By Matt Walker

Editor, Earth News



Snakes without fathers: one of the unusual baby boas

A female boa constrictor snake has given birth to two litters of extraordinary offspring.

Evidence suggests the mother snake has had multiple virgin births, producing 22 baby snakes that have no father.

More than that, the genetic make-up of the baby snakes is unlike any previously recorded among vertebrates, the group which includes almost all animals with a backbone.

Details are published in the Royal Society journal Biology Letters.

Our finding up-ends decades of scientific theory on reptile reproduction

Biologist Dr Warren Booth

Virgin births do occur among animals.

Many invertebrates, such as insects, can produce offspring asexually, without ever having mated. They usually do this by cloning themselves, producing genetically identical offspring.

But among vertebrate animals, it remains a novelty, having been documented among less than 0.1% of vertebrate species.

In 2006, scientists discovered that two komodo dragons (Varanus komodoensis), the world's largest lizard species, had produced eggs that developed without being fertilised by sperm - a process called parthenogenesis.

Then in 2007, other scientists found that captive female hammerhead sharks (Sphyrna tiburo) could also reproduce without having sex.

But vertebrates generally reproduce sexually.

Not including genetic material from the father - essentially having just a single biological parent - reduces genetic diversity and makes it more difficult for organisms to adapt to, for example, changed environmental conditions or the emergence of a new disease.

Novel beginnings

Now, a team of scientists and snake experts based in the US has identified the first case of a boa snake having a virgin birth.

"Although parthenogenesis has been documented in a few snake species, our findings are truly novel for a number of reasons," says Dr Warren Booth of North Carolina State University in Raleigh, US.

He led the team that made the latest discovery, and also worked with the researchers who documented a virgin birth in a hammerhead shark.

"The female [boa] has had not one virgin birth, but actually two, in spite of being housed with and observed to be courted by multiple males.

"All offspring are female. The offspring share only half the mother's genetic make-up," he told the BBC.

What is more, the female snake in question has produced offspring the like of which have never been seen before.

Special babies

In the two years following 2007, the captive-born female Boa constrictor produced two litters of live offspring, at the same time as being housed with four male snakes.

First impressions suggested there was something special about these babies: all were female and all had a particular, rare caramel colouration.

This colour is a rare recessive genetic trait, which is carried by the mother but not by any of the potential fathers.

So Dr Booth and colleagues conducted a series of genetic tests on the snakes to solve the enigma.

What they found was astonishing.

DNA fingerprinting revealed that the offspring had a number of genetic differences from any of their potential fathers, which ruled out all the males as sires of the litter.

That confirmed the first instance of a known virgin birth among boa snakes.

Half clones

All the offspring also had very unusual sex chromosomes.

Sex chromosomes are packages of DNA that drive the development of sexual characteristics; they essentially make animals genetically male or genetically female.

Humans for example have X or Y sex chromosomes; females have two X chromosomes and males have a combination of an X and a Y chromosome.

In place of X and Y, snakes and many other reptiles have Z and W chromosomes.

In all snakes, ZZ produces males and ZW produces females.

Bizarrely, all the snakes in these litters were WW.

This was further proof that the snakes inherited all their genetic material from their mother, as only females carry the W chromosome.

"Essentially they are half clones of their mother," says Dr Booth.

That is because the baby snakes have inherited two copies of one half of their mother's chromosomes, including one W chromosome.

More astonishing though, is that no vertebrate animal in which the females carry the odd sex chromosome (in this case the W chromosome) has ever been recorded naturally producing viable WW offspring via a virgin birth.

"For decades WW has been considered non-viable" says Dr Booth.

In such species, all known examples of babies that are the product of parthenogenesis are male, carrying a ZZ chromosomal arrangement.

The only previously known animals to carry this WW chromosome pairing were created by scientists in the laboratory, using intricate genetic techniques to artificially alter the way animal eggs develop.

"Essentially our finding up-ends decades of scientific theory on reptile reproduction," says Dr Booth.

One other mystery is what prompted the female snake to give birth this way.

"This female has given birth to sexually produced babies in the past, and only in years that she was housed with males has she produced offspring," Dr Booth explains.

"It appears that some interaction with a male is required.

"However, why she does not utilise his sperm is at present unknown."

Boas snakes are kept and bred all over the world as pets.

But, Dr Booth adds, "this study tells us we have much more to learn when it comes to reproduction in these primitive reptiles".