Lecture 18 Plant Reproduction 2

Pollen Dispersal (“Plant Sex” Phase I)

• Fundamental requirement: male gametes (pollen) must reach female gametes (stigmas)
• Three broad vectors
  • Wind (ancestral, energetically cheap, random)
  • Water (aquatic species; pollen rides water surface)
  • Animals (most diverse & specialized)

Wind-pollination specifics

• Common in gymnosperms, many grasses, homogeneous tree stands
• Favourable when plants grow densely ⇒ short travel distance to mates
• Backup often exists: efficient asexual reproduction if wind fails
• Adaptive traits
  • ♂ flowers placed high for maximum exposure (e.g.
    tassels in maize)
  • Large pollen quantities
  • Sticky/stigmatic surfaces on ♀ organs (maize silks) to snag airborne grains

Water-pollination example

• Water celery (Apium nodiflorum)
  • ♀ flower raised to surface on long pedicel; stigma sits in 2-D plane of water
  • ♂ flowers release pollen rafts that float and contact stigma more efficiently than in 3-D water column
  • After capture, pedicel coils pulling fertilised flower under water for seed development

Animal-pollination and rewards

• Key idea ≈ mutualism; plants pay couriers
• Nectar
  • High-energy sugar solution; collected by honeyeaters, bats, hummingbirds, bees
• Pollen
  • Protein-rich; harvested by bees & small mammals (e.g.
    mammals on Banksia cones)
• Visual “runways”
  • Contrasting patterns or UV guides (violets, passionfruit, daisies, Hepatica)
  • Humans may see uniform yellow; bees see UV bull’s-eye
• Chemical cues
  • Bucket plant secretes sex-pheromone analogues used by male bees → males roll in flower to gather scent, unintentionally dusting themselves with pollen
• Deception (no reward)
  • Ophrys (bee-orchids) mimic female solitary bees morphologically & spectrally
  • Males attempt copulation (pseudocopulation), leave with pollinia, fertilise next orchid before real females emerge

Complex obligate mutualisms

• Yucca ↔ yucca moth
  • Each Yucca spp. partnered with one moth spp.
  • Female moth actively collects pollen, deposits it on stigma, then oviposits into developing ovules
  • Larvae consume a subset of seeds; if too many eggs laid (all seeds threatened) plant abscises flower ⇒ both lose ⇒ stabilising selection for moderate oviposition
• Fig tree ↔ fig wasp
  • Syconium = inverted inflorescence
  • Gravid female enters via ostiole carrying pollen & eggs
  • Pollinates all flowers, lays eggs in some
  • Wingless males emerge first, mate with sisters, then die inside fig
  • Females collect pollen and exit to new figs
  • If female departs without pollinating, plant aborts syconium ⇒ sanctions
  • Both parties gain by mutual fidelity (limited seed predation, safe nursery)

Preventing Self-Fertilisation & Inbreeding

• Problem: sessile lifestyle ↔ high risk of pollen from same or close individual
• Mechanisms
  • Temporal separation (dichogamy)
    • Protandry: male first, then female
    • Protogyny: female first, then male
  • Spatial/structural separation
    • Monoecy ("one house"): separate ♂ & ♀ flowers on same plant (e.g.
    maize)
    • Dioecy ("two houses"): separate male and female plants (e.g.
    willows)
    • Heterostyly
    – Distylous Oxalis: “long-style/short-anther” vs “short-style/long-anther”; cross-pollination only between morphs
    – Tristyly exists in some spp.: three style/anther levels
  • Biomechanical/physiological self-incompatibility (SI)
    • Female controls hydration & nutrition of pollen tube
    • Tube growth can be blocked within stigma, style, or at germination
    • Genetically based (S-locus alleles) discriminating “too related” or wrong species

Alternation of Generations & Gametophyte Reduction (Angiosperms)

• Trend: sporophyte dominant; gametophyte miniaturised
• Male pathway
  • Microsporocyte (diploid) → meiosis → 4 microspores (haploid)
  • Each microspore → pollen grain (vegetative/tube cell + generative cell)
  • Generative cell divides → 2 sperm; total male gametophyte = 3 cells
• Female pathway
  • Megasporocyte → meiosis → 4 megaspores; 3 degenerate, 1 survives
  • Megaspore → 3 mitoses ⇒ embryo sac with 7 cells \,(8\,nuclei)
    • 3 antipodals
    • 2 synergids guiding pollen tube
    • 1 egg cell (female gamete)
    • 1 central cell with 2 polar nuclei

Double fertilisation (angiosperm synapomorphy)

• Pollen tube enters ovule via micropyle
• Sperm #1 + egg → diploid zygote → embryo
• Sperm #2 + 2 polar nuclei → triploid (3n) endosperm (nutritive tissue)
• Advantages
  • Endosperm forms only after successful fertilisation → conserves resources vs gymnosperm “pre-pay” nutritive tissue
  • Triploidy may boost nutrient content & genetic buffering
  • Endosperm closely related to embryo ⇒ reduced maternal immune conflict
• Human relevance: endosperm = grain, seed, nut flesh → global staples

Seed Dispersal Strategies (Phase II)

• Rationale: escape competition, pathogens, parasitism at maternal site; colonise new niches
• Seed as dispersal unit
  • Protective coat enables dormancy, environmental waiting period
• Wind mechanisms
  • Achaene + pappus (“parachute”) e.g.
    dandelion; can stay aloft in slightest air movement; long-distance (kms)
  • Whole-plant tumbleweeds roll scattering seeds
  • Tall stalks (poppies) sway releasing seeds farther
• Ballistic mechanisms
  • Mistletoe: hydrostatic pressure shoots sticky seeds onto branches
  • Impatiens: coiled fruit valves explode when disturbed, flinging seeds
• Adhesive/protective seeds
  • Puncture vine & Chenopodium produce spiny burrs piercing tyres, bedrolls, attaching to fur
  • Chemical deterrents (cyanogenic compounds, bitterness) in unripe seeds

Fruit Types & Functions

• Enclosing seed(s) in additional maternal tissue expands dispersal & defence options
• Three primary categories
  • Simple fruit: from single carpel or fused carpels (pea pod)
  • Aggregate fruit: many carpels of one flower merge (strawberry, raspberry – visible individual carpels; hairs = styles)
  • Multiple fruit: fusion of entire inflorescence’s ovaries (pineapple; each diamond = one flower)
• Accessory fruit
  • Non-ovary tissues contribute significantly (apple: fleshy receptacle surrounds ovary core)
  • Humorous taxonomy: tomato, banana = botanical berries; strawberry not a berry

Protective & signalling features

• Mechanical: thick, woody, or spiny exocarps (peach pit, Hakea follicles opening post-fire)
• Chemical/unpalatable when unripe (astringent starch, alkaloids)
• Ripeness indicators: colour change, aroma, sweetness advertise to animals when seeds ready
• Vectors
  • Birds, reptiles, mammals (bears, domestic dogs inadvertently), insects; ingestion & defecation = long-distance endozoochory

Evolutionary Significance & Exam Flags

• Double fertilisation + triploid endosperm = hallmark of angiosperms; appears in only one gymnosperm lineage ⇒ key evolutionary innovation
• Mutualisms (yucca–moth, fig–wasp) illustrate coevolution, sanction mechanisms, and stability through mutual dependence
• Self-incompatibility systems safeguard genetic diversity despite sessile life
• Seed/fruit structural diversity underpins global agriculture, ecosystem dynamics, and many human foods