7. History of Heterosis Concept Pre - Mendelian 1766 Kolreuter – Hybrid vigour in Nicotiana 1799 A. Knight – Principle of anti-inbreeding 1865 Mendel – Hybrid vigour in peas 1877 Darwin – Cross fertilization is beneficial 1880 Beal – First published report of 51% increase in yield over parents 1891 Johnson – Crossing gave better off-springs 1892 Mc Cleur – Inbreeding imparted sterility Crossing imparted vigour
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9. Academic influence pedigree chart from Darwin to Jones England Darwin 1859 Natural selection Boston Gray 1860s Michigan Beal 1880s Illinois Holden, Davenport 1890s Illinois Hopkins 1890s Illinois, CT East 1900 Connecticut Jones 1918 Double cross
10. Academic influence pedigree chart from Buffon to Shull France Buffon 1760s France Lamarck 1800s De Candolle Germany Nageli 1850s Germany Correns,De vries 1900 Holland Von Tshermack Mendel Newyork Shull 1908
11. ‘ We know what inbreeding does and I do not propose to spend people’s money to learn how to reduce corn yields’ -Hopkins
12. ‘ A recent paper by Dr. G.H. Shull has given, I believe the correct interpretation of this vexed question. His idea, although clearly and reasonably developed, was supported by few data’ - East,1908 ‘ Since studying your paper, I agree entirely with your conclusion, and wonder why I have been so stupid not to see the fact myself’ - East wrote to Shull in 1952
13. Nomenclature Contd... Heterozygosis (East & Hayes, 1912) Crossing produces heterozygosis Selfing leads to Homozygosis Heterosis (Shull, 1914) Used the term for the same phenomenon Hybrid Vigour (Jones, 1918) Synonymous to Shull’s Heterosis Heterobeltiosis(Blitzer, 1968) Increased performance of hybrid over BP Euheterosis (Dobzhansky, 1950) Hybrids possessed higher fitness than their parents Luxuriance(Dobzhansky, 1950) Extreme heterosis for morpho- characters, but no fitness
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15. Measuring heterosis a) Standard heterosis c) Heterobeltiosis b) Midparent heterosis d) Commercial heterosis
17. Genetic basis of heterosis 1. Heterosis is not due to heterozygosity per se Dominance Hypothesis (Davenport, 1907) 2. Superiority of dominant alleles over recessive alleles 3. Heterosis is due to masking of deleterious recessive alleles by dominant alleles 4. Heterosis is directly proportional to the number of dominant genes contributed by each parent
23. Haploid having Single set of genes in each nucleus Within the same nucleus dominance and heterozygosis can’t exist Higher growth representing heterosis came from heterocaryon between strains of N. tetrasperma Over dominance or single locus heterosis Strain A Strain B Heterocaryon Evidence in heterokaryon of Neurospora
24. The heyday of overdominance ‘ In 1950 the estimates of total mutation rate, mainly from Drosophila, were about 0.05 per haploid genome. This argued that the dominance hypothesis could account for an increased performance of 5% or less, but not the 15 to 20% that was observed’ J.F. Crow ‘ It would appear that the total elimination of deleterious recessive alleles would make less difference to the yield of cross-bred commercial crops than the total mutation rate would suggest. Perhaps no more than a 1% improvement could be looked for from this cause. Difference of the order of 20% remain to be explained’ R.A. Fisher
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26. ‘ In 1950s I argued that heterosis is largely due to overdominance but when more evidences for dominance began to appear I changed my mind and have retained the view that heterosis is mainly due to the loci that are dominant or partially so’ - J.F.Crow
27. Epistasis hypothesis Heterosis is mostly attributable to favourable epistatic interaction between non-allelic genes -Powell 1944, Williams 1959
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32. Biochemical basis of heterosis Bottleneck concept (Manglesdorf) (i) Excellence of genotype depents not upon its strongest link, but upon its weakest link (ii) Emphasis is laid not on the superiority of the hybrid, but on the inferiority of its parents (iii)Inferiority of the parents are thought to be the bottlenecks represented by inefficient alleles (iv)It supports the dominance hypothesis
33. Evidence by Robbins Roots of Red currant had bottleneck for Pyridoxin production Roots of Johannesfeur had bottleneck for Suppy of Nicotinamide Hybrid was able to produce both vitamins In the roots of Tomato varieties
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35. Models of gene to gene product interactions 2. Alternative pathway Ao Ao X in E1 A1 A1 Y in E2 A1 Ao X in E1 or Y in E 2 Heterozygote can produce both depending on environment Eg., Temperature sensitive alleles of plants R 1 max expression of red pigment at 80 0 F R 2 max expression of red pigment at 50 0 F R 1 R 2 produces red pigment at low or high temp
36. Ao Ao 0.1 X A1 A1 2X A1 Ao 1 X Heterozygote produces optimal amount 3. Optimal amount Models of gene to gene product interactions Eg., Drosophila lethal alleles No active allele – lethal (Low) One active allele – Heterotic (Optimum) Two active alleles – Wild type (Excess)
37. Models of gene to gene product interactions 4. Hybrid substance Ao Ao X A1 A1 Y A1 Ao X+ Y+Z eg., E 1 esterase enzyme in maize enzyme molecule composed of polypeptides specified by two different alleles A 1 A o may be more active than the homo multimers specified by same alleles Ao Ao or A1 A1. Heterozygote produces entirely new product
50. Although the mechanism of heterosis is not fully understood yet, the phenomenon has been widely exploited in crop plants. The present century is being viewed, as an era of hybrids and hybrid culture in agriculture will be the order of the day.
