Plant Improvement- Classical Breeding I Flashcards
Where do crops derive from?
Wild plant ancestors.
improvements for human usage are basically a specialised type of directed evolution.
What’s the genetic difference between wild progenitors and modern elite cultivated lines?
Much less genetic variation in cultivars than their wild ancestors. We have selected a narrow gene pool that we can used for growing cultivated crops.
Different combinations of genes found in sets- haplotypes.
How have cultivars been selected?
Cultivars have been selected by removing most of the variable gene alleles from their ancestors and preserving particular allelic combinations which maximise traits desired by us.
What is pedigree breeding?
Self- chromosomes go through one round of recombination and gametes are produced.
Each recombinant is different from another.
Cross with each other to produce assortments.
A cultivar will be extremely homozygous- very little variation.
Carried through to different genes to homogenise and reduce heterozygosity of progeny.
The more you self- the less heterozygous you get (reduces by a factor of 2/half each time).
How can pedigree breeding be used to select for useful genes?
Useful gene in pedigree- will become very heterozygous, but will reduce as you go through.
Need thousands of progeny cause it is a rare event to get good/useful alleles in plant.
Genes will segregate differently, but some plants will have good alleles from both parents. These will be selected.
How can pedigree breeding be accelerated?
This process can be accelerated by the use of single-seed descent (SSD) (no scientific input) and/or Doubled Haploid (DH) approaches (cell culture approaches).
How does single seed descent work?
Instead of selecting individual plants early in the process (F2-F5) and propagating these in rows, a bulk sample is propagated by taking a single seed per plant then re-sowing.
Putting effort into reducing heterozygosity rather than looking for good plants.
Roughly 1 year is saved by avoiding the scoring of traits early in the process, when heterozygosity is high and phenotype is less reliable.
How does double haploidy work?
Instead of selecting individual plants early in the process (F2 – F5) and propagating these in rows, haploid gametes in original F1 plants are converted into diploid plants.
Removing heterozygosity in about 3 months.
How do you convert a haploid gamete into a diploid organism?
One common way is to fertilise a plant with pollen from a different species. In many plant species this results in elimination of the entire parental chromosome set and doubling of the maternal set (at a rate of a few %).
(Pollen from a different but related species- genetically incompatible with plant.
Entire paternal set is removed and maternal set doubles to compensate.
Needs to be done to around 50x more plants than you want to produce.)
Labour intensive at start, but save a couple of years in the end.
This is a popular method.
What are the main traits that are important?
Yield- this is the main one that farmers want.
Disease resistance.
Stress tolerance.
Product quality- differentially important depending on species, may not matter if used for animal feed, but does if its for human consumption.
Traits can be divided into subtraits- eg. yield: seed size, number of seeds on ear, number of ears on plant, number of plants in field etc.
What are the components of yield for cereals?
Individual Seed weight.
Seeds per ear.
Ears per plant.
Plant density.
Traits can and do interact. For example, in a population there is a negative correlation between parameters that tend to cancel each other out. For example, lines with high TGW tend to show low numbers of seeds per ear and/or ears per plant.
(Plants with higher seed weight will likely have fewer seeds per ear.)
How are quantitative traits in a random population described?
By the Normal frequency distribution.
Traits that matter for yield are quantitative. Quantitative trait loci.
How are traits affected?
How can heritability be calculated?
The trait phenotype exhibited by a plant is the result of its genotype and the environment in which it has grown:
VP = VG + VE
The heritability of a trait (h2) is the proportion of phenotypic variability that is due to genetic variability:
h2b = VG/VP
Heritability = Genotypic variation (VG)/Genotypic + environmental variation (VP)
How are traits affected?
How can heritability be calculated?
The trait phenotype exhibited by a plant is the result of its genotype and the environment in which it has grown:
VP = VG + VE
The heritability of a trait (h2) is the proportion of phenotypic variability that is due to genetic variability:
h2b = VG/VP
Heritability = Genotypic variation (VG)/Genotypic + environmental variation (VP)
(Traits with high heritability include simple physical dimensions such as plant height, grain weight, ear length.
Traits with low heritability tend to include complex characters such as crop yield and quality (e.g. flavour).)
How can traits be measured?
Traits are measured most easily in a well-controlled setting like a greenhouse or an incubator.
However, such measurements impose a strong environmental bias to the results and distort them
If you want to predict the performance of a crop plant in the field you need to score the traits in a field. Need to correct for problems in the field too.
