Parthenogenesis << `pahr` thuh noh JEHN uh sihs >> is a type of reproduction in which an egg completely develops into a new organism (living thing) without fertilization . The term comes from the combination of two Greek words: parthenos, meaning virgin, and genesis, meaning creation. While many simple organisms reproduce asexually (from one organism), parthenogenesis may occur in more complex organisms that usually reproduce sexually, where male and female gametes (sex cells) unite to form a new individual. Parthenogenesis is common in insects. It has also been observed in birds, reptiles, and some fish, including sharks.
In animals, chromosomes are threadlike structures found in cells of all organisms. Chromosomes are the carriers of inheritance—that is, the physical or behavioral characteristics offspring receive from parents. Most animal cells are diploid—that is, they have two sets of chromosomes. During the production of male and female gametes, the number of chromosomes in each cell is halved through division, producing haploid cells. When male and female gametes join at fertilization, the diploid chromosome number is restored. The resulting embryo can begin to grow into a new individual offspring. Egg development in female animals normally begins when a single egg cell divides twice to produce four daughter cells. One of the four daughter egg cells is large and contains all organelles (cell structures) and nutrients needed for an embryo to grow. The other three daughter cells are small and eventually die. These small cells are called polar bodies and carry only a single haploid set of chromosomes.
For an egg alone to develop into a new animal, it must double its chromosome number to form a diploid cell. In parthenogenesis, this most commonly occurs when an egg cell fuses with one of the polar bodies. This makes the egg diploid, and it can develop into an embryo. This embryo has chromosomes only from the female. However, the embryo is not identical to its mother, nor is it a clone . The parthenogenic embryo has only a subset of the mother’s genetic (hereditary) material.
Many parthenogenic animal species can produce only female offspring through parthenogenesis. Usually, males develop only with the contribution of male reproductive cells, called sperm. Sperm cells are small and carry few of the necessary developmental factors that an egg cell contains. They cannot undergo parthenogenesis.
Parthenogenesis is difficult to detect in wild animals. Scientists must collect genetic material of both the mother and the offspring to determine if it has occurred. Most cases of parthenogenesis have been documented in animals kept in zoos or aquariums. Among hammerhead and bamboo sharks , for example, female sharks held in an aquarium for several years without a male present have produced normal pups. These pups were shown to be parthenogenotes, a term for offspring created through parthenogenesis. While many vertebrates (animals with a backbone) undergo parthenogenesis, this process cannot occur successfully in mammals . Reproduction in mammals requires a set of chromosomes contributed from each parent.
