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Summary PBR22303- Plant breeding
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Plant BreedingE-module 1 Modes of reproduction
Plant breeding is the genetic improvement of plants for human benefit. Course of ac-
tion:
1. Determine objectives
2. Find/create relevant genetic variation in germplasm (collection of genetic re-
sources from which genes can be transferred and recombined)
3. Cross parents
4. Select offspring
5. Finish into new cultivar
6. Multiply cultivar & market
Selection methods must be determined based on the plant’s reproductive mode, to
understand the genetic consequences of a cross. Modes are autogamy, allogamy
and clonal reproduction. Plants often belong to two modes.
Autogamous crops Self- fertilization, pollination with pollen from the same flower
Chance to receive other pollen (to prevent cross pollination) is reduced by:
Cleistogamy: flowers do not open (pea, peanut)
Chasmogamy: flowers open after pollination has taken place (wheat, lettuce)
The majority of
plants are ho-
mozygous for all
loci because of
selfing. All prog-
eny will be iden-
tical to the par-
ents. Progeny is
inbred.
,A single nucleotide polymorphism (SNP) is a difference in one single base pair in
otherwise identical DNA sequences between individuals. Can be used to ID het-
erozygous and homozygous states. Marker notation:
● AA - a/A
● Aa - h/H
● aa - b/B
You can only calculate with generalized gene proportions if genes do not affect fit-
ness, there is no genetic drift, and no linkage.
Genetic drift : random fluctuations in allele frequencies due to relatively few plants
being grown in each generation.
In case of linkage the increase in homozygosity will be faster than in case of no link-
age. The closer the linkage, the more they inherit as if they are one gene.
Autogamous crops will reach homozygosity. Therefore, populations are uniform
and genetically stable. In a mixture of homozygous genotypes:
1. Original genotypes continue to occur over generations, if large enough popu-
lation without selection; (no genetic drift)
2. The percentage of each genotype depends on its fitness (rate of reproduc-
tion).
This is why it is nice, in a landrace (mixture of pure lines), to have different geno-
types, as they will all react differently to environmental stress and you’ll probably not
lose all your crops to a drought, for example. This results in a certain yield security.
Modern cultivars consist of one single pure, homozygous line.
Outcrossing (artificially or accidental) will result in heterozygosity for all loci in which
the parents differ. Further cycles of self-fertilisation may lead to full homozygosity
again.
Clonal reproduction Asexual propagation, production of new plants by somatic tis-
sue.
Asexual reproduction:
1. Vegetative propagation: an organ for propagation or a somatic tissue sam-
ple.
2. Apomixis: a plant reproduces asexually through seed.
Clonal reproduction can occur either spontaneously (potatoes) or through human
intervention (grafting).
, Clonally reproduced plants have the following characteristics:
1. Often polyploid : increases the number of possible genotypes
2. Often (semi)sterile or poor-flowering : because of ploidy or the plant favours
clonal propagation.
3. High degree of heterozygosity when crossing occurs -> segregation of traits
4. Relatively low reproduction
5. High risk of acquiring pathogens, which may be transmitted through the gen-
erations
6. Mutations may accumulate and remain unnoticed. They can change the uni-
formity and the characteristics of the original genotype.
Selection procedure is shorter than the selection procedure in autogamous crops. No
need of selfing. The desired combination of alleles in a genotype can be selected di-
rectly in the progeny of a cross. However, finding the best individual is less easy.
Autogamous and clonal crops are immortal.
Allogamous crops Cross fertilization, pollination with pollen from another plant
These crops often have mechanisms to prevent self-pollination:
1. Different maturation times for anthers and pistil.
Protandry: polling shedding first.
Protogyny: stigma first.
2. Morphological adaptations to attract pollinators or to shed/catch pollen
3. Monoecy: seperate male and female flowers on the same plant.
4. Dioecy: seperate male and female plants.
5. Self-incompatibility. (=genetically controlled)
Plant engage in random mating by open pollination, causing new genotypes to
arise in every generation. Each of these genotypes is mortal. Recessive genes are
important! Each zygote is a novel combination of genes (=unique genotype).
Genetic characteristics of allogamous populations:
1. All plants are genetically distinct from their parents
2. All plants are genetically distinct from each other
3. There is a high degree of heterozygosity in each plant
4. For every locus which has more than one allele, some plants will be homozy-
gous and other heterozygous
5. Recessive alleles may be hidden behind dominant alleles in heterozygous
plants
6. Is it not possible to obtain a uniform and genetically stable population
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