Heterosis is a term used in genetics and selective breeding. The term heterosis, also known as hybrid vigor or outbreeding enhancement, describes the increased strength of different characteristics in hybrids; the possibility to obtain a genetically superior individual by combining the virtues of its parents.

Heterosis is the opposite of inbreeding depression, which occurs with increasing homozygosity. The term often causes controversy, particularly in terms of the selective breeding of domestic animals, because it is sometimes believed that all crossbred plants or animals are genetically superior to their parents; this is not necessarily true. When a hybrid is seen to be superior to its parents, this is known as hybrid vigor. When the opposite happens, and a hybrid inherits traits from their parents that makes them unfit for survival, the result is referred to as outbreeding depression. Typical examples of this are crosses between wild and hatchery fish that have incompatible adaptations.

Genetic basis of heterosis

Two leading hypotheses explain the genetic basis for fitness advantage in heterosis.

The overdominance hypothesis implies that the combination of divergent alleles at a particular locus will result in a higher fitness in the heterozygote than in the homozygote. Take the example of parasite resistance controlled by gene A, with two alleles A and a. The heterozygous individual will then be able to express a broader array of parasite resistance alleles and thus resist a broader array of parasites. The homozygous individual, on the other hand, will only express one allele of gene A (either A or a) and therefore will not resist as many parasites as the heterozygote.

The second hypothesis involves avoidance of deleterious recessive genes (also called the general dominance hypothesis), such that heterozygous individuals will express fewer deleterious recessive alleles than its homozygous counterpart.

The two hypotheses will have different consequences on the gene expression profile of the individuals. If over-dominance is the main cause for the fitness advantages of heterosis, then there should be an over-expression of certain genes in the heterozygous offspring compared to the homozygous parents. On the other hand, if avoidance of deleterious recessive genes is the cause, then there should be fewer genes that are under-expressed in the heterozygous offspring compared to the parents. Furthermore, for any given gene, the expression should be comparable to the one observed in the best of the two parents.

Hybrid corn

Nearly all the field corn now grown in the United States and most other developed nations is hybrid corn. Modern corn hybrids substantially outyield conventional cultivars and respond better to fertilization.

Heterosis in maize was famously demonstrated in the early 20th century by George H. Shull and Edward M. East after hybrid corn was invented by Dr. William James Beal of Michigan State University based on work begun in 1879 at the urging of Charles Darwin. Dr. Beal's work led to the first published account of a field experiment demonstrating hybrid vigor in corn, by Eugene Davenport and Perry Holden, 1881. These various pioneers of botany and related fields showed that crosses of inbred lines made from a Southern dent and a Northern flint, respectively, showed substantial heterosis and outyielded conventional cultivars of that era. However, at that time such hybrids could not be economically made on a large scale for use by farmers. Donald F. Jones at the Connecticut Agricultural Experiment Station, New Haven invented the first practical method of producing a high-yielding hybrid maize in 1914-1917. Jones' method produced a double-cross hybrid, which requires two crossing steps working from four distinct original inbred lines. Later work by corn breeders produced inbred lines with sufficient vigor for practical production of a commercial hybrid in a single step, the single-cross hybrids. Single-cross hybrids are made from just two original parent inbreds. They are generally more vigorous and also more uniform than the earlier double-cross hybrids. The process of creating these hybrids often involves detasseling.

Hybrid livestock

The concept of heterosis is also applied in the production of commercial livestock. In cattle, hybrids between Black Angus and Hereford produce a hybrid known as a “Black Baldy.” In swine, “blue butts” are produced by the cross of Hampshire and Yorkshire. Other more exotic hybrids such as “beefalo” are also used for specialty markets. Within poultry, sex-linked genes have been used to create hybrids in which males and females can be sorted at one day old by color. Specific genes used for this are genes for barring and wing feather growth. Crosses of this sort create what are sold as Black Sex-links, Red Sex-links, and various other crosses that are known by trade names.

Commercial broilers are produced by crossing different strains of White Rocks and White Cornish, the Cornish providing a large frame and the Rocks providing the fast rate of gain. The hybrid vigor produced allows the production of uniform birds with a marketable carcass at 6-9 weeks of age.

Likewise, hybrids between different strains of White Leghorn are used to produce laying flocks that provide the majority white eggs for sale in the United States.

See also

• F1 hybrid

References

• HYBRID VIGOR IN PLANTS AND ITS RELATIONSHIP TO INSECT POLLINATION - a section from Insect Pollination Of Cultivated Crop Plants by S.E. McGregor, USDA
• Hybrids & Heirlooms - an article from University of Illinois Extension
• http://www.nwfsc.noaa.gov/publications/techmemos/tm30/lynch.html
• Birchler JA, Auger DL, Riddle NC. (2003) "In search of the molecular basis of heterosis". Plant Cell 15(10):2236-9 http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=pubmed&cmd=Retrieve&dopt=AbstractPlus&list_uids=14523245&query_hl=2&itool=pubmed_docsum
• Roybal, J. (July 1, 1998) “Ranchstar” beefmagazine.com, http://beefmagazine.com/mag/beef_ranchstar/
• Sex-Links http://www.feathersite.com/Poultry/CGP/Sex-links/BRKSexLink.html
• Winfridus Bakker (2006) “Enhanced Hybrid Vigor Benefits Breeder and Broiler” Cobb Focus Issue 2, 2006. http://www.cobb-vantress.com/Publications/documents/Cobb_Focus_Two_2006.pdf