Monday, 17 December 2012

Virgin Births in the Animal Kingdom

Louise Brown

Christmas is fast approaching, a tradition celebrating the tale of the ‘miraculous’ virgin birth of Jesus Christ. However in the animal kingdom, ‘virgin’ births are actually commonplace. This was brought into context with the now classic report of a captive female Boa constrictor giving birth to 22 offspring in two different litters; offspring that did not have a father. What makes this story so extraordinary is that although asexual reproduction is common in invertebrate species, such as insects and many marine creatures, it only occurs in less than 0.1% of vertebrate species.

There are a number of amazing forms of animal reproduction, and most of these are far from the “Male+Female=Baby” convention we humans are used to.  Asexual reproduction in invertebrates is simply a cloning process, with offspring being genetically identical to the parent. However, the way in which the Boa constrictor mother produced her young was by a special form of asexual reproduction known as Parthenogenesis. This term comes from Greek mythology; the goddess Artemis, called Parthenos ‘the Virgin’, was associated with nymphs who became pregnant. Parthenogenesis occurs as a female egg sex cell develops into an embryo without being fertilised by a male; although this is well documented in bees, wasps and ants, for this to happen in a vertebrate species is something very unusual indeed.

The Boa offspring were atypical in themselves; every single one was a female and bore a rare type of pale-sand coloured scales. This colouration is a genetic trait that is only passed down from the mother. This was the unexplained fact that alerted her keepers that something different was going on. During sexual reproduction, one copy (allele) of each gene is passed to offspring, one from each parent, so they end up with two copies of every single gene in their DNA. Therefore, for the new-born snakes to carry the sand-colouration, they must receive two rare forms of the gene causing it from their mother and father. The female Boa had been living with males, so this would be a plausible explanation for unusual coloured-scales in young produced by typical sexual offspring. However, there was a complication; the males present were known not to carry the rare form of the gene. This means that they could not have passed it onto offspring, meaning there is no way that the young could ever have carried that colouration via simple sexual reproduction. The only way the new snakes could have been that colour was if they have received two copies of that gene from their mother, meaning they must have received two copies of every single gene making up their DNA from their mother. This left only one surprising explanation; Parthenogenesis.

Further proof for this was the fact they had unique sex chromosomes; instead of the X and Y that humans carry, snakes have Z and Y. Usually, males carry ZZ and females carry ZW, but the young produced in this case had WW. This meant they must have inherited both of these chromosomes from a female parent.  No vertebrate in which the female carries the odd copy of the sex chromosome (in this case, W) has ever been recorded to carry both copies of this, meaning this story disproved what was thought to be impossible.

This is not the only piece of evidence that Parthogenesis amongst vertebrates is more common than ever believed. Other examples exist, such as the case of a Hammerhead shark female who gave birth to one pup despite there being no males or sperm present. There have also been cases of female Komodo dragons laying clutches of eggs independently.

Overall, Parthenogenesis has been described in about 70 species of vertebrates, including snakes, fish, lizards and even a turkey! However, it has never been documented in mammals and until not long ago, cartilaginous fishes. It was first suggested that it must never occur in animals that have a womb. Then, the Hammerhead shark, a species of cartilaginous fish, case that appeared a few years disproved this idea, illustrating further that we still have a lot yet to discover and learn about reproduction in the animal kingdom. Mary still has a lot to answer for…