Heterosis

1. 1 Heterosis

2. 2 Heterosis “ In my opinion, hybrid corn is the most far reaching development in applied biology in this quarter century” Mangelsdorf, 1951 A plant breeding phenomena that we very successful exploit commercially, the biological basis of which remains poorly understood

3. 3 Heterosis‘d’ > ‘a’ Defining a locus in terms of scale bb mp BB Bb -a +a d

4. 4 Heterosis • The converse or complement to inbreeding depression in which the value ‘d’ ≥ ‘a’ • Inbreeding can result in loss of vigor, size, etc…. • The restoration of phenotypic performance by crossing inbred lines to produce a hybrid is called heterosis.

5. 5 History • Fundamentally about the effects of inbreeding and outbreeding • When did man first observe and take advantage of this phenomena? • Is this only a modern “scientific” development?

6. 6 Pre- History • Tantalizing anthropological evidence • Religious rituals associated with maintenance of maize lines • Helentjaris - 700 year old Anasazi cobs from SW USA observed molecular marker fragment patterns that more closely resembled F1 hybrid than an O.P. variety

7. 7 Modern - History • Koelreuter (1766) • investigated hybrid vigor in Nicotiana, Dianthus, Datura, et. al. • Darwin (1876) • discussed hybrid vigor in his book “The effects of cross and self-fertilization in the vegetable kingdom” He demonstrated that cross fertilization frequently resulted in increased size, vigor and productiveness when compared with self-fertilization. He did not attribute the differences to the uniting of different gametes. • Mendel (1865) • Wrote “ in repeated experiments, stems of 1 foot and 6 feet in length yielded plants with varied in length from 6 to 7.5 ft.”

8. 8 Modern -History • Beal - 1880 • Described how he planted in alternate rows to stocks of the same variety, one row was detasseled and the hybrid seed was more productive than either parent. • Shull - (1908-1914) • Shifted emphasis from the negative effects of inbreeding to the positive effects of hybridization. • Coined the word “heterosis” to describe the increased vigor observed from heterozygosity.

9. 9 Modern -History • East - (1908-1909) • Studied the effects of inbreeding and outbreeding • His work led to the formulation of the modern heterosis concept.

10. 10 Yield and types of populationsForest Troyer - 1991

11. 11 Mating system and heterosis • Heterosis has been reported for a wide range of crops including both self and cross pollinated species • Commercial application is via F1 hybrids • Commercialization • Added value > cost of hybrid seed production

12. 12 Estimated percentage of hybrids for selected vegetables Crop % hybrid Method Tomato (fresh) 60 Hand (Processing) 75 Hand Sweet pepper 40 Hand Onion 60 CMS Broccoli 100 S.I. Snap beans 0 - Lettuce 0 -

13. 13 Attributes of F1 hybrids • Maximum performance under optimal conditions • Stability of performance under stress • Proprietary control of parents • Often, reduced time to cultivar development • Joint improvement of traits

14. Hybrid cultivars • Hybrid cultivars are first generation offspring after cross between different inbred parent lines • Major steps in breeding • develop inbred homozygous lines • find good F1 combination between inbreds • produce F1 seed in large scale for growers • Hybrids are uniform, reproducible and ”protected” if parents are homozygous.

15. F1 hybrid population Parent population Rare desirable genotypes YIELD Inbred line population Genotypes cannot be reproduced Each hybrid can be produced large scale from its two parental inbreds Inbreeding Inbreeding and hybridisation to produce desirable hybrids

16. Major types of hybrid cultivars • Single cross hybrids (F1) A x B = F1 • Three way hybrids (A x B) x C = Three Way Hybrid • Double cross (Four way hybrid) (A x B) x (C x D) = Double Cross Three way and double cross hybrids are used to reduce seed costs when parentals are weak

17. Effects of inbreeding Selfing, full/half-sib pollination Reduced height, seed set, disease resistance, etc Increased lodging, Increased homozygosity

18. Hybrid vigour or heterosis • Heterosis: the increase in size, vigour or productivity of a hybrid plant over the average or mean of its parents. • Midparent heterosis • High parent heterosis • Standard heterosis

19. 19 Measurement of Heterosis • Mid-parent heterosis • Hybrid performance is measured relative to mean of the parents (MP) • (F1 - MP) / MP * 100 • High-parent heterosis • Comparison of hybrid to performance of best parent (HP) • (F1 - HP) / HP * 100

20. 20 Real data from dry beans% heterosis above HP • 9 x 9 Diallel of bean cultivars, evaluated in two locations A132 A476 B1222 A359 X122 A457 A231 Toche A375 A476 5 * B122 14 12 Yield of Toche = 2.38 T/Ha. A359 25 0 Yield of A476 = 2.46 T/Ha. X122 60 30 F1 = 4.96 A457 10 24 (4.96 - 2.46)/ 2.46 *100 A231 30 20 !! Told me that favorable combinations do exist!! Toche 70 102 ? How to capture this genetic effect? A375 25 33

21. 21 Genetic basis of heterosis • Three possible genetic causes: • Partial to complete dominance • Overdominance • Epistasis • The issue for plant breeders - What is the Ideal genotype? • Partial to complete dominance - Homozygote • Overdominance - Heterozygote

22. 22 Dominance Hypothesis • Davenport (1908) • Hybrid vigor is due to action and interaction of favorable dominant alleles • Hypothesizes decreased homozygosity for unfavorable recessive alleles (covering up) • Conversely, inbreeding depression is due to exposure of these recessive alleles during inbreeding

23. 23 Dominance HypothesisExample • Model AA = Aa > aa - AA=10 Aa=10 and aa=0 Parent 1 Parent 2 aaBBccDDee = 20 AAbbCCddEE = 30 F1 AaBbCcDdEe = 50 Also note that AABBCCDDEE = 50

24. 24 Discussion of Dominance hypothesis • Theoretically, plants homozygous for all favorable alleles could be developed (AABBCCDDEE….) • Why then are there no inbred equal in performance to hybrids?? • This was considered a until it was recognized that only 1 in 4n individuals in a population would be homozygous for all loci - • For 10 loci that would be 410 = one plant in a million.

25. 25 Dominance hypothesisLinkage • Recombination among loci could result in plants homozygous for all favorable alleles, but… • Repulsion phase linkages, either slow or preclude the development of such lines • Empirical evidence supports dominance hypothesis, as inbred line are improving in performance. A b a B

26. A b a B 26 Repulsion phase linkage • In 70’s investigators were interested in the relative magnitude of s2A and s2D • In F2 crosses the ratio of s2D / s2A was >1 indicating large amounts of dominance variance, but once the populations were random mated for several generations the ratio of s2D / s2A was became <1, this was likely due to recombination among repulsion phase loci

27. 27 Overdominance • First proposed by Shull (1908) and late expanded by Hull (1945) • It states that the heterozygote (Aa) at one or more loci is superior to either homozygote (AA or aa) • Model would be Aa > aa or AA • They recognized importance of dominance, but it alone cannot account for observed heterosis.

28. 28 Overdominance • Superiority of heterozygotes may exist at the molecular level, if the products of two alleles have different properties, e.g. heat stability, or advantages at different environments or maturities - thus may result in stability. • But, “single locus heterosis” difficult to observe and detect if populations are not in linkage equilibrium.

29. A b a B 29 Pseudo- Overdominance • In which nearby loci which have alleles that are dominant or partially dominant are in repulsion phase • If the populations are not in linkage equilibrium, this could mimic the effects of overdominance

30. 30 Epistasis • Epistasis - interaction among loci, may also contribute to heterosis Internode Generation No. nodes length Height Parent 1 3 1 3 Parent 2 1 3 3 Hybrid ( add) 2 2 4 Hybrid ( Dom) 3 3 9

31. 31 Epistasis • Estimates bases on mating designs to estimate the relative magnitude of add, dom and epistatic components of variance indicate that the magnitude of epistatic variance is small compared to additive and dominance components. • Yet, the magnitude of epistatic variance is difficult to estimate, and may play a very important role in heterosis.

32. 32 Prediction of heterosis • The ability to predict heterosis of “Specific combining ability” has been an elusive goal of plant breeders • Combining ability - Testing of hybrids • Diallel crosses n(n-1) / 2 • General (GCA) - Average performance - additive effects • Specific (SCA) - ability of lines to combine in specific combinations Due to dominance effects and heterosis.

33. 33 Genetic distance and heterosis • Moll (1965) showed a relationship between genetic distance and heterosis for yield in maize Heterosis Genetic distance

34. 34 Relationship between genetic distance and heterosisSmith et. Al. TAG 1990 Note, r2 of 0.76

35. 35 Relationships between genetic distance and Heterosis No relationship

36. 36 Heterosis for yield in self -pollinated vegetables Crop Mean % Range Tomato (fresh) 41 -59 to 168 (solids) -10 -45 to 53 Sweet pepper 15 -16 to 52 Eggplant 80 -29 to 242 Beans (dry) 29 -38 to 146 Peas 28 116 to 218 Lettuce 6 -6 to 119

37. 37 Hybrid Rice in China • Hybrid rice yields about 20% more than the best commercial varieties • 8.4 Million Ha. was hybrid in 1988 • Based on CMS system • ? If you believe the dominance hypothesis, is this the best investment of plant breeding effort? • ? What is the ideal genotype in rice?