Why reproduce? Asexual reproduction

Parthenogenesis is the ability of an egg to develop into a new individual without being fertilized by a male gamete. It is counted as a special kind of asexual reproduction because the offspring come from the mother only. It occurs in some worms, crustaceans and insects. In the honeybee the queen lays some eggs that develop without fertilization and become drones, which are haploid (N). Other eggs are fertilized and become diploid (2N) queens or workers, depending on the food the larvae are given. In the aphid, eggs made by mitosis develop without fertilization into diploid females, but when the aphid reproduces sexually its eggs are made by meiosis and are fertilized before they develop. Scientists have also started the development of eggs of the frog and the sea star without sperm, by exposing them to heat, an electric shock, radiation, certain salts, shaking or pricking, and early stages of rabbit embryos were obtained in a similar way. This is told here as the history of an idea; it is not something for a student to try. Tissue culture is carried out on plants. Tissues are grown in a semi-natural nutrient medium and followed until a whole organism develops. In a well-known experiment small pieces of carrot were placed in flasks of coconut milk, which supplies plant hormones and nutrients, and grew into whole carrot plants. Later, separated single cells of the same plant were grown in the same way and also gave whole plants, and a whole tobacco plant was obtained from isolated leaf cells. The experiments showed that every somatic cell of a plant carries all the genetic information, and that it can become a whole plant in a medium with plant hormones in the right ratio. The method is used to propagate rare plants, plants of a desirable kind and plants more resistant to disease.