HeterosisHeterosis

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HeterosisHeterosis

“ 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

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HeterosisHeterosis‘d’ > ‘a’‘d’ > ‘a’

Defining a locus in terms of scale bb mp BB

Bb

+a-a

d

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HeterosisHeterosis

• 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.

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HistoryHistory

• 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?

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Pre- HistoryPre- 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

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Modern - HistoryModern - 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.”

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Modern -HistoryModern -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.

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Modern -HistoryModern -History

• East - (1908-1909)– Studied the effects of inbreeding and

outbreeding– His work led to the formulation of the

modern heterosis concept.

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Yield and types of populationsYield and types of populationsForest Troyer - 1991Forest Troyer - 1991

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Mating system and heterosisMating system and heterosis 1111

• 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

Estimated percentage of hybrids Estimated percentage of hybrids

for selected vegetablesfor selected vegetables

Crop % hybridMethod

Tomato (fresh) 60 Hand(Processing) 75 Hand

Sweet pepper 40 HandOnion 60 CMSBroccoli 100 S.I.Snap beans 0 -Lettuce 0 -

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Attributes of F1 hybridsAttributes of F1 hybrids

Maximum performance under optimal conditions

Stability of performance under stressProprietary control of parentsOften, reduced time to cultivar

development Joint improvement of traits

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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.

Inbreeding and hybridisation to produce desirable hybrids

YIE

LD

Parent population

Inbreeding

F1 hybrid population

Inbred linepopulation

Each hybrid canbe produced largescale from its twoparental inbreds

Genotypes cannot be reproduced

Rare desirablegenotypes

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

Effects of inbreedingSelfing, full/half-sib pollination

Reduced height, seed set, disease resistance, etcIncreased lodging, Increased homozygosity

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

Measurement of HeterosisMeasurement 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

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Real data from dry beansReal data from dry beans% heterosis above HP% heterosis above HP

2020

• 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 = 4.96A457 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

Genetic basis of heterosisGenetic 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

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Dominance HypothesisDominance 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

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Dominance HypothesisDominance HypothesisExampleExample

• 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

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Discussion of Discussion of Dominance hypothesisDominance 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.

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Dominance hypothesisDominance hypothesisLinkageLinkage

• 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.

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A bA b

a Ba B

Repulsion phase linkageRepulsion phase linkage

• In 70’s investigators were interested in the relative magnitude of s2

A and s2D

• In F2 crosses the ratio of s2D / s2

A was >1 indicating large amounts of dominance variance, but once the populations were random mated for several generations the ratio of s2

D / s2A was became <1,

this was likely due to recombination among repulsion phase loci

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A bA b

a Ba B

OverdominanceOverdominance 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.

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OverdominanceOverdominanceSuperiority 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.

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Pseudo- Pseudo- OverdominanceOverdominance

• 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

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A bA b

a Ba B

EpistasisEpistasis

• Epistasis - interaction among loci, may also contribute to heterosis

Internode Generation No. nodes length HeightParent 1 3 1 3Parent 2 1 3 3Hybrid ( add) 2 2 4Hybrid ( Dom) 3 3 9

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EpistasisEpistasis• 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.

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Prediction of Prediction of heterosisheterosis

• 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

combinationsDue to dominance effects and heterosis.

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Genetic distance and heterosisGenetic distance and heterosis

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

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Hete

rosi

s

Genetic distance

Relationship between genetic Relationship between genetic distance and heterosisdistance and heterosis

Smith et. Al. TAG 1990Smith et. Al. TAG 1990

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Note, r2 of 0.76

Relationships between genetic Relationships between genetic distance and Heterosisdistance and Heterosis

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No relationship

Heterosis for yield in self -Heterosis for yield in self -

pollinated vegetablespollinated vegetables

Crop Mean % RangeTomato (fresh) 41 -59 to 168

(solids) -10 -45 to 53Sweet pepper 15 -16 to 52Eggplant 80 -29 to

242Beans (dry) 29 -38 to 146Peas 28 116 to 218

Lettuce 6 -6 to 119

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Hybrid Rice in ChinaHybrid 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?

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