In the early 1930s, a husband and wife research team at the University of Michigan made a strange discovery while studying a small species of freshwater fish native to Mexico and South Texas that has researchers today wondering: What's the point of sex?
The Austin American-Statesman reports after collecting pregnant fish from the wild and raising them in their aquaria, Carl and Laura Hubbs were surprised to find that the fish gave birth to a large brood of about 20 fish each comprised entirely of females.
It seemed especially strange, given that almost all of the fish they caught in the wild were also female. Perplexed by their observations, the Hubbs set about trying to determine how a species without males could produce offspring. Were they reproducing asexually somehow? Virgin fish in their tanks never became pregnant through any type of immaculate conception, so that couldn't entirely be the case.
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The researchers already knew that the female species lived with other closely related fish in the wild. In Mexico, their range overlapped with Atlantic mollies from the Yucatan through the Gulf state of Tamaulipas, while in Texas -- near Brownsville -- they lived with sailfin mollies. Both of those species had males.
When Carl and Laura Hubbs recreated that dynamic by putting males of the Atlantic and sailfin mollies in with the strange females, they successfully mated. That solved one piece of the puzzle. The Hubbs had figured out who the fathers were, but they still couldn't explain why all of the offspring were female.
That's where the story gets weird -- because not only were all of the juvenile fish female, they also looked nothing like their "fathers."
It turns out, the researchers concluded, the all-female species needed sperm from another fish to kick-start the development of embryos. But sperm and egg never fused. This meant that none of the DNA from the fathers ever made it into the offspring, and without a Y chromosome passed down from a male, no male offspring could ever be produced. Instead, these fish give birth to female clones.
This new species of fish was fittingly given the name "Amazon molly," after the Amazons of Greek mythology, a tribe of all-female warriors who would welcome men from distant areas to mate, but would kill any male children they gave birth to.
Were it not for the persistent curiosity of scientists, that might have been the end of the story. But fast forward to the early 1990s, and Carl and Laura's son, Clark Hubbs, was working as a biology professor at the University of Texas, where he attracted the attention of a new faculty member, Michael Ryan.
"My question to Clark Hubbs," said Ryan, "was why would a male sailfin or Atlantic molly mate with a female Amazon molly. He said, `Because they're males,' meaning they're horny and stupid."
But Ryan wasn't entirely satisfied with this line of reasoning, which seemed to fly in the face of evolutionary theory. How could male Atlantic and sailfin mollies continue to exist if they never got to pass down any of their genes? If this were the whole story, they should have been weeded out by the process of natural selection a long time ago.
So, working with colleagues, Ryan set up some very simple but clever experiments that showed mollies aren't all that different from humans when it comes to relationships.
A female sailfin molly put in a tank with two sailfin males typically developed a preference for one male over the other. In the next step, the researchers allowed that same female to see the male she had rejected consorting with an Amazon molly. When allowed to interact with the same two males again, the female sailfin almost always changed her mind and became smitten with the previously rejected male.
This phenomenon is known as "mate choice copying," and it's pretty common in the animal kingdom. This type of behavior has been observed in insects, fish, birds, and, yes, humans. If you've ever hoped to make someone jealous by being seen with another partner, you've participated in a type of mate choice copying.
This makes sense from a biological standpoint for the male sailfin and Atlantic mollies.
"So it seems then," said Ryan, "that when a male sailfin is courting and mating with an Amazon molly, even though he doesn't get any direct evolutionary advantage, he becomes more attractive to his own females."
For the next several years, Ryan and his colleagues conducted research that found male mollies weren't as stupid as originally presumed.
By using several large tanks at UT's Brackenridge field lab, Caitlin Gabor, a postdoctoral researcher at the time, was able to grow all three species of mollies separately.
"There were seven populations that I worked with," Gabor said. "Instead of having to go back to Mexico to get them, you have outdoor tanks that allow them to breed, then have access to young fish."
Part of the research showed that male mollies could tell the difference between females of their own species and that of Amazon mollies and preferred to mate with the former. But given that Amazon mollies are widespread in streams and rivers on the western side of the gulf, they're evidently not having much trouble finding mates.
There's still at least one big unanswered question when it comes to Amazon mollies: How are they able to exist? Biologically speaking, any species that only reproduce asexually should become extinct after a certain amount of time.
Our bodies are constantly acquiring mutations in our DNA, some of which are passed down to our offspring. Because we reproduce sexually, our genes mix with those of our partner, creating variation. This means that if you have a bad mutation, it might not necessarily get passed down to your children -- and even if it does, it might not get expressed.
But in species that reproduce asexually, those mutations have a 100% chance of being passed on. After several generations, an asexual species should be so full of harmful mutations that it is ultimately unable to survive.
But that doesn't appear to be the case with Amazon mollies. Is this species really as successful as it seems? That's the question Ph.D. student Allison Davis is trying to answer.
"My question is why did sexual reproduction even evolve?" Davis said. "If asexual reproduction can be successful in these sorts of habitats, then why would you need sexual reproduction, and to look at that, I'm really looking at variation."
By putting all three molly species through a series of tests involving mirrors and mazes, Davis can compare the results to see if one species varied in their response more than the other. Amazon mollies should all solve an underwater maze in relatively the same way, for example, since they're all clones, whereas Atlantic and sailfin mollies should display more individuality.
Researchers say studies being conducted on this native Texas fish promise to help scientists understand the importance of sex and diversity in the biological world.