Are males necessary? Not if you’re an aquatic snail, a crayfish, a python or a host of other species of insects, crustaceans and reptiles. In every major group of vertebrates and many invertebrates, females occasionally or always manage to reproduce without an assist from males—every major group except, that is, mammals and so-called cartilaginous fish, which include sharks. Now the roster of animals able to produce offspring by virgin birth has a new addition. According to a genetic analysis released this week, a bonnethead (a small species of hammerhead shark) born in December 2001 at a Nebraska aquarium contained only female DNA. Her mother was her father, too, so to speak.
In 1861 Charles Darwin wrote, “We do not even in the least know the final cause of sexuality; why new beings should be produced by the union of the two sexual elements, instead of by a process of parthenogenesis.” Ever since, biologists have been asking, who needs sex, anyway?
The standard answer has been that sex is nearly ubiquitous in both the plant and animal kingdom because it creates genetic diversity. When DNA from two parents mix it up inside a fertilized egg, the resulting offspring are more likely to have novel genetic combinations that will stand them in good stead in a variety of conditions (it’s like packing for a trip with snow boots as well as sandals—you never know what you’ll find out there). But there is a competing argument. Evolution is all about getting your genes into the next generation—the “selfish gene,” as British biologist Richard Dawkins called it. And what better way to do that than to get all your genes into your offspring rather than half of them, which is what happens when you let a mate contribute 50 percent of the DNA to junior? Offspring that come from just an egg, or perhaps two eggs, would be all Mom.
That’s what the bonnethead apparently did. The virgin mother had been captured in Florida, then kept with only females for three years at the Henry Doorly Zoo in Omaha. As far as biologists can figure, a cell that normally matures into an egg, and which contains half the female chromosomes, fused with another cell that also contains half the female chromosomes. One half plus one half equals one, so presto: the baby-to-be had a full complement of female DNA. (She died soon after birth, but not because parthenogenesis—virgin birth—produces defective offspring: she was apparently killed by another fish.)
The bonnethead was only the latest creature to surprise biologists with its ability to reproduce without males. Two Komodo dragons kept in captivity gave birth without their eggs being fertilized by a male, scientists reported last December. One female later produced additional offspring the standard way, so she didn’t seem to have anything in particular against sex (or males). “This reproductive plasticity indicates that female Komodo dragons may switch between asexual and sexual reproduction, depending on the availability of a mate,” the scientists wrote in the journal Nature. That raises hopes that this threatened species can be bred—maybe we should say propagated?—in captivity without all the bother of finding her a mate, let alone gathering sperm and then attempting artificial insemination. As one curator remarked of the parent who was both mother and father, “When the first of the babies hatched, we didn’t know whether to make her a cup of tea or pass her the cigars.”
Some flatworms are mostly parthenogenic, as are a grasshopper and a lizard from the Australian desert, whiptail lizards, the aquatic snail Melanoides tuberculata (in which individuals partial to sex and to virgin reproduction both exist in natural populations), the ant Cataglyphis cursor (which produce queens by parthenogenesis but workers by normal sexual reproduction; infer from that what you like), the marbled crayfish Marmorkrebs (a popular aquarium species; and you wondered how they managed to go forth and multiply so enthusiastically?). A female python living in the Artis Zoo in Amsterdam produced eggs five years in a row despite being isolated from males. At first biologists thought she had stored up sperm (for a rainy day?) from her last tryst, but a genetic analysis showed that the babies were the products of virgin births.
One of the most bizarre forms of parthenogenesis is caused by a germ. A parasitic bacterium called Wolbachia infects testes and ovaries, but only infected females pass Wolbachia on to their offspring. It is therefore in Wolbachia’s interest to have females, but not males, reproduce and—even better—produce all daughters and no males, which are dead ends as far as Wolbachia are concerned. Solution: induce parthenogenesis. That way the offspring are all daughters—exactly the hosts Wolbachia needs to spread to the next generation. The clever bacterium works this trick in a number of insects, including mites and wasps.
Parthenogenesis may be about to graduate from the “isn’t nature weird” bin. Scientists trying to work with human stem cells, but without triggering the moral objections that come with getting them from normal human embryos, think immaculate conception might offer a solution. The idea is to coax eggs to develop into embryos without being fertilized by sperm. The embryos are not viable. That might placate opponents of standard research on human embryonic stem cells, since the cells would be derived from something that could never develop into a baby. In 2006 scientists in Milan, Italy, announced that they had managed to produce human embryos containing only maternal genes. The ball of cells—a blastocyst—stopped developing after a few days. But stem cells taken from the blastocyst were still dividing in a lab dish two years later, and have characteristics of cells able to differentiate into any kind of cell in the body.
A certain famous virgin birth 2,000-some years ago notwithstanding, parthenogenesis in mammals is not in the cards. Mammalian genes don’t just merge in a fertilized egg and carry on. Instead, at least 100 genes in both sperm and eggs are turned off. But it’s never the same gene; if dad’s is a dud, mom’s takes up the slack, and vice versa. But obviously that doesn’t portend well for parthenogenesis. If you merge the DNA in two eggs, you’ll still have too many deactivated genes to produce a viable offspring. Some truly bizarre laboratory manipulations have gotten around this—scientists in Japan managed to disable to genetic “off” switches in a mouse egg, so all of the maternal genes were working, producing mice without a father in 2004. But don’t expect the technique to come to a fertility clinic near you anytime soon.