In this article we will discuss about:- 1. Manifestation of Heterosis 2. Genetic Basis of Heterosis 3. Applications.
Manifestation of Heterosis:
Performance or expression of any character or trait is influenced by many genetic factors — some are positive (stimulating) and others are negative (decreasing). Expressivity of the genes or the degree of manifestation of a character is the result of genetic balance in the action of differently directed factors.
The various manifestations of heterosis may be summarised as follows:
1. Increased Yield:
Increase in yield which may be measured in terms of grain, fruit, seed, leaf, tuber or the whole plant is one of the most important manifestations of heterosis.
2. Increase in Size and General Vigour:
Heterosis results in more vigorous growth which ultimately leads to healthier and faster growing plants with increase in size than the parents.
3. Better Quality:
In many cases heterosis yields better quality which may be accompanied with higher yield.
4. Greater Adaptability:
Hybrids are generally more adapted to environmental changes than the inbred lines due to heterozygosity.
5. More Disease Resistant:
Heterosis sometimes results into development of more disease resistant character in the hybrids.
6. Increased Reproductive Ability:
Hybrids exhibit heterosis by expressing high fertility rate or reproductive ability, which is ultimately expressed in yield character.
7. Increase in Growth Rate:
In many cases the hybrids show faster growth rate than the parents, but that does not always produce larger plant size than the parents.
8. Early Flowering and Maturity:
In many cases the hybrids may show early-ness in flowering and maturity than the parents, for some crops these are the desirable characters for crop improvement. All these manifestations of heterosis can be traced at all levels of hybrid plant organisation.
Molecular Level:
Heterosis is manifested in increased rate of DNA reduplication, transcription and translation influencing the formation of genetic information, enzymatic activity, other regulatory mechanisms and also hybrid protein molecule formation.
Functional Level:
Heterosis is expressed as an effective regulation in metabolic processes and morphogenesis in hybrid organism.
Cellular Level:
Due to change in electro-kinetic properties of hybrid cell nuclei, the heterosis is manifested by increased mitosis.
Organism Level:
Heterosis is expressed as increased growth and differentiation of vegetative organs, synthesis and accumulation of nutritional substances and utilisation of metabolic process for yield formation.
Genetic Basis of Heterosis:
There are two main theories to explain the genetic cause of heterosis.
(A) Dominance Hypothesis:
This hypothesis was proposed by Davenport and further expanded by others. This hypothesis suggests that at each locus dominant allele has the favourable character, whereas the recessive allele has the unfavourable character.
When they are combined together; i.e., in heterozygous condition in the hybrids, the favourable characters get expressed whereas the unfavourable characters are masked. So the heterosis results from the masking of harmful effects of recessive alleles by their dominant alleles.
Dominance Hypothesis has Assumptions:
(a) Dominant genes are beneficial and recessive genes are deleterious.
(b) The loci show addition effects, non-allelic interactions are absent.
(c) No recombination barrier between the genes.
With the help of following example heterosis can be explained:
In a cross between Inbred A (AAbbCCdd) with Inbred B (AAbbCCdd), there will be no heterosis in F1 hybrid, there is no masking of recessive gene in hybrid. But in another cross, Inbred A (AAbbCCdd) is crossed with Inbred D (aaBBccDD), where the F1 hybrid is (AaBbCcDd) with all the genes having dominant allele.
As a result the harmful effects of a, b, c, d are hidden by the dominant alleles A, B, C and D. Thus some parents produce heterotic progeny while others do not. Generally parents of diverse or different origin are more likely to produce heterotic progeny than those of similar origin.
Objection:
1. Failure in Isolation of Inbreds as Vigorous as Hybrids:
According to dominance hypothesis it should be possible to get the inbred line with all the dominant genes. Such inbreds should be as vigorous as the F1 hybrids, but such inbreds have not been isolated.
2. Symmetrical Distribution in F2:
According to dominance hypothesis, the quantitative characters should not show symmetrical distribution as because dominant and recessive alleles should segregate in the proportion of 3: 1, but generally the F2 shows symmetrical distribution.
Above two objections can be explained by linked genes. Many of the quantitative characters are governed by linked genes together, so to get the inbred line with all dominant genes require several precisely placed crossovers. In another explanation it can be showed that if the number of genes governing the quantitative characters is large, symmetrical distribution would be obtained even without linkage.
(B) Over-dominance Hypothesis:
This hypothesis was independently proposed by East and Shull. This is sometimes known as single gene heterosis, super-dominance, cumulative action of divergent alleles and stimulation of divergent alleles. According to this hypothesis, heterozygotes are superior to both the homozygotes.
So the heterozygote Aa would be superior to both the homozygotes AA and aa. Consequently, heterozygosity is essential for the cause of heterosis. In case of maize, the gene ma affects maturity. The heterozygote Ma/ma is more vigorous with late maturity than the homozygotes Ma/Ma or ma/ma.
Another proposal by East was that there are several alleles, e.g., a1, a2, a3, a4………………….. etc. with increasingly different functions. Heterozygotes between more divergent alleles would be more heterotic than those involving less divergent genes, e.g., a1a4 is more heterotic than a1a2, a2a3, a3a4, etc. In these cases due to presence of divergent alleles the hybrids have the capacity to perform different functions which is not possible by any of the heterozygotes.
Objection:
1. There are many examples where the superiority is due to the epistatic affect of several non-allelic genes, not due to over-dominance (which is the interaction between allelic genes).
2. There is another objection against over-dominance hypothesis that there are many examples where the homozygotes are superior to the heterozygotes.
Physiological Basis of Heterosis:
Hybrid vigour, the product of heterotic mechanism, is essentially a physiological manifestation.
This better physiological efficiency of hybrids is derived chiefly from:
1. Better initial growth.
2. Greater uptake followed by better utilisation of nutrients by hybrids.
The initial growth activities include the different physiological processes during germination:
(a) Efficient water absorption,
(b) Better activity of enzymes,
(c) Rapid mobilization and utilization of stored food matter,
(d) Transformation and building up of active protoplasmic synthesis.
To explain all these processes different hypotheses have been put forwarded:
1. Initial Capital and Physiological Stimulus:
Large embryo and seed size in hybrids provide initial advantage to the hybrid during germination and early growth of seedlings. This hypothesis is debatable due to two reasons: the greater seedling vigour always not associated with maturity and also hybrid seeds are need not to be always with large size to attain hybrid vigour.
2. Balanced Metabolism and Heterosis at Molecular Level:
The hybrids are endowed with a more balanced metabolism than their inbred parents. Many of the enzymes of heterotic plants exhibit greater efficiency over those of their better parents. The hybrids show better and rapid unfolding of balanced metabolic processes.
3. Mitochondrial Complementation and Heterosis:
ATPase activity of the mixture of mitochondria from different inbred lines of maize sometimes exceed that of the mitochondria of individual lines. This heterotic effect is called as mitochondrial complementation.
The mitochondria of heterotic hybrids absorb more O2 and have high P/O index, i.e., phosphorylation/oxidation ratio than those of inbred lines and non-heterotic hybrids. This suggests that high level of oxidising phosphorylation and synthesis of high energy ATP bonds create favourable conditions for biosynthetic processes and important requisite for heterotic development.
4. Greater Ability for uptake and Utilisation of Nutrients:
Heterosis in post germination seedling growth is associated with better absorption and assimilation of several specific substances essential to the fundamental growth processes of the organism; such as nutritional factors, water absorption and other factors.
Efficient uptake and assimilation of nutrients by heterotic hybrid seedlings confer the following advantages:
1. Larger number of leaf primordia.
2. High carboxylase and photophosphorylation activity.
3. Greater leaf area and larger number of leaves.
4. More branches per panicle and more grains per branch.
5. High grain weight, etc.
Applications of Heterosis in Plant Breeding:
Heterosis is observed in almost every crop species studied, the application of this phenomenon for its commercial exploitation depends on the expression of the degree of heterosis. This phenomenon is commercially used to produce hybrid or synthetic varieties, which needs the maximisation of its expression and also fixation.
Maximisation of hybrid vigour (HF1) can be achieved by increasing either ‘d’ (directional dominance) or ‘y’ (initial differences in gene frequency between parents), i.e., choosing genetically divergent parents, HF1 = ∑dy2.
Fixation of hybrid vigour is needed for its commercial application which can be done by vegetative propagation, or by stable apomictic reproduction, or by transferring heterozygosity to polyploid or fixation by obtaining structural heterozygotes.
Fixation of heterosis in crops like potato, sweet potato, sugarcane, sugarbeet and many ornamental plants can be achieved by vegetative propagation as there seeds are not essential. Incorporation of genes conditioning vegetative apospory or diploid parthenogenesis in hybrid seed crops may lead to a permanent heterozygote advantage.
Heterozygosity can be maintained or saved from being lost due to segregation by converting diploid heterozygotes into tetraploid or hexaploid. Incorporation of gaudensvelans combination helps to survive only the heterozygotes not the homozygous combination — such mechanism may be introduced in crop plants, thereby hybrid vigour can be fixed with great success.
Heterosis or hybrid vigour have been commercially utilised in both cross pollinated and in some self-pollinated species. In most of the cases the utilisation of this heterosis phenomenon is not successful because of difficulty in production of large quantities of hybrid seeds. This is particularly difficult in self-pollinated species.
Few examples where the heterosis has been utilised for improvement of the crop plant are:
Crop Species:
Asexually propagated species and also cross pollinated species like maize, jowar, bajra, sunflower, legume, cotton, etc.
Vegetable Crops:
Tomato, brinjal, onions, cucurbits, etc.
Fruits:
In almost all the fruit trees.