What is the Amazon molly and how did it get its name?

The Amazon molly (Poecilia formosa) is a small, inconspicuous freshwater fish. It belongs to a genus of around 40 fish species, which includes the well-known guppy. Amazon mollies are omnivores and spend their days feeding on small invertebrates, algae and plant matter in rivers and streams along the Gulf Coast of Mexico and Texas.

So why call it the ‘Amazon molly’? It’s not a geographical reference, but a mythological one. According to Greek mythology, The Amazons were an all-female tribe of warriors. This legend was mentioned in Homer’s The Iliad and the cultural reference continues today with the Amazonian princess comic book hero, Wonder Woman.

An Amazon molly, Poecilia formosa. By Dr. Manfred Schartl.
An Amazon molly, Poecilia formosa. By Dr. Manfred Schartl.

What’s the connection?

Do Amazon mollies form fierce tribes? Not exactly. The connection is that, just like the mythological warriors, they are female. All of them! They are able to reproduce themselves asexually. While this occurs frequently among invertebrates, very few vertebrates have evolved in this way.

As for the Amazon River, it was named by Charles I of Spain in the 16th century. The Spanish explorer Francisco de Orellana had just led the first known navigation of the entire length of the river. During this expedition, as they made their way across the continent, they were ambushed by native warriors, mostly women. On hearing this story, the king was reminded of the Greek legend and gave the river its name.

Guppy (Poecilia reticulata). The most famous close relative of the Amazon Molly fish. By Mirko Rosenau | Shutterstock.com
Guppy (Poecilia reticulata). The most famous close relative of the Amazon Molly fish. By Mirko Rosenau | Shutterstock.com

How do they reproduce?

In asexual reproduction, of which there are many types, all the offspring’s genetic material comes from a single parent. However, in the type of reproduction that Amazon mollies perform (gynogenesis), a male is still required. As there are no males of this species, the female must mate with males of a related species. Sperm is needed to stimulate reproduction in the female but, normally, none of the male’s DNA is passed on.

Asexual reproduction has some clear advantages, at least in the short term. Unlike sexual species, all members of an asexual population are capable of reproduction, so the reproductive output is greatly increased. As a result, Amazon mollies have a widespread, stable population.

DNA sequencing result sheet. By SINITAR | Shutterstock.com
DNA sequencing result sheet. By SINITAR | Shutterstock.com

Why is this a surprise to scientists?

The short term advantages of asexual reproduction are clear. But, this repeated cloning of the same genes, from generation to generation, is expected to take its toll.

If an Amazon molly mother is well-adapted to her environment, she will reach adulthood, reproduce and pass on those same successful genes. But what happens if environmental conditions in the ecosystem change? In sexually reproductive species, genetic variation means that some individuals are better adapted and these are ‘selected’. In Amazon mollies this kind of natural selection can’t take place.

Another issue is mutation. Changes in DNA can be harmful for any species. With sexual reproduction there is a chance for genetic mixing, which dilutes or nullifies the effects of these mutations. With asexual reproduction, however, this isn’t possible. It’s supposed that the accumulation of harmful mutations will, in time, lead to extinction. This is the basis of scientific principle known as Muller’s ratchet.

According to Muller’s ratchet, the Amazon molly should have died out after around 20,000 generations of asexual reproduction. However, a recent study used genome sequencing to reconstruct a family tree for the species and found it to be around 100,000 years old!

The story of this inconspicuous little fish represents another exciting advancement in evolutionary genetics.

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