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Describe Hybrid cultivars:
- A hybrid cultivar is the F1 offspring of a
- planned cross between inbred lines, cultivars,
- clones, or populations
- Dominant alleles mask deleterious recessives in
- offspring of diverse parents
- Inbreeding reveals deleterious alleles
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What is Heterosis?
- Heterosis: hybrid offspring of genetically
- distinct parents performs better than either
- parent
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What is Hybrid Vigor?
- Hybrid vigor = heterosis = the increase in
- overall productivity of a hybrid plant over the
- midparent value (only of practical value if it
- greatly exceeds midparent value)
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What is Inbreeding Depression?
- The reduction in plant performance as
- homozygosity is reached
- Tolerance to inbreeding varies by crop
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What is the Dominance Theory?
- Vigor in plants is conditioned by dominant
- alleles
- Crossing of parents with complementary
- dominant alleles will concentrate more in
- the offspring than either parent
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What is the overdominance Theory?
- Heterozygote is superior to the
- homozygote
- Assumes that the alleles of a gene (A, a)
- are contrasting, but each has a different
- favorable effect in the plant
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What makes up a heterotic relationship?
- Heterosis is an expression of the genetic
- divergence among cultivars
- Most of time spent figuring which parents
- make the best hybrid
- Define genetic diversity, classify parental
- lines, determine GCA and SCA, and define
- heterotic groups
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What is a Heterotic Relationship?
- A group of related or unrelated genotypes
- from populations that show similar
- combining ability when crosses with
- genotypes from other germplasm groups
- Established using pedigree, geographic
- isolation, and combining ability
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Process of Developing heterotic groups:
- Pedigree analysis
- Geographic isolation
- Measurement of heterosis
- Genetic analysis
- Combining ability analysis
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What is a Heterotic Pattern?
- A specific pair of heterotic groups, which
- express in their crosses high heterosis and
- consequently high hybrid performance.
- Helps breeders in a more efficient and
- consistent manner through exploitation of
- complementary lines for maximizing
- outcomes of hybrid breeding
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What are the Types of Hybrids?
- Single Cross (A x B)
- Double Cross [(A x B) x (C x D)]
- Three-way cross [(A x B) x C]
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How to obtain desired genes in a hybrid:
- Germplasm is obtained from different
- heterotic groups
- These germplasm are selected for the
- genes needed in the hybrid
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How to select parents for a hybrid:
- Characterize parents as much as possible
- Select based on phenotypic performance
- Perform combining ability test:
- [n(n-1)]= number of crosses needed
- May cross to common tester lines
- Select best GCA lines and intercross
- Select lines with best SCA
- Select best hybrid combination
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Considerations when developing a hybrid line:
- Field preparation
- Planting time
- Synchronization of flowering
- Field layout
- Plant density
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What is needed for Commercialization of hybrids:
- High heterosis
- Pollen control (CMS?)
- High F1 yield
- Economic seed production
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Breeding Clonally Propagated Species Overview:
- Clonally propagated crops use parts of the
- plant as the propagule
- Usually high level of heterozygozity in
- plant
- Clones are homogeneous (identical
- genotypes) because they reproduce via
- mitosis not meiosis
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Categories of propagated species:
- Normal flowering and seed set
- Hybridization generates recombination, clones maintain heterozygosity
- Normal flowers but low seed set
- Can still hybridize to transfer genes
- Produce seed through apomixis
- Are used as males, then select apomictic
- Non-flowering species
- Variation arise through mutations
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What are some Genetic issues in asexual
breeding?
- Clones are genetically identical and
- products of mitosis; variation occurs from
- environment
- Highly heterozygous and highly heterotic.
- Heterosis is fixed for the life of the cultivar
- Known species are interspecific hybrids or
- have high ploidy
- Chimeras may occur as natural variation
- A chimera occurs when an individual consists of
- two or more genetically different types of cells
- These are heritable changes, but can only be
- maintained through vegetative propagation
- Useful in horticultural crops
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Breeding Implications of propagated species:
- Heterosis is fixed
- Only one individual needs to be identified,
- then increased
- If vegetative products are of economic
- importance, fertility is not an issue
- Genotype can be determined early in the
- breeding process
- Difficult to obtain large amounts of plants
- Maintaining disease free plants
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Breeding Approaches of propagated species:
- Variation by either hybridization, mutation, or
- introduction
- Goal: disease free clones and new cultivars
- Can eliminate pathogens through tissue culture,
- heat treatment, chemical treatment, or
- apomictic seed
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Selection of propagated species:
- Introduce variation (either natural population,
- mutation, or through hybridization)
- Evaluate F1 population (or natural population)
- Select vigorous plants and space plant; bulk best progeny
- Conduct preliminary yield trials
- Conduct advanced yield trials
- Release cultivar
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Advantages of propagated species:
- Sterility is not a factor
- Commercial product is uniform
- Micropropagation can rapidly multiply
- planting material
- Heterozygosity and heterosis fixed
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Disadvantages of propagated species:
- Clonal propagules are often bulky
- Clones are susceptible to devastation by
- an epidemic
- Clonal propagules are difficult to store
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