同时，蛭形轮虫已经孤雌生殖了近 6000 万年。
它们可能是通过吸收外来的遗传物质以避免变异熔断。
实际上，它们大概 10% 的基因来自其它生物，像真菌、细菌、藻类等。
这其中的过程无人了解，但不论如何，这种方法似乎有作用。
为了完全揭开繁殖的奥秘，我们需要更多研究——以及像 Ispera 这样的意外。
孤雌生殖
Parthenogenesis
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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. 
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