evolution of flowers and pollinators

Question 1

Jurassic period cycad (gymnosperm) pollinators

Answer 1

• male pollen cone, female seed cone
• extant cycad example, Lepidozamia (Australia)
• Tranes weevils breed in pollen cones and visit female cones (eat pollen and sac), transfer pollen

Question 2

angiosperm radiation in Cretaceous (146-65mya)

Answer 2

• rare to find fossils of early flowers as delicate
• early flowers were small with radial symmetry
• evolution of protected pod around seeds
• radiation related to change
• Cretaceous became hotter and more humid than Jurassic, plate movement meant plants were distributed across different latitudes
• evidence that angiosperms have higher reinvention and trait flexibility than gymnosperm so were more adaptable to changes
• 3 phases of angiosperm radiation split into 30 distinct bursts

Question 3

other angiosperm innovations

Answer 3

• xylem with vessels for increased transpiration and photosynthesis
• high vein density and densely packed stomata for improves rates of transpiration and photoysnthesis
• secondary chemistry and metabolites enabled plant defences
• whole genome duplication allowed for flexibility, evolution of gene pairs, may have triggered bursts of evolution
• shed deleterious non-functional genes to have a small genome size, enabled small cell size

Question 4

evolution of pollinator insects

Answer 4

• radiation of highly specific pollinators in the Cretaceous that evolved alongside angiosperms
• Coleoptera (beetles), Diptera (flies), Lepidoptera (butterflies), hymenoptera (bees and wasps)

Question 5

angiosperm flowers

Answer 5

• bisexual flowers in majority of angiosperms, allowed efficient pollination with animals
• male stamen with anther (where pollen grains are formed) and filament (holds up anther, length varies)
• female carpel (stigma, style, ovary, physically protected ovule)
• anther often below stigma to reduce risk of self pollination
• petal structure species specific to attract pollinators
• sepals, modified leaf structures that protest closed bud when undergoing meiosis

Question 6

Arabidopsis thallana

Answer 6

• abundant model angiosperm
• very simple flower
• anther right next to stigma to allow efficient self pollination
• also insect pollinated, sticky pollen

Question 7

pollen recognition

Answer 7

• stigma can identify compatible vs incompatible pollen grains
• some also have self-incompatibility mechanisms
• specific proteins and signalling molecules on the surface of pollen grain and stigma that must interact properly
• prevents hybridisation between incompatible species and promotes genetic diversity

Question 8

floral rewards for pollinators

Answer 8

• pollen is a rich source of sugar and protein (40% each)
• bees visit lots of different plants to eat diverse protein profiles
• some also have a nectary at base of flower
• nectar, sugar rich solution to attract pollinators
• at base of flower so pollinator brushes past anthers and stigmas
• some pollinators have developed specialised structures to access nectar (proboscis)

Question 9

angiosperm life cycle

Answer 9

• alternation of sporophyte and gametophyte generations
• sporophyte dominates, microscopic short lived gametophytes in ovule and pollen

Question 10

gametophyte life stage

Answer 10

• meiosis in sporophyte phase and then further mitosis gives rise to male pollen grain and female embryo sac
• meiosis gives rise to generative cell and tube cell
• further mitosis of generative cell gives rise to 2 sperm cells
• pollen grain has pollen tube with tube nucleus and 2 more nuclei/sperm cells
• meiosis gives rise to egg and polar bodies
• further mitosis of further bodies gives rise to antipodal and synergid cells and 2 polar bodies

Question 11

pollen germination

Answer 11

• compatible pollen grain lands on stigma
• pollen grain germinates, pollen tube extends down style and enters an ovule
• synergid cells at bottom of ovule send chemical signals to attract pollen tube

Question 12

double fertilisation

Answer 12

• unique to angiosperms, believed to have evolved only once
• one sperm fertilises egg to form a diploid zygote (distinct embryonic phase)
• other sperm and 2 haploid polar bodies fuse forming a triploid nutritive endosperm (provides nutrition to embryo)

Question 13

evidence for insect pollination of ancestral angiosperms

Answer 13

• 86% basal genera (Amborella, water lily, star anise etc) insect pollinated, mostly by bees or flies (not specially adapted)
• 76% Cretaceous fossilised pollens show insect pollination features (large, not smooth, clumped)

Question 14

Insect pollination of angiosperms today

Answer 14

• 2/3 insect pollinated
• hymenoptera most common pollinators
• DNA barcoding to identify flowers visited by bees, brush pollen baskets off bees and sequence them

Question 15

Floral features that attract bees

Answer 15

• scented
• tubular petal structures to encourage entry
• landing platforms and guide lines
• zygomorphic (bilaterally symmetrical) instead of actinomorphic (radial symmetry)

Question 16

Floral symmetry and pollinator insects

Answer 16

• beetles and lepidoptera prefer actinomorphic flowers
• bumblebees have an innate preference for zygomorphy, but not a requirement
• stamens and carpels oriented for specific contact

Question 17

Flowers to attract butterflies

Answer 17

• yellow, pink or purple (not red)
• tubular or spur petals
• clustered flower heads or.wide landing pad (not as good fliers)
• scented

Question 18

Flowers to attract moths

Answer 18

• strong sweet scent at night
• tubular, no lip

Question 19

Co-evolution, sole pollinators

Answer 19

• few species only fertilised by a specific species
• Yucca plant only fertilised by Yucca moth, female moth also deposits eggs into Yucca ovary
• brimstone butterfly and bush vetch
• fig tree and fig wasp

Question 20

Bird pollinators

Answer 20

• subtropical and tropical regions
• nectivorous birds with adapted tongues and/or bills
• hummingbirds, sunbirds, flowerpeckers, honeyeaters, lorikeets

Question 21

Flowers colours, birds and bees

Answer 21

• birds detect a wide range of colours
• bee vision receptors detect green, blue and ultraviolet
• red doesn’t stand out for bees and other insects, takes longer to detect
• optimal foraging theory, bees will not compete with birds for red flowers

Question 22

Ruby-throated hummingbird

Answer 22

• 19 Eastern US plants with red tubular flowers
• hummingbird migrations coincide with flowering

Question 23

Costus species, convergent evolution

Answer 23

• many species lost floral scent following transitions from bee to hummingbird pollination
• not associated with gene loss, just deregulation
• be pollinated species gained more diverse scents

Question 24

Mammal pollinators

Answer 24

• fruit bats pollinate 173 tropical and subtropical eastern hemisphere flowers
• other mammalian pollinators include marsupial honey possum (Australia)

Question 25

Wind pollinated flowers

Answer 25

• 10% angiosperms
• grasses and forest trees possibly too numerous for insects to effectively pollinate
• small, light pollen, low nutritional value
• non-showy flowers often no petals
• trees often have unisexual flowers to reduce self pollination and flower early so leaves don’t impede pollen