lecture 7

• Angiosperms


  Flowering plants, container for seeds
• Angiosperms are the most dominant plant group today, comprising ~80% of all plants outside
• Angiosperms originated in the Triassic (~250 mya) but had a huge explosion in diversity in the Cretaceous (75-125 mya)
• Three major angiosperm innovations


  • Flowers
  • Endosperm
  • Fruits
• Flower


  A determinate structure with parts arranged in a sequential order in whorls
• Floral organs


  • Sepals
  • Petals
  • Stamens
  • Carpels
• Sepals


  Offer protection to the bud
• Petals


  Often bright, large and attractive to pollinators
• Stamens


  Fertile organs that bear pollen
• Filament


  Stalk that bears the anthers
• Anther


  Sac that contains the microsporangia where microspores form and germinate to become pollen (gametophyte)
• Carpels


  Sporophyll fused into a container within which the ovules are found
• Pistil


  Single carpel or collection of fused carpels
• Stigma


  Sticky surface where pollen lands and pollen tube germinates
• Style


  Tract down which the pollen tube migrates to reach ovules
• Ovary


  Protective container around the ovules
• Variations in the number, shape, size, arrangement, fusion, color, patterning, and symmetry of floral organs and inflorescences are the basis for the breadth of angiosperm floral diversity
• From meiosis to fertilization - pollen


  1. Sporophyte holds onto microspores in microsporangium

  2. Microspore germinates to become pollen grain (male gametophyte)

  3. Tube cell becomes pollen tube, generative cell becomes two sperm cells

  4. Pollen tube germinates on stigma and grows down style, two sperm travel down tube to ovule
• Pollination


  The transfer of pollen to the part of a seed plant containing the ovules
• Self-pollination is the surest bet but can lead to inbreeding depression, while cross-pollination maintains higher genetic diversity
• Strategies for attracting pollinators


  • Floral scent
  • Color and color patterning
  • Nectar and pollen rewards
  • Flower shape, size, number
• Strategies for depositing and withdrawing pollen


  • Floral shape
  • Organ placement
  • Precise "triggers"
• Coevolution


  The evolution of two or more species that are so closely related that the evolution of one depends on the evolution of the other(s)
• Nectar guides


  Markings that attract and orient pollinators
• Attracting Pollinators


  • Floral Scent
  • Color and Color Patterning
  • Nectar and Pollen Rewards
  • Flower Shape, Size, Number
• Depositing and Withdrawing Pollen


  • Floral Shape
  • Organ Placement
  • Precise "Triggers"
• Appearance and attraction determined not just by pigment but also by cell structure
• Sometimes pollination is deceptive
• Specialist Pollination


  Coevolution - the joint evolution of two interacting species in response to selection imposed by the other
• Pollination Syndromes


  Suites of plant traits that suggests coevolution with a particular group
• Pollination services are shrinking due to honey bee collapse disorder and other causes of declines in native bee populations
• Pollinators are essential for many grain and fruit crops (incl. >150 US crops)
• Pollinator services are valued at $10B in the US and $3T globally
• The California almond industry spends the most compared to any other crop to obtain commercial hives each year
• Understanding how to breed and manage crops that improve pollinator attraction, pollinator efficacy, pollinator health, and pollinator networks is essential
• Three Major Angiosperm Innovations


  • Flowers
  • Endosperm
  • Fruits
• Pollination


  Seedling Establishment
• From meiosis to fertilization - pollen


  1. Carpel (megasporophyll)

  2. Stamen (microsporophyll)

  3. Microsporangium

  4. Microsporocytes (2n)

  5. MEIOSIS

  6. Microspore (n)

  7. Generative cell

  8. Tube cell

  9. Male gametophyte (in pollen grain) (n)

  10. Pollen tube

  11. Tube nucleus

  12. Sperm nuclei (n)
• From meiosis to fertilization - ovules


  1. Megasporangium (2n)

  2. Four megaspores (n)

  3. One megaspore survives, germinates to become female gametophyte (n)

  4. Antipodal cells

  5. Polar nuclei in central cell

  6. Synergid cells

  7. Egg cell (n)
• Double Fertilization


  1. 1 egg nucleus (n) + 1 sperm nucleus (n) = 1 zygote (2n)

  2. 1st central cell nucleus (n) + 2nd central cell nucleus (n) + 1 sperm nucleus (n) = 1 endosperm cell (3n)