4.1 Methods of inducing heterosis

When two hybrid lines of corn are crossed, very often hybrids F1 produce twice as much seed yield as the parents. The use of hybrid seed is now the main method of growing corn for grain and for silage. In order to produce hybrid seed, inbred 1 lines of good varieties are obtained which meet the requirements of the given climatic region (an inbred line is created for 5–6 years by means of random pollination). In selection of lines, their qualities are estimated in connection with the properties which should be obtained in a future hybrid organism. Inbreeding cannot be effective if not accompanied by selection. Having created a great number of lines, crossing is begun. Interlinear hybrids of the first generation are estimated by a heterosis effect; proceeding from this, the lines of better combining ability 2 are selected and then reproduced at a greater rate for production of hybrid seed. At selection stations, work on production of inbred lines and estimation of their combining ability is carried out continuously. The more valuable lines are created as soon as it is possible to select better hybrid matching with necessary combination of characters. It has already been said that at present double hybrids of corn are used. The process of matching common hybrids for the purpose of obtaining the most productive double hybrids is a very important stage in the process of selection. The best results are achieved in crossing the lines originating from different varieties or strains. Corn is an excellent example to show that, for the successful development of hybrid seed growing, it is first necessary to investigate how long inbreeding should be carried on in order to achieve homozygosity by the group of genes which are of interest to us; and second, to develop methods for rapid estimation of their combining ability.

The above facts concerning corn are likely to be true; to a certain extent, in obtaining hybrids of other cross-pollinating plants and animals as well. Crossing of inbred lines from one or various breeds has been widely used now in the field of poultry and pig breeding. It is necessary to note that wide scale utilization of hybrids in cattle breeding is possible only at the highest level of pure-strain stock farming where there is an availability of valuable breeds.

It is clear that in most cases the inbred lines will always have lower indexes than the strains. Heterosis is evident only when the interlinear hybrid exceeds not only its parent lines but also the varieties or breeds from which these lines generate.

1 Genetic nature of inbreeding in this case is the process of segregation of population in line with different genotypes.

2 Combining ability of a line or a species is heterosis development in hybrids obtained from their crossing.

4.1.1 Combining ability

In crossing aimed at obtaining heterosis effect particular attention should be paid to combining capacity, distinguishing between general and specific combining ability. The first is characterized by an average amount of heterosis, observed in all hybrid combinations; the second, by deviation from this amount in one or another separate combination. For determination of general and specific combining ability, the form under test and the corresponding analyses (tester) are crossed. Moreover, for determination of general combining ability it is better to use analyses with a wide genetic basis (random pollination grade or corresponding animal population). The specific combining ability of the form being tested is estimated in relation to any form with which it is to be crossed later. This second form is an analysis. Choice of analyses depends on the purpose of the lines tested, i.e. whether they will be used to substitute the line in the existing hybrid combinations, or for the production of new hybrids.

The analysis for determination of combining ability of the material being selected is used for a system of crossings. For this purpose various systems of crossing are used, which are in fact various methods of determination of combining ability.

In plant growing, the following four methods are used: diallelic crossing, topcross, poly-cross, and random pollination. Of these methods only top-cross can be used in cattle breeding.

It is evident that the combining ability (general and specific) can be improved as a result of selection of recombinant forms having new combinations of genes. In this way it is possible to obtain new genotypes of a higher combining ability. Hybridization and selection are the means with which favourable complexes of hereditary factors are concentrated in a population. Effectiveness of the above means, in practice, that selection depends to a great extent on the methods applied for detection of genotypes which differ in combining ability. The complete programme of selection for determination of combining ability, the purpose of which is to obtain new components for crossing or for improving the existing lines, must provide for certain alternations of ways to produce new genetic combinations, their breeding estimation for combining ability, and selections of the best genotypes.

Depending on the direction of selective breeding of new lines, improvement of existing lines, or selection for determination of general or specific combining capacity, there are various selection programmes in existence. Various kinds of periodic selection are used, as well as convergent improvement, cumulative selection and gamete selection.