Lecture 18 Plant Reproduction 2

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)

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 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

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

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)

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

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

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

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

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

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