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April 29, 2008

No sex means tricky survival for fish species

Posted: 05:17 PM ET

Not everyone in the animal kingdom needs to have sex to reproduce, but asexual species tend not to last as long as sexuals because, as the theory goes, asexuals are more susceptible to accumulating harmful mutations over many generations. That is why scientists are so fascinated by the Amazon molly fish, whose longevity has mysteriously defied evolutionary expectations.

This fish species consists of only females and, in fact, was the first unisexual vertebrate species ever discovered. The Amazon molly lives in a small range from the Nueces River in southeast Texas to the mouth of the Rio Tuxpan in Mexico. There are well over 100,000 alive today, and there are no signs that their fertility is less than that of their sexual sister species, said biologist Laurence Loewe at the University of Edinburgh.

While Amazon mollies do not reproduce sexually, their eggs can only begin developing when triggered by sperm from males of related species. Scientists think the Amazon molly probably evolved as an asexual species about 70,000 years ago.

Loewe and collaborator Dunja Lamatsch at the University of Wuerzburg, now at the Austrian Academy of Sciences, applied mathematical models to the Amazon molly’s genetic history, and found that the species should exist for less than 20,000 years before becoming extinct. The models examined a concept called Muller’s ratchet, which assumes asexual populations tend to accumulate harmful mutations over time that lead to extinction. They recently published their findings in BMC Evolutionary Biology.

Since the species is probably much older than 20,000 years, this creates what scientists call a genomic decay paradox. How, then, could the species have defied evolutionary models, and survived so long?

One mechanism at work to help the fish survive may be what is called "paternal leakage" of undamaged DNA. In other words, when sperm from males of other species trigger egg development in the Amazon molly, DNA may occasionally leak to the female and repair or restore genes gone awry through mutations. Further research is needed to determine if this or still other processes slow down the extinction predicted by Muller’s ratchet.

The results could have implications for the conservation of other so-called ancient asexuals, which include one species closely related to the Amazon molly.

Amazon mollies are also at a disadvantage because sailfin mollies, their parental species, prefer to mate with females of their own species rather than giving sperm to the Amazon mollies, research from Texas State University shows. Sailfin mollies also produce more sperm before mating with sailfin females than with Amazon mollies.

"Our results suggest that Amazon mollies have it doubly hard; they both have limited genetic variation (as per the paper above) and males of their parental species generally avoid mating with them and providing them with sperm," said Caitlin Gabor, associate professor of biology. "Yet, they clearly have persisted for a long time and possibly longer than any other vertebrate asexual species."

–Elizabeth Landau, Associate Producer,

Filed under: Animals • environment

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Charles Jarman   April 29th, 2008 9:29 pm ET

I believe the photos are of the "Sailfin Mollie," which does reproduce via male and female like Guppies.

M   April 29th, 2008 10:04 pm ET

So does the photo show a pair of another species of molly? An Amazon female and a male of another species? Only one of those fish is female.

dianehawk   April 29th, 2008 10:54 pm ET

The photo shows a male sailfin molly checking out an Amazon molly.

ravioli   April 30th, 2008 7:11 am ET

if you read the article your question will be answer

Brian   April 30th, 2008 10:11 am ET

I think the article is mis-leading.... If they require sperm from a close relative for egg development, then how can they be considered "asexual"? Are the offspring truly "clones" of the mother, or is their DNA different from their mothers?

If, as the article says, there is DNA LEAKAGE, than I would argue they are probably not ASEXUAL, because some of the"father fish"'s DNA is being exchanged to make up the new daughter fish....

jane goldberg   April 30th, 2008 3:49 pm ET

this is so interesting and well written- I always find it fascinating to read about other species and their sexual behaviors- I could learn about this topic forever

Eric B   May 1st, 2008 7:07 am ET

The article mentions the possiblitiy that greater understanding of this species of fish and its longevity could help to preserve other asexual species as well. I have to ask if we should be trying to prevent the extinction of these species. While I agree that we should definitely do everyything we can to not cause the exticntion of any species, the process of natural selection provides for the inception and extinction of species all the time. If these (asexual) species usually only last 20,000 years, then clearly they are still coming into existence. People are so concerned about us playing God when it comes to genetic manipulation and the creation of new species. Is that any worse than playing God to preserve a species that nature has chosen for extinction?

me   May 2nd, 2008 3:25 pm ET

No sex... how boring. I feel sorry for them.

clion   May 4th, 2008 12:23 pm ET

it would suck to be an amzon molly. What is the point ..what do they do for fun? do they at least have interesting diets

vertexia   May 5th, 2008 11:20 am ET

The shark do not have enough food in the ocean anymore because man kind has druge most of there food out even little fish like krill and sardine,etc so the shark are waiting for dumb mankind to jump in they never loved you any way. You tied to make them pets you are deluded
and now you make them enmies and wipe them off the plant. You don't control the planted, what you do to others will be done to you.!

lily hoch   May 6th, 2008 10:35 pm ET

what dose that mean for us

Irene Steffen   July 26th, 2009 5:50 pm ET

In another research article I read on the Internet, it said that it is a status this when regular or sailfin male mollies provide sperm to Amazon female mollies so they can spawn. When regular or sailfin female mollies find out that a male molly has done this, she is more likely wanting to breed with him. This information can be found in one of these website links:

Amazon Mollies Unusual Sexual Reproduction

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molly   April 9th, 2011 1:12 am ET

i did not know about a fish spices the same sex i thought they all where male and female not just female

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Irene Steffen   September 6th, 2011 3:05 am ET

Let's stick with the topic at hand which is asexual reproduction in mollies. There are too many people moving off topic here- herbal medicines / sex pills have nothing to do with the main topic here. Whomever wants to share about their grandma taking such pills or the like should start a new website or social network devoted to this! Come on now– again...this discussion is about asexual reproduction of mollies. Please consider this when sharing your comment!

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