Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
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