These females don’t need a male to reproduce.

A Chinese water dragon hatched from an egg at the Smithsonian National Zoo in the United States, and her keepers were shocked. Why? Her mother had never been with a male Chinese water dragon. Through genetic testing, zoo scientists discovered that the newborn female, who came into the world on August 24, 2016, had been produced through a reproductive mode called parthenogenesis.

Parthenogenesis is a Greek word meaning “virgin creation” but specifically refers to female asexual reproduction.

While many people might assume that this behavior is the domain of science fiction or religious texts, parthenogenesis is surprisingly common throughout the tree of life and is found in a variety of organisms, including plants, insects, fish, reptiles, and even birds.

Because mammals, including humans, require certain genes to come from sperm, they are incapable of parthenogenesis.

Creation of sperm-free offspring

Sexual reproduction involves a female and a male, each contributing genetic material in the form of eggs or sperm to create unique offspring. The vast majority of animal species reproduce sexually, but females of some species are capable of producing eggs containing all the genetic material necessary for reproduction.

Females of these species, which include some wasps, crustaceans and lizards, reproduce only by parthenogenesis and are called obligate parthenogens.

A large number of species experience spontaneous parthenogenesis, best documented in animals kept in environments such as zoos, such as the Chinese water dragon at the National Zoo or the blacktip shark at the Virginia Aquarium, USA . Spontaneous parthenogens usually reproduce sexually, but they may have occasional cycles that produce eggs ready for development.

Scientists have learned that spontaneous parthenogenesis may be an inherited trait, meaning that females who experience parthenogenesis are more likely to have daughters who can do the same.

How can females fertilize their own eggs?

For parthenogenesis to take place, a chain of cellular events must successfully unfold. First, females must be able to create reproductive cells (oogenesis) without sperm stimulation or mating. Second, the eggs produced by the females need to start developing on their own, forming an early-stage embryo. Finally, the eggs must hatch successfully.

Each step of this process can easily fail, especially step two, which requires the DNA chromosomes inside the egg to bend, ensuring a full complement of genes for the developing offspring. Alternatively, the egg can be “artificially fertilized” by cells left over from the egg production process known as polar bodies. Any method that initiates embryo development will ultimately determine the level of genetic similarity between the mother and her offspring.

The events that trigger parthenogenesis are not fully understood, but appear to include environmental changes. In species that are capable of sexual reproduction and parthenogenesis, such as aphids, stressors such as crowding and predation can cause females to switch from parthenogenesis to sexual reproduction, but not the other way around. In at least one type of freshwater plankton, high salinity appears to cause this shift.

Advantages of self-reproduction

Although spontaneous parthenogenesis appears to be rare, it offers some benefits to the female who is able to perform it. In some cases, it can allow females to generate their own mating partners.

The sex of parthenogenetic offspring is determined by the same method as the sex is determined in the species itself. For organisms where sex is determined by chromosomes, such as the female XX and male XY chromosomes in some insects, fish and reptiles, a parthenogenetic female can produce offspring only with the sex chromosomes she has in her hands – which means she always will produce XX female offspring.

But for organisms where females have ZW sex chromosomes (as in snakes and birds), all living offspring produced will be ZZ and therefore male or, much more rarely, WW, therefore female.

Between 1997 and 1999, a checkered garter snake kept at the Phoenix Zoo, USA, gave birth to two male offspring that survived to adulthood. If a female were to mate with her parthenogenetically produced offspring, that would constitute inbreeding.

While inbreeding can result in a number of genetic problems, from an evolutionary perspective this is better than having no offspring at all. The ability of females to produce male offspring through parthenogenesis also suggests that asexual reproduction in the wild may be more common than scientists realize.

Biologists have observed, over long periods of time, that species that are obligatory parthenogens often die from disease, parasitism, or changes in habitat. The inbreeding inherent to parthenogenetic species seems to contribute to their short evolutionary timetables.

Current research on parthenogenesis seeks to understand why some species are capable of both sexual reproduction and parthenogenesis, and whether occasional sexual reproduction may be sufficient for a species to survive.

Editor’s Note: Opinions expressed are those of the writers alone. CNN features the work of “The Conversation,” a collaboration between journalists and academics to provide analysis and commentary for news. The content is produced exclusively by “The Conversation”.

Mercedes Burns is an assistant professor of biological sciences at the University of Maryland in Baltimore County, USA. Burns has already received funding from the National Science Foundation.

Text republished under a Creative Commons license from “The Conversation”.

*Translated text. Read the original in English here.