Regarding understand heterosis and unfolding classical Theories

1. Understanding Heterosis:
unfolding the classical theories
Pardeep Beniwal
L-2022-A-38-D
Dr. Gurpreet Kaur
Major Advisor

2. Contents
• What is heterosis?
• Heterosis with genetic diversity
• Utilization of heterosis in Agriculture
• Pre mendelian ideas of heterosis
• Post mendelian ideas of heterosis
• Classical theories of heterosis
• Molecular basis of heterosis
• Case study

3. What is heterosis?
Heterosis or hybrid vigour, describes the superior performance of heterozygous
F1-hybrid plants in terms of increased biomass, size, yield etc.
Heterozygosis
Hybrid vigour
Heterobeltiosis
Euheterosis
Luxuriance
Positive heterosis
Negative heterosis
Adaptive heterosis Selective heterosis

4. East (1936) Increasing genetic diversity boosts heterosis, with
interspecific crosses showing superior results.
Heterosis with genetic diversity
 Inter-subspecific heterosis
 Wide- hybridization-wheat
development 9000 years ago
 Intraspecific heterosis
(Rehman et al 2021)

5. Utilization of heterosis in Agriculture
 Maize yields increased by more than six-fold
Hochholdinger, F. et al 2023
Li, J., Xin, Y., & Yuan (2009)
From 1976 to 1991 rice production increase in china is 56%

6. History
Tylor (1865)
Koelreuter (1763)
Studied social structure
first to report hybrid vigour
Mendel (1865)
Observed heterosis in Pea crosses
Darwin (1876)
Reported- crossing increases
hybrid vigour
Beal (1877-1882)
concluded that F1 hybrids yield as much as 51 percent
more of the parental varieties.
Johnson (1891)
crossing usually gave better offspring
Post
Mendelian
Ideas of
Heterosis
Pre Mendelian Ideas of Heterosis

7. History
Simpson (1907)
Davenport (1908)
Rejuvenation by hybridization
Law of dominance
East and Shull (1908, 1912)
Law of Over dominace
Bruce and Pellew (1910)
Support and further explain
Dominance
Jones (1917)
Dominance of linked gene
hypothesis
Collins (1921)
Explain objection in term of population size
Power (1944)
Inter allelic interactions
Wright (1968)
Observed net like
structure
Post Mendelian Ideas of Heterosis

8. Heterosis
Genetic Basis
Dominance
Hypothesis
Overdominance
Hypothesis
Epistasis
Hypothesis
Biochemical
Basis
Molecular
Basis
Epigenetic
Basis
Physiological
Basis

9. Dominance Hypothesis
Overdominance Hypothesis
Epistasis Hypothesis
Genetic basis of
heterosis

10. Charles Davenport Edward East Caroline Pellew G. H. Shull
Source- Wikipedia (accessed on 31 October 2023)
Classical theories of heterosis

11. This hypothesis proposed by Davenport
(1908), Bruce (1910), Keeble and Pellow
(1910)
AAbbCCdd x AAbbCCdd
AAbbCCdd
(No heterosis)
AAbbCCdd x aaBBccDD
AaBbCcDd
(Heterosis)
Dominance Hypothesis
 Objection of this hypothesis are arise on
two grounds
 Mask deleterious recessive mutations from
the other parent in the heterozygous
 Heterozygosity per se is not a prerequisite
 Isolation of inbreed for all the dominant genes
 Why there are not Skew distribution in F2

12. Dominance Hypothesis
Explanation- Objection 1
 Jones (1917) linkage
 Collins (1921) large number of genes influencing vigor it would be
impossible in practice, even without assumption of linkage
Explanation- Objection 2
 Collins 1921 shown that if number of genes
under observation are 20 or more then
distribution becomes near normal

13. • Heterozygosity at single loci is superior to either
homozygote
Overdominance Hypothesis
• Shull (1910) and East (1908) advocating as major
genetic basis of heterosis
• ‘Single gene heterosis’; ‘Super dominance’ or
‘Cumulative action of divergent alleles’
• *B is an allele variant of B (irrespective of
dominance in this case).
Objection
Pseudo-overdominace

14. Epistasis Hypothesis
 Dominance and over-dominance (both proposed in
1908) remained the major genetic understandings
 Both dominance and over-dominance concepts are
based on single-locus model.
 Wright (1968) proposed that most of the quantitative
traits are conditioned by many loci and genes
invariably do interact with each other.
 Hallaur et al 1978

15. Dominance Is the Major Genetic Basis of Heterosis in Rice as
Revealed by QTL Analysis Using Molecular Markers
 All the traits studied were subjected to QTLs
analysis by single point basis and interval
mapping.
(Xiao et al 1995)
 Most of the QTL’s (73 %) were detected in only
one of two backcross generations. In most
cases heterozygotes had higher phenotype.
 23 % of QTLs were detected in both backcross
populations and each pair was mapped to same
chromosomal location. In all these cases
heterozygotes fell between two homozygotes.

16. Correlation Coefficients between Genome heterozygosity and trait value
 Heterozygosity per se to the
expression of following traits
 This finding suggested that
complementation of dominant (or
partially dominant) alleles at
different loci in F1 was major
contributor to F1 heterosis for
different traits.
(Xiao et al., 1995)

17. Overdominant Epistatic Loci Are the Primary Genetic Basis of
Inbreeding Depression and Heterosis in Rice
 studied the genetic basis of heterosis and inbreeding depression in
rice by using five interrelated mapping populations. comprising a
Lemont (japonica)/ Teqing (indica) RIL, two BC and two test cross
populations using Zhong 413 and IR64 as testers.
 Hybrid breakdown values of RILs for BY and GY were used as
input data to identified QTLs associated with inbreeding depression
and HMP values were used for to identify QTLs contributing to
heterosis
Li et al (2001)

18. Inbreeding depression
 The hybrid breakdown values of individual RILs
were negatively correlated with their heterosis
values across all four F1 populations
 Results suggested that hybrid breakdown of
the RILs and heterosis of their F1 hybrids
indeed shared a partially overlapping genetic
basis.
Li et al (2001)
Between RIL values and HMP
r Lemount Teqing Z413 IR64
GY -0.466 -0.633 -0.567 -0.490
BY -0.386 -0.587 -0.500 -0.519
P<0.0001
 They found that most of the QTLs (~ 90%) contributing to heterosis were
over-dominant especially for grain yield, biomass, panicles per plant and
grains per panicle.

19. Importance of epistasis as the genetic basis of heterosis in
an elite rice hybrid
 A classical study in rice by Yu et al 1997 using F3 population derived from
bagged F2 plants from a cross between Zhenshan 97 and Minghui 63
(Parents of Shanyou 63)
 The lack of correlation between genotype heterozygosity and trait
expression suggests that the effect of heterozygosity made only limited
contributions to the observed heterosis
(Yu et al 1997)

20. Summary of the significant interactions identified in 1994 and 1995 by
searching all possible two-locus combinations
 Total 32 QTLs were detected in two years 12 were observed in both year and
remaining 20 were detected in only one year.
 All the three types of interactions i.e. A x
A, A x D and D x D occurred among
various two-locus combinations.
 Multiple interaction terms were found in a
considerable proportion of interacting
two-locus combinations in all traits.
(Yu et al 1997)

21. Heterosis at genome organization level
 Genetic collinearity
 Differences in gene content
among maize inbred lines
 Among 72 genes in B73 and
Mo17 inbred lines, 27 genes were
absent in one of them
 Gene deletions in inbred lines
might have only minor quantitative
effects
 Hemizygous complementation of
multiple genes with minor effects in
hybrids can significantly enhance
hybrid plant performance (Hochholdinger and Hoecker 2007)

22. Heterosis at genome organization
level (Hochholdinger F. and Hoecker N. 2007)
 Hemizygous gene loss through
successive rounds of hybrid self-
pollination can elucidate the
concept of inbreeding depression
 The high degree of non-colinearity in
maize inbred line genomes may
account for the remarkable heterosis
Inbreeding depression
(Hochholdinger and Hoecker 2007)

23. Gene expression and heterosis
 At the level of gene expression, complex
transcriptional networks specific for
different developmental stages and
tissues were monitored in maize, rice and
Arabidopsis.
 In various tissues, 4-18% of the DEGs
were observed between the two inbred
lines and the hybrids, whereas no DEGs
were identified in these tissues between
the reciprocal hybrids.
 no direct link between the classical
genetic hypothesis and these gene
expression profiles.
Fu et al 2014
(Hochholdinger and Hoecker 2007)

24. Allele-specific gene expression and heterosis
 allele-specific gene expression of
32 genes whose expression in the
hybrid deviated from the mid
parent value was analyzed, 18
genes cis, one gene trans and 13
genes were find as a combination
of cis- and trans-regulation
 Interestingly, parent of origin
effects apparently play only a
minor role for allele-specific gene
expression whereas environmental
factors might have a more severe
influence on allelic contribution to
gene expression
(Hochholdinger and Hoecker 2007)

25. What they did
 Comprehensive genome analysis of 2,839 rice hybrids and 9,839 F2 individuals from 18 elite
hybrids for different aspects, encompassing 50 years of Chinese rice breeding
 Investigated the genetic basis underlying strong heterosis in intersubspecific rice hybrids.
We lack a comprehensive analysis of genome-wide breeding footprints in rice hybrids to identify
dynamic heterotic loci
Unrevealed the basis of heterosis at genomic level
(Zhoulin et al 2023)

26. F1
F1
F2
F1&F2
All 2839 rice hybrids were sequenced at 35X depth
Compared with reference genome and find 5222902 SNPs and 171091
InDels and select 18 reprehensive hybrids
Sequenced at 0.2X and high density parental SNPs used for
genotyping
Phenotyping for F1 and F2 individuals for heading date,
morphological characters and yield related traits
(Zhoulin et al 2023)

27. Phenotypic
changes in
rice hybrids
across
improvement
breeding
Large source
organ
correspond
with larger sink
organs
Improvement

28. Among the indica–japonica hybrids, 65.47% genome-wide segments were indica/japonica
genotype
Genetic basis of intersubspecific heterosis
 The genetic basis of intersubspecific heterosis investigated using 68
indica-japonica F1 hybrids and 5,342 F2 individuals from eight elite
hybrids
 Indica-Japonica hybrids offer high yield potential and robust heterosis
(Zhoulin et al 2023)

29. (Zhoulin et al 2023)
QTNs) in 17 agronomically important genes, bi-parents
for most indica–japonica hybrids contained
differentiated genotypes
Genetic basis of intersubspecific heterosis In F2 indica- japonica
association studies, 86 % loci
are present in indica-japonica
type, includes important
genes
 These results indicated genetic complementation was prevalent in
intersubspecific hybrids.

30. Genetic basis of intersubspecific heterosis
Further evaluation in Quanjungyou No. 1 hybrid-F2 population
Four major loci, controlling total grain number per plant and all resided at
the indica/japonica complementary genomic region
(Zhoulin et al 2023)

31. Genetic basis of intersubspecific heterosis
(1.4)
(0.09)
(0.24)
(0.01)
 Most F2 individuals possessing the heterozygous genotypes across all the four loci had more grain
number per plant than both parental lines
 Genetic complementation of the four master loci largely accounted for transgressive
segregation of grain number per plant in the F2 population.
(Zhoulin et al 2023)

32. Conclusions
The bulk of available data are highly consistent with the
dominance (complementation) hypothesis as the primary basis
of heterosis.
Heterosis is a genome-wide phenomenon, which reflects global
changes at both expression levels of genes and proteins
Transgenic and genomic editing technologies can significantly
improve the efficiency of research on heterosis.
Therefore, heterosis in plants remains a topic requiring keen
scientific investigations to develop a deep understanding and
manifestation of the phenomena.

33. Thank Y u