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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
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)
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
Stages of seed development
• I. Histodifferentiation (embryo differentiation)
• II. Cell expansion
• III. Maturation drying
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
Apomixis
• defn: the production of an embryo that bypasses the
usual process of meiosis and fertilization
• types:
– gametophytic apomixis (diplospory or apospory)
– adventitious embryony
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
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
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’
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)
III. Maturation drying
• physiological maturity is reached
• seed (embryo) reaches max. dry weight
• after this stage, seeds are either quiescent or dormant