Meiosis of megaspore mother cell (Angiosperms)

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Meiosis of megaspore mother cell(Angiosperms)

• pistil contains the megaspore (2n) mother cell
• meiosis yields 4 haploid (1n) gametic cells (3
  usu. die)
• 3 mitoses yield 8 (1n) nuclei in the embryo sac

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Meiosis of microspore mother cell(Angiosperms)

• stamens contain microspore (2n) mother cells
• meiosis yields 4 haploid (1n) microspores
• the tetrad of microspores are released as mature
  pollen grains
• 2 mitoses yield 3 (1n) nuclei (1 vegetative, 2
  sperm nuclei from mitosis of the generative
  nucleus)

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Pollination and double fertilization(Angiosperms)

• 1 sperm nucleus fuses with 2 polar nuclei to
  yield the 3n endosperm
• the other sperm nucleus fuses with the egg
  nucleus to form the 2n zygote

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Stages of seed development

• I. Histodifferentiation (embryo differentiation)
• II. Cell expansion
• III. Maturation drying

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I. Histodifferentiation (embryo differentiation)

• cell divisions and differentiation form the
  embryo and endosperm
• the embryo reaches the cotyledonary stage
• Differentiation differs among dicots, monocots,
  gymnosperms
• sexual and apomictic embryos develop differently

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Apomixis

• defn the production of an embryo that bypasses
  the usual process of meiosis and fertilization
• types
• gametophytic apomixis (diplospory or apospory)
• adventitious embryony

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Apomixis in Citrus

• cell in the nucellus develops into an embryo,
  outside the embryo sac (adventitious embryony)
• nucellar embryos are clones of the maternal plant
• multiple embryos are often present in 1 seed
  (polyembryony)
• polyembryony and apomixis are unrelated, but
  often occur in the same seed

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Apomixis in Kentucky bluegrass

• an embryo sac forms from a 2n cell (apospory, a
  form of gametophytic apomixis)
• the apomictic embryo is a clone of the maternal
  plant
• apomixis in Kentucky bluegrass is facultative

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Apomixis - agricultural significance

• cloning virus-free Citrus rootstocks
• breeding new turf types of Kentucky bluegrass
• highly sexual seed parent x highly apomictic
  pollen parent
• select a seedling which exhibits highly apomictic
  seed yield and important agronomic traits
• cultivars developed this way America,
  Eclipse, Princeton 104

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II. Cell expansion

• an active period of DNA, RNA, protein synthesis
• major food reserves accumulate carbohydrates
  (starch), storage proteins, and lipids (oils and
  fats)
• many food reserves are made into useful products
  (foods, detergents, lubricants)

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III. Maturation drying

• physiological maturity is reached
• seed (embryo) reaches max. dry weight
• after this stage, seeds are either quiescent or
  dormant