1.1 Meaning of Heterosis:
When two homozygous individuals inbreeds (a true breeding line obtained by continuous inbreeding) genetically unlike constituents are crossed together, the resulting hybrids obtained from the crossed seeds are usually robust, vigorous, productive and taller than the either parents.
This increased productivity or superiority over the parents is known as heterosis or hybrid vigor.
Heterosis can be defend as the superiority of F hybrid over both the parents in terms of yield and some other character.
Heterosis is a multigenic complex trait and can be extrapolated as the sum total of many physiological and phenotypic traits.
1.2 History of Heterosis:
Heterosis has been known since the art of hybridization came into existence.
- Koelreuter (1763) was the first to report hybrid vigour in the hybrids of tobacco, Datura etc.
- Mendel (1865) observed this in pea crosses.
- Darwin (1876) also reported that inbreeding in plants results in deterioration of vigour and the crossing in hybrid vigour.
- On the basis of his experiments Beal (1877-1882) concluded that F hybrids yield as much as 40 percent more of the parental varieties. From subsequent studies on inter-varietal crosses in maize, it was observed that some of the hybrids show heterosis.
- Dr. G.H. ShuII (1914) proposed the term heterosis (Gr. heteros different and osis = condition).
- Poweri (1944, 45) reported that the crossing, however, may result in either weak or vigorous hybrids as compared to parental inbreeds.
Hybrid vigour is used as synonym of heterosis. It is generally agreed that hybrid vigour describes only superiority of the hybrid over the parents while heterosis describes the other situation as well i.e., crossing over may result in weak hybrids e.g., many hybrids in tomato are earlier (vegetative phase is replaced by reproductive phase).
Earliness in many crops is agriculturally desirable so, it is argued that F( hybrid vigour), shows faster development in which vegetative phase is replaced by the reproductive phase more quickly than in the parents. On the basis of this explanation it was justified to use the term hybrid vigour as synonym of heterosis.
- However, Whaley (1944) has the opinion that it would be more appropriate to term the developed superiority of the hybrids as hybrid vigour and to refer to the mechanism by which the superiority is developed as heterosis.
- Smith (1955) suggested that the use of heterosis and hybrid vigour as synonyms is highly desirable on the basis of their long usage.
1.3 Types of Heterosis:
Heterosis is of two types:
- True heterosis (euheterosis)
It is inherited i.e, from parents to offsprings during reproduction. It can be further divided into two types:
(a) Mutational true Heterosis: It is the sheltering or shadowing of the deleterious, un-favourable, often lethal, recessive mutant genes by their adaptively superior dominant alleles.
(b) Balanced true heterosis: It arises out of balanced gene combinations with better adaptive value and agricultural usefulness.
Crossing of the two parental forms brings in an accidental, excessive and un-adaptable expression of temporary vigour and vegetative overgrowth. It is also called luxuriance.
1.4 Causes of Heterosis:
The phenomenon of heterosis can be explained on the basis of the causes:
- Genetic causes
- Physiological causes
There are two possible causes of heterosis,
- Dominance hypothesis:
This theory was proposed by Davenport (1910), Bruce (1910) and Keable and Pellew (1910).This theory is based on the assumption that hybrid vigour results from bringing together female dominant genes. According to this theory, genes that are favourable for vigour and growth are dominant, and genes that are harmful to the individual are recessive. The dominant genes contributed by one parent may complement the dominant genes contributed by the other parent, so that F will have the more favourable combination of dominant genes, than either parent e.g., Dominant genes ABCD are favourable for good yield. Inbred A has the genotype AA BB cc dd (AB dominant) and inbred B has the genotype aa bb CC DD (CD dominant).
The genotype of the F hybrid is described assince the F hybrid contains dominant genes at all the loci represented here (ABCD) and exhibits more vigour than either of the parent inbred lines.
- Over dominance hypothesis:
This hypothesis was given by Shull (1903) and East (1908) independently. According to the supposition hybrid vigour on the basis of heteozygosity is superior to homozygosity. According to this hypothesis there are contrasting alleles for example a1 and a2 , for a single locus. Each allele produces favourable yet different effects in the plant. In a heterozygous plant (a1 , a2 ) a combination of the effects is produced which is more favourable in the plant than the effect produced by either of the alleles alone. This phenomenon of heterozygote(a1 a2 ) being superior to the homozygotes (a a1 or a2 a2 ) is termed over dominance. Various names have been given to this idea e.g., super dominance (Fisher 1930), interaction of alleles at a single locus (East, 1930) over-dominance (Hull, 1945) etc., but the term over-dominance is widely accepted.
- Greater initial capital hypothesis:
This hypothesis was proposed by Ashby (1930). He studied the physiology of inbreeds and hybrids of maize and tomato and concluded that hybrid vigour is due to an increased initial embryo size. He termed it as ‘Greater initial capital.’
- Cytoplasmic-nuclear interactions:
Michelis, Shull, Lewis, and others suggested that hybrid vigour is the interaction of cytoplasmic and nuclear systems. Cytoplasm is a transparent fluid rich in RNA and mitochondria, which is usually transmitted through the female parent to the offspring.
1.5 Effects of Heterosis:
Whatever may be the cause (genetical or physiological), heterosis is a well-known phenomenon. It is basically the result of the increased metabolic activity of the heterozygote. Its effects are well established or manifested in the following tree ways:
- Increase in size and genetic vigour: Hybrids are generally more vigorous i;e larger, healthier and faster growing than the parents e.g., head size in cabbage ,jowar, cob size in maize, fruit size in tomato etc.
- Increase in yield: Yield may be measured in terms of grain, fruit, seed, leaf tuber or the whole plant. Hybrids usually have increased yield.
- Better quality: Hybrids show improved quality e.g., hybrids in onion show better keeping quality.
- Greater resistance to diseases and pests: Some hybrids show greater resistance to insects or diseases than parents.
- Greater flowering and maturity: Earliness is highly desirable in vegetables In many cases, hybrids are earlier in flowering and maturity than the parents, e.g. tomato hybrids are earlier than their parents.
- Greater Adaptability: Hybrids are usually less susceptible to adverse environmental conditions.
Hybrids exhibiting heterosis show an increase in biological efficiency i.e., an increase in fertility (reproduction ability) and survival ability.
Heterosis in animals:
- Mule is a hybrid from a cross between Jack (Equus hemicus) and Mare (Earns equus) which has been known since ancient times for its well-known qualities of strength and stubbornness.
- Cross between red Sindhi breed of Indian Cattle and Jersey breed of America contams 30% more butter fat in milk.
- Increased pork yield in pigs, more egg laying hens, silk production in silk worms etc.
1.6 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:
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.
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.
In many cases heterosis yields better quality which may be accompanied with higher yield.
Hybrids are generally more adapted to environmental changes than the inbred lines due to heterozygosity.
More Disease Resistant:
Heterosis sometimes results into development of more disease resistant character in the hybrids.
Increased Reproductive Ability:
Hybrids exhibit heterosis by expressing high fertility rate or reproductive ability, which is ultimately expressed in yield character.
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.
Early Flowering and Maturity:
In many cases the hybrids may show early-ness in owering 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.
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.
Heterosis is expressed as an effective regulation in metabolic processes and morphogenesis in hybrid organism.
Due to change in electrokinetic properties of hybrid cell nuclei, the heterosis is manifested by increased mitosis.
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.
1.7 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:
- Dominant genes are beneficial and recessive genes are deleterious.
- The loci show addition effects, non-allelic interactions are absent.
- 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 F 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 F 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.
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 F hybrids, but such inbreds have not been isolated.
Symmetrical Distribution in F :
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 F 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, superdominance, 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., a1 a4 is more heterotic than a1 a2 , a2 a3 , a3 a4 , 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.
- 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).
- 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:
- Better initial growth.
- 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:
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.
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.
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 O 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 oxidizing phosphorylation and synthesis of high energy ATP bonds create favourable conditions for biosynthetic processes and important requisite for heterotic development.
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:
- Larger number of leaf primordia.
- High carboxylase and Photophosphorylation activity.
- Greater leaf area and larger number of leaves.
- More branches per panicle and more grains per branch.
- High grain weight, etc.
1.8 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 (HF ) can be achieved by increasing either ‘d’ (directional dominance) or ‘y’ (initial differences in gene frequency between parents), i.e., choosing genetically divergent parents, HF = Σdy . 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 fxation 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 utilized 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.
1.9 Heterosis in Dairy Production
In addition to the positive effects of individual genes from the breeds used in crossbreeding, dairy crossbreeding producers can expect a boost in performance due to heterosis. Heterosis is a term used in genetics and breeding and is also known as hybrid vigor.
Heterosis is an additional gain above the average genetic level of the two parent breeds. The bonus from heterosis should be approximately 5% for production and 10% for mortality, fertility, health and survival. Therefore the impact of heterosis on profit should be substantial for commercial dairy producers.
As relationships between individuals rise, it becomes more and more likely that bulls and cows that are mated to each other will be closely related. Unfortunately the use of genomically selected sires appears to be increasing inbreeding in some dairy breeds. Inbreeding robs dairy producers of income by increasing stillbirths, meanwhile reducing cow fertility, disease resistance and shortening herd life. As highly inbred embryos are not viable, this leads to a reduction in cow fertility.
Crossbreeding eliminates concerns about inbreeding. When breeds for a crossbreeding scheme are chosen well, the effects of heterosis are the opposite of the effects of inbreeding depression. Heterosis will be an additional benefit on top of the parent (breed) advantage and is especially valuable for traits such as vitality, fertility, disease resistance and health. Effective crossbreeding begins with two superior breeds. These breeds must two breeds with desirable genes is called complementarity.