Are you familiar with such a concept as the effect of heterosis? In our article, we will consider its causes, areas of application and significance for humans.
Hybridization systems
In order to improve existing and develop new varieties of plants, animal breeds, and human bacterial strains, there are two main selection methods. This is artificial selection and hybridization. First, he selects from the source material individuals with the most valuable properties. These may be indicators of egg laying of chickens, the amount of starch in the seeds, muscle mass of cattle, and the size of the fruit. To increase the efficiency, the genotype and condition of the alleles of the initial forms are taken into account.
The result largely depends on the system of crosses - hybridization. The essence of this method is to combine the genetic material of germ or somatic cells.
Crossbreeding
There are several types of hybridization. Crossbreeding, or inbreeding, is the result of combining the genetic apparatus of individuals of one species. It is possible among self-pollinated plants and hermaphroditic animals capable of self-fertilization.
What are the consequences of this hybridization? In each subsequent generation, the number of homozygous individuals increases. Their phenotypic characters gradually weaken, which leads to degeneration of the line.
Remote hybridization
Do you know who the original forms of the mule are? This is a donkey and a mare. And if you combine the genotypes of one and two-humped camels, you will get a much stronger and hardier hybrid. These are all examples of distant hybridization.
With this crossbreeding system, there is only one problem. Interspecific hybrids are barren. This can be corrected only by a multiple increase in the chromosome set.
Unrelated Crossing
The phenomenon of heterosis is associated with the hybridization of different varieties, breeds and lines that belong to the same species. Such a cross is called unrelated, or outbreeding.
It has the opposite effect. With each subsequent generation, homozygosity increases. At the same time, mutations with negative consequences do not occur. They become heterozygous and do not affect the viability of species. Therefore, this type of hybridization is the basis of selection.
Heterosis phenomenon
Scientists have long noticed that combining the genotypes of unrelated individuals of the same species, the resulting hybrids are characterized by increased viability. They are more prolific, resistant to the action of pathogens. This phenomenon is called heterosis. This is the result of unrelated crosses. Translated from Greek, this term means "reincarnation."
The effect of heterosis is due to the fact that lethal and sublethal alleles can go into a heterozygous state. Due to this, their negative consequences do not appear externally. Moreover, the genotype of first-generation hybrids often contains dominant alleles of favorable characters.
Heterosis effect
Hybrid strength is the opposite of inbreeding, or closely related mating, which results in phenotypic depression of individuals.
Heterosis is manifested not only in the form of increasing heterozygosity. In this state, several types of enzymes are formed. Their combined effect also increases the hybrid strength of the descendants.
Heterosis was first discovered back in the 19th century by the German botanist Kelreiter. He owns the first experiments on artificial hybridization of plants. And this happened even before Gregory Mendel formulated his famous laws on hereditary variability. At the beginning of the 20th century, corn heterosis was described in detail. The yield of its hybrid samples was many times higher than that of ordinary forms.
Scientists distinguish several forms of heterosis. But one way or another, they all manifest themselves in the form of a hybrid force. With the reproductive form, an increase in fertility and productivity is observed, therefore, it is inherent in plants. Hybrid power is also manifested in the ability of organisms to withstand adverse environmental conditions. This form is called adaptive, or adaptive. Somatic heterosis leads to an increase in the linear dimensions and mass of unrelated hybrids obtained in the first generation.
Application area
Since heterosis is a phenomenon in which the productive qualities of plants and animals increase significantly, it is widely used in agriculture.
In what areas is its practical application possible? First of all, in poultry farming. The most effective are two lines. This is the breeding of egg and broiler breeds. Heterosis is used in such areas of animal husbandry as fishing, sheep breeding, beef cattle breeding, camel breeding.
Scientists have found that hybrid power can be a consequence of interspecific crossbreeding. Mules are much more resilient and stronger compared to their parental forms. At the beginning of the last century, the famous Santa Gertrude meat breed was bred in the United States. This is a hybrid of a wild zebu bull with shorthorns - meat cows. The result was simply amazing. The weight of adults of the obtained hybrids is close to 700 kg.
Another example of the manifestation of hybrid strength is the sheep breed archaromerinos. It was bred in Kazakhstan in the middle of the 20th century as a result of crossing thin-wooled sheep with mountain sheep argali. The obtained hybrids acquired valuable characteristics in several directions at once: cutting wool, early maturity, fertility, slaughter yield, adaptability to mountain conditions, strong immunity.
Heterosis is a method that is widely used in crop production. This technique allows you to increase the yield of individual varieties up to 30%. Important aspects are obtaining hybrid seeds. Heterosis in this case makes this process economically viable.
A promising direction of selection is to consolidate the effect of heterosis in the genotype of plants capable of vegetative propagation. Most often, fruit and berry and tuber crops, which are obtained from hybrid seeds, are used for this.
The leading place among heterotic crops is still occupied by corn. Hybrid varieties of this plant were developed, which are distinguished by drought tolerance, high immunity, and good indicators of the duration of preservation of seeds. Hybrids of sugar beets, sorghum, and wheat were also widely distributed.
Significant flaws
The phenomenon of heterosis depends on the degree of kinship between the parental individuals: the more distant relatives the parental individuals are, the more pronounced the effect of heterosis in first-generation hybrids.
What is the main problem with hybrid power? Its manifestation is observed only in the first generation. In the second, it weakens, and in the subsequent, it completely disappears. Currently, breeders are trying to find ways out of this situation. One of them is variable crossbreeding. Its scope is livestock farming. During this process, hybrids of two different breeds are first obtained. Then the resulting individual also crosses. But at the same time, an individual of a completely different breed is used.
So, the effect of heterosis is due to an increase in the heterozygosity of organisms and the transition of lethal alleles to a recessive state. Phenotypically, its effect is manifested in an increase in the quantitative and qualitative indicators of first-generation hybrids in comparison with parental forms.