Parthenogenesis - Reproduction Without Sex

Parthenogenesis is a type of asexual reproduction in which a female gamete or egg cell develops into an individual without fertilization. The term comes from the Greek words parthenos (meaning virgin) and genesis (meaning creation.) Animals, including most kinds of wasps, bees, and ants, that have no sex chromosomes reproduce by this process. Some reptiles and fish are also capable of reproducing in this manner. Many plants are also capable of reproducing by parthenogenesis. Most organisms that reproduce by parthenogenesis also reproduce sexually. This type of parthenogenesis is known as facultative parthenogenesis, and organisms including water fleas, crayfish, snakes, sharks, and Komodo dragons reproduce through this process. Other parthenogenic species, including some reptiles, amphibians, and fishes, are only capable of reproducing asexually. Key Takeaways: Parthenogenesis In parthenogenesis, reproduction occurs asexually when a female egg cell develops into a new individual without fertilization.Many different types of organisms reproduce by parthenogenesis including insects, amphibians, reptiles, fish, and plants.Most parthenogenic organisms also reproduce sexually, while others reproduce only by asexual means.Parthenogenesis is an adaptive strategy that allows organisms to reproduce when sexual reproduction is not possible due to environmental conditions.Parthenogenesis that happens by apomixis involves the replication of an egg by mitosis resulting in diploid cells that are clones of the parent.Parthenogenesis that happens by automixis involves the replication of an egg by meiosis and the transformation of the haploid egg to a diploid cell by chromosome duplication or fusion with a polar body.In arrhenotokous parthenogenesis, the unfertilized egg develops into a male.In thelytoky parthenogenesis, the unfertilized egg develops into a female.In deute rotoky parthenogenesis, a male or female may develop from the unfertilized egg. Advantages and Disadvantages Parthenogenesis is an adaptive strategy to ensure the reproduction of organisms when conditions are not favorable for sexual reproduction. Asexual reproduction can be advantageous to organisms that must remain in a particular environment and in places where mates are scarce. Numerous offspring can be produced without costing the parent a great amount of energy or time. A disadvantage of this type of reproduction is the lack of genetic variation. There is no movement of genes from one population to another. Since environments are unstable, populations that are genetically variable are able to adapt to changing conditions better than those that lack genetic variation. How Parthenogenesis Happens Parthenogenesis occurs in two main ways: apomixis and automixis. In apomixis, egg cells are produced by mitosis. In apomictic parthenogenesis, the female sex cell (oocyte) replicates by mitosis producing two diploid cells. These cells have the full complement of chromosomes needed to develop into an embryo. The resulting offspring are clones of the parent cell. Among the organisms that reproduce in this manner are flowering plants and aphids. Dorling Kindersley / Getty Images In automixis, egg cells are produced by meiosis. Normally in oogenesis (egg cell development), the resulting daughter cells are divided unequally during meiosis. This asymmetrical cytokinesis results in one large egg cell (oocyte) and smaller cells called polar bodies. The polar bodies degrade and are not fertilized. The oocyte is  haploid  and only becomes diploid after it is  fertilized by male sperm. Since  automictic parthenogenesis does not involve males, the egg cell becomes diploid by fusing with one of the polar bodies or by duplicating its chromosomes and doubling its genetic material. Since the resulting offspring are produced by meiosis, genetic recombination  occurs and these individuals are not true clones of the parent cell. Sexual Activity and Parthenogenesis In an interesting twist, some organisms that reproduce by parthenogenesis actually need sexual activity for parthenogenesis to occur. Known as pseudogamy or gynogenesis, this type of reproduction requires the presence of sperm cells to stimulate egg cell development. In the process, no genetic material is exchanged because the sperm cell does not fertilize the egg cell. The egg cell develops into an embryo by parthenogenesis. Organisms that reproduce in this manner include some salamanders, stick insects, ticks,  aphids,  mites,  cicadas, wasps, bees, and  ants. How Sex Is Determined In some organisms such as wasps, bees, and  ants, sex is determined by fertilization. In arrhenotokous parthenogenesis, an  unfertilized egg develops into a male and a fertilized egg develops into a female. The female is diploid and contains two sets  of chromosomes, while the male is haploid. In thelytoky parthenogenesis,  unfertilized  eggs develop into  females.  Thelytoky parthenogenesis occurs in some ants, bees, wasps, arthropods, salamanders,  fish, and reptiles. In  deuterotoky parthenogenesis, both males and females develop from unfertilized eggs. Other Types of Asexual Reproduction In addition to parthenogenesis, there are several other types of asexual reproduction. Some of these methods include: Spores: Reproductive cells develop into new organisms without fertilization.Binary fission: An individual replicates and divides by mitosis creating two individuals.Budding: An individual grows out of the body of its parent.Regeneration: The detached part of an individual forms another individual. Sources Allen, L., et al. â€Å"Molecular Evidence for the First Records of Facultative Parthenogenesis in Elapid  Snakes.†Ã‚  Royal Society Open Science, vol. 5, no. 2, 2018.Dudgeon, Christine L., et al. â€Å"Switch from Sexual to Parthenogenetic Reproduction in a Zebra Shark.†Ã‚  Nature News, Nature Publishing Group, 16 Jan. 2017. Parthenogenesis.  New World Encyclopedia.