Parthenogenesis: Virgin Births in Nature
Parthenogenesis in Sharks
What is Parthenogenesis?
The word parthenogenesis is derived from Greek and literally means “virgin birth.” An unfertilized egg will develop into a new individual –the new individual contains genetic information from its mother, and does not have a father. This phenomenon is observed in nature among some animals (insects, frogs, and sharks have been recorded in history).
Parthenogenesis was first described by Charles Bonnet, in the 18th century. By pricking frog eggs with a needle, Jacques Loeb was able to produce parthenogenetic frogs: some of the resulting embryos developed into completely healthy, adult frogs.
Parthenogenesis often results in a partially formed (or malformed) animal when attempted in mammals, though Gregory Pincus was able to induce parthenogenesis in rabbit eggs in 1936, using chemicals and temperature changes.
The terms Haploid and Diploid refer to the number of chromosome sets a species carries. Humans are diploid, as we have two of each chromosome. Some insects are haploid, such as male honeybees (drones). Haploid animals only have one copy of each chromosome. Gametes (egg and sperm cells) are typically haploid, with single chromosomes: this allows the sperm and egg cell to merge and form a diploid cell. Some plants and insects are tetraploid, which means they carry four copies of each chromosome.
Bee Colony Collapse
The Way Honeybees Reproduce
While parthenogenesis may sound like an odd or rare event in nature, it is actually the preferred form of reproduction for many species. Honeybees, for example, are able to sustain their population only through the ability of unfertilized eggs to develop. In honeybee colonies, the fertilized eggs become females, and the unfertilized eggs will develop into male drones. This is a process known as haploid parthenogenesis: the unfertilized egg has only half the number of chromosomes of a fertilized egg. The haploid bee will have the sex chromosomes XO, which causes the bee to become a male drone. Female bees have twice the number of chromosomes, with two X chromosomes to induce the development of female worker bees (or a Queen, if sufficient nutrition is provided to the larva).
Honeybee colonies that lack a male drone will eventually die out, as all of the larvae produced by the queen will be haploid and develop into drones. This is known as a drone brood, and the bee colony will degenerate and collapse without a sufficient supply of female worker bees.
Another way that drone broods form is when the colony lacks a breeding queen. The worker bees are unable to mate and will not typically produce young. In the absence of a fertile queen, however, the worker bees will begin producing eggs. These eggs are not fertilized, and will produce only male honeybees. These colonies are also doomed to collapse.
Types of Parthenogenesis
In haploid parthenogenesis, the unfertilized egg cell develops into an organism with half the number of chromsomes. This may result in a male (honeybee) or female (sheild bug).
Honeybees, rice, and wheat.
In diploid parthenogenesis, an unfertilized egg combines with a polar body or another cell nucleus and develops into an organism with two copies of each chromosome. Diploid parthenogenesis is more common than haploid parthenogenesis.
Roundworms, fluke, and dandelions.
This term refers to an occurrence of parthenogenesis in a species that does not typically reproduce in this manner.
Sharks, frogs, mayflies
Normal or Physiologic
This term refers to parthenogenesis when it is the typical method of reproduction for an organism.
Honeybees, aphids, gall wasps, and many other insects.
Komodo Dragon Virgin Birth
Komodo Dragon Virgin Births
Rare Occurrences in Nature
While parthenogenesis is common in insects, it is less common in fish and mammals. There have been documented cases of parthenogenesis in sharks, for example: Blacktip, Hammerhead, and White-Spotted Bamboo sharks have been reported to reproduce with this method.
The first documented case of a shark "virgin birth" was in Omaha, Nebraska in 2001. A female Hammerhead shark became pregnant, which was rather surprising since she had not been in contact with male sharks for over three years. The resultant offspring was confirmed to contain only the mother's DNA. A short time later, a Blacktip shark at a Virginia aquarium also became pregnant without the presence of males.
Both events resulted in a single pup from each mother - sharks typically deliver relatively large litters, so parthenogenesis is not a particularly good form of reproduction for sharks. In addition, all pups produced through this rare event will be female, as a Y chromosome is required from a fertilizing male shark to produce any male pups.
Komodo Dragons have also demonstrated the ability to reproduce using parthenogenesis. Unlike sharks which use an X and Y chromosome to determine gender, the reptiles have a ZW gender determination system. Female dragons are ZW and male dragons are ZZ. When a female Komodo Dragon's eggs develop parthenogenetically, the eggs are either ZZ or WW - the ZZ embryos develop into males, and the WW embryos fail to develop at all.
Due to this interesting ability, a female Komodo Dragon could create a breeding colony in isolation, as she would be able to lay a clutch of eggs - the developed male offspring could then mate with the mother and produce a colony of breeding dragons.
The use of parthenogenesis to breed Komodo Dragons is not advised, however, as the population would suffer from a condition known as a genetic bottleneck. When a breeding population lacks sufficient genetic diversity, it can become unstable as mutations increase through inbreeding.
Parthenogenesis was originally thought to be impossible for mammals, as the resulting offspring could never develop to term. Mammals are all diploid and require a sufficient amount of genetic information to develop into healthy, fully developed animals. Mammalian parthenogenesis has never been observed in the natural world, but scientists have managed to artificially induce it and to grow a healthy adult mouse, with the use of two female nuclei.
Inducing parthenogenesis in mammals requires the use of two cell nuclei, as all mammals are diploid and require two copies of each chromosome. Scientists at the Tokyo University of Agriculture in Japan fused two egg nuclei and managed to create a parthenogenetic mouse. The process is extremely difficult, however, as one of the egg nuclei had to be manipulated to contain the necessary genetic information for embryonic and fetal development. For example, a growth factor called IGF-2 is required for the development of the fetus, and the genetic information for this growth factor is provided in the sperm cell, not the egg cell. Mice were genetically modified to carry the genes for this growth factor in their egg cells, as the mouse embryos would have been unable to develop without it.
Parthenogenesis in Humans
Human eggs have the potential to become “activated,” or to begin division through parthenogenesis. An enzyme found in sperm, phospholipase-C-zeta (PLC-zeta), will induce the division of a human female's egg. There have been no scientifically documented cases of a human parthenogenetic egg cell developing into a fetus – these “activated eggs” simply develop to the blastocyst stage and become cysts or benign tumors. The blastocysts formed by the activated eggs look like very early embryos, and contain stem cells. As humans are diploid creatures, the use of the PLC-zeta enzyme would not ever allow for the development of a baby: the egg cell would remain haploid and only carry half the number of chromosomes required for normal development.
Parthenote Stem Cells
Uses of Parthenogenesis
Parthenogenetic human eggs might have a future for the growth of embryonic stem cells. No human egg cell has ever been able to develop into a fetus through parthenogenesis, but it is possible for these “activated eggs” to create new embryonic stem cell lines without the controversy endemic to embryonic stem cells gathered from early embryos. These stem cells are called parthenote stem cells.
Gynogenesis and Androgenesis
Some salamanders reproduce in a method that is similar to parthenogenesis. These salamanders, however, require the presence of sperm for the egg to activate. The sperm does not contribute any genetic material to the egg, but certain enzymes are required to trigger the egg to divide. This process is known as gynogenesis -all of the animals of a gynogenetic species are female, and must seek out a closely related species for mating to provide the necessary spermatic enzymes to activate the eggs.
The opposite of parthenogenesis is androgenesis, where an organism is able to fully develop from the male gamete. The resulting offspring are clones of their fathers - this phenomenon is observed in clams and other mollusks.
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