Host Plant ResistancePlant Breeding

1
Host Plant ResistancePlant Breeding Review Corn
Ear Worm (Helicoverpa zea) Breeding techniques
applicable to genetic improvement Understand the
problem Basic Plant Breeding Terminology
Basics of Gene/Trait Segregation Effects of
Selection in Cross Pollinated Crops Breeding
Strategies/Methods in Cross Pollinated
Crops Nursery Screening Strategies/Methods for
CEWR Finding Resistant Parents
2
Host Plant ResistancePlant Breeding Insect pest
Corn Ear Worm (also called the tomato fruitworm
and the cotton bollworm) Crop Corn (Zea mays
L.) tomato, cotton, green beans, clover, vetch,
lettuce, peppers, soybeans, and sorghum Insect
Heliocoverpa zea Distribution native to North
and South America commonly found wherever corn is
grownestimated to infest at economically
damaging levels of up to 35 million acres of
cornpresent in an estimated 98 of Texas corn
fields with essentially none treated (except for
food corn or sweet corn) General effects on
corn reduced yield losses in corn can be as
high as 50 in sweet corn and range from 1.5 to
16.7 for other corn types, representing 315,000,
000 to 3,501,000,000 loss to U.S. field corn
producers reduced grain quality opportunities
and challenges
98 of Texas corn fields infested annually
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Host Plant ResistancePlant Breeding
Adults yellowish-brown moth 1-1/2 inches (37
mm) wingspand same group as the common armyworm
moth vary in colorgenerally front wings are a
light tannish brown, marked with dark gray
irregular lines and a dark area near the tip of
the wing.
Eggs laid at night individually on underside
of leaves by the first generation or in the
cornsilks by the second generation white,
hemispherical, about one-half the diameter of a
pin head eggs hatch in 2-10 days.
Larvae 1-1/2 inches (37 mm) tan head
prominent alternating light and dark lengthwise
stripes vary in color from a light green or pink
to brown or nearly black
Pupae full grown leave the corn ears and drop
or crawl to the ground burrow moth will
emerge 10-25 days later depending on temperature.
The development from egg to adult usually takes
about 30 days in midsummer.
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Host Plant ResistancePlant Breeding
newly hatched worms pass through the silk channel
of the developing ear, feeding along the way to
the kernelleave clusters of moist fecal matter
at the tip in the husk and in the silk
channelkernels, especially near the tip of the
ear, will be eaten down to the cob.
By cutting off the silks, pollination is
prevented and poor ear fill may result.
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Host Plant ResistancePlant Breeding
Control problems larvae are protected from
insecticides once they enter the ear insecticide
timing is critical, and typically targets the
egg and young larval stages. Insecticides must
be present on silks at egg hatch often too
costly to treat field corn with
insecticides Another option for growers is
Bt-transgenic field cornexpresses an
insecticidal toxin throughout the plant, and
corn earworm is killed before causing damage
Bt field corn also preserves natural
enemies, further reducing earworm
populations.
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Host Plant ResistancePlant Breeding
So why should breeders devote time to genetic
control? yield losses range from 1.5 to 16.7
for field corn types, representing 315,000,000
to 3,501,000,000 in lost income to U.S. field
corn producers in 2005 in sweet corn losses can
be as high as 50 and result in lower value or
lost sales 2007 favorite means of control is
Bt or GMO Bt-corn is also expensive, and is
seldom warranted for treating corn earworm
alone (there are other pests that warrant use of
this technology)
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Host Plant ResistancePlant Breeding
Management plant resistant hybrids altering
planting dates to avoid high densities of corn
earworms resistant hybrids limit the amount of
injury to both the leaf and the ear A
combination of silks that are antibiotic to
larvae husks that are tight around the ear
alter larval behavior most effective
resistance feeding is limited to the ear tip,
resulting in small larvae or larvae that leave
the ear before completing development Bt
hybrids suppress corn earworm populations and
reduce the amount of injury to the ear Crop
rotation nor tillage significantly influences
corn earworm survival moths prefer corn to
beans, tomatoes, and other crops, borders or
strips of corn planted as a trap crop around or
within fields of other vegetables
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Chemical control of the corn earworm can be
expensive most spraying occurs in sweet corn
fields where a majority of the market value is in
the quality of the ears. Since larvae move down
the silk channels as soon as they hatch, the
timing of insecticide applications is very
important. As the larvae move down the silks and
under the husk of the ear, insecticide sprays
become ineffective. For insecticides to work
effectively,spray residues need to be present on
the silks where the eggs hatch. There will be no
insecticide residue on new silk growth. Many
producers follow a regular spray schedule based
on the number of captured moths from pheromone
traps. Others base the spray schedule on the
injury sustained to the whorl or tassel. To the
right is a spray schedule for corn earworm in
sweet corn (from the University of Minnesota)
based on the number of corn earworm moths caught
in a pheromone baited cone trap. (Counts are from
a nylon trap a full size Hartstack wire trap
catches more moths, so multiply counts in the
first column by 1.5 to get the approximate for a
wire trap.)
Typically fields are most vulnerable when in the
brush and brown silking stages. Control of corn
earworm is difficult because larvae are protected
from insecticides once they enter the ear. For
this reason, insecticide timing is critical, and
typically targets the egg and young larval
stages. Insecticides must be present on silks at
egg hatch to kill young larvae. It is often too
costly to treat field corn with insecticides.
Another option for growers is Bt-transgenic field
corn. This corn expresses an insecticidal toxin
throughout the plant, and corn earworm is killed
before causing damage to the ears. Another
benefit of Bt field corn is that it preserves
natural enemies, further reducing earworm
populations. However, Bt-corn is also expensive,
and is seldom warranted for treating corn earworm
alone.
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Host Plant ResistancePlant Breeding
10
Host Plant ResistancePlant Breeding
Breeding Methods in Cross-Pollinated Crops
Mass Selection Recurrent Selection
Reciprocal Recurrent Selection Synthetic
Cultivars Hybrids When would each type be
appropriate? Revisit later
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Host Plant ResistancePlant Breeding Example
Number 2 of cross pollinated species alfalfa,
Medicago sativa Alfalfa grown for seed production
requires good pollinator (bees) activity during
bloom since plants grown from self -fertilized
seed frequently lack vigor and produce less
forage. When alfalfa flowers are self -pollinated
a low percentage of the ovules are fertilized,
few pods set, and the pods are small since only 1
or 2 seeds form. Alfalfa flowers have a special
mechanism that must be tripped in order to be
cross-pollinated and to produce seed in
commercial quantities. When a bee inserts its
proboscis into the flower, it separates the keel
petals which bears the stamens and pistil. When
released (or tripped) the sexual column strikes
the bee, pollen is deposited on the bees head,
and in the same action, the stigma usually
receives pollen brought by the bee from another
plant.
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Host Plant ResistancePlant Breeding
(Left) Close-up of yellow-flowered alfalfa,
Medicago sativa subsp. falcata (Right) Some of
the 100 falcata accessions evaluated at Ames from
1997-1999.
Two subspecies of Alfalfa Medicago sativa
subsp. sativa subsp. falcata
Autotetraploid Opportunity heterosis
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Host Plant ResistancePlant Breeding
Yield is essential for inclusion of alfalfa in
intensive farming systems.  In the dairy
industry, where much alfalfa is used, the
alternative to alfalfa is corn silage. Alfalfa
is often desirable to as hay, silage, AND erosion
control. Alfalfa must be high yielding, or it
will be replaced in the cropping system, to
the detriment of the environment. Alfalfa
must persist for multiple years.  A major
determinant of persistence in Iowa and the
northern US and Canada is winter
hardiness. Yield stability is important
relative to corn and soybeans as are both
dependable and alternative crops.
Insect (as well as diseases and nematodes)
resistance are important but most breeders
work with yield per se since yields of alfalfa
over the past 25 years have been stagnant or
decreasing! One other issue! Pollination is by
bees
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Host Plant ResistancePlant Breeding
Alfalfa Weevil, Hypera postica, and Damage
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Interesting Note Seed production is secondary to
hay production The alfalfa flower must be
tripped to set seed. This is done best by
pollinating insects, primarily wild bees and
honeybees. Producers must take care when
applying insecticides not to diminish the bee
population. Protecting nesting places of wild
bees. Use 3 to 6 hives of honeybees per acre.
Controlling flowering weeds and competitive
crops that may attract bees away from the
alfalfa during the flowering period.
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Breeding Cross Pollinated Crop Species Self
fertilization Homozygosity,
i.e. cant maintain heterozygosity because each
gene is segregating to homozygous dominant or
homozygous recessive AND THUS heterozygosity is
reduced by half for each generation of
selfing Cross fertilization Heterozygosity is
maintained in the absence of selection because
every gamete has an equal probability of
combining with every other gamete We described
the genetic makeup of a selfed population through
the understanding that a gene containing two
different alleles will always segregate in a
121 ratio. Hardy-Weinberg Equilibrium (HWE)
describes the genetic makeup of a cross
pollinating population a population of a diploid
species, considering 2 alleles or 1 allele versus
all other alleles at that locus, can be described
by the expansion of (a b)2 or in our terms (p
q)2 p2 2pq q2.
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Breeding Cross Pollinated Crop Species Hardy-Weinb
erg Equilibrium (HWE) describes the genetic
makeup of a cross pollinating population a
population of a diploid species, considering 2
alleles or 1 allele versus all other alleles at
that locus, can be described by the expansion of
(a b)2 or in our terms (p q)2 p2 2pq
q2. Genotypic and allelic freq. will not change
in a cross pollinating species under the
following assumptions - random mating - no
selection is practiced (naturally or otherwise)
- no differential migration - mutation rates are
equal, i.e. A?a a?A - species in question is
diploid - usually considering 1 gene and 2
alleles or 1 allele of an allelic series
against all other alleles in the series
Actually HWE can be use with multiple genes
(not in 610)
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Effects of Selection on Hardy-Weinberg Consider
a population with 2 alleles, A1 and A2, such
that A1 p 0.4 A2 (1-p) 0.6 So, p2
2p(1-p) (1-p)2 .42 2 (.4)(.6) .62 .16
A1A1 .48 A1A2 .36 A2A2 ? 1.0
20
Effects of Selection on Hardy-Weinberg Select
against the recessive, A2A2, HW p2 2p(1-p)
(1-p)2 .42 2 (.4)(.6) .62 .16 A1A1 .48
A1A2 .36 A2A2 Select against the recessive,
A2A2,
21

• Effects of Selection on Hardy-Weinberg
• Select against the recessive, A2A2,
• HW p2 2p(1-p) (1-p)2
• .42 2 (.4)(.6) .62
• .16 A1A1 .48 A1A2 .36 A2A2
• First recalculate allelic frequencies
• (.16 A1A1 .48 A1A2) 0.64
• Freq. of A1 .08 .08 (.5)(.48) / .64
  0.625 p
• Freq. of A2 by calculation or subtraction
  0.375 (1-p)
• Second recalculate HW .6252 2(.625)(.375)
  .3752
• .39 A1A1 .47 A1A2 .14 A2A2

22

• Effects of Selection on Hardy-Weinberg
• Select against the recessive, A2A2,
• HW p2 2p(1-p) (1-p)2
• .42 2 (.4)(.6) .62
• .16 A1A1 .48 A1A2 .36 A2A2
• .390625 A1A1 .46875 A1A2 .140625 A2A2
• Is this population in HW Equal.?

23

• Effects of Selection on Hardy-Weinberg
• Select against the recessive, A2A2,
• HW p2 2p(1-p) (1-p)2
• .42 2 (.4)(.6) .62
• .16 A1A1 .48 A1A2 .36 A2A2
• .390625 A1A1 .46875 A1A2 .140625 A2A2
• Is this population in HW Equal.?
• Determine by recalculating HW frequencies!
• p.390625 (.5)(.46875) 0.625 A1 previous
  generation
• Therefore this generation is in HWE AND we had
  decreased the proportion of A2A2 from 0.36 to
  0.14

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Effects of Selection on Hardy-Weinberg Selection
After Pollination In the previous example, we
made selections before pollination, i.e. we
removed some plants and allowed the remainder to
intermate without restriction (population
size??) What happens to HW if we cant see A2A2
until after pollination? Well, lets start with
the same population .16 A1A1 .48 A1A2 .36
A2A2 Essentially, our selections are going to be
maternal selections i.e., the pollen can and
will come from any plant in the nursery but we
will attempt to select on A1A1 and A1A2 females
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• Effects of Selection on Hardy-Weinberg
• Selection After Pollination
• Therefore
• let pf and (1-pf) the freq. of A1 and A2 for
  the female gametes
• and pm and (1-pm) the freq. of A1 and A2 for
  the male gametes
• Such that for the female gametes
• .16 A1A1 .48 A1A2 .36 A2A2 (same original
  population)
• First recalculate allelic frequencies
• (.16 A1A1 .48 A1A2) 0.64
• Freq. of A1 .08 .08 (.5)(.48) / .64
  0.625 pf
• Freq. of A2 by calculation or subtraction
  0.375 (1-pf)

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Effects of Selection on Hardy-Weinberg Selection
After Pollination Therefore let pf and (1-pf)
the freq. of A1 and A2 for the female gametes and
pm and (1-pm) the freq. of A1 and A2 for the
male gametes Since we do not select male
gametes they will have the same allelic
frequencies as the original population, i.e pm
.4 A1m 1-pm .6 A2m Such that HW
becomes pmpf pm(1-pf) pf(1-pm)
(1-pm)(1-pf)
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Effects of Selection on Hardy-Weinberg Selection
After Pollination Therefore If pm .4 AND 1-pm
.6 AND pf .625 AND 1-pf .375 (SEE NEXT
DUPLICATE SLIDE) Then HW becomes pmpf
pm(1-pf) pf(1-pm) (1-pm)(1-pf) (.4)(.625)
A1A1 .4(.375) .625(.6) A1A2 (.6)(.375)
A2A2 .25 A1A1 .525 A1A2 .225 A2A2 ? 1.0
28

• Effects of Selection on Hardy-Weinberg
• Select against the recessive, A2A2,
• HW p2 2p(1-p) (1-p)2
• .42 2 (.4)(.6) .62
• .16 A1A1 .48 A1A2 .36 A2A2
• First recalculate allelic frequencies
• (.16 A1A1 .48 A1A2) 0.64
• Freq. of A1 .08 .08 (.5)(.48) / .64
  0.625 p
• Freq. of A2 by calculation or subtraction
  0.375 (1-p)
• Second recalculate HW
• .6252 2(.625)(.375) .3752
• .390625 A1A1 .46875 A1A2 .140625 A2A2

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Effects of Selection on Hardy-Weinberg Selection
After Pollination .25 A1A1 .525 A1A2 .225
A2A2 ? 1.0 Is this population in HWE? First
recalculate p and (1-p) p .25 (.5)(.525)
.5125 1-p .4875 Therefore (.5125)2
2(.5125)(.4875) (.4875)2 .262656 A1A1
.499688 A1A2 .237656 A2A2 ? 1.0
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Effects of Selection on Hardy-Weinberg Selection
After Pollination .25 A1A1 .525 A1A2 .225
A2A2 ? 1.0 Is this population in HWE? First
recalculate p and (1-p) p .25 (.5)(.525)
.5125 1-p .4875 Therefore (.5125)2
2(.5125)(.4875) (.4875)2 .262656 A1A1
.499688 A1A2 .237656 A2A2 ? 1.0
NO WHY NOT?
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• Effects of Selection on Hardy-Weinberg
• Original cross pollinating population in HWE
• .16 A1A1 .48 A1A2 .36 A2A2
• Selection against A2A2 before pollination
• .390625 A1A1 .46875 A1A2 .140625 A2A2
• Selection against A2A2 after pollination
• .25 A1A1 .525 A1A2 .225 A2A2
• So selection before pollination (i.e. control
  both male and female gametic selection) is about
  twice as effective as after pollination!

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Effects of Selection on Hardy-Weinberg What if
this had been a self pollinating population? What
would have been our progress with 1 generation of
selection against A2A2? Original genotypic
frequencies .16 A1A1 .48 A1A2 .36
A2A2 Therefore the new genotypic frequencies
are .16/(.16 .48) .25 A1A1 .48/(.16 .48)
.75 A1A2
33
Effects of Selection on Hardy-Weinberg What if
this had been a self pollinating population? What
would have been our progress with 1 generation of
selection against A2A2? Original genotypic
frequencies .16 A1A1 .48 A1A2 .36
A2A2 Therefore the new genotypic frequencies
are .16/(.16 .48) .25 A1A1 .48/(.16 .48)
.75 A1A2
.25(.75) A1A1 .5(.75) A1A2 .25(.75) A2A2
.1875 A1A1 .375 A1A2 .1875 A2A2
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Effects of Selection on Hardy-Weinberg What if
this had been a self pollinating population? What
would have been our progress with 1 generation of
selection against A2A2? Original genotypic
frequencies .16 A1A1 .48 A1A2 .36
A2A2 Therefore the new genotypic frequencies
are .16/(.16 .48) .25 A1A1 .48/(.16 .48)
.75 A1A2
.25(.75) A1A1 .5(.75) A1A2 .25(.75) A2A2
.1875 A1A1 .375 A1A2 .1875 A2A2
.25 .1875 A1A1 .375 A1A2 .1875 A2A2 ..4375
A1A1 .375 A1A2 .1875 A2A2 ? 1.0
35

• Effects of Selection on Hardy-Weinberg Versus
  Selfing
• Original cross pollinating population in HWE
• .16 A1A1 .48 A1A2 .36 A2A2
• Selection against A2A2 before pollination
• .390625 A1A1 .46875 A1A2 .140625 A2A2
• Selection against A2A2 after pollination
• .25 A1A1 .525 A1A2 .225 A2A2
• Selection in the same population through natural
  or forced selfing
• ..4375 A1A1 .375 A1A2 .1875 A2A2

36

• Breeding Methods in Cross-Pollinated Crops
• Mass Selection
• Recurrent Selection
• Reciprocal Recurrent Selection
• Synthetic Cultivars
• Hybrids

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Host Plant ResistancePlant Breeding
Breeding goal selection of improved cultivars by
increasing the frequency of desirable genes in
the population of plants. In cross-pollinated
crops like corn and alfalfa mass selection was
the first type of genetic improvement
undertaken corn, the development of vigorous
inbred lines and the ease of pollen control
(i.e., detasseling) led to the development of
hybrids. alfalfa, in contrast, is predominantly
accomplished through recurrent phenotypic
selection methods because of economic limitations
Recurrent selection is the identification of
and intermating of individual plants with
superior performance for a given trait or group
of traits. Once a desirable population of
plants is identified, plants are allowed to
randomly intermate and seed are collected from
the group of plants and identified as a
synthetic cultivar.
38

• Breeding Methods in Cross-Pollinated Crops
• Mass Selection
• Same form as with self-pollinated crops
• essentially a form of maternal selection since
  no pollination control
• select desirable plants
• bulk seed
• repeat cycle
• with strict selection breeder will reduce popul.
  Size
• slow genetic gain since lack pollination control
• must be able to ID superior phenotypes
• Not suitable for quantitative traits

39

• Breeding Methods in Cross-Pollinated Crops
• Mass Selection
• Recurrent Selection
• designed to increase the freq. of desirable
  genes within a population
• developed in the 1940s for developing inbred
  lines
• original corn inbreds came from OP varieties
• selection of new inbreds from these 1st
  generation hybrids did not result in improved
  2nd gen. hybrids
• so the theory evolved that the proportion of
  superior inbred lines depended on the proportion
  of superior alleles within the population
• so recurrent selection was devised to increase
  the frequencies of superior alleles

40

• Breeding Methods in Cross-Pollinated Crops
• Mass Selection
• Recurrent Selection
• Yr 1 self a number of plants
• select and harvest only superior plants at
  maturity (keep separate
• Yr 2 plant as progeny row nursery and allow to
  intermate thus re-establishing HWE
• Repeat cycle

41

• Breeding Methods in Cross-Pollinated Crops
• Mass Selection
• Recurrent Selection (Cycle 1)
• Year 1

x selfed x selected at maturity (superior
performing plant)
Year 2 Plant in an intercross block and allow
intermating to re-establish HWE


42

• Breeding Methods in Cross-Pollinated Crops
• Mass Selection
• Recurrent Selection (Cycle 2)
• Year 3

x selfed x selected at maturity (superior
performing plant)
Year 4 Plant in an intercross block and allow
intermating to re-establish HWE


43

• Breeding Methods in Cross-Pollinated Crops
• Mass Selection
• Recurrent Selection (Cycle n of continuing
  cycles)
• Year n

x selfed x selected to initiate inbred line
development MAY self and cross with a tester.


Year n1 Plant in an intercross block and allow
intermating to re-establish HWE AND performance
test hybrids
44

• Breeding Methods in Cross-Pollinated Crops
• Mass Selection
• Recurrent Selection
• Reciprocal Recurrent Selection
• Proposed by Comstock, Robinson, and Harvey
  (1949) to select for both general and specific
  combining ability
• General combining ability (GCA) is the ability
  of a breeding strain to combine favorably with
  many other strains or inbred lines. Analogous to
  ADDITIVE gene action
• Specific combining ability (SCA) is the ability
  of a genotype to combine favorably with one or a
  few other genotypes. Analogous to DOMINANT gene
  action

45
Yields for 45 single crosses (complete diallel
without selfs)
Which lines show GCA? SCA?
46

• Breeding Methods in Cross-Pollinated Crops
• Mass Selection
• Recurrent Selection
• Reciprocal Recurrent Selection General Outline
  for Improvement of Population A and Population B
• Yr. 1 Select individual plants before
  pollination in each popul.
• Self selected plants in each popul.
• Cross selected A plants to random B plants
• Cross selected B plants to random A plants
• Thus producing AB hybrids and BA hybrids
• Harvest selfed seed per plantplace in storage
• Harvest hybrid AB and BA seed and condition for
  planting

47

• Breeding Methods in Cross-Pollinated Crops
• Mass Selection
• Recurrent Selection
• Reciprocal Recurrent Selection General Outline
  for Improvement of Population A and Population B
• Yr. 2a. Plant AB hybrids in a performance trial
  (loc? reps?)
• Plant BA hybrids in a performance trial (loc?
  reps?)
• At maturity, harvest and determine superior AB
  and BA hybrids

48

• Breeding Methods in Cross-Pollinated Crops
• Mass Selection
• Recurrent Selection
• Reciprocal Recurrent Selection General Outline
  for Improvement of Population A and Population B
• Yr. 2b. Analyze data and determine the selfed A
  plants that produced superior AB hybrids and
  the selfed B plants that produced superior BA
  hybrids.
• Yr. 3. Retrieve selfed seed from storage and
  plant in an intermating nursery the A plants
  that produced superior AB hybrids to
  re-establish HWE
• Same for selfed B plants
• End first cycle of Reciprocal Recurrent Selection

49

• Breeding Methods in Cross-Pollinated Crops
• Mass Selection
• Recurrent Selection
• Reciprocal Recurrent Selection General Outline
  for Improvement of Population A and Population B
• NOTE Sometimes referred to as Reciprocal
  Half-sib Selection
• AND you can develop Reciprocal Full-sib Selection
  with cross pollinated species where you can
  produce both selfed and hybrid seed on the same
  plant (wheat, rice, corn lines that produce gt 1
  ear, grasses, others?) Hallauer, 1967 Lonnquist
  Williams, 1967

50

• Breeding Methods in Cross-Pollinated Crops
• Mass Selection
• Recurrent Selection
• Reciprocal Recurrent Selection
• Although Recurrent and Reciprocal Recurrent
  Selection methods were originally designed and
  used to improve the chances of developing
  superior inbreds of corn, these procedures are
  not used in private industry for that purpose
  today.
• Most new inbreds in corn and sorghum are
  developed through pedigree or pedigree type
  procedures

51

• Breeding Methods in Cross-Pollinated Crops
• Mass Selection
• Recurrent Selection
• Reciprocal Recurrent Selection
• Synthetic cultivars
• First Generation Synthetic (Syn 1)
• Advanced Generation Synthetic

52

• Breeding Methods in Cross-Pollinated Crops
• Synthetic cultivars
• First Generation Synthetic (Syn 1)
• First generation progenies derived by
  intercrossing (polycross nursery) a specific
  set of seed-propagated lines or clones
• Usually used with cross pollinated crop spp.
• Can be used with self pollinated spp. if
  genetic (usually) mechanism have been
  introduced to maximize cross pollination,
  e.g. male sterility
• Limited to the 1st generation progeny and
  therefore can not be reproduced from seed of the
  1st generation.
• Examples include pearl millet, rye, alfalfa,
  turf grasses

53

• Breeding Methods in Cross-Pollinated Crops
• Synthetic cultivars
• First Generation Synthetic (Syn 1)
• Advanced Generation Synthetic
• Consist of seed harvested from successive
  generations beyond the 1st generation of a
  synthetic
• The Syn 2 is derived from the Syn 1 Syn 3 from
  Syn 2 ,etc
• Examples include grasses and forage crops
• Advanced generation synthetics are stable usually
  for a limited number of generations and then must
  be reconstituted.

54
Breeding Methods in Cross-Pollinated
Crops Synthetic cultivars (additional
comments) Once superior clones or lines are
identified then they can be mated in various
combinations AND the performance of synthetics
MAY be predicted from the performance of
individual clones/lines. To predict the
performance of synthetic cultivars in a diploid
spp. Wright (1922) proposed the following
formula F2 MF1 (MF1 MP) / n, where F2
predicted performance of the synthetic MF1
mean performance of all possible single crosses
of n parents MP mean performance of n parents
55

• Breeding Methods in Cross-Pollinated Crops
• Synthetic cultivars (additional comments)
• Performance of Synthetic cultivars (contid)
• Random mating and a lack of epistasis are
  necessary to obtain a good relationship between
  actual and predicted values
• Formula suggest that Syn or F2 performance can
  be improved by
• increasing the combining ability of the parents
• increasing the number of parents
• increasing the performance of the parents
• However, as n increases, MP and MF1 tends to
  decrease

56

• Breeding Methods in Cross-Pollinated Crops
• Synthetic cultivars (additional comments)
• Performance of Synthetic cultivars (contid)
• The number of Synthetic Cultivars increases with
  the number of potential parents since a Syn can
  be composed from 2 to many lines/clones
• Parents possible synthetics
• 4 11
• 8 57
• 12 4,083
• n 2n n 1

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• Breeding Methods in Cross-Pollinated Crops
• Synthetic cultivars (additional comments)
• Performance of Synthetic cultivars (contid)
• The number of Synthetic Cultivars increases with
  the number of potential parents since a Syn can
  be composed from 2 to many lines/clones
• Parents possible synthetics
• 4 11
• 8 57
• 12 4,083
• n 2n n 1

with parents 1, 2, 3, and 4 1234 12 13 123 14
124 23 134 24 234 34
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• Breeding Methods in Cross-Pollinated Crops
• Synthetic cultivars (additional comments)
• Factors to consider in developing a polycross
  nursery
• clones must flower synchronously
• adequate isolation
• replication is essential to random pollination
  (consider that adjacent clones are more likely to
  intermate that more distant clones
• most common designs are RCB and Latin Square
• harvest seed from each clone within each rep
  separately and bulk a similar quantity from each
  to help insure random mating
• Similar to a blend but a blend or multi-line but
  these have to be reconstituted each year

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