The document discusses the history and concepts of heterosis or hybrid vigor in plant breeding. It covers pre-Mendelian observations of hybrid vigor in the 1700s and 1800s. It then discusses the early 20th century work of scientists like Shull, East, and Jones who studied heterosis and coined related terms. The document also summarizes various theories for the genetic and physiological basis of heterosis, such as dominance, overdominance, and epistasis hypotheses. It discusses evidence from studies of embryos, seedlings, biochemistry, and gene interactions that help explain the mechanisms behind heterosis. While the full basis is still unknown, heterosis continues to be widely used in crop breeding.

1. Concepts of heterosis- Genetic, Physiological and Molecular basis

3. Hybridization in plants

4. breeding

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

8. History of Heterosis Concept Simpson – Rejuvenation by hybridization East and Hayes – Stimulus of heterozygosis Shull – Coined the term heterosis 1918 Jones – Practical utilization of heterosis

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

14. Positive and Negative heterosis(Powers, 1944) To include inferiority of hybrid over parents Adaptive Heterosis (MacKay, 1976) Heterosis for adaptability Selective Heterosis (MacKay, 1976) Heterosis for Competitive ability Luxuriant Heterosis (MacKay, 1976) Increased vigour in Size, yield but not in fitness Labile Heterosis (MacKay, 1976) Heterosis which is not fixable Fixed Heterosis (MacKay, 1976) Fixed by various mechanisms

15. Measuring heterosis a) Standard heterosis c) Heterobeltiosis b) Midparent heterosis d) Commercial heterosis

16. Heterosis- theories

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

18. Dominance hypothesis - An illustration

19. Objections True breeding homozygous individuals for all dominant genes in F 2 F 2 Curve should be skewed towards dominant genes Explanations Dominance of linked gene hypothesis (Jones, 1917) Smooth and symmetrical as yield is governed by polygenes Genetic Basis of Heterosis

20. Genetic Basis of Heterosis Over Dominance Hypothesis (Shull & East, 1908) Heterozygosity per se is the cause of Heterosis Heterozygote is superior to either of the homozygotes Increase in vigour is directly proportional to amount of heterozygosis Superiority of heterozygote may arise due ● production of superior hybrid substance in heterozygote ● cumulative action of divergent alleles More divergent alleles will exhibit more heterosis than less divergent ones

21. Shull’s Concept of Heterosis 1. Self-fertilization purifies the strain and releases new variation Conclusions based on his investigations 2. Self-fertilized plants are weaker than the cross-fertilized ones 3. Generations of close inbreeding reduces the strains into simple constituent bio-types which are weaker than the hybrid combinations 4. Crossing the weakened pure strains would cancel the effects of inbreeding Interpretation of increased vigour, size, fruitfulness, resistance to diseases and insect pests or climatic rigour of any kind, manifested by Cross-bred organisms as compared with corresponding inbreds, as the Specific results of unlikeness in the constituents of the uniting parental gametes Shull (1952)

22. Overdominance hypothesis - An illustration

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

25. Objections Evidences of single locus heterosis may not hold good for quantitative characters like yield Quantitative estimation of overdominance is difficult due to linkage bias Genetic basis of heterosis

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

28. Physiological Basis of Heterosis Ashby’s Hypothesis of Greater Initial Capital Heterozygosity results from the greater initial weight of the embryo Hybrid vigour is nothing more than the maintenance of initial advantage in embryo size Hybrids do not differ from their parents in relative growth rate

29. Physiological Basis of Heterosis Stages of physiological manifestation of heterosis 1. Seed and Embryo development High positive correlation was recorded between hybrid vigour and embryo weight Copland noted that hybrid embryo exhibited greater vigour in the earlier stages of development Ashbey found significant differences between embryo weight of reciprocal crosses Early differences in embryo size in favour of hybrids resulted from rapid growth rate of multi cellular zygote

30. Physiological basis of heterosis Early seedling growth Hybrid shows higher growth rate than inbreds immediately after germination but not later Hybrids do have a larger maximum growth rate period Ashby says embryo size is more significant than reserve food material

31. Later Seedling Growth Hoffer found root dry matter content of hybrids to be intermediate between parents Hybrids found to absorb and utilize more amount of N ,P than the inbred lines Catalase activity was higher in heterotic hybrids Physiological basis of heterosis

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

34. Models of gene to gene product interactions Supplementary action model Ao Ao X A1 A1 Y A1 Ao X+Y Alleles produce entirely new products Heterozygotes produce both the products eg., Rust resistance in flax

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

38. Molecular understanding of heterosis QTL analysis

39. Molecular understanding of heterosis

44. Transcriptomics- RNA amount polymorphism (RAP)

45. Proteomics- Protein amount polymorphism (PAP)

46. Metabolomics- Metabolic flux theory

47. Unraveling the basis of heterosis

48. Unraveling the basis of heterosis

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.