bio 130 ovule dev. ist sem 2013 2014

X-section of an ovary Ovary contains a cavitylined with an epidermal layer. Ovules develop from the epidermal cells and are contained within the cavity of the ovary, attached to its inner surface by a short stalk- funiculus

Ovary bears ovules on a ridge on the ovary wall called placenta

Parts of ovule

1. nucellus- central body with vegetative cells enclosing thesporogenous cells

2. 1 or 2 integuments (unitegmic or bitegmic) enclosing the Nucellus

3. funuculus-stalk connecting ovule with the placenta.

4. chalaza-where nucellus, integuments and the funiculus merge

Megaspore mother cell dev. from surrounding nucellar tissue and undergoes meiotic division to form megaspore. Nucellus considered as a megasporangium

Funiculus

nucellus

Chalaza- region where integuments fuse with funiculus

Development of embryo sac and female gamete (in an anatropous ovule)

•A hypodermal cell of the nucellus enlarges and becomes differentiated into a megaspore mother cell or megasporocyte.

•This diploid megaspore mother cell increases in size and undergoes meiosis to form a linear tetrad of 4 haploid megaspores,

•3 of which degenerate and the 4th becomes the functional megaspore in monosporic types,

•all 4 become functional in tetrasporic types

Female Gametophyte

• The nucleus of the megaspore undergoes three successive mitotic divisions forming eight nuclei.

•The megaspore enlarges into an oval shaped structure called the embryo sac. The eight nuclei of the embryo sac arrange themselves in 3 groups.

Darkly stained cluster of cellsis the nucellus

Inner and outer integumentsbegin as ridges of tissue

Inner integument

Outer integument

Differential growthcauses them to curve so micropyle is bent around funiculus

micropyle

funiculus

funiculus

placenta

Outer and inner integuments completely overgrow the nucellus

Megaspore mother cell dev. from surrounding nucellar tissue and undergoes meiotic division to form megaspore. Nucellus considered as a megasporangium

Funiculus

nucellus

Chalaza- region where integuments fuse with funiculus

Outer and inner integuments completely overgrow the nucellus

Developing ovules of Lilium. Ovule emerges from the placenta as conical protuberance with the first sporogenous cell,called archesporial cell. Integuments formed by periclinal div. of epidermis

Or megaspore mother cell

Micropyle

Inner integument

Outer integument

placenta

funiculus

Outer and inner integument completely overgrow the nucellusExcept for the micropyle. (nucellus=central portion of ovule inside the integuments )

--Begins with elongation of the functional megaspore, usually at chalazal end.-- initially megaspore is non-vacuolate but later small vacuoles appear which may fuse to form large vacuole.

Development of embryo sac

Embryo sac After 3 mitotic divisions

Egg and synergids

Sporogenous cell develops directly from hypodermal nucellar cell- large dense cytoplasm, large nucleus.In others, hypodermal cell divides into outer parietal and inner primary sporogenous cell.

meiosis

Hypodermalarchesporialcell

meiosis

meiosis

Chalazal region

Post meiosis: survivingMegaspore will divide bymitosis

A, D. (bel mutant) exposed nucellus and a single integument

F. Larger mutant ovuleOuter integument has many cells.

A. First megaspore mitosis yields binucleate embryo sac. Spindle of first nuclear div oriented along the long axis of the cell. Wall formation Does Not follow the nuclear division. Both nuclei divide 2x, forming 4 in B then 8 in C

B. Large vacuole appears between the two daughter nuclei. As cell expands, nuclei are pushed toward opposite poles of the cell. Both nuclei from each pole divide twice

D. The 8 nuclei arrange themselves in two clusters of 4 nuclei one at each opposite ends. One nucleus from each end migrates towards the middle, called polar nuclei (named for where they came from, not where they end up).

C.8-nucleate state . All 8 nuclei are present in a common cytoplasm, they move around probably from remnants of spindle fibers from earlier divisions.

Chalazal trio called antipodals( Latin “against the foot”) at opposite end of the egg and antipodals

Egg apparatus consists of larger egg flanked by two smaller cells called synergids (greek for “helpers” or cooperators

The large binucleate

Chalazal=basal part of ovulewhere integument and nucellusare joined

megasporogenesis

Megagameto-genesis

Mutants in ovule determination

1. bell (bel1)- ovule lacks inner integument

2. Aberrant testa shape (ats)- no clear distinction between inner and outer integument

3. Extreme types of integument mutations:

aintegumenta (ant)

huellenhos (hll)Do not develop integument and embryo is disrupted.

highly vacuolatedamount of cytoplasm is

limited cytoplasm spread as a thin layer surrounding vacuole

cytoplasm contains very little ER, limited no. of plastids, mitochondria and dictyosomes but high ribosomes which are randomly distributed rather than aggregated as polyribosomes

cell wall does not extend over the entire cell

EGG

Synergids- limited life span, wilt after fertilization. Probably involved in nutrition of egg. has extensive wall ingrowth at micropylar region called filiform apparatus

metabolically active

Antipodals-transient existence , minimal cytoplasmic organelle, show nuclear abnormalities like

endoreplication polar nuclei-metabolically active, extensive ER, numerous plastids, mitochondria, dictyosomes and polysomes, has large quantities of starch, proteins and lipids

Megasporogenesis and microsporogenesis

megasporocyte microsporocyte

Generates megaspore and microspores

meiosis