Some of us start to get cranky after a day or two without sex. So what would it be like to remain celibate for forty million years? Perfectly fine, if you’re a microscopic creature called a “bdelloid srotifer,” according to a recent study by two Harvard biologists. Yet by renouncing sex for so long without any ill-effects, bdelloids are thumbing their tiny noses at a central dogma of biology: that sexual reproduction is vital for the long-term survival of a species.
If we humans collectively abandoned sex, we would of course go extinct in short order. In that sense, sex is more than a pleasure, it’s a duty at least for the heterosexuals among us. But why shouldn’t females reproduce non-sexually, through the process of parthenogenesis or “virgin birth”? Several species of vertebrates, including a whiptail lizard living in the American Southwest, do exactly that.
So why do most animals and plants resort to sexual reproduction? Sex mixes the genes of different individuals. According to one popular theory, this mixing is useful because it produces new and advantageous traits. In other words, it promotes evolutionary progress. Another theory and one that I favor argues that the function of sexual reproduction is to prevent the gradual accumulation of harmful genetic mutations; in other words, to maintain the evolutionary status quo. With sexual reproduction, the mixing of genes leads on occasion to the appearance of individuals with more than their share of mutations. These unfortunate offspring “runts” die before or soon after birth and take the mutations to the grave with them, while their luckier siblings with fewer mutations survive to perpetuate the family tree.
In parthenogenesis, however, once a harmful mutation becomes established in a given individual’s germ line, the mutation must persist in all the descendants of that individual there is no way to get rid of it short of eliminating the entire family. Thus mutations accumulate and gradually degrade the species’ genome. So species that renounce sex, such as the aforementioned whiptail lizards, should not last long. In fact, those lizards did reproduce sexually until a few thousand years ago, and they are probably doomed to go extinct within the next few thousand years.
Here’s where the bdelloids present such a puzzle. These tube-like creatures are less than a millimeter in length and move around like leeches (“bdelloid” means “leech-like”). They can be found in almost any freshwater environment, in damp tree litter and the like. They are not just one species, but an entire class of invertebrates with at least 352 member species. They are known to have existed for at least forty million years, because they have been found trapped in amber of that age. Yet there is no evidence that bdelloids reproduce sexually and, in fact, no one has come across a male or even hermaphrodite bdelloid they’re all female.
Of course, researchers might simply have failed to detect the microscopic organisms having sex. (An anthropologist from Mars might need some time before she actually spotted any humans having sex, after all.) There are many species that usually reproduce parthogenetically but occasionally engage in sex, and many scientists suspected that the bdelloids might do the same. According to this idea, the one-in-a-thousand sexual births might be enough to reap the benefits that sexual reproduction is supposed to confer.
That’s where Harvard biology professor Matthew Meselson and David Mark Welch, his postdoctorate student, come in. (Meselson, incidentally, is the scientist who determined that the so-called “yellow rain,” thought by the Reagan administration to be a chemical warfare agent sprayed by Communists in Laos, was nothing more than bee excrement.) Meselson and Welch analyzed the DNA of several species of bdelloids and found exactly the patterns that would be expected in animals that stopped mixing their genes tens of millions of years ago. Essentially, they proved that bdelloids have been totally celibate all this time and are none the worse for it. Thus, if the function of sex is to get rid of harmful mutations, the bdelloids must have found some way to prevent such mutations from happening in the first place, or else they have developed a method to get rid of them once they occur. No small discovery, for if bdelloids have a trick for preventing or repairing mutations, it could be of great use in the prevention of human diseases that involve mutations, such as birth defects and cancer.
So why do humans, along with most animals, ignore the example of the bdelloids and engage in sex to propagate ourselves? We really don’t know why we do it, but at least we’re enjoying the ride.