Many organisms dedicate their entire adult lives to finding a mate and producing offspring. The rhythms of sex govern the actions and choices of so many animals that it seems to be a rule of biology: Sex is important.
But life’s multifariousness yields some exceptions. A small percentage of animals reproduce asexually, though many of these also resort to sex now and then. These asexual outliers have different techniques for reproducing: Some small invertebrates create offspring by budding, growing small versions of themselves that eventually detach; others, like some corals, can divide themselves in two. Some fish species need males around only because they require sperm to kickstart their reproductive process, even though they only rarely incorporate any genetic material from those males.
And then there are the parthenogenetic lizards: entirely female species that produce eggs with no males required. These unusual lizards — there are a few dozen such species — avoid many of the pitfalls of sex.
But asexual reproduction comes with its own problems, as evolutionary biologist Sonal Singhal of California State University, Dominguez Hills, and her colleagues describe in an article about parthenogenesis in the 2020 Annual Review of Ecology, Evolution, and Systematics. By studying these lizards, the researchers hope to understand how parthenogenetic lizards evolved the ability to reproduce asexually, and to uncover hidden truths about biology and sex itself. Singhal spoke with Knowable Magazine about what these exceptions to the rule can teach us. This conversation has been edited for length and clarity.
Why study these lizards that don’t have sex?
Most animals reproduce sexually. If we want to understand how sexual reproduction works and why it’s important, it’s hard to study it by looking at animals generally, because everyone’s doing it. So we have to look at the outliers, the organisms that opted out and are doing something different. And, in particular, parthenogenesis is in some ways the most extreme form of asexuality in animals.
So parthenogenetic lizards are almost a control group for sex.
Yeah! I think people would quibble with that, but I’m going to say yes.
Biologists have puzzled over the existence of sexual reproduction for generations. Why? Is there something wrong with sex?
Sex is inefficient, quite simply. From a practical standpoint, you have to find a mate; it takes resources to find a mate. Finding a mate can be costly, because often, when you’re mating, you’re exposing yourself to predators. And it can also be costly because doing all the things to attract a mate can be expensive. You have to invest all that energy into making a peacock’s tail, or whatever other trait you might be using to attract a mate. And then, of course, there’s a risk of catching disease while you’re mating.
On top of that, it’s also inefficient because the rate of population growth is just slower. Let’s say we start out with one female, and let’s assume she has two offspring. In an asexual population, they’re both going to be female. Each of those offspring will have two female offspring. You go from one, to two, to four. So you have this exponential population growth in asexual populations. Whereas in sexual populations, the female produces a male and a female. In your second generation, the male and female will mate, and again, only the female will reproduce. Under those conditions, the sexual population stays the same size.
Of course, this is more of a thought experiment, but it shows why sexual reproduction can be such a downer.
All else being equal, asexual species should reproduce much faster than sexual ones. That’s because in an asexual species, every individual is female and can bear young, while about half of most sexual species’ offspring are males.
Considering how common sex is, there must also be some significant benefits to it, evolutionarily speaking.
The main reason why we think sex is good is that it creates variation. If environmental conditions change, sex allows organisms to quickly respond to that change because they have the genetic variation to do so. So that is, on a very fundamental level, why we think sex is good.
Another reason is on a much more microscopic level. Let’s imagine a case where you have a good mutation. And it’s sitting right next to a bad mutation on a chromosome. If there’s no sex, those two mutations are always going to travel together, because they’re on the same chromosome. Mom is going to pass on both the good mutation and the bad mutation to her kid, and so on.
But because of sex, you can actually break apart those two mutations. And that’s through this process called recombination, where the mutations can, through sex, get broken apart, and we can then get a new individual that only has the good mutation. Natural selection will weed out the individuals with the bad mutation, and then the good mutation can start to increase. Before, they were trapped by each other. When you separate them, they can do their own thing.
Can you explain a little more about this process of how sex creates variation?
Sex creates variation in two ways. Independent assortment is the process by which you mix and match genetic variation in creating the next generation. Most organisms have two sets of chromosomes, and an individual inherits one set of chromosomes from one parent and one set of chromosomes from another parent. So for every chromosome, the offspring will essentially inherit one or the other chromosome from each of its parents. So the chromosomes get divvied up into the gametes — egg or sperm cells — differently.
The second way in which variation is introduced is through crossing over, which to me is still one of the craziest things that happens in the entire biological world. In meiosis, the process by which organisms produce gametes, the chromosomes from parent one match up with the chromosomes from parent two. And literally, the DNA breaks in half in the same place in each chromosome, and then they swap bits of the chromosome.
So you create new chromosomes that have never existed before. People who have actually calculated it have said that a single individual can create more genetically different gametes than there are stars in the sky. That’s how powerful recombination is.
An Australian gecko, Heteronotia binoei, with orange parasitic mites attached near its eyes. This gecko has both sexual and asexual populations, and the asexual populations have more mites. This demonstrates one advantage of sexual reproduction: It produces more variation, which may help species keep pace in an evolutionary arms race with predators and parasites.
CREDIT: MICHAEL KEARNEY
So at the end of the day, the benefits of sex outweigh its costs?
If you think about it, the arguments for asexual reproduction are pretty strong. But it’s pretty rare. And when parthenogenesis does evolve, it disappears. It dooms the species to extinction. All the asexual species of lizards are fairly young, evolutionarily — they evolved in the last million years. It’s very rare to find an old lineage of asexual lizards, where a species has been reproducing asexually for a long time.
Organisms are constantly getting new mutations. And it’s very rare for mutations to be beneficial. Most have no effect on your fitness, or they decrease your fitness. Fitness is basically how good you are at life — surviving, making babies. So if you’re an asexual organism, you’re constantly getting new mutations every generation, and you’re just going to accumulate these slightly negative mutations — up until the point where your genome is a disaster zone.
There’s this really compelling metaphor for this phenomenon called Muller’s ratchet. A ratchet is basically a process that can’t be reversed. In this case, that refers to the escalation of bad mutations that negatively affect your fitness through time. And there’s no way of reversing or purging these mutations from your genome, because you don’t have sex, and sex is considered a really effective way to get rid of bad mutations, through recombination.
It’s hard to quantify benefits versus costs, but clearly the benefits of sex outweigh its costs.
Sexual species can purge themselves of harmful mutations through genetic recombination. Asexual species lack the opportunity for recombination and thus accumulate new harmful mutations with each generation. In the long run, this buildup, known as Muller’s ratchet, may drive the asexual species to extinction.
What are some of the questions you’re hoping to answer by studying these lizards?
In sexual reproduction, you have to make your sperm and your eggs, and then you have to combine them, and then that leads to an organism starting to form, a zygote becoming an embryo. Those are all fundamental steps in the beginning of an organism’s life. Once you shift from doing the form of reproduction that most species on Earth do, to this form that very few do, what are the evolutionary and physiological consequences of that? These lizards don’t go through meiosis in the normal way, so they’re breaking one of the main things organisms do to persist. That’s an evolutionary novelty, and that’s just generally interesting, from both a cellular perspective and from an evolutionary perspective.
And with the lizards, it’s cool, because it’s evolved multiple times. I think studying them will tell us more about the physiological process of meiosis. By understanding what happens when we break it, we can understand how it works.
Also, because the genomes of these asexual lizards are so closely related to their sexual ancestors, we can compare the genomes and look at regions that may be showing signs of recent change. Then we can understand what types of genes are being affected in the transition to asexuality. That, to me, is a really cool question.
I would also be really interested in maintaining a colony of these lizards in the lab, so that we can sequence their genomes through the generations and look at how mutations accumulate. We have some hypotheses for how things should look in asexual genomes. The first is this idea of Muller’s ratchet I mentioned earlier, where we expect more and more of these bad mutations every generation. On top of that, we should see less evidence of adaptive evolution. Ecological change often does not allow you to just sit and wait for good mutations to line up. So that’s another idea that you could test by looking at asexual genomes: Is there evidence that they can’t adapt as fast to changing ecological conditions?
Parthenogenesis isn’t unique to lizards — there are plenty of insects and other invertebrates that are also parthenogenetic. So why study lizards?
The genomes of water fleas or stick insects are a lot different from vertebrate genomes. So we’re studying genomes that look more like genomes that we care most about. But honestly, the number one answer is that they’re cool. We’re lizard biologists at heart.
