Download BASIC FEATURES OF BREEDING

BREEDING BASED ON
THE GENETIC
RECOMBINATION
PRINCIPLE
Taryono
Faculty of Agriculture
Gadjah Mada University
Hybridization
 It is a combining in the future strains the useful traits and characters
inherent in each parent separately
 it has become the predominant method of improvement of
cultivated strains due to hybrid vigor
 It is known as combination breeding
 Distinction is made between intra- and interspecific crossing
 The successful of hybridization is to great extent dependent on the
selection of parental materials and how well the breeder is familiar
with the genetic processes occurring in the segregating progenies of
hybrid populations
 The choice of parents might seem the best starting point, but in
practice, it is better to expound crossing and selection schemes first
and then to consider the source and choice of parents in the wider
context of breeding strategy
Hybridization
 The material resulting from hybridization is used in different
ways
1. For selection in the breeding of selection strains (open pollinated
strains)
2. Directly in the production as first generation hybrids (hybrid strains)
 The improvement of cross pollinated organisms cannot be
based on isolation of homozygous genotypes
1. As a consequence of outbreeding, any strain of outbreeding
organism has heterogenous genotypes, each being to some
extent different from the other ones in a given population
2. Induced selfing of outbreeding organisms leads to inbreeding
depression of the offspring
Hybridization
 Sexual recombination in fish might occasionally happen
in nature, but most were conducted artificially
 The principle of artificial sexual recombination is to
impose the male and female gametes of different fishes
to fuse together as a zygote by artificial methods that
does not happen in natural conditions
 Those hybrid zygotes will develop into hybrids with
some improved characteristics
 The modified phenotypes appeared in those hybrids is
the result of “hybridity expression” of the newly
reconstructed genomes of the individuals
 Sexual recombination is very useful for producing
better F1 hybrids
Hybridization
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Sexual recombination only generally can be done between
taxonomically closely related species
In fish, sexual recombination not only can be successfully
made among closely related species, but also can be applied to
those fish species which belong to rather distantly related
species
1. Inter-genus sexual hybridization
Cyprinus carpio x Carassius auratus
2. Inter-subfamily sexual hybridization
Ctenophry idellus (sub-fam. Leucinae) x Megalobrama amblycephala
(Sub-fam. Abramidinae)
3. Inter-order combination
Carassius auratus (order Cypriniformes) x Oreochromis nilotica
(order Perciformis)
Natural barrier in distance recombination
1. The failure of foreign sperm to penetrate the egg
2. Foreign sperm can enter the eggs but it will be degenerated and
disappear in the egg cytoplasm without performing any vital
function
3. Foreign sperm can enter the egg and enlarge as a male
pronucleus but it can not fuse with the egg to form a zygote
nucleus
4. Foreign sperm can enter the egg and enlarge as a male
pronucleus but it can fuse with the egg to form a zygote nucleus
and divide in coordinating with egg cytoplasmic division,
however due to unknown natural incompatibility which existed
between the sperm and egg, the hybrid zygote nucleus will
become heteroploid. The embryo develops abnormally
Open pollinated populations (OPP)
 The improvement of OPP depends essentially upon the
changing gene frequencies towards fixation of favorable
allele while maintaining a high degree of heterozygosity
 Uniformity is impossible and trueness to type is a
statistically feature of the population as a whole, not a
characteristic of individual organism
 Two types
1. Population improvement
a population is changed by the chosen selection procedures based on
purely phenotypic selection (mass selection)
2. Synthetic
population improvement which has to be reconstructed from
parental lines. It can be constructed from combining inbred, clones
as a parent
Heterosis
1.It implies increased vigor of the first generation
hybrids as compared to the parental forms
2.It manifests itself fully in the first generation,
whereas in the subsequent generation the hybrid
vigour goes down substantially
3.Heterotic hybrid seed can be sown commercially
once
4.Practical utilization of heterosis involves annual
crossing to produce heterotic hybrid seed
Attributes of F1 hybrids
Maximum performance under optimal
conditions
Stability of performance under stress
Proprietary control of parents
Often reduced time to strain development
Joint improvement of traits
Hybrid Strains
Hybrid strains 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.
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
Hybrid vigour or heterosis
Heterosis
The increase in size, vigour or productivity of a
hybrid plant over the average or mean of its
parents.
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Midparent heterosis
High parent heterosis
Standard heterosis
Measurement of Heterosis
Mid-parent heterosis

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Hybrid performance is measured relative to mean of the
parents (MP)
(F1 - MP) / MP * 100
High-parent heterosis

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Comparison of hybrid to performance of best parent
(HP)
(F1 - HP) / HP * 100
Genetic basis of heterosis
Three possible genetic causes:
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Partial to complete dominance
Overdominance
Epistasis
The issue for breeders - What is the Ideal genotype?
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Partial to complete dominance - Homozygote
Overdominance - Heterozygote
Dominance 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
Dominance Hypothesis
Example
Model AA = Aa > aa Parent 1
aaBBccDDee = 20
AA=10 Aa=10 and aa=0
Parent 2
AAbbCCddEE = 30
F1
AaBbCcDdEe = 50
Also note that AABBCCDDEE = 50
Discussion of
Dominance hypothesis
Theoretically, 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 individual in a
million.
Dominance hypothesis
Linkage
Recombination among loci could result in plants
homozygous for all favorable alleles
Repulsion phase linkages, either slow or preclude the
development of such lines
A
b
a
B
Empirical evidence supports dominance hypothesis,
as inbred line are improving in performance.
Overdominance
 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.
Overdominance
Superiority 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.
“single locus heterosis” difficult to observe
and detect if populations are not in linkage
equilibrium.
Pseudo- Overdominance
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
A
b
a
B
Epistasis
Epistasis - interaction among loci, may also
contribute to heterosis
Generation
Parent 1
Parent 2
Hybrid ( add)
Hybrid ( Dom)
Internode
No. nodes
length
3
1
1
3
2
2
3
3
Height
3
3
4
9
Epistasis
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.
The magnitude of epistatic variance is difficult
to estimate, and may play a very important role
in heterosis.
Prediction of heterosis
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
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General (GCA) - Average performance - additive effects
Specific (SCA) - ability of lines to combine in specific combinations
Due to dominance effects and heterosis.
Genetic distance and heterosis
Heterosis
Moll (1965) showed a relationship between genetic
distance and heterosis for yield in maize
Genetic
distance
Nuclear transplantation
 It is a diploid nucleus into enucleated egg.
 It is to combine the nucleus and cytoplasm of different
species to produce nucleo-cytoplasmic hybrids
1. Inter-genus combination
Nucleus of common carp and cytoplasm from crusian carp.
Adult fish with essentials of common carp phenotype and
cytoplasmic influenced character at morphological, physiological
and biochemical levels were obtained. Both male and female
hybrids are fertile
2. Inter –subfamily combination
Nucleus from grass carp and cytoplasm of blunt snout bream.
Adult fishes were obtained.