Dr. Tatiana's Sex Advice to All Creation

by Olivia Judson

  “For us bacteria, reproduction is reproduction and sex is sex. Unlike you ‘higher’ creatures, we’re not so vulgar as to do both at once. For us bacteria, reproduction is asexual: we simply divide into two genetically identical cells. This way, sex—by which I mean the acquisition of extra genes—is something that we reap the benefits of during our lives. If humans could do this, which they can’t, it would be like suddenly adding a few genes for longer legs or bluer eyes.”

  At this, one of the homing pigeons muttered wryly, “Lucky bacteria. Getting new genes beats a tummy tuck for a midlife crisis. Wish I could get some new genes.”

  Before I could turn the discussion back to Miss Philodina, the bacteria said, “So, how we do it? We have several ways. We pick up DNA that’s loose in the environment. We gather DNA from passing viruses. We even plunder the genes of dead bacteria—the cognoscenti call it necrophilia:”

  The thought of necrophilia—even if only bacterial necrophilia—produced gasps of horror from the audience, and someone yelled, “Perverts!”

  The bacterium blithely went on: “We also indulge in bestiality, getting genes from bacteria of other species. But above all, when we’re in the mood, we have sex with each other. We’ll show the rotifer how it’s done—I’m going to give my friend here a useful set of antibiotic resistance genes. Look, Miss Philodina, watch how we do it!”

  As the crowd roared, one of the bacteria started to extend a tube toward the other.

  Luckily, at that moment my technician flicked the microscope switch and plunged the screen back into darkness. I tell you, it was a close call. Talking about sex is one thing; as you can imagine, showing sex would land me in all sorts of hot water with the network and thoroughly upset my commercial sponsors (though PlayBeast would’ve been thrilled). It could’ve been the end of the show.

  The dirty little germs did do me one favor, though. They reminded everyone what sex is—and that it’s not the same as reproduction. Sex is any process that mixes genes from different individuals.

  To my amazement, Miss Philodina took up the cue. Like a pubescent boy, she knows an awful lot about the theory of sex; unlike a pubescent boy, she obviously finds the whole idea disgusting. I guess that’s bound to happen if you’ve got eighty-five million years of celibacy behind you.

  “Bacteria,” she said, sighing. “Always exaggerating their sex lives.”

  She was right, of course. Despite the impression the speaker tried to give, bacteria are not nature’s libertines.

  “Most bacteria don’t have sex of any sort very often,” she went on, “and E. coli are among the most abstemious.” She spun her wheels indignantly. “I wouldn’t want anyone to think that we bdelloids are merely a sort of bacteria. Or worse, a sort of virus.”

  She was worried that many in the audience might not really know the difference between viruses, bacteria, and all the rest of Mother Nature’s children. And that if folks didn’t understand the difference, they wouldn’t appreciate how truly unique she is.

  “Viruses cannot reproduce by themselves. Instead, they invade a cell and hijack its machinery to make more viruses. In fact,” she said with a sniff, “viruses are not even proper organisms. They are little more than bands of rogue genes traveling in a tiny capsule.”

  I pointed out that although viruses are mainly famous for causing disease—from polio to AIDS—some of them do merit a place in the Kama Sutra. For example, the reason that humans must make a new flu vaccine each year is that influenza viruses sometimes have sex, getting new genes that help them sidestep the human immune system. All the same, if Miss Philodina were a virus—or a bacterium—no one would be making a fuss about her being asexual. The problem is that, just like any mammal or bird, Miss Philodina is a eukaryote.

  Unlike bacteria, we eukaryotes keep our genes sequestered in a special place—that is, a nucleus. Eukaryotes come in many shapes and sizes—some have only one cell, others, like bdelloid rotifers and humans, have lots of cells. But for all this diversity, eukaryotes are properly puritanical when it comes to sex. If the mood takes them, bacteria and viruses have lots of different ways to mix genes. Eukaryotes have only one. And when scientists talk of sex being essential, it’s eukaryotic sex they mean.

  In eukaryotic sex, you get half your genes from your mother and half from your father. But which half? That’s decided through the lottery known as meiosis. Say you’re playing a game with two decks of fifty-two cards. Each card represents a chromosome—a string of genes. The only rule is that each of your offspring gets one complete deck. It doesn’t matter whether they get the queen of spades you got from your father and the jack of diamonds you got from your mother. It doesn’t even matter if you cut up both aces of spades and stick them back together again as jumbles of the originals. Indeed, genetic cutting and pasting of this sort is an integral part of eukaryotic sex. It is an internal shuffling of each chromosome, and it is called recombination. Thus, at the end of meiosis, each sperm and each egg carries one complete but unique mixture of genes—a complete but shuffled deck. When egg and sperm from two individuals fuse, they therefore produce a new gene combination. Bdelloid rotifers haven’t seen a new gene combination in years—eighty—five million years.

  “Eighty-five million years without meiosis. Without genetic exchange. Without men,” said Miss Philodina. “For eighty-five million years, we’ve done nothing but clone, and we’re jolly proud of it. What’s more, we think everyone should follow our example.”

  She looked all set to go on advocating abstinence, but the belligerent ram in the front leapt to his hooves. He had fixed to his fleece a large badge exhorting everyone to “Save Our Sex.” He didn’t believe for a moment that bdelloid rotifers are genuine ancient asexuals—or, indeed, that any ancient asexuals exist at all.

  Punctuating his speech with vigorous nods of his woolly head, he bleated: “Baaaa. Miss Philodina, you claim to be the descendant of a militant virgin who got rid of men and abandoned sex millions of years ago. If what you say is true,” he paused to emphasize his skepticism, “it would be sensational. Baaaa. Perhaps you are not aware that other organisms have made this claim—and that it has never stood up to scrutiny.”

  The ram clearly knew what was at stake. If ancient asexuals really walk the planet, the ramifications—here I bleated respectfully—are dramatic. Again, to spell it out: if they can do without sex or men, maybe the rest of us can too. So you can see why he tried so hard to paint her as a fraud.

  “Some of you will remember the case of the chaetonotid gastrotrichs—microscopic animals much like you, Miss Philodina, who also live in puddles and mosses,” he said. “Like you, they claimed to be ancient asexuals. But when scientists looked at them more closely, they were caught making sperm, an activity not exactly consistent with asexuality, I think you’ll agree.

  “And who could forget the aphids of the Tramini tribe? Baaaa. What liars! Those fat little insects, too, said they were ancient asexuals. Another hoax! Genetic tests showed they weren’t as virtuous as they pretended. And sure enough, scientists found that they kept their males hidden among the roots of the weeds where they live.

  “What can we learn from this?” He paused dramatically. “Besides the bdelloid rotifers, a handful of other groups still profess to be ancient asexuals. The darwinulid ostracods—a score of species of small freshwater shellfish—claim to have been without sex for a hundred million years. Baaaa. Certain families of oribatid mites also insist that they got rid of men aeons ago. And there are other self-styled supercelibates: shrimp that live in Old World salt flats, two species of North American fern, and a species of clam. But the evidence for these claims is flimsy, to say the least.

  “I put it to you,” the ram thundered, “that all claims of ancient asexuality will turn out to be bogus, the supposed celibates unfrocked! Baaaaaaa. Miss Philodina, your chastity is a sham! Like the others before you, you are hiding males, and sooner or later you will be exposed!”

  As the ram sat down, the audie
nce burst into applause.

  I had to say, he was on the mark. Over the years, various organisms have claimed to be ancient asexuals; and many of them have indeed been exposed as imposters. Until now, all claims of ancient asexuality have rested on negative evidence—mainly that no one has ever seen a male of that species. Negative claims are weak and easy to dismiss. After all, biology is full of species where the males and females look so different from each other that for decades they were not recognized as being each other’s other halves.

  Well, I thought the audience was ready to tear up the seats. I must take my hat off to Miss Philodina. She kept her cool—and stunned the audience by producing strong evidence that the bdelloid rotifers are not charlatans but genuine ancient asexuals. To everyone’s dismay, she made a credible case that she and her foremothers have indeed managed to live without men or meiosis for millions of years. They are wholly virgin, purer than pure, the nun’s nuns, ultimate maidens, poster children for abstinence.

  Her proof rests on the fact that cloning for millions of years has dramatic effects on the way that genes evolve. “Being asexual for generations leaves an unmistakable mark, a molecular tattoo on your genes,” she said smugly. “If you always clone, there’s only one source of genetic novelty, only one thing that could cause my genes to differ from my mother’s, grandmother’s, or great-great-great grandmother’s: mutation.”

  I reminded everyone that mutations are nothing more than sporadic mistakes made by the cell’s genetic copying mechanism.

  Miss Philodina went on, “Let’s go back to my ancestor of eighty-five million years ago, the last child of original sin in my family. Say she inherited two copies of a gene for, I don’t know, wheel number. One copy came from her father, the other from her mother. And for the sake of argument, let’s say they were identical. But now it’s eighty-five million years later. Since a bdelloid lives about three weeks, that’s about 1.5 billion bdelloid rotifer generations. So you would expect that my two copies of the wheel number gene would be extremely different from each other. Each will have accumulated different mutations.”

  The best way to understand this process is through an analogy. Imagine an ancient manuscript had been copied again and again by monks in two distant and lonely monasteries. If each new version of the manuscript is copied from the previous one, more and more mistakes will creep in. And unless the monks are telepathic from scriptorium to scriptorium, the mistakes they make will be different. As time goes by, the manuscripts owned by the two monasteries will diverge more and more. In contrast, if you were having sex, it would be as if the monks in the two monasteries were regularly copying from each other, as well as frequently replacing their versions with manuscripts from monasteries elsewhere. The communication between all the monks would ensure that the manuscripts resembled each other closely.

  Extensive divergence between the two copies of any given gene, Miss Philodina explained, is the molecular stamp of ancient asexuality.

  The homing pigeon, his wings twitching with such excitement that he accidentally lifted off, shouted from the air above his perch, “But an ancient text copied 1.5 billion times by two independent groups would change beyond recognition! I don’t believe these patterns are detectable!”

  “Identifying the two copies of a gene can certainly be difficult. But luckily, in our case, they hadn’t changed beyond recognition,” said Miss Philodina. And then she played her trump card. Triumphantly spinning her wheels in the ram’s direction, she put an end to the accusations of imposture, revealing that genetic tests had shown that bdelloid rotifers have the predicted pattern of divergence. Brandishing a copy of Science magazine, Miss Philodina said with a bounce, “The evidence is conclusive. We bdelloids are celibate. Male bdelloid rotifers do not exist.”

  The radical feminists at the back greeted this news with a rousing chorus of “That’s all right, that’s OK, nobody needs them anyway!”

  The rest of the audience, however, didn’t seem at all happy. As I looked around the studio, I saw long face after long face, and the room hummed with angry murmuring. No one could any longer dispute Miss Philodina’s ancient asexual credentials, so instead the crowd became abusive, insinuating the bdelloid rotifers’ success was a momentary aberration and that they were surely heading for extinction like other asexuals. A python coiled in a corner raised a large placard that read “The Bdelloids Are Bdoomed” and hissed menacingly, “You’re going extinct, you spineless spinster, you’re going extinct.”

  “In the long run we’re all going extinct,” said Miss Philodina tartly. “Sex won’t save you from extinction! The dinosaurs had rampant sex, and look what happened to them. You can have sex till you’re blue in the face, but if your habitat vanishes, it’s you and the dodo. Asexuals—”

  The pocket mouse bravely interrupted: “But surely if you don’t have sex, you can’t adapt to the future? If you can’t adapt to the future, you haven’t got a future.”

  “Who says asexuals can’t adapt?” Miss Philodina spluttered. “I’ll have you know that the bdelloid rotifers are one of the most versatile groups on earth. We make up a sisterhood of more than 360 species. We live in the moss, damp soil, funeral urns, gutters, and puddles of seven continents. You’ll find us in the wastes of Antarctica and the jungles of Sumatra. We live in sulfurous hot springs and in the purest dew. Compare that with our distant cousins, the seisonid rotifers. They’ve always had sex, and a fat lot of good it’s done them. There are only two species, and they both live on the bodies of one type of shrimp. Call that evolutionary success? Humph. I call it a miserable failure.”

  I stepped in to stop all the snarling. “The thing to focus on,” I said, “is how exceptional the bdelloid rotifers are. They are the only asexual group with lots of species. After eighty-five million years, nobody rational could suppose their extinction is looming. But most asexuals don’t survive. Understanding why they don’t—and how the bdelloids have—can give us important clues as to why we need sex.”

  Hallelujah! I’d managed to get the audience back to the matter at hand. I explained that there are more than twenty theories that purport to explain the role of sex, and I briefly summarized the three front-runners, popularly known as Muller’s ratchet, Kondrashov’s hatchet, and the Red Queen. According to both the ratchet and the hatchet, asexuals are driven extinct by the accumulation of harmful mutations—in other words, asexuals eventually die of genetic diseases. The Red Queen, in contrast, invokes a more traditional horseman of the apocalypse: pestilence, also known as infectious disease.

  Moby the puffer fish splashed straight into the subject of harmful mutations: “Miss Philodina, without sex, how can asexuals get rid of harmful mutations? And if you’ll pardon my saying so, your wheels are looking a little wonky—my eyes aren’t too good, so perhaps it’s just the light, but the one on the left actually looks square. I bet it’s those mutations you were saying you’ve accumulated.”

  Miss Philodina hit right back. “I may be square, but my wheels are not.” She sounded confident enough, but I swear I saw her spinning them, just to check. ‘And if you’ll pardon my saying so, dear puffer fish,”she went on, “mutations are greatly overrated as an evolutionary force. Geneticists think mutations are bad because their methods are so crude. All they can see are the bad mutations. Obviously, having no head is bad for you. And if you’re a fly, having no wings isn’t much good. For a start, you’d have to be called a ‘walk’. But, in fact, most mutations are neutral. They have no effect. They change the DNA sequence of a gene, sure. But they don’t affect the information. It’s like switching from the English spelling of a word to the American spelling. “P–l–o–u–g–h” and “p–l–o–w” look different on paper, but they mean the same thing and sound the same when said aloud.”

  Well, well, well. Given all her mutations, I guess I should have known that Miss Philodina would be a neutralist—that she would subscribe to a controversial, even radical school of thought that holds that most mutations are
neither helpful nor harmful, just irrelevant. I couldn’t let her get away with it, though. First of all, there’s a vigorous debate going on over whether most mutations are neutral. And second of all, it’s generally agreed that when a mutation does have an effect, the effect is usually bad: small random changes are likely to harm, not help. Or to put it more starkly, many different mutations will kill you or make you sick, but none guarantees success in life. Which brings us to the ratchet and the hatchet.

  According to Muller’s ratchet (named for its inventor, the geneticist Hermann Muller, who won the Nobel prize for demonstrating that X rays cause mutations), asexuals are evolutionarily short-lived because, over time, the number of harmful mutations they carry will irrevocably and inevitably ratchet upward. Imagine a population that has just become asexual. For the sake of argument, imagine that all members of the population are free of harmful mutations. Over time, copying errors will lead to mutations among their descendants, and gradually the population will consist of individuals who carry several mutations. Then one day the last mutation-free individual will fail to leave children, and the ratchet will have clicked forward one notch. This process continues until eventually all the individuals are so sick that they die and the population goes extinct. Sexuals avoid this fate because the shuffling of genes in each generation produces individuals who carry few mutations.

  Muller’s ratchet is an elegant idea. But it works only if a number of assumptions are met. The most important of these is that the asexual population is small. In large populations, you see, there may always remain some individuals bearing few mutations. Kondrashov’s hatchet (also named for its inventor, a Russian geneticist), however, is another story: it holds regardless of population size.

  Suppose there’s a threshold number of slightly harmful mutations that any individual can carry. Above that threshold, the hatchet falls—and you’re dead. In a population that has sex, the shuffling of genes creates some lucky creatures with few harmful mutations. But it also creates some unlucky ones with many. The unlucky ones fall under the hatchet, taking their mutations to the grave. This quickly and efficiently purges the population of harmful mutations. Asexuals, however, have no such recourse. Far more asexual individuals will cross the threshold of having one bad mutation too many. According to the theory, if the harmful mutation rate is high enough, there is no way to survive without sex.