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.
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.