An Asian water dragon hatched from an egg at the Smithsonian National Zoo and her keepers were shocked. Why? Her mother had never been to a male water dragon. Through genetic testing, zoo scientists discovered that the newly hatched female, born on August 24, 2016, had been produced through a reproductive mode called parthenogenesis.
Parthenogenesis is a Greek word meaning “virgin creation,” but it specifically refers to female asexual reproduction. While many people 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, need certain genes to get out of sperm, mammals are incapable of parthenogenesis.
Making offspring without sperm
Sexual reproduction involves a woman and a man, each contributing genetic material in the form of eggs or sperm, to create a unique offspring. The vast majority of animal species reproduce sexually, but females of some species can produce eggs that contain all of the genetic material necessary for reproduction.
Females of these species, including some wasps, crustaceans and lizards, reproduce only by parthenogenesis and are called obligate parthenogens.
A greater number of species experience spontaneous parthenogenesis, best documented in animals kept in zoos, such as the Asian water dragon at the National Zoo or a blacktip shark at the Virginia Aquarium. Spontaneous parthenogens usually reproduce sexually, but they can occasionally have cycles that produce development-ready eggs.
Scientists have learned that spontaneous parthenogenesis can be an inherited trait, meaning that women who suddenly experience parthenogenesis are more likely to have daughters who can do the same.
How can females fertilize their own eggs?
For parthenogenesis to occur, a series of cellular events must unfold successfully. First, females must be able to make eggs (oogenesis) without sperm stimulation or mating. Second, the eggs produced by females must begin to develop on their own and form an early-stage embryo. Finally, the eggs should hatch successfully.
Any step of this process can easily fail, especially step two, which requires the chromosomes of the DNA in the egg to double, ensuring a full complement of genes for the developing offspring. Alternatively, the egg can be “faux fertilized” by leftover cells from the egg production process known as polar bodies. Whichever method initiates the development of the embryo will ultimately determine the degree of genetic similarity between the mother and her offspring.
The events that cause parthenogenesis are not fully understood, but appear to involve environmental changes. In species capable of both sexual reproduction and parthenogenesis, such as aphids, stress factors 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 trigger the switch.
Benefits of Self Reproduction
Although spontaneous parthenogenesis appears to be rare, it offers some benefits to the woman who can achieve it. In some cases, this allows females to generate their own mating partners.
The sex of parthenogenetic offspring is determined in the same way, the sex is determined in the species itself. For organisms whose sex is determined by chromosomes, such as the XX female and XY male chromosomes in some insects, fish and reptiles, a parthenogenetic female can only produce offspring with the sex chromosomes she has on hand – meaning she will always have XX females. will produce offspring. But for organisms where females have ZW sex chromosomes (as in snakes and birds), any living offspring produced will either be ZZ, and therefore male, or much more rarely, WW and female.
Between 1997 and 1999, a checkered garter snake at the Phoenix Zoo gave birth to two male offspring that eventually matured. If a female mated with her parthenogenetically produced son, there would be inbreeding. While inbreeding can lead to numerous genetic problems, from an evolutionary perspective it is better than having no offspring at all. The ability of females to produce male offspring through parthenogenesis also suggests that asexual reproduction is more common in nature than scientists have ever realized.
Biologists have observed over long periods of time that species that are obligate parthenogens often become extinct due to disease, parasitism or changes in habitat. The inbreeding inherent in parthenogenetic species seems to contribute to their short evolutionary timelines.
Current research on parthenogenesis seeks to understand why some species are capable of both sex and parthenogenesis, and whether incidental sexual reproduction may be sufficient for a species to survive.
This article is republished from The Conversation under a Creative Commons license. You can read the original article here.