Hybrid vigor

Hybrid vigor may be defined as the increase in size, vigor, fertility, and overall productivity of a hybrid plant over the mid-parent value. It is calculated as the difference between the crossbred and inbred means:

The synonymous term, heterosis, was coined by G.H. Shull. It should be pointed out immediately that, as it stands, heterosis is of no value to the breeder, if a hybrid will only exceed the mid-parent in performance. Such advantageous hybrid vigor is observed more frequently when breeders cross parents that are genetically

diverse. The practical definition of heterosis is hybrid vigor that greatly exceeds the better or higher parent in a cross. Heterosis occurs when two inbred lines of outbred species are crossed, as much as when crosses are made between pure lines of inbreeders.

          Heterosis, though widespread in the plant kingdom, is not uniformly manifested in all species and for all traits. It is manifested at a higher intensity in traits that have fitness value, and also more frequentlyamong cross-pollinated species than self-pollinated species. All breeding methods that are preceded by crossing make use of heterosis to some extent. However, it is only in hybrid cultivar breeding and the breeding of clones in which the breeder has opportunity to exploit the phenomenon to full advantage.

Hybrids dramatically increase yields of non-hybrid cultivars. By the early 1930s, maize yield in the United States averaged 1250 kg/ha. By the early 1970s, maize yields had quadrupled to 4850 kg/ha. The contribution of hybrids  to this increase was estimated at about 60%.

Inbreeding depression

Heterosis is opposite to inbreeding depression.In theory, the heterosis observed on crossing is expected to be equal to the depression upon inbreeding, considering a large number of crosses between lines derived from a single base population. In practice, plant breeders are interested in heterosis expressed by specific crosses between selected parents, or between populations that have no known common origin. Reduction in fitness is usually manifested as a reduction in vigor, fertility, and productivity. The effect of inbreeding is more severe in the early generations.Just like heterosis, inbreeding depression is not uniformly manifested in plants. Plants including onions, sunflower, cucurbits, and rye are more tolerant of inbreeding with minimal consequences of inbreeding depression. On the other hand plants such as alfalfa and carrot are highly intolerant of inbreeding.

Genetic basis of heterosis

Two schools of thought have been advanced to explain the genetic basis for why fitness lost on inbreeding tends to be restored upon crossing. The two most commonly known are the dominance theory, first proposed by C.G. Davenport in 1908 and later by I.M. Lerner, and the overdominance theory, first proposed by Shull in 1908 and later by K. Mather and J.L. Jinks. A third theory, the mechanism

of epistasis,has also been proposed. A viable theory should account for both inbreeding depression in cross-pollinated species upon selfing and increased vigor in F1, upon hybridization.