Like a Virgin (13 page)

It is among insects, though, that virgin births probably exhibit the most diverse array of genetic strategies of animal reproduction, including some extremely rare mechanisms. Take the electric
ant,
Wasmannia auropunctata
, a tiny ant that lives deep in the rainforests of Brazil and French Guiana, though it has now spread around the globe. Commonly known as the ‘little fire
ant’, it is ranked among the world’s most invasive species. Some populations of the ant reproduce through normal sex. But for others, the females can make babies without the males
– through a rather unusual process. The virgin females produce males, which then fertilize the eggs, though biologists do not understand how they do so. In any case, the fertilized eggs grow
into more females – workers that are all sterile.

What is especially odd about the little fire ant is that it isn’t just the females that clone themselves; the males can do it too. This indicates that virgin birth in little fire ants
probably
developed spontaneously, as a result of mutations in their DNA. The males of the species are able to clone themselves by eliminating their maternal genes, so that
they only ever pass on their father’s genes – the reverse of the scenario when females produce offspring through virgin birth. When scientists first discovered this detail, they assumed
that the female DNA was discarded from the egg during fertilization, just as happens in reverse with sperm in female virgin births. It seemed as though the ants were waging a war of eugenics
– the females dumping male DNA, with the males doing it right back.

In further studies, it has been revealed that things are not so simple (or so very human). The male little fire ants appear to be clones, without any of their mother’s DNA, but this trait
appears to be controlled by the females – or more specifically, by the queen ant. Only males mated to the queen can become clones of their fathers, which is to say that, if a son ditches its
maternal DNA, it is because the mother has dictated it to do so.

For the little fire ant, it seems there would be no male clones if the females themselves were not able to have virgin births. When the cells in a queen divide to produce parthenogenetic eggs,
there is not an equal division. One of the two ‘daughter’ cells takes the DNA-containing nucleus in its entirety, while the other takes just the cell’s cytoplasm. So a queen lays
some eggs that have a complete set of DNA and require no fertilization (and which become sterile daughters) and lays eggs that are ‘empty’, with no nucleus or DNA. Should these empty
eggs be fertilized by sperm, the only DNA to be donated to the embryo comes solely from the male – there simply is no genetic information from the mother ant in the egg.

In other species, parthenogenesis is triggered by the sort of hazard that some scientists believe gave rise to sex in the first place: a contagious infection. Believe it or not, this means that
you might be able to ‘cure’ some animals from the plague
of virgin births – animals like stingless wasps (
Trichogramma
). These tiny parasites make
their living with some rather cunning tricks. The females attack the eggs of moths and other species, injecting their own eggs into those of their unsuspecting victims. In some cases, they find
newly hatched eggs via some quite sophisticated chemical espionage. The wasps can sense anti-aphrodisiacs – the pheromones that many male insects pass to females to signal that mating has
taken place, and which makes these females less attractive to other males, in an attempt to keep other males’ DNA from competing in a race to fertilize the egg. This may be a useful tactic to
employ when there is competition for females, but it’s also a communication system prone to sabotage by the stingless wasp. By following the scent of these pheromones, the wasps can locate a
female butterfly, moth, or other insect that has just mated and is poised to lay a clutch of eggs. The female wasp then hitches a ride on her hostess insect, until the host lands on a plant to lay
its eggs. The parasitic passenger hops off and quickly injects its own eggs inside the fresh host eggs, so that when the wasp’s larvae develop, they are perfectly placed to eat the contents
of the victim eggs. And these host insects are not the only victims. For the female wasps themselves play host to another parasite, one yet more brilliant. When this parasite infects its target,
the males of its host species become infertile, or are killed while they are still developing as embryos. And as the males are eradicated, their females begin having virgin births. This is probably
how stingless wasps began to reproduce in this way – and why they may yet be kept from transforming into an all-female species.

The parasite responsible for infecting the stingless wasp is the
Wolbachia
bacterium.
Wolbachia
live in the eggs of the host wasp, through them passing from mother to young. But
these tiny wasps are not their only targets:
Wolbachia
are among the most abundant and remarkably widespread of all parasitic
bacteria, and the number of species known
to be infected by the bacteria is increasing rapidly. Just how far afield
Wolbachia
are distributed is uncertain, but so far, they have been found in over seventy-five percent of arthropods,
which include eighty insect species, seventeen isopods (a category of crustaceans that includes the woodlouse), many spiders, and one type of mite. It is also likely that there is probably an
equivalent proportion of infections among nematode worms. And there is a good chance that it will spread.

If this doesn’t seem like much to be concerned about, bear in mind that between them, arthropods and nematode worms comprise something in the order of 99.99 percent of animal species in
the world. From cockroaches to termites, dragonflies to ladybirds, woodlice to worms,
Wolbachia
wreaks havoc with ovaries, testes, eggs, and sperm in many ways – not all of which are
entirely clear. What is clear is that
Wolbachia
need eggs; they live in them. This means that the bacteria must be assured of a good supply of eggs – and in this respect, males are
dispensable. So
Wolbachia
has found a way to make the eggs of stingless wasps develop without any sperm involved.

When the wasps reproduce through parthenogenesis, they produce only daughters, of course. But in 1990, scientists found that they could make females and males start having sex again, and that
they could induce mothers once again to have sons. All it took was a dose of antibiotics.

Wolbachia
may be the most prevalent bête noire of male insects, but it is by no means the only parasite to play this game. There is a parasitic fungus,
Ichthyophonus hoferi
,
that causes parthenogenesis epidemics in fish worldwide. Among its targets is the green swordtail (
Xiphophorus helleri
), a species in which the female fish is able to turn into a
sperm-producing, fully functional male, indistinguishable from a true male. In the fish, the fungus causes haemorrhages, destroys muscle, and rots fins and
skin – so
there is reason to worry about its effects. One of the toxins it produces also seems to make eggs develop without sperm. In addition, there is a large and diverse array of micro-organisms that
alter the ratio of males to females in their host populations, usually by killing or feminizing the males. These include protozoa, which affect mosquitoes and amphipods, an order of tiny
crustacean, most usually less than ten millimetres in size; spiroplasms, which affect fruit flies; enterobacteria in wasps; and
rickettsiae
in ladybird beetles. For most of the
planet’s species, the sexes and reproduction are not always what they seem.

Then, there are those few animals that normally would have sex to reproduce, but don’t strictly have to do so. In recent years, the list of these animals has been steadily
getting longer. It’s not that virgin birth as a phenomenon is necessarily increasing in the natural world; rather, that scientists have simply started to realize that some very large animals
– not just insects – are able to reproduce without mates, should they need to.

On 14 December 2001, the list was extended in an exciting new way. Before then, virgin births had been documented in all of the jawed vertebrates – that is, animals with backbones and
jaws. The exceptions, therefore, were the mammals and cartilaginous fishes, the latter of which include sharks and rays but also the aptly named chimaeras – peculiar-looking deep-sea
rat-fishes and egg-laying ghost sharks that have glowing eyes and snouts like an elephant. But that December, an adult bonnethead shark gave birth to a normal, live female pup at the Henry Doorly
Zoo in Nebraska.

The adult shark had been captured from the Florida Keys when she was not yet a year old. For the bonnethead shark
(
Sphyrna tiburo
), that meant it was two years away
from hitting puberty. Once captured, it had been raised in a tank at the zoo, in the company of only two other sharks – both female. Now, sharks of the hammerhead family, of which the
bonnethead is a member, normally can store up sperm from a previous mating for as long as five months. But that would not have been possible for this female, as it was so young when it was caught.
The other possibility that the zookeepers considered was that it may have both male and female genitals, as some humans do. But there again, it didn’t have any claspers, a kind of penis that
a shark that is biologically both male and female could use to fertilize itself. There was only one other way a pup could have ended up in a tank full of adult female bonnethead sharks. And genetic
tests later confirmed that the pup was in fact a virgin-born animal, the first of its type to be identified.

For obvious reasons, it is extremely difficult to spot a virgin birth in a wild animal population. Unless you are very lucky, and stumble on a female-only group that cannot possibly be accessed
by males – the shark in a tank – reproduction by parthenogenesis in an animal that usually has sex is easy to miss. So it’s not surprising that the next reported virgin shark
mother was once again found in captivity, seven years later, at the Virginia Aquarium, in Virginia Beach. This time the mother was an eight-year-old blacktip shark (
Carcharhinus limbatus
)
named Tidbit; unfortunately, her parthenogenetic offspring was only discovered during Tidbit’s necropsy. Like the bonnethead shark mother, Tidbit had been caught in the wild when very young,
less than a year old, and had reached sexual maturity in a tank at the aquarium. The shark had died during a routine physical examination – nothing had seemed amiss. When the body was cut
open, it was found to be carrying a pup that was thirty centimetres (one foot) long and nearly full term. Like the bonnethead baby, DNA analysis revealed
that the female
blacktip foetus did not have a shred of genetic material from a father. There couldn’t have been, as there were no male blacktip sharks in the tank for the entire eight years that Tidbit had
lived there.

Sharks are not the only new animals joining the parthenogenetic ranks. Two years before Tidbit’s examination, two virgin births in Komodo dragons at zoos in the United Kingdom were
announced; since then, a third has occurred in Kansas. Komodo dragons (
Varanus komodoensis
) are the largest of all lizards; the adults can grow to a length of three metres (ten feet) and can
weigh more than ninety kilograms (two hundred pounds). Yet, as wild Komodo populations have become smaller and more fragmented, these legendary creatures have come under threat of extinction.
Today, there are fewer than four thousand Komodo dragons remaining in the wild, of which fewer than a thousand are believed to be mature females.

Because of these worrisome figures around the species’ future, at least fifty zoos had begun participating in an international breeding programme by the time the virgin births were
discovered; it could be said that quite a few people were paying quite a bit of attention to Komodo sex life. In all of Europe, there were only two sexually mature female lizards: Flora at Chester
Zoo and Sungai at London Zoo. Both had been bred in captivity, and were therefore crucial to the success of the European breeding programme. But what the zoo staff involved had not realized is that
female Komodos can reproduce without a male.

It’s not that Komodos need males to lay eggs – much like chickens, it has long been known that some of the eggs Komodos lay will be unfertilized. But in 2006, the then zoo-keeper in
Chester, Kevin Buley, took a clutch of twenty-five eggs that Flora had laid and incubated them ‘on a zoo-keeper’s whim’. It was a serendipitous moment for science, and, indeed,
for Komodo conservation. Flora had never been with a
male dragon. Yet of the twenty-five eggs that Buley fostered, eleven looked just like normally fertilized eggs would. And
in January 2007, eight hatched into healthy male Komodo babies. The three eggs that didn’t make it had collapsed early during the incubation, but they proved useful in providing embryonic
material for genetic fingerprinting. Through such analyses, Flora was proven to be both mother and father to the surviving eight sons. Similarly, Sungai in London Zoo laid twenty-two eggs –
two and a half years after her last contact with a male. Nearly eight months later, four of these eggs hatched, producing healthy sons. Sungai subsequently successfully mated with Raja, a male also
housed at the zoo. Two months later, Sungai laid a second clutch of six eggs, only one of which hatched. Sungai has since died, but at Chester Zoo, Flora has been set up with Norman, a two-metre
(seven-foot) male in whom the female has so far showed no interest whatsoever.