J.D. Lewis Fordham University Plants and pollination biology at’s a nice plant like you doing in a place like th
May 14, 2015
J.D. Lewis
Fordham University
Plants and pollination biology(what’s a nice plant like you doing in a place like this?)
The importance of flowers
Flowers evolved relatively recently, yet ~ 80% of all plant species are flowering plants• e.g, of the > 4600 plant species in the
NE US, roughly 4000 are flowering plants
The rapid diversification of flowering plants suggests that pollination is a key, if not the key, factor limiting plant populations
Reproduction in plants
Plants may reproduce sexually or asexually Most asexual reproduction in plants
involves vegetative structures Agamospermy: asexual production of seeds
• is found in a variety of species• generally results in clones
Sexual reproduction
Sexual reproduction involves the combining of two gametes, sperm cells and eggs, to form a new zygote (embryo)
Floral structure Pollen grains are produced in the anthers,
part of the stamen, and carry sperm cells Eggs are produced in the ovary,
which is part of the carpel
Floral structure In addition to anthers and carpels, flowers
typically have petals and sepals
The parts of a flower
Calyx: the sepals collectively Corolla: the petals collectively Perianth: the calyx and corolla collectively Androecium: the stamens collectively Gynoecium: the carpels collectively
Arrangement of flower parts Parts may be in spirals along an elongated
receptacle or may be in whorls Floral parts of same whorl may be fused, and
parts of different whorl(s) may be fused Ovary position may not always be at the base
of the flower
Variations in flower structure
Regular: flowers that are radially symmetrical• parts of whorl are similar in shape and
radiate from the center of the flower• color patterns may be irregular
Irregular (zygomorphic): flowers that are bilaterally symmetrical• e.g., snapdragons
Inflorescences
Arrangement of flowers in aggregations Is another key aspect of pollination ecology
• Aggregations of flowers may increase the attractiveness to pollinators
• The form of the inflorescence may affect the behavior of pollinators
Flower form and pollination
Pollen transfer usually occurs by wind, water or animals
Flower form generally reflects the mode of pollen dispersal• Animal-pollinated flowers are showy;
wind-pollinated flowers are not• Evolutionarily, showy flowers may be a
disadvantage in wind-pollinated plants
Wind pollination No need to produce
showy flowers, scents or nectar
Anthers and stigmas are well-exposed
Males and females are separated on the same plant
Plants often grow in dense clusters
Aquatic plants
Plants in 31 genera of 11 families produce flowers underwater
Aquatic plants often produce large pollen that moves on the surface of the water• pollen often is sticky and forms rafts
In some marine species, pollen release coincides with low tide
Stigmas often are modified for pollen contact
Visual displays Animal-pollinated plants
coevolved with their pollinators Selection favored floral
modifications that promoted consistency in pollinator type
Beetles and flies likely were among the earliest pollinators
Beetles and flies are attracted by large flowers with fruity or musty odors
Bees The most important group of
pollinators Flowers are showy, blue or
yellow and brightly colored Often are scented Honey guides: distinctive
patterns that aid recognition• may include UV patterns
May have landing platforms• e.g., snapdragons
Butterflies and moths Diurnal flowers often are similar to bee flowers, but
unscented and have long, slender corollas Nighttime flowers often are sweetly scented, light
colored, and have a long, slender corolla Butterflies can see red but
have a weak sense of smell Moths can detect scents
Birds and bats Bat flowers often are large, produce
copious nectar, are dull colored and are only open at night; scents often are very strong and fruity or musty
Bird flowers produce copious but thin nectar, are odorless and are red or yellow
Odors and acoustic guides
Many flowers have odors Odors can attract pollinators over longer
distances, but are harder to pinpoint Odors can differ independently of flowers,
and may attract one group but repel another Some plants use acoustic guides for bats Much less is known about odors and
acoustic guides than about colors
Floral rewards
The two key rewards are pollen and nectar Pollen provides protein Nectar provides sugar Flowers may also serve as egg-laying sites
Limiting unwanted visits
Plants limit pollinator visits so that pollen is likely to go to a plant of the same species
Changes in flower shape are the main way• e.g., long tubes, spurs and small cones
Reward type and timing are also common Nonetheless, many flowers are visited by
large numbers of inefficient pollinators Some insects are nectar robbers
Pollination syndromes
Color, odor, reward type and timing, and shape all influence the type of floral visitor
Pollination syndromes: certain combinations of traits associated with specific pollinators
However, the syndromes are just general patterns
Also, generalists may be favored under some fairly broad conditions
Mating systems
Mating systems refers to biological factors that determine who mates with whom
Some species can self-fertilize Others are obligate out-crossers Some are a combination Inbreeding often has fitness costs, so there
is selection for outcrossing in many species Outcrossing often is enhanced by negative
assortative mating
Assortative mating
Plants with similar phenotypes are more likely to mate with one another
Negative assortative mating - individuals that tend to mate with dissimilar phenotypes• Causes obligate outcrossing• The two primary forms
are heterostyly and self-incompatibility
Self-incompatibility
There are two general types of systems Gametophytic SI - haploid pollen carries
an allele shared by diploid stigma Sporophytic SI - diploid pollen parent
carries an allele shared by diploid stigma• Sporophytic SI is more restrictive
Self-compatibility
May be favored where a particular genotype is successful in a particular location
Cleistogamous flowers may partly be explained by this, but the data are equivocal• Flowers that open are chasmogamous
Pollen to ovule ratio is often much lower in self-compatible species
Plant gender
Cosexual plants - function as both males and females simultaneously
Perfect flowers - hermaphroditism Monoecy - separate male and female
flowers on the same plant Dioecy - male and female flowers on
separate plants
Plant gender (cont.)
These three systems can exist in combination
Gynomonoecy (andromonoecy is more common)
Andromonoecy Gynodioecy Androdioecy (gynodioecy is more common) Sequential hermaphroditism - flower
usually begins as male
The genetics of plant gender
Cosexuals may self-fertilize, or not Perfect flowers can function as if they were
unisexual (Cryptic dioecy) Fitness maximization is thought to drive
variation in the ratio of female to male function
Pollen competition
Most plants have more ovules than ever produce seeds
Extra ovules may exist for three reasons• Pollen is limiting• Flowers have another function• The plant is bet-hedging
Sexual selection
Competition for pollinators within a species is analogous to competition for mates
Variation in pollination success may be one driver of sexual selection
Pollen donors may compete through visual displays (male-male competition)
Competition between pollen grains may be regulated by the style (female choice)
Pollen dispersal
Pollen typically does not move very far Inbreeding is common in many populations
• Most matings occur between neighbors
In animal-pollinated species, plants are clumped, animals usually focus on patches, and most pollen ends up on the next visited flower
Animal dispersal
Wind dispersal
Wind moves pollen in short bursts, plants occur in dense stands, and pollen may be clumped and mostly released near the ground