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Classical breeding methods June 10 th 2008. Source: Bioethics. Few terms (1).

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Classical breeding methods June 10 th 2008

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Classical breeding methods june 10 th 2008 l.jpg

Classical breeding methodsJune 10th 2008


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Source: Bioethics

Few terms (1)

Dominance: Effects of different versions of a gene on a trait. Many plants have two copies of each gene, one inherited from each parent. Genes are located on specific locations called alleles. If the two copies of a gene are different, and their combined effect is determined by one of the genes, than we call that gene dominant.

Recessive: The other gene, which does not determine the outcome, is said to be recessive.

Source: Model horse reference

Meiosis

Source: University of Central Arkansas


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Few terms (3)

Qualitative trait: A qualitative trait is expressed qualitatively, which means that the phenotype falls into different categories. The pattern of inheritance for a qualitative trait is typically monogenetic, which means that the trait is only influenced by a single gene.

Quantitative trait: A quantitative trait shows continued variation. If several gene effects are present, the phenotype values for a population will typically have a normal distribution.

Source: Knud Christensen


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Source: University of Guelph

Few terms (2)

Homozygous: In genetics, zygosity describes the similarity or dissimilarity of DNA between homologouschromosomes at a specific allelic position or gene.

Homogeneous

Heritability: Proportion of phenotypic variation in a population that is attributable to genetic variation among individuals. Variation among individuals may be due to genetic and/or environmental factors.


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Few terms (4)

Polyploidy: occurs in cells and organisms when there are more than two homologous sets of chromosomes. Polyploid types are labelled according to the number of chromosome sets in the nucleus.

Most organisms are normally diploid; polyploidy may occur due to abnormal cell division. It is most commonly found in plants. Haploidy may also occur as a normal stage in an organism's life. A haploid has only one set of chromosomes.


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Overview: Cultivars

Pure inbred lines (species that are highly self-pollinated)

homogeneous, homozygous

series of self- pollinations

Open-pollinated populations (species that are naturally cross-pollinated)

heterogeneous, heterozygous

recurrent selection

Hybrids (crossing inbred lines)

homogeneous, heterozygous

Clones (asexually propagation)

homogeneous, heterozygous


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Note:

You often have a combination between different breeding methods depending on the breeding objective and the stage of your breeding program!!

Often limited to the environment in which is was developed.

As a general rule modifications of breeding methods are possible.


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Breeding self-pollinated species


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Mass selection

Key features:

Oldest method for self-pollinated species

Variable population

Self- and cross-pollinated species

Solely selection (no variability)

Recurrent

Based on phenotype

Applications:

Maintain purity of a cultivar

Develop cultivar from base population

Initial inexpensive screening


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Generalized steps in breeding by mass selection for (a) cultivar development, and (b) purification of an existing cultivar

Mass selection

Procedure:

Plant heterogeneous population

negative mass selection

positive mass selection

Disadvantages:

should have > heritability

Uniform environment

Cannot distinguish homozygous from heterozygous

Advantages:?


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Pure line selection

Key features:

Lines that are genetically different may be isolated from within a population of mixed genetic types.

Variation is due to environmental factors and ???

Applications:

Cultivars

Parents for hybrid production (uniform etc.)

Generalized steps in breeding by pure-line selection


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Mendel’s laws of inheritance

(1) The Law of DominanceIn a cross of parents that are pure for contrasting traits, only one form of the trait will appear in the next generation.  Offspring that are hybrid for a trait will have only the dominant trait in the phenotype.

(2) The Law of SegregationDuring the formation of gametes (eggs or sperm), the two alleles responsible for a trait separate from each other.  Alleles for a trait are then "recombined" at fertilization, producing the genotype for the traits of the offspring.


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Pedigree selection

Key features:

Records of the ancestry of the cultivar

Hybridization is used to generate variability

Followed by selection of a segregating population

Applications:

Mainly crop plants,

Improvement of quantitative traits


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Pedigree selection

Procedure:

Establish base population

Space plant progenies of selected plants

Keep records of generations

Generalized steps in breeding by pedigree selection


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Pedigree selection

  • Advantages and disadvantages:

  • + Selection is not only on phenotype but also genotype (plants are usually observed and described over several generations)

  • +/- High degree of genetic purity

  • Expensive

  • Takes 10 – 12 years


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Increasing homozygosity in each subsequent generation via self- hybridization

Broman W. 2005


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Bulk population breeding

Key features:

Delay artificial selection to allow natural selection pressure

Pure line cultivars

Mostly self-pollinated crops (soybean)

Generalized steps in breeding by bulk selection


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Bulk population breeding

Key features:

Delay artificial selection to allow natural selection pressure

Pure line cultivars

Mostly self-pollinated crops (soybean)

Advantages and disadvantages:

+ Simple and convenient

+ Less labor intensive and less expensive

+ Natural selection may increase frequency of desirable traits

- Selecting in off-season may favor genotypes that are undesirable in the production region


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Backcross breeding

Key features: ?

Replace a specific undesirable gene with a desirable alternative, while preserving all other qualities of an adapted cultivar

Backcrossing to ‘recurrent parent’

Applications:

Improvement of established cultivars

Introgress genes from wild relatives

Isogenic lines


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Backcross breeding

Procedure:

Cross between donor parent and recurrent parent

Repeated backcrossing to recurrent parent

MOVIE!!!!!


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Backcross breeding

  • Advantages and disadvantages:

  • + Reproducible

  • + Predictable (no new recombination events)

  • + Allows introgressions from wild relatives

  • + Applicable to breeding self and cross- pollinated species

  • Only qualitative traits

  • Quantitative traits (major genes)

  • Linkage


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Breeding cross-pollinated species

Focus on the improvement of plant populations

Change the gene frequency such that desirable genotypes predominate

Maintain genetic variability for future improvements.


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Recurrent selection

Key features:

One of the oldest methods in plant breeding

Gradually concentrating desirable alleles in a population

The concept of recurrent selection


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Concept of recurrent selection

Cyclical and systematic technique in which desirable individuals are selected from a population and mated to form a new population

Improvement of a population without losing the variability

Parents should be highly diverse

Applications:

Establish broad genetic base, add new germplasm

Since possible intermating between multiple families add new germplasm

Used in Legumes and grasses


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Emasculating Zinnias:

Collecting Pollen:

Images kindly provided by Goldsmith Seeds


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Fertilization

Images kindly provided by Goldsmith Seeds


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Breeding vegetatively propagated crops

Maintenance of desired genotypes


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  • Through:

  • bulbs and corms (tuplips, gladioli)

  • Budding or grafting (?)

  • Cuttings (pelargoniums, carnations)

  • Division (hardy herbaceous perennials)


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Basic scheme for breeding clone cultivars.


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“Grafted before planting”

“Field grafting”


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  • Mass selection

  • Polyploidy breeding

  • Hybrid breeding


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Thank you for your attention!Next week: Basic genetics!


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