by James Morris

For Ting Wu, who introduced me to fruit flies and taught me to pay careful attention to exceptions.

There is a well-known saying in genetics: “Treasure your exceptions.” What this means is that we should really take time and learn as much as we can from things that are different, that don’t fit the mold.

Geneticists often look for mutants, an organism like a fly, worm, or mouse that is different from all of the rest. These mutants sometimes harbor a defect in a key gene. And, seeing what happens when a gene doesn’t function properly can often help us to understand what the gene usually does.

It’s a bit like a car. You quickly learn what something does when it stops working. Without a battery, the car doesn’t start. With a broken clutch, it’s hard to shift gears. You get the idea.

An exceptional fly. This fly is truly one in a quarter million.

An exceptional fly. This fly is truly one in a quarter million.

I learned this lesson the hard way when I was in graduate school. I worked in a fruit fly lab and looked for a rare mutation, a mutation that I didn’t know was even possible when I started the experiment. I found it – but only after I looked through a quarter of a million flies. That’s a lot of flies. The fly I found was truly exceptional, and I certainly took good care of it and learned as much as I could from it. I treasured it.

Exceptions not only help us understand the normal function of something, but also teach us what’s truly important. Consider the strange case of the bdelloid rotifers. These small, freshwater organisms might have escaped notice, except for one peculiarity. No, it’s not the unusual “bd” in their name, nor the fact that the “b” is silent.

It’s that they don’t have sex.

The reason why this is unusual is that there is a basic premise in biology that sexual reproduction is the key to success. That is, if you survey all kinds of organisms – like animals, plants, and fungi – a striking pattern emerges: All of the successful lines of organisms reproduce sexually at least some of the time. Some organisms, like us, only reproduce sexually. Some organisms reproduce without sex – asexually – but these either occasionally reproduce sexually, or they are not very old in evolutionary terms. Put another way, exclusive asexuality is an evolutionary dead-end.

Except for the bdelloid rotifers.

What can we learn from these little creatures? They are either teaching us that we are wrong about the importance of sexual reproduction, or it is not really sexual reproduction that’s important – it’s something else, perhaps something related to sexual reproduction.

Sexual reproduction has certain benefits, but from a biological view, it’s actually a lot of work – it takes a good deal of time, resources, and energy. Wouldn’t it be quicker and easier if you could just reproduce on your own without sex? The answer is yes, but there is a cost. Asexual reproduction produces clones – offspring that are genetically identical to the parent.

Clones are fine in an unchanging environment. But imagine if the environment changes, or if a new pathogen emerges. In this case, the last thing you want to be is just like everyone else. For evolution to work, for organisms to adapt to new environments, genetic variation is required. Genetic variation is the raw material on which evolution acts. Natural selection picks and chooses from this vast reservoir of genetic variation, allowing organisms to adapt to new environments.

Put another way, genetic uniformity is the Achilles’ heal of asexual reproduction.

Sexual reproduction, by contrast, creates genetic diversity. By mixing and combining the genetic material from two different individuals, sexual reproduction produces offspring that are genetically different from each other and from the parent. It’s this genetic diversity that is thought to be the key to success of sexually reproducing organisms.

But what about the bdelloid rotifers? What’s the secret to their success? How do they escape the cost of long-term abstinence?

Could it be that they reproduce sexually, but we have just missed it? No.

Perhaps they are not very old in evolutionary terms? No, that’s not the case either.

It turns out that they are great collectors of genetic material from other organisms – bacteria, fungi, and algae, for example. Recent counts estimate that they harbor genetic material from at least 500 different organisms. In other words, they found a novel way to create genetic diversity, one that doesn’t require sexual reproduction.

So, looked at from the vantage point that all successful groups of organisms reproduce sexually, they are a glaring exception. But what they teach us is that it’s not sexual reproduction that’s key. Instead, it’s genetic diversity. They just found another way to promote genetic diversity, that’s all.

We are all genetically different from one another. If you stop and think about it for a moment, each and every one of us is the product of the fusion of a unique egg with a unique sperm. The egg and sperm that came together were different from all of the other eggs and sperm present at the time of conception. In fact, they were unique in the entire history of life on Earth. The product of this fusion – each of us – is also one of a kind, a genetic instance in a long history stretching back billions of years.

What this means, more simply, is that we are all different from one another. We are all mutants. We are all exceptions. And you know what geneticists say…

© James Morris and Science Whys, 2015.

8 thoughts on “Exceptionalism

  1. James Morris Post author

    I just learned from Harvard professor and geneticist Dan Hartl that “Treasure your exceptions” comes from a lecture by William Bateson on “The Methods and Scope of Genetics” in 1908. The full quote is, “Treasure your exceptions! When there are none, the work gets so dull that no one cares to carry it further. Keep them always uncovered and in sight. Exceptions are like the rough brickwork of a growing building which tells that there is more to come and shows where the next construction is to be.”

  2. Emily Koch

    Thanks Jamie! Really interesting…It reminds us that if all of our ducks don’t line up in a row, well, that’s OK–in fact, that that is where the adventure begins.

  3. Jacqueline Baikovitz

    Dear Professor Morris,

    I am fascinated by your article. You mention that this fly is around one in a quarter million. How exactly did you find the mutant fly: Were wildtype flies scored and this mutant naturally occurs in rare circumstances? Does the fly have any other distinugishing features besides no sexual reproduction? Is this the only fly that you have found of this kind? Is this mutation seen in both sexes? Has an associated gene been discovered? Sorry for bombarding you with a lot of questions. Thank you for the article, I enjoy reading your posts. Have a great summer.

    – Jacqueline

    1. James Morris Post author

      Hi Jacqueline, Thanks for reading. Those are great questions. I identified the exceptional fly by doing a screen, where flies were mutagenized with a chemical agent and crossed with other flies. The progeny flies were all yellow. I was looking for a fly that would be darkly pigmented, closer to normal (wild-type). The screen was designed in such a way that a darkly pigmented fly would likely carry a mutation that allows two genes on separate chromosomes to communicate with each other, which is a process I was (and continue to) study. Because the gene is X-linked, I could only identify darkly pigmented flies that were female, not males. The exceptional fly was fully fertile, which was useful in that I could cross it and, in this way, keep the mutation (in flies). Hope this answers all your questions.

      1. Jacqueline Baikovitz

        Thank you so much for your informative response. Just a quick follow up question: referencing pigmentation, is there a balancer effect or is the chromosomal communication potent enough to display different abdominal color phenotypes?

        Best regards,


        1. James Morris Post author

          Hi Jacqueline, Pigmentation in flies can indeed vary quite a bit, from fully yellow to dark brown. Pigmentation can be seen in the wings, body, and even the bristles of the fly. Scientists can measure the degree of pigmentation by simply looking at the fly, or by measuring the amount of messenger RNA produced by genes involved in pigmentation. In the system I studied, the degree of pigmentation could indeed be used to as an indication of how well the two genes communicated with each other. And, chromosomal rearrangements (but not balancers) could in some cases disrupt the communication between the two genes.


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