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——=, , _ Heterosis Breeding _, , , , INTRODUCTION , Heterosis refers to the[ superiority of F, 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 F, over parents in general vigour, yield and adaptation, whereas latter, , Sd refers to increase of F, over parents in Wegetative growtpbut hotyin 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 F, : Heterosis is confined only to the F, generation of a cross. It declines and, disappears in F, and subsequent generations of a cross as a Consequence of segregation and, , recombinations. Thus heterosis is related to F, 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., , i the manifestation of heterosis is more pronounced in the area of adaptation of a, , ses ne SCA : Heterosis has positive association with specific combining ability, ce. The SCA is a measure of dominance variance and existence of a significant, , amo i i i i, amount of dominance variance is essential for undertaking heterosis breeding programme., , 4, , , , , , , , Scanned with CamsScanner

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fleterosis Breeding, , , , 6. Effect of Heterozygosity : The magnitude of heterosis is associated with heterozygosity,, pecause 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 ws, {his reason, occurrencé Of heterosis is more in cross pollinated crops than in self-pollinated ones. oe, , 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 F, crossés onl ly few desirable heterotic combinations are identified. All, the F, crosses do not exhibit desirable heterosis. , GENETIC BASIS OF HETEROSIS, , , , There are two main theories which have been adavanced to explain the mechanism of heterosis.One, is the dominance hypothesis and the second is overdominance hypothesis. The epistasis is also, considered to be associated with heterosis. Thus there are three possible genetic causes of heterosis., viz. (1) dominance, (2) overdominance, and (3) epistasis. These are briefly discussed below :, , 1. DOMINANCE HYPOTHESIS, This theory was proposed by@avenport (1908)) Bruce (1910) and Keeble and Pellew (1910)., , This is the most widely accepted explanation of heterosis. According to this hypothesis, heterosis av), is the result of the superiority of dominant alleles when recessive alleles are deleterious. Here the ~, déleterious 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 1s 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, , , , , , , , WY, eS, AABBccdd x aabbCCDD ——-——» AaBbCcDd, Parent 1 Parent 2 Hybrid with, with 2 dominant — with 2 dominant 4 dominant, genes genes genes, , , , , , “FIG. 19.1. Explanation of dominance hypothesis. In the hybrid, ~ all the four recessive genes are masked by the respec, , , , , , , , , , Objections, , There are two objections to dominant gene hypothesis. First, if the hypothesis is true, itl, Should be possible to obtain pure heterotic individuals in F, which are homozygous for all the, domi es Canes COD) 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, F, generation. He proposed dominance of linked gene hypothesis to explain heterosis., i i, , , , Scanned with CamScanner

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SS aL PLR, , , , (56: :, The second objection is that if the heterosis is due to dominance, the curve should be, xkewed towards dominant genes, but the curve of F, is found always smooth and symmetrical not, am TCallins (1921 provided explanation for this objection. He suggested that trait like yield, , is “is governed E by_large_numper of genes_or polygenes, which exhibit continuous variation resulting, in “in symmetrical d distribution of genes. ——, , ——, , 2. OVERDOMINANCE HYPOTHESIS, , This theory was independently proposed by(Shull and East in 1908yand 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 overdominance,, , Though this theory was proposed by Shull and East in 1908, the term overdominance was coined, , uy 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 hetérozygosis. The superiority of heterozygote over b both”, homozygotes may arise either due to (1) production of(superior hybrid substance i 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 allélés., , East in 1936 further elaborated this theory by proposing a series of alleles Q,, Ay Ay a,—> of, gtadually increasing divergence in function. Thus a combnination of more divergent alieles will, exhibit higher heterosis than less divergent combinations. For example, combination of a a, will, , exhibit higher heterosis as compared to combinations of a,a, a,d, and a,q,., , Overdominance has been reported in Garley) In maize, availaule evidences suggest that if, overdominance occurs, it is either infrequent in occurrence or small in magnitude. Dominance, and overdominance hypotheses have some similarities and some dissimilarities. A brief comparison, , of these two theories is presented in Table 19.1. ‘, , ;, , . TABLE 19.1, Comparison of dominance and overdominance theories of heterosis, , , , , , , , | Outbreeding restores vigour. ce, | Heterosis is. pssocaiod with parental, , || diversity. Ho, , , , , , , , , , , , Scanned with CamsScanner

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(leterosis Breeding a, , , , 157, 3. 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 (Moll and Stuber. 1974). ~~, Epistasis can be detected or estimated by various biometrical models., , Out of above three genetic explanations of heterosis, the dominance hypothesis_is_most a, widely accepted. Overdominance and epistasis also operate in the manifestation of heterosis., Some other explanations such as gene cytoplasm ineraction and mitochondrial complementation, , have also been suggested to account for heterosis. For detailed description of these aspects refer, Rai, 1979 and Frankel, 1983., , FIXATION OF HETEROSIS, , There are four principal ways of fixation of heterosis in crop plants. These are : Ady 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. E, , 2. Apomixis : Apomixis is another important method of fixing heterosis. In apomixis, the w—, seed develops without fertilization Apomitic seeds generally develop from maternal diploid cellls\\ ww, Thus the apomictic progeny is identical to the mother plant. The fixation of heterosis by apomixis, ig common in citrus fruits, blackbernes, roses, bluegrasses and many other flowering plants., Apomixis is also common in’ Hieracium (Hawck weed). As a result of apomixis, Mendel could, not confirm his findings of garden peas on Hieracium. In Hieracium, the F, and later generation, progeny were similar to their F, hybrid., , 3, Balanced Lethal System : Balanced lethal system also leads to fixation of heterosis in, some plants. For example, in many species of @vening primrose Ocnothera 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 ystem has developed due to complex translocations. wl _, , 4. Polyploidy : Heterosis can also be fixed by(chromosome doubling polyploidy'e specially, ifGinterspecifio andGntergeneric hybrids. For example, the heterosis in wheat-rye cross can, be conserved in amphidiploid hybrids through chromosome doubling. The F, 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. : 4, , , , Mr, Scanned wl CamScanner

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sie Essentials of Plant Breeding, , ‘FACTORS AFFECTING HETEROSIS, , There are four main genetic factors which affect magnitude of heterosis in corp plants. These are, mode of pollination, genetic deversity 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 W), , characters and flowering time, and also with respect to geographical origin of parents. In alfalfa, , and cotton, giéatér 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 adai tability of, the parents, because there is close association between adaptability and genetic base., , 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 = [(F, - MP)/MP] x 100, where, F, is the mean valu, , 2. Heterobeltiosis :, Teferred to as heterobelti, , le of F, and MP is the mean value of two parents involved in the cross., , When the heterosis is estimated over the superior or better parent, it is, losis. It is worked out as follows :, , Heterobeltiosis = [(F, - BP)/BP] x 100, wh i «ath, ere, BP is the mean value (over replications) of the better parent of the particular cross., , 3. Useful Heterosis : The term i, : useful i in pe, Tefers to the Superiority of F fe sting Oo check variety. Ii ao elle, , , Over the standard commercial check variety. It is also called as, €conomic heterosis. Thi 2 OF henge Sommercial check variety. 2 :, ost mpS- ‘This type of hete Fi _ It is, estimated as follows YP Tosis is of direct practical value in plant breeding. I, , Useful heterosis = [, - COV/CC] x 100, © over replications of the local commercial cultivar., , Scanned with CamsScanner, , where, CC is the mean valu