Amazon molly

I got 99 problems, but a male ain’t one

When I first heard of Amazon mollies, I thought their name derived from the region where they live. I had assumed they would be from the Amazon. But they are not, they live much further north in central America, the largest chunk of their distribution being Mexican territory. At the time I came across Amazon mollies, Jeff Bezos wasn’t quite as (in)famous as he is today, but I still figured that these fish had probably not been named for an online retail company. Unfortunately, that was the end of my immediate references to ‘Amazon’ – because I never received a proper classical education. Had I been given even a half-decent background in Greek and Latin and their respective mythologies, the actual namesakes of Amazon mollies would have likely been obvious to me: that tribe of all-female warriors from Greek mythology that had but one use for men… and this is exactly where the parallels between Amazon mollies and Amazon warriors lie. Both are female-only species/peoples and both need sperm of other species/peoples for reproduction. However, there is one major difference: Amazons actually used the sperm they… ahm… ‘acquired’ to make a new baby. Amazon mollies do not! All they need sperm for is to kick-start the ripening of their eggs. No male DNA makes it into a baby Amazon molly. Lucky that there is no alimony in mollies – paternity tests would be an iffy task. Amazon mollies literally have no father.

Lucy, I’m not your father, for you are an Amazon molly

Now that’s some cool and weird biology already. Usually we do not associate such modes of reproduction with vertebrates. A plant that engages in self-fertilisation? No big deal! A hermaphroditic worm that needs no partner? Sure thing. A bacteria multiplying without support? Absolutely natural! But a vertebrate baby without a biological father? That’s enough to start a religion!

"My child was born without a father! It will be our saviour!" - "Calm down, Mary, we all were!"

So how come Amazon mollies get to make babies in ways that 99% of other animals cannot? As we currently understand it, Amazon mollies have their origin in a hybridization event between two closely related molly species. In other words: there once were two mollies, a sailfin and an Atlantic one, who got a little confused over whom to mate with and they fell in love with each other despite the fact that they technically belonged to different species. And then their daughters gained the power to produce clones of themselves. The exact molecular and ecological mechanisms leading to this situation are not fully understood yet, largely because we have been unable to produce asexual mollies from crossing individuals of the parental species. Whenever researchers have tried, this resulted either in fertile, but sexual hybrids. Or in infertile offspring. Whatever that Atlantic molly female and her sailfin lover did right roughly 125.000 years ago to give birth to a whole new species that has existed without males ever since, we have not yet managed to unravel it completely. And that may be the key to the rarity of unisexual vertebrates: a lot seems to have to go right in order for such individuals to be viable and fertile.

Some among you may think: well, isn’t Amazon molly life just paradise? Either you fancy that a female-only society should be a true utopia. Or you think that males of the two ancestral species are well off, getting to sleep around without any risk of unwanted paternity. But things aren’t quite so easy. First of all, male sailfin and Atlantic mollies should do their best to not fall for Amazons. While there is no direct risk of harm to them (a stark contrast to the mythological Amazons), any sperm a male ‘donates’ to an Amazon female is evolutionarily speaking ‘wasted’ – it does nothing to propagate that male’s genes into the next generation. And these males can become ‘sperm depleted’, making them less successful in those matings with females of their own species that would count in natural selection. Not to mention the time and energy each interaction with an Amazon costs. Amazons on the other hand are not as free as you might suspect. Yes, they do not genetically depend on males, but physiologically they still do. Thus, they can never live far from access to males and their sperm, nor can they completely outcompete their sister species – if they did, they would go extinct within one generation. So there you have it: even in a system where technically no males exist, the sexes are still at conflict. The lesson learned for our own species and societies? Never look to biology to make moral, ethical, political or social decisions. Biology is just too weird. Or rather: humans are just too weird!