In this article we will discuss about:- 1. Meaning and Features of Hetrosis Breeding 2. Fixation of Heterosis 3. Factors Affecting 4. Genetic Basis 5. Estimation 6. Inbreeding and Heterosis.
Meaning and Features of Hetrosis Breeding:
Heterosis refers to the superiority of F1 hybrids in one or more characters over its parents. The term hybrid vigour is used as synonym for heterosis. Heterosis differs from luxuriance. The former refers to increase of F1 over parents in general vigour, yield and adaptation, whereas latter refers to increase of F1 over parents in vegetative growth but not in yield and adaptation. The term heterosis was first used by Shull in 1914.
Some important features of heterosis are given below:
1. Superiority Over Parents:
Heterosis leads to superiority in adaptation, yield, quality, disease resistance, maturity and general vigour over its parents. Generally, positive heterosis is considered as desirable. But in some cases negative heterosis is also desirable.
For example, negative heterosis for plant height, maturity duration and toxic substances is desirable in many cases because it shows superiority over the parents. For yield, heterosis of 40% and above over the better parent is considered significant from practical point of view in most of the crop plants.
2. Confined to F1:
Heterosis is confined only to the F1 generation of a cross. It declines and disappears in F1 and subsequent generations of a cross as a consequence of segregation and re-combinations. Thus heterosis is related to F1 generation only.
3. Genetic Control:
The expression of heterosis is governed by nuclear genes. In some cases, heterosis results due to interaction between nuclear genes and cytoplasm.
4. Reproducible:
Heterosis once identified can be easily reproduced in a definite environment. However, the manifestation of heterosis is more pronounced in the area of adaptation of a hybrid.
5. Association with SCA:
Heterosis has positive association with specific combining ability (SCA) variance. The SCA is a measure of dominance variance and existence of a significant amount of dominance variance is essential for undertaking heterosis breeding programme.
6. Effect of Heterozygosity:
The magnitude of heterosis is associated with heterozygosity, because the dominance variance is associated with heterozygosity. The dominance effects are expected to be maximum in cross pollinated species and minimum in self-pollinated species. For this reason, occurrence of heterosis is more in cross pollinated crops than in self-pollinated ones.
7. Conceals Recessive Genes:
In case of heterosis, deleterious recessive genes are covered by the favourable effect of dominant genes. Thus, the recessive mutant genes are concealed in heterozygous condition.
8. Low Frequency:
The frequency of desirable heterotic combinations is very low. After screening thousands of F1 crosses only few desirable heterotic combinations are identified. All the F1 crosses do not exhibit desirable heterosis.
Fixation of Heterosis:
There are four principal ways of fixation of heterosis in crop plants.
These are:
(1) Asexual reproduction,
(2) Apomixis,
(3) Balanced lethal system, and
(4) Polyploidy.
These are briefly discussed below:
(1) Asexual Reproduction:
Heterosis can be easily conserved in vegetatively propagated corps such as sugarcane, potato, sweet potato, banana etc. Most of the currently cultivated varieties of sugarcane, potato and banana are hybrids.
(2) Apomixis:
Apomixis is another important method of fixing heterosis. In apomixis, the seed develops without fertilization. Apomitic seeds generally develop from maternal diploid cells. Thus the apomictic progeny is identical to the mother plant.
The fixation of heterosis by apomixis is common in citrus fruits, blackberries, roses, bluegrasses and many other flowering plants. Apomixis is also common in Hieraciwn (Hawck weed). As a result of apomixis, Mendel could not confirm his findings of garden peas on Hieracium. In Hieracium, the and later generation progeny were similar to their F1 hybrid.
(3) Balanced Lethal System:
Balanced lethal system also leads to fixation of heterosis in some plants. For example, in many species of evening primrose (Oenothera spp.) genetic segregation is almost completely suppressed by a balanced lethal system. The homozygotes are lethal, hence they die. Only heterozygotes survive. This results in the fixation of heterosis in Oenothera. In Oenothera, this balance lethal system has developed due to complex translocations.
(4) Polyploidy:
Heterosis can also be fixed by chromosome doubling or polyploidy especially in interspecific and intergeneric hybrids. For example, the heterosis in wheat-rye cross can be conserved in amphidiploid hybrids through chromosome doubling.
The F1 of wheat-rye cross is sterile, which becomes fertile after doubling of chromosomes through colchicine treatment. The doubled species hybrids are often fully fertile and their progeny exhibit heterosis due to combination of genes from two parent species. In cultivated species, about 50% are amphidiploids.
Factors Affecting Heterosis:
There are four main genetic factors which affect magnitude of heterosis in crop plants. These are mode of pollination, genetic diversity of parents, their genetic base and adaptability.
A brief description of these factors is given below:
1. Mode of Pollination:
The magnitude of heterosis differs depending upon the mode of pollination of a species. The level of heterosis is generally higher in cross pollinated species than in self-pollinated species.
2. Genetic Diversity of Parents:
The expression of heterosis is influenced by genetic diversity of parents. For example, in wheat higher heterosis is associated with crosses of more distantly related parents. In fescue, heterosis increased with genetic divergence in morphological characters and flowering time, and also with respect to geographical origin of parents.
In alfalfa and cotton, greater heterosis was associated with greater parental diversity. In maize, the level of heterosis increased with the increase in parental diversity upto some limits and decreased with further increase in parental diversity. Thus maximum heterosis occurs at an optimal or intermediate level of parental diversity.
3. Genetic Base of Parents:
The manifestation of heterosis is affected by the genetic base of the parents. For example, in cotton higher heterosis is associated with broad genetic base of the parents.
4. Adaptability of Parents:
The magnitude of heterosis is also affected by the adaptability of the parents. In cotton and many other crops, heterosis is associated with wider adaptability of the parents, because there is close association between adaptability and genetic base.
The following factors are important in the commercial exploitation of heterosis:
1. Enough Magnitude of Heterosis:
There should be enough magnitude of heterosis for its commercial exploitation. The level of heterosis differs from crop to crop. It is generally higher in cross pollinated species than in self- pollinated species. Heterosis of 40% and above is considered significant in most of the crops.
2. High Percentage of Outcrossing:
There should be high percentage of outcrossing. This is essential for good seed setting.
3. Floral Biology:
The floral biology should permit large scale production of hybrid seed with less expenditure.
4. Availability of MS and SI System:
Availability of male sterility (MS) and self incompatibility (SI) systems help in reducing the cost of hybrid seed by eliminating the process of hand emasculation. There should be proper nicking in the flowering time of A and R lines. The A line should have good stigma receptivity and R lines should have high pollen production efficiency and pollen longevity. In case of self incompatibility, the modifier genes which break self incompatibility should be absent.
Genetic Basis of Heterosis:
Thus there are three possible genetic causes of heterosis, viz.:
(1) Dominance,
(2) Over dominance, and
(3) Epistasis.
These are briefly discussed below:
i. Dominance Hypothesis:
This theory was proposed by Davenport (1908), Bruce (1910) and Keeble and Pellew (1910). This is the most widely accepted explanation of heterosis. According to this hypothesis, heterosis is the result of the superiority of dominant alleles when recessive alleles are deleterious.
Here the deleterious recessive genes of one parent are hidden by the dominant genes of another parent and the hybrid exhibits heterosis. Both the parents differ for dominant genes. Suppose genetic constitution of one parent is AABBccdd and that of another as aabbCCDD.
A hybrid between these two parents will have four dominant genes and exhibit superiority over both the parents which have two dominant genes each. Thus heterosis is directly proportional to the number of dominant genes contributed by each parent.
Objections:
There are two objections to dominant gene hypothesis. First, if the hypothesis is true, it should be possible to obtain pure heterotic individuals in F2 which are homozygous for all the dominant genes. Jones (1917) provided explanation for this. He suggested that there may be linkage between some favourable dominant genes and some unfavourable recessive genes and as a result it is not possible to obtain true breeding homozygous individual for all dominant genes in F2 generation. He proposed dominance of linked gene hypothesis to explain heterosis.
The second objection is that if the heterosis is due to dominance, the F2 curve should be skewed towards dominant genes, but the curve of F2 is found always smooth and symmetrical not skewed. Collins (1921) provided explanation for this objection. He suggested that trait like yield is governed by large number of genes or polygenes which exhibit continuous variation resulting in symmetrical distribution of genes.
ii. Over Dominance Hypothesis:
This theory was independently proposed by Shull and East in 1908 and supported by East (1936) and Hull (1945). This theory is called by various names such as stimulation of heterozygosis, cumulative action of divergent alleles, single gene heterosis, super-dominance and over-dominance.
Though this theory was proposed by Shull and East in 1908, the term over dominance was coined by Hull in 1945 working on maize. This term is now in common use. According to this hypothesis; heterosis is the result of superiority of heterozygote over its both homozygous parents.
Thus heterosis is directly proportional to the heterozygosis. The superiority of heterozygote over both homozygotes may arise either due to (1) production of superior hybrid substance in heterozygote which is completely different from either of the homozygous products or due to (2) greater buffering capacity in the heterozygote resulting from cumulative action of divergent alleles or stimulation of divergent alleles.
East in 1936 further elaborated this theory by proposing a series of alleles a1, a2, a3, a4 → of gradually increasing divergence in function. Thus a combination of more divergent alleles will exhibit higher heterosis than less divergent combinations. For example, combination of a1a4 will exhibit higher heterosis as compared to combinations of a1a2, a2a3 and a2a4
Over dominance has been reported in barley. In maize, available evidences suggest that if over dominance occurs, it is either infrequent in occurrence or small in magnitude. Dominance and over dominance hypotheses have some similarities and some dissimilarities. A brief comparison of these two theories is presented in Table 22.1.
iii. Epistasis:
Epistasis refers to interaction between alleles of two or more different loci. It is also known as non-allelic interaction. The non-allelic interaction is of three types viz. additive x additive, dominance x dominance and additive x dominance. It is well established that the incidence and magnitude of heterosis has positive association with the presence and magnitude of non-allelic interaction.
Epistasis, particularly that involves dominance effects (dominance x dominance) may contribute to heterosis. This has been observed in cotton and maize. Epistasis can be detected or estimated by various biometrical models.
Out of above three genetic explanations of heterosis, the dominance hypothesis is most widely accepted. Over-dominance and epistasis also operate in the manifestation of heterosis. Some other explanations such as gene cytoplasm interaction and mitochondrial complementation have also been suggested to account for heterosis.
Estimation of Heterosis:
Heterosis is estimated in three different ways, viz.:
(1) Over mid parent,
(2) Over better parent, and
(3) Over commercial cultivar/hybrid.
Thus on the basis of estimation, heterosis is of three types as given below:
1. Average Heterosis:
When the heterosis is estimated over the mid parent, i.e, mean value or average of the two parents, it is known as average heterosis, which is estimated as follows:
Average heterosis = [(F1 – MP)/MP] x 100
where, F1 is the mean value of F1 and MP is the mean value of two parents involved in the cross.
2. Heterobeltiosis:
When the heterosis is estimated over the superior or better parent, it is referred to as heterobeltiosis.
It is worked out as follows:
Heterobeltiosis = [(F1 – BP)/BP] x 100
where, BP is the mean value (over replications) of the better parent of the particular cross.
3. Useful Heterosis:
The term useful heterosis was used by Meredith and Bridge (1972). It refers to the superiority of F over the standard commercial check variety. It is also called as economic heterosis.
This type of heterosis is of direct practical value in plant breeding. It is estimated as follows:
Useful heterosis = [(F1 – CC/CC] x 100
where, CC is the mean value over replications of the local commercial cultivar.
Sometimes, heterosis is worked out over the standard commercial hybrid. It is estimated in those crops where hybrids are already available for comparison. This type of heterosis is known as standard heterosis. This is also of direct practical importance in plant breeding.
It is estimated as follows:
Heterosis = [(F1 – SH)/SH] x 100
where, SH is the mean value over replications of the standard (local commercial) hybrid.
Heterosis leads to increase in yield, reproductive ability, adaptability, disease and insect resistance, general vigour, quality etc. For most of the characters, the desirable heterosis is positive. But for some characters, for example, earliness, height in cereals, micronaire value in cotton and toxic substances like neurotoxin in Lathyrus sativus, negative heterosis is important.
Inbreeding and Heterosis:
Inbreeding depression refers to decrease in fitness and vigour due to inbreeding. The degree of inbreeding is measured by the inbreeding coefficient.
The main differences between inbreeding and heterosis are as follows:
1. Inbreeding results from matings between closely related individuals, whereas heterosis results from crossing between unrelated strains.
2. Inbreeding depression is the decline in fitness and vigour with decreased heterozygosity, whereas heterosis is the increase in fitness and vigour with increased heterozygosity.
3. Inbreeding depression results due to fixation of unfavourable recessive genes in F2, while in case of heterosis the unfavourable recessive genes of one line (parent) are covered by favourable dominant genes of other parent. The fixation of all favourable dominant genes in one homozygous line is impossible due to linkage between some unfavourable recessive and favourable dominant genes.
4. The heterosis will be the highest when some alleles are fixed in one parent and other alleles in the other parent.
5. The genes with lack of dominance will not exhibit heterosis in F1 but may show increase in performance in F2, due to fixation of genes, i.e. additive action.
6. If some genes have dominance in one direction and some in other direction there will be no heterosis due to mutual cancellation effects of such genes.
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