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

譯者: Lilian Chiu 審譯者: Helen Chang

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

2021 年，

撒丁島一間水族館的員工相當驚訝，

因為一隻星貂鯊誕生了， 他們稱牠為伊斯佩拉。

of a smoothhound shark, who they called Ispera.

What shocked them was that, for the last decade,

他們之所以驚訝是因為在過去十年，

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

伊斯佩拉的母親 只和其他母鯊魚生活在一起。

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

但其實完全有可能

伊斯佩拉根本沒有父親——

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

而那背後的原因，

也能解釋其他生物學上的奇特現象。

like the existence of an all-female lizard species.

比如，有一種全雌的蜥蜴物種存在。

Usually sexual species have sex cells that contain

通常，有性繁殖物種的 性細胞所包含的

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

染色體數量，只有製造可發育胚胎

所需要的染色體數量的一半。

So an egg cell must be fertilized by a sperm cell

因此，卵細胞必須被精子細胞受精，

to form two full sets of chromosomes.

才能形成兩組完整的染色體。

But some species that have sex cells can undergo

但有些物種的性細胞可以進行一種

a type of asexual reproduction called parthenogenesis—

無性生殖，稱為孤雌生殖——

meaning “virgin origin” in Greek.

希臘文的原意是「處女起源」。

In parthenogenesis, an embryo develops from an unfertilized egg cell

孤雌生殖是指從未受精的 卵細胞發展出胚胎，

that doubles its own chromosome count.

該細胞會將其染色體數變為兩倍。

In fact, some animals only ever undergo parthenogenesis,

事實上，有些動物 只會進行孤雌生殖，

while others can reproduce both sexually and parthenogenetically.

也有些動物有性生殖和孤雌生殖皆可。

It's actually more common than previously thought.

孤雌生殖其實比我們以前 認為的還要更常見。

More than 80 different sexual vertebrate species—

有超過八十種不同的 有性生殖脊椎動物——

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

包括科摩多巨蜥，和某幾種火雞、

蟒蛇和鯊魚——

have surprised us by occasionally reproducing this way.

偶爾會用這種方式生殖， 讓我們相當驚訝。

These discoveries were usually made when females

會發現這個現象，通常 是因為被圈養雌性動物

unexpectedly gave birth in captivity.

在未預期的情況下生出寶寶。

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

舉例來說，伊斯佩拉的出生，

可能就是最早關於星貂鯊

of parthenogenesis in smoothhound sharks.

孤雌生殖的記載。

Scientists also confirmed that parthenogenesis was taking place

科學家也證實了

有些野生蛇類也會進行孤雌生殖。

in some wild snake populations.

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

但到底有多少

沒有父親的生物在世界上 亂跑、亂滑行、亂游，

around out there is unknown:

是個未知數。

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

若沒有做整個群體的 基因分析，就很難去追蹤。

So, why is it happening at all?

那麼，到底為什麼會發生？

Scientists think parthenogenesis could be evolutionarily beneficial

科學家認為，在某些情況下，

孤雌生殖在演化上 是有益處的，因為，

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

嗯，性事可能很拖拉。

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

交配和其相關的需求及儀式

可能會很耗費時間和精力，

leave individuals vulnerable to predators, and even be fatal.

很可能受到捕食性動物攻擊，

甚至有致命的危險。

Parthenogenesis, meanwhile, requires only one parent.

而孤雌生殖只需要單親。

Mayflies can sometimes default to parthenogenesis

蜉蝣有時會進行孤雌生殖，

if there are no males available,

如果找不到雄性的話。

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

這樣就非常方便，

因為牠們在死前只有 一天左右的時間繁殖。

to reproduce before dying.

It can also help rapidly expand a population.

孤雌生殖也能協助快速 增加一個群體的數量。

In the summer, when food is abundant,

夏天，食物豐足時，

pea aphids can rely on parthenogenesis,

蜿豆蚜可以仰賴孤雌生殖，

allowing their population to explode under favorable conditions.

讓牠們群體的數量 在有利的條件下爆增。

And in the autumn, they switch back to sex.

到了秋天，牠們又換回有性生殖。

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

但有些蚜蟲、紡織娘、 蜥蜴、壁虎，和蛇

only ever reproduce via parthenogenesis.

就只靠孤雌生殖來繁殖。

So, why do other animals bother with sex?

那麼，為什麼其他動物 還要這麼麻煩進行性事？

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

科學家假設，性事的長遠益處可以

補足它的短處。

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

它讓動物可以混合牠們的基因，

促成更高的基因多樣性。

That way, when the going gets tough,

這麼一來，艱困的時候，

beneficial mutations can be selected and harmful ones can be removed

有利的突變種可以被天擇留下，

有害的突變種則會消失，

without ending the entire population.

不會讓整個物種滅絕。

In a parthenogenetic population, on the other hand,

另一方面，如果是完全 靠孤雌生殖的動物，

individuals can only reproduce using their own genetic material.

每隻動物都只能用牠們 自己的遺傳物質來繁殖。

According to a theory called Muller’s ratchet,

根據「穆勒棘輪」這個理論，

that’s not good.

那並不是好事。

The theory predicts that parthenogenetic lineages will accumulate harmful mutations

該理論預測，

靠孤雌生殖的世系會隨時間 累積有害的突變種，

over time and eventually, after thousands of generations,

最終，在經過了數千個世代之後，

will reach a point of so-called mutational meltdown.

就會達到所謂的「突變崩潰」點。

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

在這個階段，

動物會受到嚴重的損害， 到了無法繁殖的程度，

so the population will nosedive, leading to extinction.

群體數量會爆跌，導致絕種。

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

我們尚未在大自然中看到 有這個完整的過程發生。

But scientists have observed an accumulation of harmful mutations

但科學家已經觀察到 有害突變種的累積，

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

發生在孤雌生殖的竹節蟲身上，

而牠們的有性生殖親屬物種並沒有。

Only time will tell whether this will cause their extinction.

只有時間才能證明 這是否會造成牠們的絕種。

Otherwise, some parthenogenetic species appear to have ways of circumventing

要不然，有些孤雌生殖物種顯然有辦法

避免突變崩潰。

a mutational meltdown.

New Mexico whiptail lizards came about

新墨西哥鞭尾蜥蜴的出現，

when two different lizard species hybridized,

是因為兩個不同的 蜥蜴物種進行雜交，

creating this new all-female species.

創造出了這種全雌的物種。

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

身為雜種，牠們的基因組 是不同組染色體的組合，

from their two parent species.

分別來自雙親的物種。

This gives them a high level of genetic diversity,

這讓牠們的基因多樣性非常高，

which may allow them to survive long into the future.

讓牠們能長期生存。

Bdelloid rotifers, meanwhile, have been reproducing parthenogenetically

至於雙尾輪蟲，

六千萬年來一直靠孤雌生殖來繁殖。

for 60 million years.

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

牠們之所以能辦到，可能 是有取得外來的遺傳物質。

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

的確，牠們的基因大約有 10%

來自其他生物，

like fungi, bacteria, and algae.

如菌類、細菌，和水藻。

How exactly they do this is unclear,

還不清楚牠們怎麼做到的，

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

但不論是什麼招術，似乎管用。

To totally untangle the mysteries of reproduction,

若要完全揭開繁殖的謎團，

we’ll need more research—

我們需要更多研究——

and probably a few more surprises like Ispera.

可能還需要再多幾個 像伊斯佩拉這樣的驚喜。