BIOL120 Lecture 16 – Angiosperm Dispersal

Angiosperm Dominance

• Flowering plants are the most diverse land-plant group; rapid Cretaceous radiation linked to biotic pollination
• Success hinges on effective movement of two life stages:
  • Male gametophyte (pollen)
  • Sporophyte (seeds)

Pollen (Male Gametophyte) Dispersal

• Vectors: wind, water, animals (insects, birds, bats)
• Steps: release → transport → capture on stigma → pollen hydration/filtering → tube growth toward ovule
• Global pollination modes (Stephens 2023): $\sim80\%$ biotic, $\sim19.6\%$ wind, $\sim0.04\%$ water

Pollination Syndromes

• Wind (anemophily): excess dry pollen, reduced perianth, flowering before leaves, small clustered flowers, stands of plants
• Animal-mediated traits (overview)
  • Bees – UV/yellow/blue, open bowls, nectar guides, scented, nectar + pollen
  • Butterflies – bright pink/lavender, landing platform, nectar tubes
  • Moths – white, nocturnally scented tubes
  • Flies – sweet (myophily) or carrion-like (sapromyophily) odours
  • Beetles – large dull flowers, spicy/fruit/carrion scents, pollen reward
  • Birds – red/orange tubes, no scent, dilute nectar
  • Bats – large white night flowers, musty scent, copious nectar/pollen

Co-evolution with Pollinators

• Reciprocal adaptation between floral form and pollinator morphology/behaviour
• Timing correlations: diversification of “higher” Holometabola insects overlaps angiosperm radiation
• Convergent nectar feeders: hummingbirds (Americas), sunbirds (Afro-Eurasia), honeyeaters (Australasia)
• Floral signals target pollinator sensory biases (colour vision, scent, size, activity period, rewards)

Self vs. Cross-Pollination

• Advantages of outcrossing: genetic diversity, reduced inbreeding depression
• Barriers to selfing
  • Spatial: dioecy; heterostyly (pin/thrum)
  • Temporal: protandry/protogyny
  • Genetic self-incompatibility systems

Seed (Sporophyte) Dispersal

• Wind: wings (maple), plumes (dandelion), hairs (Typha)
• Water: buoyant, smooth coats (coconut, mangrove)
• Animals
  • External: hooks/burrs (Bathurst burr, cat’s claw creeper)
  • Internal: fleshy fruits/nuts/arils (red-eyed wattle) entice ingestion → endozoochory

Key Takeaways

• Two-stage dispersal strategy (pollen & seed) underpins angiosperm ecological dominance
• Majority of pollination is animal-mediated; floral traits predict vector
• Long-term co-evolution with insects, birds, and bats shapes biodiversity
• Mechanisms exist to favour cross-pollination while maintaining reproductive assurance
• Seed dispersal vectors extend range, reduce competition, and link plants to animal mutualists