Reproduction, Asexual

Reproduction, Asexual

Although sexual reproduction is more frequent, asexual reproduction also commonly occurs in the plant kingdom. The technical term for asexual reproduction in plants is apomixis, derived from apo meaning "without," and mixis meaning "mingling." Apomixis thus refers to the fact that asexual reproduction lacks the mixing of genes that occurs in sexual reproduction. In apomixis, a new individual is produced by a single parent without pollination or mixing genetic material. A familiar example of apomixis is the production of new plants by the growth of horizontal stems (runners) in strawberries (genus Fragaria ). Other familiar plants with asexual reproduction include blackberries (genus Rubus ) and dandelions (genus Taraxacum ), both of which produce asexually formed seeds. Apomixis is of great interest to plant breeders, because it allows the production of exact genetic duplicates of plants with favorable characteristics.

Asexual reproduction in plants is divided into two general types: vegetative reproduction and agamospermy. Vegetative reproduction refers to the formation of new plants by the growth of specialized structures that can survive after physical separation from the parent. Examples include growth by above- or below-ground stems (called stolons and rhizomes), and layering, in which the stem of a woody plant forms roots upon contact with the soil. Fragments of some plants can also grow to form new individuals. Poplar trees (genus Populus ), for example, often shed branches that become rooted and produce new trees below the parent. Poplar trees can be easily propagated by simply cutting off branches and planting them directly in the ground.

Asexual reproduction by seed, called agamospermy, occurs when a single parent plant forms seeds without pollination. Agamospermy thus differs from self-pollination, in which pollen produced by a plant fertilizes its own ovules. Asexually produced seeds also differ in their development from typical, sexually produced seeds. In some plants, maternal diploid cells (which, in a normal seed, do not contribute to the new embryo) divide via mitosis and overgrow the developing ovule. The seed produced is thus genetically identical to the parent plant. A number of tropical fruit trees, such as mangos (Mangifera spp.), can reproduce in this manner.

Asexual reproduction is thought to be an important adaptation for plants that colonize open areas and harsh environments and, as such, is perhaps most common in plant species in arctic and alpine environments. The advantage may be that an asexually reproducing individual reaching a new area can always reproduce, even if no other plants of that species are present. Asexual reproduction also means that a plant's offspring will share 100 percent of its genes, while sexually produced offspring share only 50 percent of their genes with each parent. Evolutionary theorists have argued that, all other things being equal, this should act to favor asexual reproduction, since a parent thereby guarantees that all of its genes are represented in the next generation.

The main disadvantage of asexual reproduction is lack of genetic variation. For example, a disease or pest that has a large effect on one individual may be able to quickly infect all other individuals that share the same exact genetic makeup. In the long run, asexual reproduction may often be an evolutionary dead-end because plants that only reproduce asexually cannot recombine genes to produce new genetic variants.

see also Propogation; Reproduction, Sexual; Stems; Tissue Culture.

Sean C. Thomas

Bibliography

Raven, Peter. H., Ray F. Evert, and Susan E. Eichhorn. Biology of Plants, 6th ed. New York: W. H. Freeman and Co., 1999.

Richards, A. J. Plant Breeding Systems, 2nd ed. London: Chapman & Hall, 1997.