Heterosis – Wikipedia
For the homonymous article, see string theory for heterotic strings in physics.
In biology of evolution, the heterosis (old Greek heterosis , “Change”) designates the increase in capacities and or the vigor of a hybrid compared to parents, whether different species, populations, or pure lines (homozygotes). The quality of the hybrid obtained depends on the heterotic qualities joined for the selected parents.
The heterosis effect, also named Hybrid vigor , results in a performance gain (or more exactly a cancellation of the “tares” of the “pure” lines) which results from the brewing of the different alleles of the different lines.
The heterosis effect designates the particularly pronounced increase in the performance of hybrid or metis individuals. This effect is used in animal selection and plant selection. We are talking about heterosis effect when the hybrid generation F1 has performance superior to the average performance of the Parente P, homozygous or not.
En 1909, Edward Murray East (in) Discover that the crossing of two pure corn lines gives descendants with high vigor.
The term “heterosis” was invented in 1914 by scientist George Harrison Shull, who widens the theory of his rival Edward Murray East (in) , by seeing not a genetic effect, but a “physiological stimulation” due to the heterozygous state. Currently, the mechanism of “physiological stimulation” of East and Shull has never been identified. This is why the genetic explanation by cancellation of the “tares” of “pure” lines remains the most likely. The latter explains in particular why the heterosis effect is all the greater since the starting populations are genetically distant.
It was not until 1917 that a third researcher, Donald F. Jones, pupil of East, set to the point of double hybrids, the technique consists in making two floors of crossing. We start from four pure lines AA, BB, CC, dd. We first make simple “hybrids” (ordinary corn plants) AB, AC, AD, BC, BD and CD. Then we meet each of these plants two by two, for example AB and BD. We castrate the Suence Plant Ab to make sure it will be fertilized by the BD plant. We then harvest the seeds on the AB plant, that is to say on an ordinary plant, which has recovered its vigor and therefore produces seeds economically. The success of Jones in 1918 is miraculous: “Jones should have devoted an entire life to finding a combination that allied with the same perfection as his first cross between the simple burr and leaming hybrids. The coin had dropped to the edge » (Crabb 1987: 86). Without knowing it Jones combined in its double hybrids of different genes of genes – different corn populations and found an improvement technique offered in 1870 by Beal at the University of Michigan! “Hybridors” will take ten years to understand that this is what they have to do [ first ] .
The Minister of Agriculture Henry Cantwell Wallace decides in that “hybridization” will be the exclusive way to improving corn. He acts at the instigation of his son Henry Agard Wallace, coach and producer of corn seeds, future Minister of Agriculture of Roosevelt in 1933 and founder in 1926 in Pioneer, the largest “seed” company. The young Wallace is impressed by the new science of genetics and the perspectives of manipulation of the living that it opens. In 1946, he compared the power of heterosis to that of the atomic bomb.
Between 1935 and 1945, the percentage of the area strewn in hybrids in the United States went from 1 to 88%.
The term heterosis is used by supporters of hybridization as by adversaries of any hybridization:
- The first (Lyssenko, Prezent) claimed that any hybridization is necessarily positive and necessarily implies an increase in yield. This interpretation of heterosis has led to a decrease, by dissemination, of the number of pure varieties of cattle and wheat seeds as well as a decrease in Soviet agricultural yields [ 2 ] ;
- The latter, starting from a close reading of Mendel’s laws, consider heterosis as a waste of time, and recommend a continuous selection without hybridization on the grounds that, in the event of appearances of Tares in the selected population, it is always Possible to add to the selection criteria, the elimination of crazy elements.
In cereals, such as corn or rye, the heterosis effect leads to a strong increase in potential yield, provided that the real production conditions allow this potential to express themselves fully. The use of F1 hybrid in the production of corn would have contributed to multiplying the average yield by 5.7 in France between 1950 and 2000 (from 15 to 86 q/ha) [ 3 ] .
The use of hybridization in agriculture has increased considerably in recent decades, especially in vegetable plants. In 1995, more than 80% of broccoli, tomato and cabbage varieties came from older varieties hybrids. In addition to their increased performance, these hybrids have greater homogeneity than the parent strains: plants from the crossing of two pure and selected lines because of their hybrid vigor have themselves been the subject of a selection so intensive that They are now genetically identical.
Parents :
A-B-C-d-e-f a-b-c-D-E-F
A-B-C-d-e-f X a-b-c-D-E-F
phénotype :
A-B-C-d-e-f X a-b-c-D-E-F
F1 :
A-B-C-d-e-f
a-b-c-D-E-F
phénotype :
A-B-C-D-E-F
Interest of heterosis [ modifier | Modifier and code ]
The hybrid varieties valuing this heterosis effect show a stronger tolerance to stress and prove to be interesting to mitigate the effects of climate change, in some cultures [ 4 ] .
Obtaining a “pure” line [ modifier | Modifier and code ]
The heterosis effect needs to be implemented, at least two varieties, breeds or distinct and relatively distant lines. The purity of these lines makes it possible to properly master the characters of hybrid products, and to ensure their regularity.
A line is deemed “pure”, when it tends towards homozygotia: for each gene, the two alleles are identical. The advantage of a “pure” line is that his descendants are homologous and therefore predictable. Its drawback is to multiply the risks of the appearance of “tares”.
We get a “pure” line by reproducing a restricted population on itself for several generations. For animals we speak of in -consanguinity. There follows a possible genetic drift and a hybrid depression which can be lifted during a population brewing.
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