Single Seed Descent
Single Seed Descent
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!!
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.
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.
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.
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.
Season 1: F
2
plants
grown. One F
3
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.
Season 2: Bulk of F
3
seed is planted. One
F
4
seed per plant is
harvested and all
seeds are bulked.
Collect a reserve
sample of 1
seed/plant.
Season 3: Repeat.
Season 4: Grow bulk of
F
5
seed and harvest
individual plants
separately.
Season 5: Grow F
5
:
6
lines in rows; select
among rows and harvest
selected rows in bulk.
Season 6: Begin
extensive testing of
F
5
derived lines.
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 F
4
plants and 70% of the seeds in
each generation will produce
plants with at least one seed.
Then, by working back to the
F
2
generation, you need to
plant 584 F
2
plants. Be sure
to take this into account when
selecting the number of F2
seeds.
Each single seed
traces back to a
single F2 plant.
Theoretically, if you
start with a large
enough F2 sample, then
by the F
5
generation
you will still have a
broad representation
of variability from
the cross.
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.
SSD’s 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!
Single Hill Procedure
It can be used to
ensure that each F
2
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.
Multiple Seed
Procedure
To avoid starting
with a large F
2
population that
compensates for loss
of seed over
generations, bulk 2-3
seeds per plant at
harvest.
Genetic Considerations
1) Additive genetic variation
among individuals increased at a
rate of (1+F)
2
A
where F=0 in F
2
.
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.
There may be a reduction in
2
G
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. 34:55-61).
Pros
• Easy way to maintain pops
during inbreeding
• Natural selection does not
influence pops
• Well suited to GH and off
season nurseries
Cons
• Selection based on individual
phenotype rather than progeny
performance
• Natural selection cannot
influence pop in a positive
manner
Doubled Haploid
Breeding
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.
Doubled Haploids
• Where do the haploids come
from?
• Naturally occurring
– Maternally derived
– Paternally derived
Maternally Derived
Haploids
• Maize - cross normal color
(recessive) female parent x
purple color male parent
• Germinate the F1 seeds
• Purple seedlings are F1’s
• Green seedlings are haploids
(or selfs)
Paternally Derived
Haploids
• Occur at a very low frequency
• Not practical for use in a
breeding progam
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
Interspecific Crosses
• Hordeum bulbosum method:
– Emasculate H. vulgare (2n=2x=14)
– Pollinate with H. bulbosum
(2n=2x=14)
– Treat with hormones
– Culture embryo
– Treat seedlings with colchicine
– Place in pots, harvest selfed seed
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
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
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?
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
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
Use of DH in Recurrent
Selection
• Griffing (TAG 46:367 -) 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