No sex please, we’re parthenogenetic

While women still need to endure courtship, females of other species have learned how to reproduce without males.

Written by      

In 2006, a female Komodo dragon called Flora laid a clutch of eggs in her enclosure at Chester Zoo in England. It would have been unremarkable but for one extraordinary detail: Flora had never mated. The eggs hatched right on Christmas, spawning the inevitable virgin birth jokes, but tests confirmed that indeed, Flora had gone this one entirely alone. While self-fertilisation,

called parthenogenesis, is routine for many invertebrates—rotifers, snails, water fleas, aphids, zooplankton and some honey bees— it’s rare in complex creatures. It has been recorded in just 0.1 per cent of vertebrates, but Flora’s feat indicated that the strategy may be more widespread than we first thought.

In 2002, a female white-spotted bamboo shark laid eggs at the Belle Isle Aquarium in Detroit, despite six years without a partner. Ordinarily, keepers throw such eggs away on the presumption that they must be infer­tile, but curator Doug Sweet recalled that a captive bonnethead shark had pulled off a virgin birth at the Henry Doorly Zoo in Omaha, Nebraska, the previous year, so he left the eggs in the tank. Fifteen weeks later, two live pups hatched.

Besides some sharks, parthenogenesis has been witnessed in salamanders, lizards, at least six species of snake, and even a breed of domestic turkey. Oftentimes, it happens in captivity, prompting some to suggest that mateless females, with no recourse to other means, resort to partheno­genesis as a conscious, if desperate, attempt at perpetuation.

Certainly, parthenogenesis can be a handy talent in the wild. Komodo dragons, for instance, are scattered about a select few Indonesian islands. Like many reptiles, they’re capable colonisers, able to survive long sea passages aboard flotsam without food or water. But what happens should a female wash up on a new island that has no males?

Well, she might take Flora’s lead and fertilise her own eggs, creating an embryo using her own two sets of maternal chromosomes. The sex of Komodo dragon babies is determined by the ovum, not the sperm as it is in mammals. In this arrangement, known as the ZW sex-determina­tion system, the female has two different kinds of chromosome; Z and W. The Z chromosome is the larger, and has more genes.

Her eggs will be haploid, which means their chromosomes contain only one member of the chromosome pairs found in the rest of her body. Ordinarily, a male’s sperm would provide the rest. The trick of parthenogenesis is that those eggs can nevertheless double their chromo­somes to become diploid, either by duplicating without cell division, or by self-fertilisation by a polar body, a sort of tiny ovum produced at the same time as her immature egg cell. The polar body contains almost no cytoplasm, but it does have a duplicate copy of the egg’s DNA.

In an environment without males, females of some species have adapted to breed without them. Very little is known about the process, but some researchers have hypothesised that a defunct body called the polar cell is employed to fertilise the egg in the absence of sperm. Parthenogenetic sharks give birth only to females because the sex is determined by the egg’s two X chromosomes, where as in birds, reptiles and snakes—which share an alternative sex-determination system—the offspring of this immaculate conception will always be male.

In sexual reproduction, the polar body is redundant: it withers and vanishes, but solo female Komodos can somehow use it to “ferti­lise” their own eggs.

Should a dragon reproduce this way, her progeny will have only one chromosome from each of her own pairs, including her two sex chromosomes. When an egg receives a Z chro­mosome, it becomes a male (ZZ); if it gets a W chromosome, it fails and dies.

And therein lies the ingenuity of partheno­genesis: our castaway Komodo will give birth only to males, allowing the next generation to reproduce sexually as normal.

There’s a cost to this sleight of cell: technically, the baby dragon isn’t a clone of its mother—there is at least some shuffling of genetic material—but it has nevertheless derived all its genes from her. That Xerox-like approach limits resistance to disease and hampers adaptability, so it makes for a flimsy genetic platform on which to build a new popu­lation—one reason many believe partheno­genesis is strictly an emergency measure. But here’s an odd thing: even among mixed-sex populations, Komodo dragons still employ parthenogenesis.

Odder still, many other creatures seem to prefer auto-replication rather than doing the wild thing. Our own stick insects are well known for it. Often, a parthenogenetic population of stick insects is separated from sexual relatives by latitude or altitude. A study of the common tea-tree stick insect, Clitarchus hookeri, last year found plenty of males in the north of the species’ range, but netted very few in the south, where partheno­genesis ruled.

Researchers introduced sexual and celibate individuals to see where their preferences lay. Females from sexual populations always mated—even though they could reproduce parthenogenetically if they needed to—whereas few asexual females, when intro­duced to males, took up the offer. But doing it for themselves had little impact on their productivity: mated and unmated females produced similar numbers of offspring. The only differences were that mated females turned out equal numbers of male and female babies that hatched out within 9 to 16 weeks, while the parthenogenetic progeny, which hatched after 21 weeks, were entirely female.

In other cases, parthenogenesis isn’t just a plan B, or even optional. Some animals are doing so well out of celibacy that evolution would appear to be favouring it.

US researchers discovered that, after a large proportion of male Campeloma snails from the Florida Gulf were rendered sterile by a parasitic trematode, females began fertilising themselves. A 2005 study compared the fortunes of those parthenogenetic individuals with those of their sexual relatives. It found that they not only managed similar fecundity and offspring size to their sexual counterparts, but enjoyed five-times greater survivor­ship, and grew 60 per cent faster.

Not surprisingly, humans have found parthenogenesis an irresistible phenomenon. In April 2004, scientists at Tokyo’s University of Agriculture induced parthenogenesis to create fatherless mice—the first documented parthenogenetic mammal. The mice, it turned out, lived significantly longer than their two-parent kin. So far, the only attempt to create a fatherless human was inadvertently discovered in South Korea, after the discred­ited scientist Hwang Woo-suk claimed he had extracted stem cells from cloned human embryos. The experiment was later shown to be a fraud, but scrutiny of the stem cells revealed that, as early as 2004, Hwang had unwittingly created the first human embryos by parthenogenesis.

More by