Breeding is the science of methods for creating new and improving existing animal breeds, plant varieties and microbial strains.
The tasks of modern breeding:
- increasing the productivity of organisms;
- improving the quality of products (taste, appearance, chemical composition);
- improvement of economically important physiological properties (resistance to diseases and pests, responsiveness to fertilizers or feed).
A variety, breed, strain is an artificially created resistant group (population) of living organisms that has certain hereditary characteristics.
All individuals of this group have similar morphological and physiological characteristics, the same type of reaction to changes in environmental factors, and a certain level of productivity.
1. Artificial selection is used to preserve and reproduce individuals with the desired combination of traits. Distinguish between mass and individual selection.
In a mass selection, a large number of individuals with the desired trait are simultaneously selected, the rest are discarded. This is phenotypic selection; it does not produce genetically homogeneous material. It is repeated many times.
With individual selection (by genotype), one individual with the necessary characteristics is isolated and offspring are obtained from it.
2. The following hybridization methods are used in breeding work: inbreeding, outbreeding and distant hybridization.
Inbreeding is a closely related cross.
When inbreeding, descendants with parental forms or descendants of the same parents are crossed. This type of crossing is used to obtain pure lines, that is, to transfer the majority of genes to a homozygous state and to consolidate valuable traits. An undesirable consequence of closely related crossbreeding is inbred depression – a decrease in the productivity and vitality of the offspring due to the manifestation of recessive mutations.
Outbreeding is an unrelated (interbreed or intervarietal) crossing.
With unrelated crossing, the effect of heterosis (hybrid strength) can be observed – an increase in the viability and productivity of hybrids in comparison with the parental forms. Heterosis manifests itself in first-generation hybrids and is caused by the transition of most genes to a heterozygous state. In this case, unwanted recessive mutations become hidden. With sexual reproduction in subsequent generations, the degree of heterozygosity decreases and the effect of hybrid strength disappears. It can only persist during vegetative propagation.
Distant hybridization is the crossing of organisms belonging to different species and genera.
It is carried out with difficulty, and the resulting hybrids are sterile due to the difficulty of conjugation of chromosomes of different species in prophase I of meiosis. Methods for overcoming infertility have been developed.
3. Artificial (induced) mutagenesis is used to increase the diversity of the starting material. Mutagenesis is caused by the action of mutagenic factors, for example, X-ray radiation. Mutations are undirected, so the breeder selects organisms with new beneficial properties.
A genomic mutation is polyploidy, i.e., a multiple increase in the number of chromosome sets. Used in plant breeding. Polyploidy avoids the infertility of interspecific hybrids. In addition, many polyploid forms of cultivated plants (wheat, potatoes, vegetables) have higher yields than related diploid species.
Artificially polyploidy is caused by treating plants with colchicine. Colchicine destroys the spindle filaments and prevents the separation of homologous chromosomes during meiosis.
Works by N.I. Vavilov
For successful breeding work, you first need a variety of source material.
The search for source material is facilitated by the law of homologous series of hereditary variation, discovered by N.I. Vavilov.
Related genera and species of living organisms are characterized by similar series of hereditary variability.
If the forms of variation of one species are known, then it can be assumed that similar forms will exist in other closely related species.
NI Vavilov also established seven centers of origin of cultivated plants and founded a world collection of seeds of cultivated plants and their wild relatives.