Bio 360/Fall 2001 September 11, 2001

Flowering and fertilization in angiosperms

I. Initiation of flowering

A. A terminal meristem changes from a vegetative shoot apex to a reproductive apex

B. Signals

1. Photoperiodism

2. Vernalization - induction of flowering by exposure to cold temperatures

a. Required temperature - varies, but 6^oC is often most effective

b. Required time of exposure - 4 days -> 8 weeks

C. Changes in the reproductive shoot

1. Increase in mitotic activity

2. Changes in dimensions and organization

3. Growth is now determinate

4. Order of formation is usually sepals, petals, stamens, carpels

5. Development is similar to leaf development

II. The morphology of the flower

A. General

A flower is a shoot tip with very short internodes -4 kinds of appendages

1. Gynoecia - ovaries, styles, stigmas

2. Stamens (androecium) - filament, anther

3. Petals = corolla, usually colored other than green

4. Sepals = calyx

together these are often called the perianth

 B. Variation in flower structure

1. Number and kinds of appendages

a. Perfect - has both gynoecia and stamens

Imperfect - missing one or the other of these

b. Complete - has all four kinds of floral appendages

Incomplete - missing one or more kinds of appendages

2. Variation in gynoecia

a. number and degree of fusion

b. shape of stigma

c. length and shape of style

d. number and position of ovules

3. Variation in stamens

a. number

b. equal or unequal in size

c. degree of fusion to each other and with other floral parts

d. method of pollen release

4. Variation in perianth

a. number of parts

b. symmetry

c. degree of fusion

5. Arrangement of flowers into various kinds of inflorescences

III. The angiosperm life cycle

A. All higher plants have essentially the same overall life cycle

meiosis mitosis fertilization

diploid --------> haploid ---------> gametes --------> diploid

sporophyte gametophyte sporophyte

The size of the diploid and haploid plants varies

The gametophyte is very tiny in angiosperms

B. Formation of pollen

1. Microsporogenesis and Microgametogenesis

a. Sporogenous cells are only a small part of the anther

b. Microsporocytes produce microspores by meiosis

c. Microspores produce pollen grains by mitosis and further development

c. Pollen grain wall

1. exine - made of sporopollenin

2. intine - made of cellulose and pectin; later forms the wall of the pollen tube

2. Microgametophytes - pollen grains

a. Tube cell - controls pollen tube growth

b. Generative cell - form two sperm cells either before or after pollination

c. The entire male gametophyte is these three cells

C. Formation of the embryo sac

1. Megasporogenesis

a. Ovule - megasporangium + integuments

one to several per ovary

b. Megasporocytes form 4 megaspores by meiosis

c. The three megaspores closest to the micropyle degenerate, leaving 1 megaspore per ovule

2. Megagametogenesis

a. Three nuclear division with no cell division -> one cell with 8 nuclei

b. Two nuclei migrate to the center, one from each end of the embryo sac

c. The 3 nuclei at the micropylar end form the egg cell and 2 synergid cells

d. The 3 nuclei at the other end form the antipodal cells

e. This kind of megagametophyte, with 7 cells and 8 nuclei is typical of about 70% of plant species

D. Pollination and fertilization

1. The pollen reaches the stigma and if it is compatible -

2. The pollen hydrates

3. The pollen grain germinates, extruding a pollen tube

4. The pollen tube grows through the style

hollow style filled with substances from the female or between the cells of the transmitting tissue in the style resources for growth come from the gynoecium

5. One of the synergids degenerates

6. Pollen tube grows through the micropyle and into the degenerated synergid

7. The sperm cells are discharged into the synergid

8. One sperm fertilizes the egg and one fertilizes the endosperm

IV. Incompatibility systems - found in at least 78 plant families

A. Gametophytic self-incompatibility, most common and most primitive

1. One locus, many alleles, called S alleles

2. Female plant has a diploid heterozygous genotype, e.g., S₁ S₂

3. Pollen phenotypes come from the male parent. Each male has 2 S alleles and so produces two kinds of pollen. The pollen is haploid, so an S₁S₃ father produces S₁, and S₃ pollen.

4. In most cases, all pollen germinates and begins growing

5. While pollen is growing through the style, if it has an S allele that matches one of the S alleles in the seed parent, pollen tube growth will stop

6. Based on an interaction between the pollen tube wall and stylar cells

7. If a mother is S₁S₂

1. One locus, many alleles, called S alleles

2. Female plant has a diploid heterozygous genotype, e.g., S₁S₂

5. Typically, incompatible pollen is arrested in growth at the stigmatic surface

6. The reaction occurs between the pollen wall chemicals and the stigmatic papillae

7. If the pollen is incompatible, the papillae produce callose that prevents pollen tube growth

8. If the mother is S₁S₂

9. This kind of incompatibility is more recently derived, less sloppy, and better at excluding relatives than is gametophytic incompatibility.

C. Heteromorphic incompatibility

a. Distyly

1. Two forms of flowers

a. Pins - long style, short anthers

b. Thrums - short style, long anthers

2. Controlled by one gene

Ss = pin, ss = thrum

a gene complex controls both compatibility and morphology

3. Typically, within morph matings are incompatible

b. Tristyly - Three floral forms

D. Dichogamy - Temporal separation of sexes

1. Protandry - male first, most common, anthers ripen before stigmas are receptive

2. Protogyny - female first, stigmas are receptive before anthers ripen

E. Spatial separation of stigma and anthers

1. Separation of anthers and stigmas within flowers

2. Monoecy - sex flowers on same plant, promotes outbreeding but may include selfing if there are no incompatibility barriers.

Most are self compatible.

3. Dioecy - separate sex flowers on separate plants, outbreeding

III. Why control level of selfing

A. Advantages of outbreeding

1. Maintains genetic variability within populations

2. Avoids inbreeding depression

B. Advantages of inbreeding

1. Reproductive assurance

after colonization;; loss of pollinators;pollinator competition; isolation

2. Selfers pass on more genes.