PBG 650 Advanced Plant Breeding
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PBG 650 Advanced Plant Breeding. Module 12: Selection Inbred Lines and Hybrids. Selection for a high mean. Success is a function of the population mean  the deviation of the best segregants from  ability to identify the best segregants Advanced Cycle Breeding = “inbred recycling”

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PBG 650 Advanced Plant Breeding

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Pbg 650 advanced plant breeding

PBG 650 Advanced Plant Breeding

Module 12: Selection

  • Inbred Lines and Hybrids


Selection for a high mean

Selection for a high mean

  • Success is a function of

    • the population mean 

    • the deviation of the best segregants from 

    • ability to identify the best segregants

  • Advanced Cycle Breeding = “inbred recycling”

    • cross best by best (often related)

    • pedigree and backcross selection

    • emphasis on high mean at the expense of G2

    • need methods for predicting 

Bernardo Chapt. 4


Probability of fixing favorable alleles during inbreeding

Probability of fixing favorable alleles during inbreeding

Standardized effect of a locus

  • Three approaches to increase chances of fixing favorable alleles

    • selection before inbreeding

    • selection during inbreeding

    • one or more backcrosses to the better parent before inbreeding

Relative fitness

A1A1A1A2 A2A2

(no dominance)

  • Recombinant inbred from an F2

    • without selection

    • with selection

(Because p=1/2)


Mean with selfing

Mean with selfing

  • Inbreeding decreases the mean if there is dominance

  • At fixation (with no selection):

Genotypic Value

A1A1A1A2 A2A2

p2+pqF

2pq(1-F)

q2+pqF

Frequency

does not depend on dominance

RI = recombinant inbred lines


Mean of recombinant inbreds from a single cross

Mean of recombinant inbreds from a single-cross

Mean of recombinant inbreds derived from F2 of a single-cross

Means of the parents

(for a single locus)

  • The mean of recombinant inbreds derived from an F2 or backcross population can be predicted as a simple function of allele frequencies (the contribution of the parents)

A = 6 t/ha

B = 4 t/ha

RI[(AxB)(A)BC1] = ¾*6 + ¼*4 = 5.5 t/ha


Selfed families from a single cross

Selfed families from a single-cross

F2=S0 plant

F3=S1 plant

F4=S2 plant

F5=S3 plant

F3=S1 family

F4=S2 family

F5=S3 family

represents S0 plant

represents S1 plant

represents S2 plant


Selfed families from a single cross1

Selfed families from a single-cross

F2

¼A1A1½A1A2¼A2A2

F3

¼A1A1⅛A1A1¼A2A2

¼A1A2

⅛A2A2

Bernardo, Chapt. 9


Variance among and within selfed families

Variance among and within selfed families

F3

¼A1A1⅛A1A1¼A2A2

¼A1A2

⅛A2A2


Genetic variance with selfing

Genetic variance with selfing


Inbreeding as a selection tool for opvs

Inbreeding as a Selection Tool for OPVs

  • More genetic variation among lines

  • Increased uniformity within lines

  • Visual selection can be done for some traits

  • Permits repeated evaluation of fixed genotypes in diverse environments, for many traits

  • Sets of inbred lines can be used to identify marker-phenotype associations for important traits

  • Best lines can be intermated to produce synthetic varieties with defined characteristics


Testcrosses

Testcrosses

  • The choice of tester will determine if an allele is favorable or not

Bernardo, Section 4.5


Effect of alleles in testcrosses

Effect of alleles in testcrosses

Tester is an inbred line or population in HWE

Genotypic Value

A1A1A1A2 A2A2

ppT

pqT+ pTq

qqT

Frequency


Testcross mean of recombinant inbreds

Testcross mean of recombinant inbreds

Testcross means of parental inbreds

Testcross mean of recombinant inbreds derived from F2 of a single-cross

  • The testcross mean of recombinant inbreds derived from an F2 or backcross population can be predicted as a simple function of allele frequencies (the contribution of the parents)

T=AxC and BxC

TA = 8 t/ha

TB = 6 t/ha

For RI derived from the F2 of AxB

TRI(AxB) = ½*8 + ½*6 = 7 t/ha


Testcross means

Testcross means

  • Testcross mean of the heterozygote is half-way between the two homozygotes

  • Cross “good” by “good”

  • But, the correlation between the performance of inbred lines per se and their performance in testcrosses is very poor for yield and some other agronomic traits


Heterosis or hybrid vigor

Heterosis or Hybrid Vigor

  • Quantitative genetics:

    • superiority over mean of parents

  • Applied definition

    • superiority over both parents

    • economic comparisons need to be made to nonhybrid cultivars

  • Various types

    • population cross

    • single-, three-way, and double-crosses

    • topcrosses

    • modified single-cross

Bernardo, Chapt. 12


Heterosis

Heterosis

  • Amount of heterosis due to a single locus = d

  • 50% is lost with random-mating

A1A1 x A2A2

A1A2

F1

F2

¼A1A1½A1A2¼A2A2


Theories for heterosis

Theories for Heterosis

  • Dominance theory: many loci with d  a

    • Should be possible to obtain inbred  single-cross

    • Expect skewed distribution in F2 (may not be the case if many loci control the trait)

  • Overdominance theory: d > a

  • Pseudo-overdominance - decays over time

+1 -2

-1 +2

+1

  • tight, repulsion phase linkages

  • partial to complete dominance

A1 B2

A2 B1

A1 B2

X

A1 B2

A2 B1

A2 B1

+2


Heterosis some observations

Heterosis – some observations

  • Experimental evidence suggests that heterosis is largely due to partial or complete dominance

  • Yields of inbred lines per se are poor predictors of hybrid performance

    • due to dominance

    • hybrids from vigorous lines may be too tall, etc.

    • due to heritability <1

  • Heterosis generally increases with level of genetic divergence between populations, however….

    • There is a limit beyond which heterosis tends to decrease

    • A high level of divergence does not guarantee that there will be a high level of heterosis


Heterosis more observations

Heterosis – more observations

  • Epistasis can also contribute to heterosis

    • does not require d>0

  • Selection can influence heterosis

  • Iowa Stiff Stalk Synthetic (BSSS)

  • Iowa Corn Borer Synthetic (BSCB1)

  • High density SNP array shows increasing divergence over time in response to reciprocal recurrent selection

Gerke, J.P. et al., 2013 arXiv:1307.7313 [q-bio.PE]


Heterotic groups

Heterotic groups

  • Parents of single-crosses generally come from different heterotic groups

  • Two complementary heterotic groups are often referred to as a “heterotic pattern”

  • Temperate maize

    • ‘Reid Yellow Dent’ x ‘Lancaster Sure Crop’

    • Iowa Stiff Stalk x Non Stiff Stalk

  • Tropical maize

    • Tuxpeño x Caribbean Flint


Identifying heterotic patterns

Identifying heterotic patterns

  • Diallel crosses among populations

  • Crosses to testers representing known heterotic groups

  • Use molecular markers to establish genetic relationships, and make diallel crosses among dissimilar groups

    • initial studies were disappointing

    • markers must be linked to important QTL


Exploiting heterosis

Exploiting heterosis

  • Recycle inbreds within heterotic groups

  • Evaluate testcrosses between heterotic groups

    • elite inbreds often used as testers

  • BLUP can predict performance of new single-crosses using data from single-crosses that have already been tested

    • fairly good correlations between observed and predicted values


What is a synthetic

What is a synthetic?

  • Lonnquist, 1961:

    • Open-pollinated populations derived from the intercrossing of selfed plants or lines

    • Subsequently maintained by routine mass selection procedures from isolated plantings

  • Poehlman and Sleper:

    • Advanced generation of a seed mixture of strains, clones, inbreds, or hybrids

    • Propagated for a limited number of generations by open-pollination

    • Must be periodically reconstituted from parents

    • Parents selected based on combining ability or progeny tests


Predicting hybrid performance

Predicting hybrid performance

Three-way crosses

Double-crosses

Wright’s

Formula

Synthetics

= avg yield of all F1 hybrids n = number of parents

= avg yield of parents


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