Parthenogenesis: what is a ‘virgin’ birth?

In school you probably learned that to make a baby you need an egg and a sperm – and if we’re talking about human reproduction, you’re right. However, in the broader animal kingdom, this is not necessarily the case. Many animals reproduce asexually, and a select few experience what is known as parthenogenesis, also called “virgin” births.

What is parthenogenesis?

The word parthenogenesis literally means ‘virgin creation’ in Greek. It is a form of asexual reproduction in which an embryo develops without the need for egg fertilization. This causes the body to have to compensate for the missing genes that are not provided by the absent sperm.

To get things started, females must produce an egg that begins developing into an embryo without any outside input. This can be done in a number of ways. First, in a version of parthenogenesis called automixis, the egg can fuse with cells called polar bodies, which are remnants of the egg production process. This produces offspring that resemble the mother, but not exact clones. Generally they are all women.

Alternatively, parthenogenesis in plants can occur by apomixis, in which reproductive cells duplicate their chromosomes via mitosis. In this case, the offspring are genetically identical to their parent.

What causes parthenogenesis in the first place is not fully understood, but it is thought to be related to environmental stress in some cases.

How rare is parthenogenesis?

It may surprise you, but virgin births in the animal kingdom are not that rare. For small invertebrates, such as bees and tardigrades, parthenogenesis is actually relatively common. And while it’s certainly not the norm among vertebrates, it’s also not as rare as you might think. In fact, it has been observed in more than 80 taxa.

The phenomenon is known to also occur in plants and algae.

What types of animals reproduce by parthenogenesis?

Various insects, fish, reptiles and even birds have been found to undergo parthenogenesis.

Females of some species, including some lizards and snakes, reproduce exclusively asexually. These are known as obligate parthenogens.

Then there are species that experience spontaneous parthenogenesis, where they generally reproduce sexually but occasionally throw a curveball and attempt asexual reproduction. This is best documented in captive animals.

Recently, a female stingray at a North Carolina aquarium became pregnant, without a male stingray involved. Despite claims of a “shark-ray hybrid,” this is most likely a case of spontaneous parthenogenesis. Similar examples have been observed in zoo crocodiles and several species of aquarium sharks, among others.

In some species that would otherwise reproduce sexually, for example fruit flies, parthenogenesis has even been artificially induced. Parthenogenic mice have also been created in the laboratory.

However, without this helping hand, mammals are incapable of parthenogenesis.

Parthenogenesis in humans

It should come as no surprise that humans cannot reproduce through parthenogenesis. As with all mammals, an important genetic process called imprinting throws a spanner in the works.

Imprinting involves specific markers, a kind of molecular stamp, that are inherited from our parents and can influence the way certain genes are expressed. Some genes can be turned on and others turned off by this process. If we had only one parent, as is the case during parthenogenesis, some essential genes would not be activated at all, making offspring nonviable.

That said, occasionally spontaneous parthenogenetic development of an egg can occur, which is believed to result in the formation of a teratoma in the ovary – but certainly not a little parthenogenetic prince or princess.

All “explainer” articles are confirmed by fact checkers to be correct at the time of publication. Text, images and links can be edited, deleted or added at a later time to keep the information current.

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