Asexual reproduction

Resources

Sexual reproduction involves the production of new cells by the fusion of sex cells (sperm and ova) to produce a genetically different cell. Asexual reproduction is the production of new cells by simple division of the parent cell into two daughter cells (called binary fission). Since there is no fusion of two different cells, the daughter cells produced by asexual reproduction are genetically identical to the parent cell.

The adaptive advantage of asexual reproduction is that organisms can reproduce rapidly and so colonize favorable environments rapidly.

Bacteria, cyanobacteria, algae, most protozoa, yeast, dandelions, and flatworms all reproduce asexually. When asexual reproduction occurs, the new individuals are called clones, because they are exact duplicates of their parent cells. Mosses reproduce by forming runners that grow horizontally and produce new stalks; the runner then decomposes, leaving a new plant, which is a clone of the original.

A starfish can regenerate and eventually produce a whole new organism from one of its severed appendages.

Duplication of organisms, whether occurring sexually or asexually, involves the partitioning of the genetic material (chromosomes) in the cell nucleus.

KEY TERMS

Binary fission

—The process in which cell division occurs and two cells are produced where only one existed before.

Blastomere separation

—Cloning by splitting multi-celled embryos.

Gamete

—A male or female sex cell capable of reproduction.

Regeneration

—The ability of an organism to reproduce wholly from a part of another one.

Replication

—The production of new cells like the original one.

Vegetative propagation

—A type of asexual reproduction in plants involving production of a new plant from the vegetative structures—stem, leaf, or root—of the parent plant.

During asexual reproduction, the chromosomes divide by mitosis, which results in the exact duplication of the genetic material into the nuclei of the two daughter cells. Sexual reproduction involves the fusion of two gamete cells (the sperm and ova) which each have half the normal number of chromosomes, a result of reduction division known as meiosis.

Bacteria reproducing asexually can double their numbers rapidly; for example, under ideal conditions of nutrients and temperature, Escherichia coli are capable of growth and division in approximately 20 minutes. This reproduction rate is offset by a high death rate, possibly as a result of the accumulation of alcohol or acids that concentrate from the bacterial colonies. Bacteria in other niches, such as infections and in oxygen-poor environments, also reproduce asexually, but at a much slower pace. The time for one bacterium to divide to form two bacteria can be on the order of weeks or even months.

Yeasts reproduce asexually by budding, as well as reproducing sexually. In the budding process, a bulge forms on the outer edge of the yeast cell as nuclear division takes place. One of these nuclei moves into the bud, which eventually breaks off completely from the parent cell. Budding also occurs in flatworms, which divide into two and then regenerate to form two new flatworms.

Bees, ants, wasps, and some other insects can reproduce sexually or asexually. In asexual reproduction, eggs develop without fertilization, a process called parthenogenesis. In some species the eggs may

or may not be fertilized; fertilized eggs produce females, while unfertilized eggs produce males.

There are a number of crop plants that are propagated asexually. The advantage of asexual propagation to farmers is that the crops will be more uniform than those produced from seed. Some plants are difficult to cultivate from seed, and asexual reproduction in these plants makes it possible to produce crops that would otherwise not be available for commercial marketing.

The process of producing plants asexually is called vegetative propagation and is used for such crops as potatoes, bananas, raspberries, pineapples, and some flowering plants used as ornamentals. Farmers plant the “eyes” of potatoes to produce duplicates of the parent. With banana plants, the suckers that grow from the root of the plant are separated and then planted as new ones. With raspberry bushes, branches are bent and covered with soil. They then grow into separate plants with their own root systems and can eventually be detached from the parent plants.

See also Buds and budding; Clone and cloning; Genetics.

Resources

Ellstrand, Norman C. Dangerous Liaisons?: When Cultivated Plants Mate with their Wild Relatives. Baltimore: The Johns Hopkins University Press, 2003.

Majerus, Michael E. N. Sex Wars: Genes, Bacteria, and Biased Sex Ratios. Princeton: Princeton Univerity Press, 2003.

O’Neill, Sherman, and Jeremy A. Roberts. Plant Reproduction. New York: Blackwell, 2001.

Vita Richman