Plants And Pollination Biology

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