How does this all-female species reproduce? - Susana Freitas and Darren Parker

翻译人员: Weihao Sheng 校对人员: Jacky He

In 2021, workers at a Sardinian aquarium were stunned by the birth

2021 年，撒丁岛一家水族馆的 工作人员惊奇地发现

of a smoothhound shark, who they called Ispera.

一条星鲨出生了， 并给它起名为 Ispera。

What shocked them was that, for the last decade,

令人震惊的是，过去十年里，

Ispera’s mother had been living only with other females.

Ispera 的母亲一直只和 其它雌性鲨鱼生活在一起。

But it’s actually entirely possible that Ispera had no father—

但 Ispera 没有父亲是完全有可能的，

and the reason why that is also explains other biological curiosities,

而其背后的原因也可以 解释其它奇特的生物，

like the existence of an all-female lizard species.

比如全雌性的蜥蜴物种的存在。

Usually sexual species have sex cells that contain

有性繁殖的物种通常有性细胞，

half the number of chromosomes required to create a viable embryo.

包含形成胚胎所需的一半染色体。

So an egg cell must be fertilized by a sperm cell

因此，卵细胞必须由精子受精

to form two full sets of chromosomes.

才能形成两套完整的染色体。

But some species that have sex cells can undergo

不过，一些具有性细胞的物种

a type of asexual reproduction called parthenogenesis—

可以进行一种无性繁殖， 即“孤雌生殖” (parthenogenesis)，

meaning “virgin origin” in Greek.

在希腊语中是“处女起源”的意思。

In parthenogenesis, an embryo develops from an unfertilized egg cell

在孤雌生殖中， 胚胎由未受精的卵细胞

that doubles its own chromosome count.

在染色体数目翻倍后发育而来。

In fact, some animals only ever undergo parthenogenesis,

实际上，有些动物只进行孤雌生殖，

while others can reproduce both sexually and parthenogenetically.

而另些则既可以进行有性繁殖， 也能进行孤雌生殖。

It's actually more common than previously thought.

这种现象比先前认为的更常见。

More than 80 different sexual vertebrate species—

超过80种不同的有性脊椎动物，

including Komodo dragons and certain kinds of turkeys, pythons, and sharks—

包括科摩多巨蜥和 一些火鸡、蟒蛇、鲨鱼等，

have surprised us by occasionally reproducing this way.

偶尔会以这种令人惊讶的方式繁殖。

These discoveries were usually made when females

这些现象往往是通过被捕的雌性动物

unexpectedly gave birth in captivity.

意外分娩发现的。

Ispera’s birth, for one, may have been the first account

Ispera 的诞生则是记录中的

of parthenogenesis in smoothhound sharks.

第一例星鲨孤雌生殖。

Scientists also confirmed that parthenogenesis was taking place

科学家们也发现，孤雌生殖还会

in some wild snake populations.

在某些野生蛇类种群中发生。

But just how many fatherless creatures are running, slithering, and swimming

但是，没有人知道 世界上到底有多少

around out there is unknown:

无父亲的生物在四处游走：

it’s a tough thing to track without population-wide genetic analyses.

由于缺乏种群范围内的遗传分析， 追踪变得非常困难。

So, why is it happening at all?

那么， 这到底为什么发生呢？

Scientists think parthenogenesis could be evolutionarily beneficial

科学家们认为，孤雌生殖有时候 在进化上更有利，

in some contexts because, well, sex can be a drag.

因为交配可以成为一种负担。

Mating and its associated demands and rituals can be time- and energy-intensive,

交配和其相关的需求规范 需要大量的时间和能量，

leave individuals vulnerable to predators, and even be fatal.

使个体易受到捕食者的伤害， 甚至会致命。

Parthenogenesis, meanwhile, requires only one parent.

而孤雌生殖只需要一位父母。

Mayflies can sometimes default to parthenogenesis

蜉蝣在没有雄性存在的条件下

if there are no males available,

可以转为孤雌生殖，

which is especially handy because they’ve only got a day or so

这非常方便，因为它们在死亡前

to reproduce before dying.

只有一天左右的时间进行繁殖。

It can also help rapidly expand a population.

这也能帮助迅速扩大种群。

In the summer, when food is abundant,

在夏季食物充足的时候，

pea aphids can rely on parthenogenesis,

豌豆蚜可以孤雌生殖，

allowing their population to explode under favorable conditions.

让种群数量在有利环境中迅速膨胀。

And in the autumn, they switch back to sex.

到了秋季，它们就会回归有性生殖。

But some aphids, katydids, lizards, geckos, and snakes

而有些蚜虫、蝈蝈、 蜥蜴、壁虎、蛇等

only ever reproduce via parthenogenesis.

只进行孤雌生殖。

So, why do other animals bother with sex?

那么，为什么还需要有性生殖呢？

Scientists hypothesize that sex makes up for its shortcomings with long-term gains.

科学家们提出假说， 认为有性生殖的长期收益能弥补不足。

It allows individuals to mix their genes, leading to greater genetic diversity.

有性繁殖能混合不同个体的基因， 从而提高遗传多样性。

That way, when the going gets tough,

这样，当环境变得不利于生存时，

beneficial mutations can be selected and harmful ones can be removed

自然选择可以保留有益突变、剔除有害突变，

without ending the entire population.

而不至于让整个种群灭绝。

In a parthenogenetic population, on the other hand,

另一方面，在孤雌生殖的群体中，

individuals can only reproduce using their own genetic material.

个体只能利用自身的遗传物质进行繁殖。

According to a theory called Muller’s ratchet,

根据穆勒棘轮效应，

that’s not good.

这不是好事。

The theory predicts that parthenogenetic lineages will accumulate harmful mutations

此理论推测，孤雌生殖的血族 会逐渐积累有害突变，

over time and eventually, after thousands of generations,

最终在几千代以后，

will reach a point of so-called mutational meltdown.

达到所谓的“变异熔断”。

At this stage, individuals will be so compromised that they can't reproduce,

到这个节点，个体已经非常受损， 无法再继续繁殖，

so the population will nosedive, leading to extinction.

种群数量将猛跌造成灭绝。

We haven’t yet seen this entire process unfold in nature.

我们还没在自然界中 观察到整个过程发生，

But scientists have observed an accumulation of harmful mutations

但科学家们已经发现， 一种孤雌生殖的粘虫

in parthenogenetic stick insects that are absent in their sexual relatives.

在缺乏性伴侣时 积累起有害的变异。

Only time will tell whether this will cause their extinction.

只有时间才能证明， 这是否会导致它们的灭绝。

Otherwise, some parthenogenetic species appear to have ways of circumventing

不过，一些孤雌生殖的物种 似乎有策略规避

a mutational meltdown.

变异熔断。

New Mexico whiptail lizards came about

新墨西哥鞭尾蜥是通过

when two different lizard species hybridized,

两种不同蜥蜴的杂交形成，

creating this new all-female species.

是一种新的全雌性物种。

As hybrids, their genome is a combination of the different sets of chromosomes

作为杂交种，它们的基因组分别是由两个

from their two parent species.

亲代物种的染色体组合而成。

This gives them a high level of genetic diversity,

这使它们具有高度的遗传多样性，

which may allow them to survive long into the future.

也许有助于未来的长期生存。

Bdelloid rotifers, meanwhile, have been reproducing parthenogenetically

与此同时，蛭形轮虫已经孤雌生殖了

for 60 million years.

近 6000 万年。

They might have managed this by taking in foreign genetic material.

它们可能是通过吸收外来的遗传物质 以避免变异熔断。

Indeed, about 10% of their genes comes from other organisms,

实际上，它们大概 10% 的 基因来自其它生物，

like fungi, bacteria, and algae.

像真菌、细菌、藻类等。

How exactly they do this is unclear,

这其中的过程无人了解，

but whatever the trick is, it seems to be working.

但不论如何，这种方法似乎有作用。

To totally untangle the mysteries of reproduction,

为了完全揭开繁殖的奥秘，

we’ll need more research—

我们需要更多研究——

and probably a few more surprises like Ispera.

以及像 Ispera 这样的意外。