BSCI 124 Lecture Notes

I. Sexual reproduction and evolution

• A. Sexual reproduction aids in evolution because sexual reproduction produces variable offspring upon which natural selection works
  B. Sexual reproduction is advantageous only if an organism mates with someone other than itself (outcrossing or outbreeding)
  • 1. Animals- most species have separate male and female individuals
    2. Potential problem for plants- most flowers have both male and female parts in them (perfect flowers)
    3. Many flowering plants have evolved special ways to assure outcrossing and prevent self-crossing
    4. Pollination, the transfer of pollen from the male anther to the female stigma, must occur before seed plants can reproduce sexually.
    
  • A primary function of flowers is to attract pollinators with colorful petals, scent, nectar, and pollen.

II. Pollination & fertilization

Excellent summary; strongly recommended for review

• A. Pollen contains a sperm nucleus and a tube nucleus. There is no antheridia.
  B. The s perm nucleus is enclosed and prote cted by the gametophyte tissue. Since the sperm is protected, fertilization is no longer restricted to damp environments. The protected sperm can blow through the air. The male and female gametophytes can live anywhere as long as the sperm can reach the egg. As a result, the gametophyte generation of seed plants became drastically reduced in size and is attached and dependent on the sporophyte.
  C. For a protected pollen sperm (generative cell) to successfully fertilize a protected egg, there must be a method to get the pollen from the male to the female. This process is called pollination.
  D. The male gametophyte (pollen) travels from the microsporangium to the megasporangium containing the female gametophyte with the egg. The pollen grain sticks to the receptive (sticky) part of the megasporangium.
  E. The process of fertilization begins when a second cell within the pollen grain, a tube cell, creates a tube through the megasporangium to the egg.
  F. When the egg has matured, the pollin's generative cell travels down the tube where it divides to produce sperm to fertilize the egg. In most gymnosperms, this can be a two year process.
  G. The gametes are never really exposed. The sperm develops within a pollen tube and is liberated at the archegonium. The egg is continuously surrounded by the gametophyte tissue. The covering around each megasporangium further s the protection, not only to the megagametophytes and eggs but also to the developing embryos.

III. How do plants get pollen from one plant to another? (plants are rooted in the ground)

• A. Wind
  • 1. Gymnosperms and some flowering plants (grasses & many trees)
    2. Hit or miss affair (very chancy)

  B. Many flowering plants rely on animals for cross-pollination

  • 1. 3 main animal pollinators- insects, birds, and mammals
    • a. Insect pollinators- beetles, bees, wasps, flies, butterflies, and moths
      b. Bird pollinators- hummingbirds, honey creepers, and honeyeaters
      c. Mammals- bats

    2. Many plants have evolved relationships with animals that are their pollinators

IV. Coevolution - the interactions between two different species affect the development of each other's characteristics during the course of evolution

• A. One of the best examples of coevolution is the relationship between flowering plants and their animal pollinators
  • 1. Plants evolved elaborate methods to attract animal pollinators
    2. Animals evolved specialized body parts and behaviors that aid plant pollination

  B. The evolution of these pollinating mechanisms have given flowering plants almost as much mobility in their mating as animals have
  C. Coevolution sometimes results in a plant and its pollinator being totally dependent on one another

  • 1. Example- a species of yucca that is only pollinated by a small moth species
    2. The yucca cannot reproduce successfully without the moth transferring pollen
    3. The moth cannot reproduce successfully without the yucca because it only lays its eggs inside the ovary of the yucca flower; the young moth larvae that hatch feed on the yucca until they are mature

V. Pollen

The pollen structure is often related to its method of pollination.

• A. In insect-pollinated species, the pollen is frequently sticky or barbed.
  B. Wind-pollinated species usually produce lightweight, small, smooth pollen.
  C. Pollen grains have a limited life span. Depending on the species, pollen may be viable for only a few hours up to several weeks after it is released from the anther.

VI. Pollinators

• A. Bees - Bees are the most important group of flower pollinators
  • 1. The bees live on the nectar and feed it to their larvae. With special mouth parts, hairs, and other body adaptations, they are especially suited to collect and carry these materials.
    2. Bees are guided by sight and smell.
    • a. Bees see yellow and blue colors. They also see ultraviolet light as a distinct color and many flowers have ultraviolet markings. Bees are not able to see the color red, so they rarely pollinate red flowers.
      b. Some flowers have "honey guides" that guide bees to the nectar. They often have a "landing platform".
      c. The nectary is usually situated at the base of the petals. The pollen is normally large and sticky.

  B. Butterflies - Like bees, butterflies are guided by sight and smell.

  • 1. Some butterflies are able to see red as a distinct color, so some butterfly-pollinated flowers are red and orange.
    2. Butterfly or moth-pollinated flowers are often in the shape of a long tube because each insect has a long proboscis.
    3. The pollen is normally large and sticky.

  C. Moths - Most moths come out only at night, so the typical moth-pollinated flower is white or pale with a strong sweet odor that often is only emitted after sunset.

  • 1. The pollen is normally large and sticky.

  D. Flies - Some flowers look and smell like dung or rotten meat. Dung and carrion flies mistakenly lay their eggs on the flower and in the process collect and pass pollen. When the eggs hatch, the maggots starve for lack of food.
  E. Birds - Birds have a good sense of color and are attracted to colorful red or yellow flowers. They do not have a keen sense of smell, so bird-pollinated flowers usually have little odor.

  • 1. Birds are attracted to red and yellow flowers.
    2. Pollen does not readily stick to the bill, but does to the feathers.
    3. Since birds lap up the nectar with their tongue, bird-pollinated flowers produce nectar that is more fluid and in greater quantity than insect-pollinated flowers.
    4. Hummingbird-pollinated flowers usually have a long, tubular corolla suited to their long beak.
    5. The pollen is normally large and sticky.

  F. Beetles - Beetle-pollinated plants are often dull in color or white, but produce strong odors.
  G. Mammals

  • 1. Bats are usually active at night, so flowers they pollinate are often white.
    2. One mouse-pol linated flower has inconspicuous green blossoms that open at night, with a well-hidden source of copious, sucrose-rich nectar. When the rodent grasps the flower and inserts its tongue to probe for nectar, the pollen is released explosively from the anthers, dusting the rodent's face.

  H. Wind - The flowers are small in comparison to animal-pollinated flowers.

  • 1. The pollen grains are light, small, and are not sticky.
    2. The stamens are exposed to the wind so that the pollen can easily blow in the wind.
    3. The stigmas are feathery and exposed to the wind so that they can catch the pollen.
    4. The flowers are often grouped in inflorescences to facilitate pollen release and capture.

VII. Why animal pollinators "service" flowering plants

• A. Rewards for the pollinator- food
  • 1. Nectar (a sugar solution) - In exchange for moving their pollen around, flowers have evolved many rewards to attract pollinators. The most common attraction is food, either nectar (a sugar solution) or pollen, which is high in protein. In many plants nectar is produced in special glands called nectaries. Nectaries are most commonly found in the flowers, but can be located on leaves or other parts of the plant (extra-floral nectaries). Regardless of where nectar is produced, it is usually protected from evaporation into the atmosphere and from dilution by rainwater. The nectar is also protected from "robbing" by other animals that do not aid in pollination.
    • a. Concentration of sugar has evolved to match the energy requirements of the specific animal pollinator
      b. Example- bees need 30-35% sugar to make honey (keeps their larvae alive through the winter); bees will not visit flowers with low sugar concentrations in their nectar
      c. Example- birds need dilute nectar because their mouthparts cannot suck up a thick, syrupy nectar

    2. Pollen (high in protein)

    • a. Beetles eat pollen directly
      b. Bees use pollen to make bee bread (mixture of pollen and nectar; used to feed larvae)
      c. Some plants (roses, peonies, and poppies) rely on pollen as a reward and don't produce any nectar
      d. Some flowers produce two kinds of pollen (normal pollen and sterile, tasty pollen for insects)

  B. Getting the pollinator's attention- advertising nectar and pollen rewards- colors and aromas

  • 1. Colors- vary depending on the pollinator- different animals perceive light differently
    • a. Recall visible spectrum 380 nm - 760 nm (violet - blue - green - yellow - orange - red)
      b. Bees see best at lower end of visible spectrum and into ultraviolet radiation; flowers pollinated by bees are most often violet, blue, or yellow and may have ultraviolet markings (invisible to human eye)
      c. Birds are particularly attracted to red flowers (poster in lab of hummingbirds and their flowers)
      d. Bats don't see well- flowers are a dusky white

    2. Nectar guides or honey guides

    • a. Once the pollinator comes closer to the flower, it sees a visual guide that pinpoints the location of the reward
      b. Examples- dots (foxglove) or lines (pansy) or contrasting color pattern (yellow center, blue petals of forget-me-nots)

    3. Aromas

    • a. Nectar guides of daffodils have a fragrance that is stronger than rest of flower
      b. Aroma is more important in insect-pollinated flowers (birds don't have a strong sense of smell)
      c. Aroma doesn't have to be pleasant- carrion plant smells like rotting flesh

    4. Examples: (Please note: The following pictures are on the IBC disk)

    • a. bees
      b. butterflies
      c. moths
      d. hummingbird
      e. ants
      f. wasp
      g. fly and mosquito (flower about 8 mm across)
      h. wind
      i. large bee - note nectary guide
      j. hummingbird and butterfly
      k. beetle
      l. bee, butterfly, hummingbird
      m. Hawkmoth
      n. tropical curved-bill birds
      o. carrion fly
      p. bat
      

    5. Position and color of stamens can vary and promote pollination

  C. Another method of attraction- flower evolved to resemble female insect

  • 1. Certain orchids mimic female wasps
    2. Mature male wasp emerges before female and attempts to copulate with orchids, accomplishing pollination; female wasp later emerges

Other Sites of Interest:
Review of pollination
Yucca and yucca moth

Back to BSCI 124 main page

Last revised: 30 Jan 1997 - Delwiche