1. BREEDING BASED ON THE GENETIC RECOMBINATION PRINCIPLE
Taryono
Faculty of Agriculture
Gadjah Mada University

2. 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

3. 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

4. Sexual recombination is very useful for producing better F1 hybrids
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

5. Hybridization
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)

6. Natural barrier in distance recombination
The failure of foreign sperm to penetrate the egg
Foreign sperm can enter the eggs but it will be degenerated and disappear in the egg cytoplasm without performing any vital function
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
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

7. 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

8. Heterosis
It implies increased vigor of the first generation hybrids as compared to the parental forms
It manifests itself fully in the first generation, whereas in the subsequent generation the hybrid vigour goes down substantially
Heterotic hybrid seed can be sown commercially once
Practical utilization of heterosis involves annual crossing to produce heterotic hybrid seed

9. 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

10. 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.

11. 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

12. Hybrid vigour or heterosis
The increase in size, vigour or productivity of a hybrid plant over the average or mean of its parents.
Midparent heterosis
High parent heterosis
Standard heterosis

13. Measurement of Heterosis
Mid-parent heterosis
Hybrid performance is measured relative to mean of the parents (MP)
(F1 - MP) / MP * 100
High-parent heterosis
Comparison of hybrid to performance of best parent (HP)
(F1 - HP) / HP * 100

14. Genetic basis of heterosis
Three possible genetic causes:
Partial to complete dominance
Overdominance
Epistasis
The issue for breeders - What is the Ideal genotype?
Partial to complete dominance - Homozygote
Overdominance - Heterozygote

15. 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

16. Dominance Hypothesis Example
Model AA = Aa > aa - AA=10 Aa=10 and aa=0
Parent Parent 2
aaBBccDDee = 20 AAbbCCddEE = 30 F1
AaBbCcDdEe = 50
Also note that AABBCCDDEE = 50

17. 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.

18. 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
Empirical evidence supports dominance hypothesis, as inbred line are improving in performance.
A b
a B

19. 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.

20. 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.

21. 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

22. Epistasis
Epistasis - interaction among loci, may also contribute to heterosis
Internode
Generation No. nodes length Height
Parent
Parent
Hybrid ( add)
Hybrid ( Dom)

23. 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.

24. 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
General (GCA) - Average performance - additive effects
Specific (SCA) - ability of lines to combine in specific combinations
Due to dominance effects and heterosis.

25. Genetic distance and heterosis
Moll (1965) showed a relationship between genetic distance and heterosis for yield in maize
Heterosis
Genetic distance

26. 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.