Asexual reproduction is a method of reproduction that does not involve meiosis or the union of gametes. As a result, the segregation and recombination of genetic material that are a normal part of sexual reproduction do not occur, and offspring are genetically identical or nearly identical with the parent organism. In normal sexual reproduction, offspring are genetically different from either of the parents, and as a result new traits may be introduced into the population. Some members of this diverse population may be able to adapt to changing conditions in ways that the parents could not, and the species as a whole is advantaged. In asexual reproduction these advantages are lost, but under some conditions the opportunity for rapid increase in numbers of individuals may compensate for this disadvantage.
Asexual reproduction is found in many types of organisms including bacteria, algae, fungi, plants, and some simple animals. Although bacteria are capable of sexual union, they commonly reproduce asexually by cell division. Many fungi and algae form asexual reproductive structures, or spores, that divide, grow, and develop into complete new organisms. Simple fragmentation of some simple forms of life may also give rise to new individuals. Higher plants have developed many mechanisms for asexual reproduction. Some plants may reproduce asexually when a mature plant is split into two or more parts and the fragments subsequently develop into new individuals. Plants like Strawberries (Genus Fragaria) and quack grass (Agropyron repens) form runners, or rhizomes, that spread out from the parent to form plantlets at their terminus. The plantlets become independent plants when they develop roots, and the connecting runners disintegrate. Other plants develop specialized structures that give rise to new organisms when released from the parent. One example is found in members of the genus Gladiolus, which form bulbs attached to the parent bulbs that are capable of producing new individuals. Members of the genus Kalanchoe, often called maternity plants, form specialized aerial plantlets attached to their leaves that eventually fall away to form new, genetically identical, individuals.
Asexual reproduction has been widely used in horticulture since ancient times. Many popular ornamental plants are reproduced almost exclusively with asexual techniques. Superior varieties of fruit and nut trees are commonly reproduced this way. The practice can be assisted with the use of synthetic plant hormones like naphthaleneacetic acid and indolebutyric acid. When applied to the base of stem or leaf cuttings these chemicals stimulate the formation of adventitious roots, allowing the cuttings to develop into normal plants. Plants also may be asexually reproduced from undifferentiated plant calluses grown in tissue culture, or even from individual cells grown in culture. By suitably treating the cultured material with plant growth substances (hormones), differentiation into complete individual plants, genetically identical with the original, can result.
Body cells of adult animals, including humans, are often asexually reproduced, or cloned, in research laboratories. Muscle cells, for example, can be removed from a donor animal and grown on suitable culture medium where they may go on growing and dividing, to produce an abundance of genetically identical descendants. Most of the cells of adult animals (and plants) contain the genetic information needed to produce a complete organism, but as cells mature into tissues and organs they tend to express only the information needed for their particular cell type. This has proven to be a particular challenge to scientists interested in the reproduction of complete organisms from adult animal cells. Nevertheless, it has been possible. By the 1950's scientists were able to clone frogs, and Scottish researchers reported successful cloning of sheep in 1996.
Unlike asexual reproduction in plants involving only somatic (non-sexually reproductive) cells, animal cloning has typically made use of enucleated egg cells to which the nuclear contents of somatic cells are added. The reconstituted egg cell is placed in suitable environment where it develops into an organism genetically identical with the organism from which the nucleus was taken. The process is difficult and complex. Despite these obstacles, the possibilities for large economic gain have led to much research in this field. Animal breeders, for instance, would welcome the opportunity to clone high quality livestock. As research advances, the possibilities for asexual human reproduction seem less remote. This has presented society with a very difficult ethical dilemma. Who will make decisions about the availability of this costly procedure? Will the advantages of genetic variation intrinsic to sexual reproduction be lost, making the human race more vulnerable to future challenges? Human society may have to face these and many similar questions in the near future./para>
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