4.1.2 Periodic selection

Periodic selection provides for alternation in crossing of the material selected with the inbred material and selection in the intermediate period. The selection is carried out on the basis of estimation of species selected by the characters which are of interest.

4.1.3 Convergent improvement

This is a method suggested for corn selection. The theoretical basis of the method involves the fact that heterosis is considered as the result of a favourable effect of dominant linkage factors. By this method, using the back-cross of common hybrid A × B for the parental lines, new self-pollinated lines A' and B' are obtained, each of them carrying a number of genes of the second parental line. In this way the accumulation of favourable dominant genes in new parental lines A' and B' is ensured.

4.1.4 Cumulative selection

Cumulative selection is also based on the theory of dominance. This method is used to accumulate favourable dominant genes by crossing the lines which have excellent combining ability, and selecting the best combinations.

4.1.5 Gamete selection

Theoretically this is related to the phenomenon of superdominance. This method provides for improvement of the selected self-pollinated line on the basis of recombination, achieved by random selection of gametes of the strain. In order to obtain a new improved line, the line selected is crossed with a definite strain or hybrid, assuming that the offspring will differ from the initial line only by such properties (or gametes) which were introduced by the strains.

4.2 Retention of heterosis

The problem of hybrid vigour retention, i.e. constant development of heterosis is a sequence of generations, has several theoretical approaches, some of which are applied in practice in plant growing. However, in animal breeding, the problem of retention of hybrid vigour remains to a considerable degree unsolved.

The theoretical basis of heterosis retention is accumulation in a population of balanced, closely-linked gene blocks, the combination of which in the heterozygote leads to heterosis. In other words, it is necessary to avoid segregation of the valuable genotype obtained as a result of crossing.

One of the approved methods of retention of heterosis is asexual reproduction of hybrids. This method of retention of hybrid vigour is widely used in selection of fruit plants.

Another form of asexual reproduction in plants is apomixis. In some cases of apomixis the seeds develop from diploid cells, both stages of meiosis being omitted, and the generation of these seeds is genotypically identical to the mother plant. Retention of heterosis through apomixis is used in growing blackberry, roses, meadow grass, citrus and many other plants.

Retention of hereditary heterosis may occur as a result of combination of lethal genes and certain chromosomic arrangements. In a specimen, heterozygous by inversion, the crossing-over inside of an inverted section leads to development of sterile gametes. As the crossovers obtained inside of an inverted section are suppressed due to sterility of crossover gametes, the heterozygosis of the inverted section becomes permanent. This type of balanced lethality is observed in hymenopteran insects.

Retention of hybrid vigour sometimes is obtained with the aid of polyploidy. The practice of plant culture shows that increased vigour of interspecific hybrids is observed, not only in diploid hybrids which can be sterile, but also in amphidiploid forms, in which the chromosomic combination is doubled. Interspecific hybrids with a double combination of chromosomes are very often fertile, and their descendants continue manifesting heterosis developed as a result of the genotypes from two species.

Especially of interest in fisheries is a very promising method of retention of heterosis with the help of gynogenesis. It has been proved that in some cases the gynogenetic development of the zygote in fishes results in the emergence of clones, i.e. forms genotypically absolutely identical to the mother organism. In other words, meiotic segregation is absent in this case. Thus, development of gynogenesis in fishes on a commercial scale might facilitate the solution of the problem of retention of heterosis. [1,9]