Instructions for printer-friendly copy.

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Credit: Brett Zimmerman on DeviantArt
Go buy his photos.

    Sexual Encounters
    of the Floral Kind

    Going from 2n

      to n

        to 2n again.

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    xxxxxxxxx When plants do it, it's pretty.

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    Animal Sex is Simple

    1. Inside adult animal ovaries or testes, diploid (2n) germline cells
    xxxproduce haploid (n) gametes (ova or sperm) via meiosis.

    2. Male and female gametes join to form a diploid zygote.

    3. Zygote grows into a new diploid individual via mitosis.

    4. The new diploid individual produces haploid gametes via meiosis.

    ...and the cycle continues.

    Simple!

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    Plant Sex is...Complicated.

    Plants insert extra steps between Animal #1 and #2.

    1. The diploid (2n) sporophyte plant produces diploid germline cells.

      1a. The 2n germline cells undergo meiosis to produce haploid (n) spores.

      1b. Each spore grows via mitosis into a haploid (n) gametophyte.

      1c. The gametophyte produces haploid (n) gametes via mitosis.

    2. Male gametes (sperm) and female gametes (ova) join to form a diploid zygote.

    3. Zygote grows via mitosis into a new diploid sporophyte.

    4. The sporophyte produces haploid spores via meiosis.

    ...and the cycle continues.

    Not quite so simple.

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The change in ploidy from parent to offspring is known as the Alternation of Generations.

sporophyte (2n) ---> gametophyte (n) ---> sporophyte (2n) ---> gametophyte (n) ---> sporophyte (2n) ---> etc.

Land plant alternation of generations is said to be heteromorphic.

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By IvoShandor via Wikimedia Commons

    Evolution of the Alternation of Generations

    A land plant is either a

    • non-vascular bryophyte (more primitive)

      • Gametophyte (n) is the dominant life cycle stage.
      • Sporophyte (2n) is small and ephemeral/seasonal.

    • vascular tracheophyte (more derived)

      • Sporophyte (2n) is the dominant life cycle stage.
      • Gametophyte (n) is small and ephemeral/seasonal.

      (The "dominant" life cycle stage is larger and more conspicuous.)

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(click on pic for source)

    Changing Morphologies

    Across plant taxa, homologous life cycle stages
    have undergone extensive evolutionary change.

    Not only do gametophytes look different from sporophytes.

    But also, both sporophytes and gametophytes often
    vary in appearance from taxon to taxon
    .

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Terminology: A typical fern (tracheophyte) will illustrate.

    Sporophyte (2n)

    (spore means "spore" and phyt means "plant")

    • The sporophyte is a diploid (2n) spore-producing plant.

    • When reproducing, the sporophyte produces specialized leaves called sporophylls.

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    Sporophyll (2n)

    (spore means "spore" and phyll means "leaf")

    • The sporophyll is a spore-bearing leaf.

    • Because it is a sporophyte organ, the sporophyll is diploid (2n).

    • The sporophyll grows clusters of specialized 2n structures called sporangia.

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    Sporangium (2n)

    (spore means "spore" and angion means "box")

    • The sporangium is a spore-producing structure.

    • Inside the"box", many diploid (2n) germline cells undergo meiosis.

    • The resulting haploid (n) daughter cells are called spores.

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    Spore (n)

    (spore means "spore")

    • Spores (n) are the propagules produced by the sporophyte.

    • The sporangium breaks open to release mature spores.

    • A lucky spore will land on a moist, hospitable spot.

    • The spore will germinate and grow via mitosis into a gametophyte.

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    Gametophyte (n)

    (gamete means "gamete" and phyt means "plant")

    • The gametophyte is a haploid (n), gamete-producing plant.

    • Gametogenesis takes place in a multicellular sex organ (n) called
      • a gametangium (generic term), which is either
        • an archegonium (female)
        • an antheridium (male)

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By I, EncycloPetey

    Multicellular Sex Organs

    Plants sex organs are somewhat analogous to ovaries and testes.
    They are present only in the gametophyte.

    • The female archegonium produces an ovum.
    • The male antheridium produces sperm.

  • Sperm are released when the environment is wet.
  • They swim to the ovum, still nestled in the archegonium.
  • Fertilization takes place inside the archegonium.
  • This produces a diploid zygote

By I, EncycloPetey

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    Which Sex?

    (And you thought it couldn't get any worse.)

    Depending on its species, a plant--whether gametophyte or sporophyte--can be

    dioecious

      and reproductive structures are on
      separate individuals

    monoecious

      and reproductive structures are on a
      single individual, but on different branches.

    bisexual/
    hermaphroditic

      and components are on a single individual
      and are combined in a single branch/structure.

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(click on pic for source)

    Sexual Prefixes

    • A megasporophyll develops megasporangia that produce female megaspores.
    • A megaspore will grow into a female gametophyte.
    • A female gametophyte has only archegonia, and produces only ova.

    • A microporophyll develops microsporangia that produce male microspores.
    • A microspore will grow into a male gametophyte.
    • A male gametophyte has only antheridia, and produces only sperm.

    • A generic sporophyll develops plain old sporangia that produce plain old spores.
    • A spore will grow into a bisexual gametophyte.
    • A bisexual gametophyte has both antheridia and archegonia.


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    The fern gametophyte is a mature, bisexual plant.

    And it's about to suffer a cruel fate because of that diploid zygote inside its archegonium.

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    Zygote (2n)

    (zygo means "yoke")

    • The zygote is diploid.

    • Inside the gametophyte's archegonium, the zygote will grow via mitosis.

    • Eventually, it will emerge from the archegonium as the new sporophyte (2n)

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    New Sporophyte (2n)

    (spore means "spore" and phyt means "plant")

    • Like its grandparent, the new sporophyte is diploid.

      • The bryophyte sporophyte is annual.
      • An annual plant lives, reproduces and dies in one year/season.

      • The tracheophyte sporophyte is perennial.
      • A perennial plant lives (and usually reproduces) for more than one year/season.
        • The growing tracheophyte sporophyte crushes its gametophyte mom out of existence.
        • Life (almost always just one season) is short and cruel for the tracheophyte gametophyte.

    • The new sporophyte will mature, produce sporophylls and spores, and the cycle will continue.

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If Animals Underwent Alternation of Generations:
A Somewhat Disturbing Analogy

It is sometimes mentally difficult to equate animal and plant life cycles.
To help clear up this confusion (or maybe make it worse),
here is an Imaginary Animal Species that Undergoes a Plant-like Alternation of Generations.

I. The Sporophyte Generation

    An Imaginary, Dioecious
    Animal Sporophyte

    In our imaginary species, the sporophytes are dioecious.
    There are separate male and female sporophytes.

    • Sporophyte Brad is diploid and male.
    • He will produce microspores

    • Sporophyte Angelina is diploid and female.
    • She will produce megaspores

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    The Sporophyte Produces
    Spores via Meiosis

    In the springtime, small lesions begin to appear
    on Sporophyte Brad and Sporophyte Angelina.

    Each lesion/bump matures into a sporangium.

    Inside each sporangium, diploid cells undergo meiosis to produce haploid spores.

    • Sporophyte Brad produces (male) microspores
      in his microsporangia.
    • Sporophyte Angelina produces (female) megaspores
      in her megasporangia.

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    Spores are Released

    Once the spores are mature, they are ready to be dispersed.
    The sporangia rupture, releasing spores to the wind.

    Remember:

    • Sporophyte Brad releases (male) microspores.
    • A microspore (n) will grow into a male gametophyte (n).

    • Sporophyte Angelina releases (female) megaspores.
    • A megaspore (n) will grow into a female gametophyte (n).

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    Sporophytes Also Can Be Monecious or Bisexual

    The Brad and Angelina Sporophytes belong to a dioecious species.

    In other species, sporophytes can be monoecious or bisexual.

    A monoecious sporophyte produces either
    • megasporangia that yield megaspores that grow into female gametophytes.
    • microsporangia that yield microspores that grow into male gametophytes.

    The RuPaul Sporophyte belongs to a bisexual species that produces spores.

    Review the "Sexual Prefixes" slide (six slides back) before you answer the Quiz question.

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    II. The Gametophyte Generation

    Carried by the wind, the spores tumble out into the world.

    Some land on rooftops and wither away in the heat.
    Some end up in Spike's food bowl and are ingested with the kibble.

    But some land on a patch of moist soil.

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    Male and Female Gametophytes

    Spores that land in a hospitable environment germinate.

    They grow via mitosis into haploid gametophytes.

    The gametophytes look completely different from their diploid sporophyte parents.

    (For one thing, they are relatively small and inconspicuous.)

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    Male Gametophyte (n): Antheridia Produce Sperm

  • A male gametophyte (n) is also known as a microgametophyte.

  • Microgametophytes have male sex organs called antheridia.

  • Each antheridium produces many haploid sperm.

  • Although they are haploid, antheridia can be considered analogous to testes.

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    Female Gametophyte (n): Archegonia Produce Ova

  • A female gametophyte (n) is also known as a megagametophyte.

  • Megagametophytes have female sex organs called archegonia.

  • Each archegonium produces one haploid ovum.

  • Although they are haploid, archegonia can be considered analogous to ovaries.

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    Sperm and Ova Ready...

    In their verdant little glade,
    the mature male and female gametophytes
    wait for that fateful event they need for reproduction:

    A GOOD RAIN STORM.

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    Go Sperm, Go!

    Rain provides the moisture sperm need to swim.

    Male gametophytes release their sperm, which follow
    the chemical trails exuded by the female gametophytes.

    Sperm swim up the female's legs and into her archegonium.
    One sperm fertilizes the waiting ovum.

    A diploid zygote is formed inside the archegonium.

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III. Return of the Sporophyte

The next sporophyte generation is growing inside the archegonium of the gametophyte.

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    A Pregnant Pause

    Our imaginary animal is now incubating a growing zygote
    inside her archegonium.

    The diploid zygote will begin to grow into a
    new, genetically unique, diploid sporophyte.

    But what does this mean for the gametophyte?

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    This Will Not End Well for Smurfette

    Lacking a muscular, innervated uterus, the female gametophyte
    cannot give birth to the new sporophyte.

    As her offspring grows, it gradually crushes her
    out of existence.

    Tell your parents about this next time they complain
    that you're always asking them for money.

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    Evolution: Gametophyte vs. Sporophyte

    In the earliest plants, the gametophyte (n) generation was long-lived and dominant.
    As plants evolved, the gametophyte became more and more reduced.

    In most extant plants, the sporophyte is the long-lived, dominant generation.

    What does this progression look like in

    • the gametophyte?
    • the sporophyte?

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(click on pic for source)

    I. Evolutionary Progression:
    Long-lived Gametophyte,
    Ephemeral Sporophyte

    The earliest land plants resembled filamentous green algae.
    The gametophytes and sporophytes looked the same.

    Eventually, the descendant sporophyte

    • began to change in appearance
    • became dependent on the gametophyte for its survival

    Modern bryophytes still exhibit this ancestral pattern.

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REQUIRED VIDEO:

    The Bryophyte Gametophyte is the
    Dominant, Perennial Generation

    Bryophytes include
    • hornworts (Anthocerophyta)
    • liverworts (Hepatophyta)
    • mosses (Bryophyta)

    When you see any of these growing in a damp place,
    you are seeing haploid gametophytes, the dominant, perennial generation.

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    <--- Watch the video for a clear illustration of a typical bryophyte life cycle.

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    Bryophyte Gametophytes

    Bryophyte gametophytes are small and close to the ground.
    They usually reproduce once a year, in the spring.

    <---- On the left, a liverwort gametophyte (Marchantia sp.)

    xxxx On the right, a moss gametophyte (Dicranum sp.) ---->

    A relatively undifferentiated plant body such as this
    is called a thallus (Greek thallus means "green shoot").


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  • Liverwort archegoniophores
    resemble tiny palm trees.
  • Archegonia are on the undersides
    of the "fronds".
    • Bryophyte Gametophytes can be
      Monoecious or Dioecious

      Mosses and liverworts can be monoecious or dioecious.

      Liverwort dioecy is so cute that we have to show you.

      <--- Female liverworts bear archegoniophores.

      xxxxMale liverworts bear antheridiophores. --->

    • Umbrella-shaped antheridiophores bear antheridia.
    • Palm tree-shaped archegoniophores bear archegonia.

      Antheridia release sperm when the environment is wet.
      These swim up the archegonial stalks, into the archegonia.
      And we have...zygote!

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  • Liverwort antheridiophores
    resemble tiny umbrellas.
  • Antheridia are on the upper surface.

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    (click on pic for source)

      II. Evolutionary Progression:
      Long-lived Sporophyte,
      Ephemeral Gametophyte

      Over the course of evolution, tracheophytes reversed
      the ancestral pattern of dominant gametophyte and ephemeral sporophyte.

      As tracheophytes became more derived,
      the gametophyte generation became more and more reduced.

      In the most derived plants (angiosperms),
      the gametophyte is a tiny blob of cells deep inside the flower's ovary.

      The little blob's only job is to make gametes.

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      Seedless Tracheophyte Gametophytes

      Like bryophyte gametophytes, seedless tracheophyte gametophytes are small and bisexual.
      Unlike bryophyte gametophytes, they live only one season.

      <---- On the left, a fern gametophyte

      xxxx On the right, a horsetail gametophyte ---->

      The resemblance of seedless tracheophyte gametophytes
      to liverwort gametophytes is no mere coincidence.

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      Seed Plant Gametophytes are Small and Dioecious

      All seed plant gametophytes are dioecious.

      Seed plant gametophytes are so reduced and highly derived
      that they no longer resemble their liverwort ancestors.

      But they are completely homologous to them.

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      Seed Plant Gametophytes: Male

      The male gametophyte of a pine or flowering plant
      is a small, haploid plant you may know as a pollen grain.


      <---- pine male gametophyte (pollen)

      xxxx anthophyte male gametophyte (pollen) ---->

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      Seed Plant Gametophytes: Female

      The gametophytes of female seed plants
      are even more reduced.

      <---- pine female gametophyte (inside ovule)

      xxxxanthophyte female gametophyte (inside ovule) ---->


      The seed plant female gametophyte remains encased
      in sporophyte tissue throughout its life.

      Sporophyte integuments + gametophyte comprise the ovule .
      After fertilization, the ovule becomes a seed .
      The sporophyte integuments form the seed coat.

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