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

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

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)

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

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 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

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 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

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

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

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 - AA=10 Aa=10 and aa=0 Parent 1 Parent 2 aaBBccDDee = 20 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 Empirical evidence supports dominance hypothesis, as inbred line are improving in performance. A b a B

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 Internode Generation No. nodes length Height Parent 1 3 1 3 Parent 2 1 3 3 Hybrid ( add) 2 2 4 Hybrid ( Dom) 3 3 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 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 Moll (1965) showed a relationship between genetic distance and heterosis for yield in maize Heterosis 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.