Pollen Development
Early Development
Germination
The Mature Microgametophyte
Announcements
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Annotated bibliography?
Lab this week – More pollen and pollen
tubes.
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Sign up for times to pollinate flowers and
fix/clear/stain styles.
Pollen Development
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Young anther tissues.
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Parietal layer → Thecae
Sporangial layer → pollen
spore mother cells (2N) → 4
microspores (1N).
Mitosis of microspores to
produce 2 cells (nuclei).
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Bi-nucleated pollen.
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Vegetative nucleus.
Generative nucleus.
Only vegetative and
generative nuclei at
dehiscence.
Tri-nucleated pollen.
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Mitosis of generative nucleus
occurs before dehiscence.
Pollen Grain
Walls
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Pollen wall (sporoderm).
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Intine – inner - (cellulose).
Exine – outer - (sporopollenin).
Microspore tetrad.
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Initially surrounded by a callose wall.
Establishment of base pattern of the exine.
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Arrangement of pores (apertures).
Free spore stage.
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Spores are released into the tapetum (fluid
inside the anther).
Deposition of sporopollenin to form exine
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Originates from the tapetum.
Probably orchestrated by the young
gametophyte.
Intine deposited over the same time period.
Exine
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Sporopollenin
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Exine sculpturing.
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Taxon specific patterns.
Function of the pollen wall.
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Extremely resistant to
chemical degredation.
Deposition of
proteins/enzymes important
for germination (originated
from the tapetum).
Pollenkitt
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Lipoidal substance – sticky.
Responsible for color/odor.
Pollen Morphology
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Size range
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5 – over 200
microns
Size variation
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Hydration
Chromosome
numbers
Genetic
variation
among plants
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Heterostyly
Environmental
factors
Pollen Quantity
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Anemophylous species generally produce
more than entemophylous species.
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Hydrophylous species the least.
Production per flower is generally fairly
constant.
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Number and size of anthers produced.
The Mature Pollen Grain
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Storage substances.
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RNA synthesis.
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Gametophytic origin.
Bi-nucleated (generative and vegetative).
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Carbohydrates or lipids.
Most plant families.
Self-incompatibility occurs after germination.
Tri-nucleated (sperm nuclei)
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Certain families (grasses, mustards, etc.)
Self-incompatibility reaction occurs on the stigma.
Pollen Germination
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Adhesion to the stigma.
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Wet stigmas – covered with stigmatic
exudate.
Dry stigmas – papillae are coated
with pectin.
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Chemical bonding.
Biochemical changes
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More common in wind pollinated
species.
Tri-nucleated grains.
Metabolic activity increases
RNA synthesis
Pollen tube emergence from the
aperture.
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Penetration of the stigma.
Grows between cells
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Releases pectinase
Pollen Tube Growth
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Autotrophic stage.
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In vitro comparison.
Heterotrophic stage.
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Mitosis of the generative nucleus.
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Changes in tube morphology.
Stylar transmitting tissue.
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Two sperm nuclei.
Solid styles (dicots).
Hollow styles (monocots).
Gene expression.
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Gametophytic genome.
Perhaps 60% overlap with the
sporophyte.
Pollen-Style Interactions
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Self incompatibility.
Sporophytic.
„ Gametophytic.
„ Heteromorphic.
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Pollen competition.
Selective fertilization.
Self-Incompatibility
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Pollen-style interaction that prevents
fertilization by self pollen.
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Gametophytic
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Occurs in a broad range of plant families.
Multiple evolutionary origins.
Reaction occurs after pollen germination.
Determined by haploid genotype.
Probably due to uptake of RNAses
(enzymes that degrade RNA in the
developing gametophyte).
Sporophytic.
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Determined by the sporophytic genotypes.
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Proteins deposited in the pollen coat.
Prevents pollen germination.
The S Locus Model
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One locus with many alleles.
Gene products:
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Style part.
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Pollen part.
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Characterized as RNAses in gametophytic systems.
Recognition sites.
Probably blocks uptake of RNAses unless recognition sites
match.
Evolution.
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Frequency-dependent selection.
Many S alleles (i.e., S1, S2, S3, etc.) are maintained in
the population.
Favors spread of new alleles.
Alleles are maintained within species over long periods
of time.
Hetermorphic Incompatibility
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Heterostylous species.
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Morphological differences.
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Long style – small pollen.
Short style – large pollen.
Incompatibility.
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Reciprocal placement of anthers
and stigmas for flowers on different
plants.
Increases efficiency of pollen
transfer?
Sporophytic in action – depends of
the genotype of the sporophytes.
Reaction occurs during pollen-tube
growth.
Are morphology and incompatibility
linked?
Pollen Competition
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Pollen development.
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Genetic control of pollen size in corn (Singleton and Manglesdorf
1940).
Grain size within tetrads (Mulcahy et al. 1992).
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Unanswered questions:
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Larger grains when fewer mature.
Do larger grains contain more resources? Do they have higher
fertilization success?
Pollen density effects for germination.
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Increase in frequency of germination at higher density.
„ In vitro and in vivo.
Position on the stigma important for germination rate (Thomson
1989).
Unanswered questions:
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What is the consequence of variation in germination time for pollen
competition?
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See Galen et al., 1986.
Do longer styles promote pollen competition?
Pollen Competition
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Can competition among
microgametophytes affect the
sporophytic phenotype?
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Numbers of pollen grains:
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Faster offspring growth (Ottaviano
and Mulcahy).
Effect of selection on the
microgametophytes?
Distance the pollen has to grow.
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Longer distances produce more
vigorous/competitive offspring in
Dianthus (McKenna and Mulcahy)
Selection on Pollen
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Selection on pollen tube growth rates.
Self pollen grows faster in self styles after
seven generations of selection (Johnson and
Mulcahy).
„ Several generations of selection does not
increase pollen competitive ability when
tested across recipients in Raphanus (Stanton,
Snow, and Young).
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Limits to selection.
Available genetic variation.
„ Selection at other life stages.
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Pollen Competition
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Pollen competitive ability varies among donors.
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Hibiscus: Snow and Spira 1996.
Erythronium: Cruzan 1989, 1990.
Viola: Skogsmyr and Lankinen 2000.
Styles can favor one donor over another.
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Self vs. outcross pollen.
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Cryptic self-incompatibility.
Among outcross donors (in Petunia).
Selection by styles may increase offspring vigor?
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In Viola?
Selective Fertilization
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Self vs. Outcross
fertilization
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Cryptic self-incompatibility.
„ Clarkia: Bowman (1987).
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Depends on the size of
the pollen load (Cruzan
and Barrett 1996).
„ Pollen growth rates.
„ Pollen-tube attrition.
Pollen mentor effects.
„ Erythronium (Cruzan
1993)
• G/O is the grain to ovule ratio.
• F is the proportion of ovules
fertilized.
• P is the proportion outcross pollen on
the stigma.
• α is the slope – indicates pollen
competitive ability.
• δ is the intercept – indicates level of
attrition.
Selective Fertilization
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Does the style influence which
microgametophytes win the
race?
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Cryptic self-incompatibility.
Adding stylar extracts to in vitro
germination media can… (Shivanna
and Sari Gorla).
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Increase the length of tubes.
Increase the variance in tube
length.
Outcome of pollen competition
depends on stylar environment.
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Maize (Jones 1928, Landi and
Frascaroli 1988, Pfahler 1967)
Raphanus (Marshall and Folsom
1992)
Petunia (Cruzan 1990)
A Critical Test of the
‘stylar influence’
hypothesis
1.
2.
3.
Test donors across recipients.
• Measure pollen
performance.
Disable styles.
Test donors across recipients
again.
• Measure pollen
performance.
Pollen Performance
Normal
High
High
Disabled
High
Low
Conclusion
Pollen
Competition
Stylar
Selection