Variation of Taraxacum officinale

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Variation of Taraxacum officinale

• Jing Luo
• Nov. 30, 2004

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The forms of variation

• Performance growth rate biomass accumulation
  germination rate survival
• Morphology leaf flower seed color
• Protein enzyme allozyme
• Genome DNA base sequence

Smaller scale
Easier to observe
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Example leaf morphological variations
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General concept what cause variation?

• Genetic difference (genotype)
• Maternal effects
• Environmental influence water temperature
  nutrients light competitors
• phenotype, biotype, ecotype.

More important
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Genetic variation of T. officinale a dilemma

• Two types of T. officinale
• Diploid 2n 16 produce sexually
• Triploid 3n 24 produce asexually
• Europe (origin area) both diploid and triploid
  are found
• North America (introduced area) only triploid is
  found
• Triploid T. officinale is an obligate apomictic
  plant, which has high variation within population

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Apomixis asexual production

• The seeds of T. officinale are developed without
  fertilization (agamospermy apomixis by seed)
• Seed developed from unreduced megaspore
  (diplospory) instead of form somatic cell
  (aposory)

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Comparison of sexual and asexual production
Sexual
Asexual
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(No Transcript)
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During megaspore meiosis, phase I is restricted
(restitution). Phase II goes on normally
2n
4n
4n
4n
2n
2n
2n
2n
n
n
n
n
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Whats the result of this asexual production?

• All the offspring from one mother plant should be
  genetically identical to each other (confirmed by
  DNA finger printing)
• A pure species (identical in genome) is
  expected in a triploid T. officinale population.

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However variation widely existsSolbrig and
Simpson (1974)

• 284 T. officinale samples from a 500m500m area
  in Ann Arbor, Michigan.
• Electrophoretic variations (allozymes) of six
  enzymes were analyzed

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Allozyme and electrophoretic variation

• Enzymes that are encoded by one allele at a
  single locus.
• Base sequence variation of that locus can differ
  the enzyme's amino acid sequence.
• These enzymes (allozymes) with different amino
  acid sequence can be functionally identical, but
  differ in electrophoretic mobility.
• Allozyme variation indicates genetic variation.

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• 4 allozyme types (A, B, C and D) were found in
  this area.

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(Lyman JC and Elstrand NC, 1984)

• Sample sites in North America
• 22 population (all are triploids)
• Over 20 individuals for each population
• Allozymes from three different enzyme systems
  were analyzed for their variations
• Seed color variation was also considered.

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• 21 allozyme types and 47 clones (allozyme
  combined with 7 seed-color type) were found among
  22 population (518 individual)
• Far from an expected pure species dilemma

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Clones per population

• Most populations had more than one clone
• Maximum 13
• Mean 5.0

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Populations per clone

• Most clones only available in one population
  (33/47)
• Widely distributed clone exists (found in 19
  population out of 22)
• This L shape distribution is also found in
  other studies

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Variations found in DNA analysis

• Restriction endonuclease site variations
• (King, 1993) Ribosomal DNA plus chloroplast DNA
  restriction endonulease site variation plus seed
  color revealed 145 distinct genotypes among 318
  plants (31 sites) sampled across North America
  (from Alaska to Mexico to New York)
• VNTR (variable number tandem repeat, Rogstad et
  al. 2001)
• AFLP (amplified fragment length polymorphism, vam
  Hulst, 2000)

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Conclusion

• Apomictic (triploid) T. officinale population is
  a mixture of different genotypes (clones). Most
  genotypes are restricted to one specific
  population, while several are widely spread.

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What cause this genetic variation?

• Mutation
• Occasion sexual production with diploid (2n) in
  Europe
• Triploid act as pollen donator
• Offspring is triploid apomixis
• ? (triploid father) ?(diploid mother) ?
  triploid offspring
• pollen (2n) egg cell (n)
  3n

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How about North America?

• Multiple genotypes are available in Europe, where
  diploid and triploid both exist.
• Repeat introductions from Europe to North America
  cause the genetic variation, although only
  triploid are found

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Comparison between sexual and asexual Taraxacum
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How different genotypes perform? (Vavrek et al.
1996)
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Solbrig and Simpson (1974)
r-stategist
K-stategist
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Question

• T. laevigatum is diploid species produce
  sexually. Is the production method (sexual vs.
  asexual) an important index for invasiveness?