Single Seed Descent

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Single Seed Descent
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SSD

• Single seed descent can be used in self or cross
  pollinated crops. It is a method of inbreeding a
  segregating population that is quite conducive to
  environments that are not typical good news for
  off-season nurseries!!

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Goulden (1941) proposed a similar system (without
calling it SSD) and it resulted from the interest
of plant breeders to rapidly inbreed populations
before evaluating individual lines. He noted
that a wheat breeding program could be divided
into the development of pure lines from a
segregating populations and selection among the
best of those lines. He emphasized that with the
pedigree method, plants had to be grown in an
environment in which genetic differences would be
expressed for the characters under selection and
thus probably limited to one generation per year.

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Also, the pedigree method is based on the premise
that progress in obtaining the lines with the
required characteristics can be made at the same
time as the lines are being selected for
homozygosity. His alternative was to separate
the inbreeding and selecting generations in order
to speed the process along.
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By doing this, the number of progeny grown from a
plant in each generation should be one or two
only, and two generations can be grown in the
greenhouse and one in the field. He proposed the
model with spring-sown cereals. In this manner,
he could attain the F6 generation in 2 years, as
opposed to 5 years as with the pedigree method.
After the desired level of homozygosity was
achieved, the lines could then be tested for
desired characteristics.
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Single Seed Procedure This is the classic
procedure of having a single seed from each
plant, bulking the individual seeds, and planting
out the next generation.
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Season 1 F2 plants grown. One F3 seed per plant
is harvested and all seeds are bulked. Collect a
reserve sample of 1 seed/plant. Brim suggested
harvesting the 2-3 seeded soybean pod and using 1
seed for planting and 1-2 for reserve.
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Season 2 Bulk of F3 seed is planted. One F4
seed per plant is harvested and all seeds are
bulked. Collect a reserve sample of 1
seed/plant. Season 3 Repeat.
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Season 4 Grow bulk of F5 seed and harvest
individual plants separately.Season 5 Grow
F56 lines in rows select among rows and harvest
selected rows in bulk.Season 6 Begin extensive
testing of F5 derived lines.
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In reality, the population size will decrease
with each generation (due to lack of germination,
lack of seed set, etc.). So if you want 200 F4
plants and 70 of the seeds in each generation
will produce plants with at least one seed.
Then, by working back to the F2 generation, you
need to plant 584 F2 plants. Be sure to take
this into account when selecting the number of F2
seeds.
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Each single seed traces back to a single F2
plant. Theoretically, if you start with a large
enough F2 sample, then by the F5 generation you
will still have a broad representation of
variability from the cross.
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As stated previously, the breeder must expect the
genotypic frequencies in a bulk population to
change during the propagation period. To
eliminate, or at least reduce, shifts in
genotypic frequencies in bulk populations, Brim
(1966) proposed using the Modified Pedigree
Method a modification of the SSD. The true
single seed descent method maintains the total
genotypic array. The modified pedigree is
similar but allows some selection during
inbreeding.
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SSDs Bonus Points Rapid generation advance,
maintenance of an unbiased broad germplasm base,
labor and time efficient, able to handle large
number of samples, and easily modified!
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Single Hill Procedure It can be used to
ensure that each F2 plant will have progeny in
the next generation of inbreeding. Progeny from
individual plants are maintained as separate
lines during each generations by using a few
seeds per hill and harvesting those to plant back
the following year.
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Multiple Seed Procedure To avoid starting
with a large F2 population that compensates for
loss of seed over generations, bulk 2-3 seeds per
plant at harvest.
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Genetic Considerations 1) Additive genetic
variation among individuals increased at a rate
of (1F)?2A where F0 in F2. There is little
natural selection, except for seed germination
potential or where the environment prevents some
genotypes from setting seed. In multiple seed
procedure, there may be a variation associated
with sampling of seed from a bulk samples to
plant the next generation. This sampling results
in exclusion of progeny from some plants, and
multiple representation of progeny from others.
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There may be a reduction in ?2G but is slight.
Multiple seed descent indicated about 18 of the
lines were due to repetitive sampling and did not
represent independent lineages. (See Keim et
al., 1994, Crop Sci. 3455-61).
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Pros

• Easy way to maintain pops during inbreeding
• Natural selection does not influence pops
• Well suited to GH and off season nurseries

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Cons

• Selection based on individual phenotype rather
  than progeny performance
• Natural selection cannot influence pop in a
  positive manner

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Doubled Haploid Breeding
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Doubled Haploids

• What are they?
• Homozygous diploid lines that come from doubling
  the chromosome number of haploid individuals.
• Heterozygous haploid individuals are produced,
  the chromosome number doubled, and an array of
  inbred homozygotes results.

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Doubled Haploids

• Where do the haploids come from?
• Naturally occurring
• Maternally derived
• Paternally derived

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Maternally Derived Haploids

• Maize - cross normal color (recessive) female
  parent x purple color male parent
• Germinate the F1 seeds
• Purple seedlings are F1s
• Green seedlings are haploids (or selfs)

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Paternally Derived Haploids

• Occur at a very low frequency
• Not practical for use in a breeding progam

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Interspecific Crosses

• Concept Make a very wide cross
• Use the species of interest as female
• Fertilization Occurs
• Chromosomes of wild species are eliminated
• Use embryo rescue to recover haploid embryo

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Interspecific Crosses

• Hordeum bulbosum method
• Emasculate H. vulgare (2n2x14)
• Pollinate with H. bulbosum (2n2x14)
• Treat with hormones
• Culture embryo
• Treat seedlings with colchicine
• Place in pots, harvest selfed seed

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Interspecific Crosses

• Wheat x maize method
• Emasculate wheat plant
• Pollinate with fresh maize pollen
• After several days maize chromosomes eliminated
• Rescue embryo and place in culture
• Treat seedling with colchicine, harvest selfed
  seed

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Anther culture

• Anthers, or in some cases, microspores (pollen
  cells) can generate haploids
• Haploids are grown in tissue culture
• Callus is induced to differentiate through
  hormone treatments
• Plantlets are obtained and treated with
  colchicine
• Selfed seed harvested

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Anther culture

• In tobacco, found that anther derived di-haploids
  (doubled haploids) were more variable and less
  fit than SSD lines
• Why?
• Residual heterozygosity?
• Alterations brought about through tissue culture?

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Pros

• Homozygosity achieved rapidly
• Selection among homozygotes more efficient than
  selection among heterozygotes
• Homozygous, homogeneous seed source available for
  release
• Dominance not a problem when selecting among
  haploids

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Cons

• Requires a well-oiled machine method of
  producing haploids
• Evaluation of inbred lines will require at least
  as much time as usual
• May be problems among the DH ( tobacco example)
• Not feasible to use with all of your populations
• Frequency of haploid production impossible to
  predict

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Use of DH in Recurrent Selection

• Griffing (TAG 46367 -) shows that if an
  efficient DH extraction method can be devised DH
  based selection will be much more efficient than
  diploid selection
• In the case of individual selection, given
  certain parameter values, theory says that
  individual DH selection can be 6 times as
  efficient as individual diploid selection