Lecture 12

  1. List the different Types of cultivars:
    •  Homozygous and homogeneous (pure-line cultivars)
    •  Heterozygous and homogeneous (hybrid cultivars)
    •  Homozygous and heterogeneous (mixed landrace types)
    •  Heterozygous and heterogeneous (synthetic and composite breeding)
  2. Mass selection of self-pollinating plants:
    • Population improvement through increasing the gene frequencies of desirable genes
    •  Improvement of the average performance of the base population
    •  Limited to the genetic variation that exists in the population
  3. Application of mass selecting self-pollinating plants:
    •  Maintain purity of an existing or soon-to-be released cultivar
    •  Develop a cultivar from a base population, hybrid population, or introduced population  Used to breed horizontal resistance—apply low density of disease inoculum so that quantitative genetic effects can be assessed
  4. Mass selection
    •  Desirable for both self and cross-pollinated species
    •  Selection can be positive or negative
    • Image Upload 1
  5. Mass selection advantages:
    •  Rapid, simple, and straightforward
    •  Large populations can be handled
    •  Inexpensive to conduct
    •  Phenotypically uniform and stable
  6. Mass selection disadvantages:
    •  Traits need high heritability
    •  Selection based on phenotypic values— need uniform environments
    •  With dominance, heterozygotes can’t be distinguished and can segregate in subsequent generations
  7. Pure-line selection:
    •  Lines that are genetically different may be isolated from within a mixed genetic population
    •  Highly homozygous selection usually from a single plant—variation is caused by the environment
    •  Useful when breeding for a specific trait (flower color, processing quality)
    •  In theory, has identical alleles at all loci
    •  Have a very narrow genetic based and tend to be uniform for traits of interest
  8. Application of Pure-line selection:
    •  Cultivars for mechanized production
    •  Cultivars with a premium on visual appeal or processing quality
    •  Advancing ‘mutants’ in the population
    •  A part of other methods such as pedigree and bulk population selection
  9. Genetic issues with Pure-line selection:
    •  With narrow genetic base are less likely to be stable across environments
    •  Variants arise through mutations and outcrossing
  10. Pure-line selection overview:
    Image Upload 2
  11. Pure-line selection advantages:
    •  Rapid and inexpensive
    •  Population size can be variable
    •  Applicable to low heritability traits because selection is based on progeny performance
    •  Only the best lines are advanced
  12. Pure-line selection disadvantages:
    •  Narrow genetic base—variants arise
    •  Promotes genetic erosion—only the superior lines are advanced and multiplied
    •  Progeny rows take up more resources
  13. Pedigree selection:
    •  Ancestry of the cultivar well documented
    •  Continuous individual selection after hybridization
    •  Must have an effective and easy to maintain system of record keeping
  14. Applications of Pedigree selection:
    •  Must allow plants to be observed, described, and harvested separately
    •  Used for readily identifiable qualitative traits of the target for improvement
  15. Pedigree selection overview:
    Image Upload 3
  16. Advantages of Pedigree selection:
    •  Provides a catalog of genetic information
    •  Selection based on phenotype as well as genotype (progeny row)
    •  High degree of genetic purity
  17. Disadvantages of Pedigree selection:
    •  Record keeping is slow, tedious, and expensive (modern equipment is making this easier)
    •  Must be able to individually select plants  More suited for qualitative rather than quantitative disease resistance
    •  Selecting early for yield traits may not be effective
  18. Bulk Population Breeding:
    •  A strategy of crop improvement in which the natural selection effect is solicited early and stringent selection solicited late
    •  Usually suitable for crops which are normally closely spaced in production and need a competitive advantage
  19. Key feature of Bulk Population Breeding:
    •  Natural selection in early generations, stringent selection in later generations
    •  Natural selection eliminates less fit genotypes
    •  Stringent selection delayed until homozygosity is reached
  20. Bulk Population overview:
    Image Upload 4
  21. Genetic issues with Bulk Population:
    •  Promotes homozygosity
    •  Mean population performance will be increased
    •  Inferior genotypes may out-compete superior lines
  22. Advantages of Bulk Population breeding:
    •  Simple and convenient to conduct
    •  Less labor intensive in early generations  Compatible with mass selection
    •  Allows for large amounts of materials to be handled
    •  Well adapted lines through natural selection
    •  High degree of homozygosity
  23. Disadvantages of Bulk Population breeding:
    •  Superior genotypes can be lost to natural selection
    •  Selecting in off-season nurseries not useful  Genetic characteristics are hard to ascertain from generation to generation
  24. Single-seed descent:
    •  F2 population is generated to maximize genetic variation
    •  Each plant is kept as an individual unit and a single seed is harvested and replanted  Done until homozygosity is reached
    •  Little selection done in previous generations
  25. Genetic issues of Single-seed descent:
    •  Wide genetic diversity is carried on to advanced generations
    •  Only one seed is selected from each plant, regardless of the number of seeds produced
  26. Advantages of Single-seed descent:
    •  Easy and rapid way to attain homozygosity (depending on the crop cycle)
    •  Small spaces are required in early generations
    •  Duration of breeding program can be reduced
    •  Great genetic diversity exists
  27. Disdvantages of Single-seed descent:
    •  Plants are selected based on individual performance and not progeny performance
    •  Selecting a single seed runs the risk of losing desirable genes from the population
    •  Natural selection has no benefit
  28. Backcross Breeding
    •  Used to replace a specific undesirable gene with a desirable alternative genes, while preserving all other qualities (both good and bad)
    •  Uses a recurrent parent and a donor parent
    •  Most useful for simply inherited genes
  29. Backcross breeding overview:
    Image Upload 5
  30. Backcross breeding Genetic issues:
    •  Number of backcrosses will determine the proportion of donor genes present
    •  1-(1/2m-1) where m is the number of generations
    •  Linkage drag an issue
    •  Heritability of the trait important
    •  Genetic distance between parents should be considered
  31. Advantages of Backcross breeding:
    •  Reduce the number of field testings needed  It is a conservative method; does not permit new recombination to occur
    •  Useful for introgressing specific genes from wide crosses
  32. Disadvantages of Backcross breeding:
    •  Not effective for transferring quantitative traits
    •  Linkage drag an issue
    •  Recessive traits are more time consuming to transfer
  33. Multiline and Cultivar Blends:
    •  Planned seed mixture of cultivars or lines
    •  Can be done by the breeder or the grower  Usually used for disease protection
  34. Advantages of Multiline and Cultivar Blends:
    •  Provides protection to a broad spectrum of races of a disease/pest
    •  Phenotypically uniform (except blends)
    •  Provide greater yield stability (over many years)
    •  Can be readily modified
  35. Disadvantages of Multiline and Cultivar Blends:
    •  It takes a long time to develop all of the isolines
    •  Most effective in areas where there is a specialized disease of major importance
    •  Maintaining the isoline is labor intensive
Card Set
Lecture 12
plant breeding