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Heterosis

Many of the portrayed examples show that a gene can exert its full power already in a heterozygous state. We have also seen that there are often several genes involved in the expression of a single feature. We will meet this phenomenon frequently, when we deal with a complex character, such as the colour of a flower or the 'yield'. As a rule, can transitions between the single phenotypes of such features fluid be found. E. BAUR, for example, characterized in long years of research into Antirrhinum majus more than 100 genes that participated in the colour moulding of more than 1000 different (exactly analyzed) varieties up until 1919. The relatively small amount of roughly 20 genes is enough for the whole range of colours that is displayed by the flowers of snapdragons. As a result of all possible crossings can a continuous row of colours be drawn up. With 20 genes namely are 2²⁰ = 1,048,576 different combinations (phenotypes) possible. The visible continuity is thus based on discrete unities (genes). This dual principle is widespread in nature. Just think of the well-known chemical fact that every substance is composed of simple particles (atoms or molecules). Continuity can always be observed, when the underlying units are below the resolution limit of the observer (human, animal or technical device). In summary, E. BAUR wrote:

" Similar to a chemist, who can produce a huge number of compounds from a few basic substances, am I, due to the exact knowledge of the fundamental differences of Antirrhinum majus able to produce every wanted variety, i.e. a certain combination of basic differences with the help of a small set of plants, whose formula I know exactly."

This statement is a program for all plant cultivators, because the exact knowledge of the basic material proves to be the decisive prerequisite for the selection of varieties with high yields.

Although the principal proceeding is founded on seemingly simple ideas, the isolation and manifestation of pure lines especially those of cross-fertilizers is enormously difficult. It is, for example, estimated that roughly 30 growth-promoting genes exist in corn. The probability to find a plant, in which they all exist in a homozygous state (AA BB CC DD EE FF.....) is 1:4³⁰ (the space needed for such an experiment would make up 2000 times the earth's surface). How can a cultivator solve this dilemma?

The magic word is heterosis (G. SHULL, 1909). Heterosis is based on the observation that the F₁ generation of two varieties with strong yields has an even stronger yield than each of the parental varieties. The average yield is also always higher than that of the following generations (F₂, F₃,.....F_n).

The aim of cultivation can therefore not only be in the production of pure lines. It is much more important to select such plants that display the most favourable characters under controlled conditions. This is the idea behind the yearly production of new hybrids that serve to feed man or animals. Contrary to traditional practices this results in the separation of the production of seed and nutrition. Today, nearly the whole breeding of corn in the United States (and also in other countries) is founded on the heterosis principle. The scientific basis for the economic success was founded by the American geneticists and plant breeders D. F. JONES, G. H. SHULL, P. C. MANGELSDORF and L. J. STADLER during the first half of the 20th century. A simple example will serve to elucidate the effect:

The height of pea plants is determined by the number of internodes in the stem and the average length of the internodes. The gene that causes the number of internodes shall be called Z that for the internode length L. The respective dominant alleles do consequently produce many long internodes. The crossing of two varieties with average height, of which many individuals have short and the few others have long internodes would have to result in a F₁ with increased height. On the assumption that the dominant allele Z causes 24 and the respective recessive allele (z) 12 internodes and the length of the average internode would be 6 cm with L and 3 with l, the crossing would have the following result:

The F₂ has thus an average length of 110.25 cm. The situation is even more disadvantageous in the F₃ generation, the average length decreases to 95.06 cm. The example shows that the achievements of following generations decline drastically. The effect is based on the continual decrease of heterozygous forms and a simultaneous increase of homozygous ones. This does not mean that only dominant alleles concentrate within the respective plants. Much more so do combinations like AA bb CC dd ee ff GG or aa bb CC DD EE ff gg or AA BB cc dd EE FF GG occur. In each of these combinations is just a part of the genes in a homozygously dominant condition, which results in rather unfavourable phenomenons. The achievement of the heterozygous Aa Bb Cc Dd Ee Ff Gg is only attained by few plants and these again are not easily spotted in the mass of the other genotypes.

There are some wild types that exist only as heterozygous forms. The classic example are Oenothera lamarckiana (and some other Oenothera-species as well) of the evening primrose family, clarkias and fuchsias (Oenagraceae). The genetic analysis shows that their genome is organized into two complexes, that were termed gaudens and velans by O. RENNER in 1924 (Universität Jena). The gaudens complex induces the formation of green buds, broad leaves and red spots on the rosette-forming leaves. The velans complex results in red-striped buds, slim leaves and green rosette-forming leaves.

These features cannot be observed at all in Oenothera lamarckiana plants and their intraspecific offspring. They become visible only after crossing with related species, like Oenothera muricata. This is, because Oenothera lamarckiana is actually a gaudens-volens hybrid. Due to a balanced lethal system, only hybrids survive. The situation is similar in Oenothera muricata: the species is characterized as a rigens-curvans hybrid. The rigens complex results in defect pollen (and fully functional egg cells), the curvans complex in an embryo sac that has lost its function. Therefore, only the combination rigens (female) x curvans (male) can survive. An interpretation of the balanced lethal system was given in 1949 after analysis of the chromosome constitution (the karyotype) of the respective complex (complex heterozygosity).