Lecture 21 - Plant Life Cycles The Alternation of Generations (Chapter 29, 35) - Evol & Biodivrsity PDF

Preview

Summary

Download Lecture 21 - Plant Life Cycles The Alternation of Generations (Chapter 29, 35) - Evol & Biodivrsity PDF

Description

The Sex Lives of Plants Plants can be classified into these major groups:  Bryophytes - The Non-Vascular Plants o Liverworts (Marchantiomorpha) o Hornworts (Anthocerophyta) o Mosses (Bryophyta)  Tracheophyta - The Vascular Plants o Club "mosses" (Lycopdiopsida) o Ferns (Polypodiopsida) o Seed Plants (gymnosperms and angiosperms) ALL of these diverse plants undergo a life cycle that includes an Alternation of Generations.    

In bryophytes, the dominant life cycle stage is the gametophyte. The bryophyte sporophyte is small and ephemeral/seasonal. In tracheophytes, the dominant life cycle stage is the sporophyte. The tracheophyte gametophyte is small and ephemeral/seasonal.

The structures and life cycle stages in all these plants are homologous and have the same names, but they may look VERY different from one another, across taxa.

Plant Life Cycle: The Alternation of Generations All Embryophytes and some algae undergo an Alternation of Generations. Each generation of offspring has a different ploidy than the parent generation before it. Generations alternate from diploid to haploid to diploid to haploid, etc. sporophyte (2n) --> gametophyte (n) --> sporophyte (2n) --> gametophyte (n) --> sporophyte (2n) etc...

In short: A diploid (2n) sporophyte plant undergoes meiosis to produce haploid spores. Each spore undergoes mitosis to grow into a haploid (n) gametophyte plant. The gametophyte undergoes mitosis to produce gametes (sperm and/or ova). Ovum and sperm join (fertilization) to form a diploid (2n) zygote. The zygote undergoes mitosis to grow and develop into a new diploid (2n) sporophyte plant.  Rinse and repeat.     

All plants go through this same basic life cycle, though the (homologous) stages may look quite different across different taxa.

Sometimes it's hard to envision how this cycle works, because plants seem to introduce a "middle man" where animals go straight from haploid gamete to diploid zygote.

To get around this sticking point, I offer you... Alternation of Generations: An Analogy Meet our model organisms, Diploid Brad and Diploid Paris:

Looks can be deceiving. Although Diploid Brad and Diploid Paris may LOOK like animals, they are not. They are highly derived plants that have evolved animal mimicry in order to get more attention and fertilizer.

As plants, they still undergo alternation of generations. An alert scientist can spot a mimic by observing its life cycle.

Life Cycle: The Sporophyte Generation In the springtime, Diploid Brad and Diploid Paris begin to feel frisky. Small lesions known as sporangia begin to develop on specialized areas of their epidermis. Inside each sporangium, diploid cells divide to produce haploid cells called spores.

Because Diploid Brad and Diploid Paris produce spores, their life cycle stage is named the SPOROPHYTE GENERATION.

New Vocabulary Alert!

 dioecious species - each individual organism is either male or female (separate sexes)  monoecious species - each individual organism has both male and female reproductive structures (bisexual) In our Brad/Paris species, the sporophyte generation happens to be dioecious:   

male organisms produce male spores called microspores female organisms produce female spores called megaspores In monoecious species, the spores are bisexual, and are just called spores.

After a period of maturation, the spores are ready for distribution into the environment. The sporangia rupture, releasing spores into the air where they can be carried by the wind to new and exciting environments.

Life Cycle: The Gametophyte Generation The tiny spores swirl out into the night, diaspora of the human vegetable mimics. Wind carries them everywhere. Some land on the tops of buildings, and there remain until the elements kill them. Some end up in Snuffy's dog food bowl, and are ingested along with the kibbles.

But a few lucky spores are carried away to land on a patch of moist soil. And here the spores germinate and grow (via mitosis) into haploid organisms that look completely different from their diploid parents. These new, haploid organisms are the GAMETOPHYTE GENERATION. In our Brad/Paris species, the gametophyte generation also happens to be dioecious: 

Brad's microspores will develop into male gametophytes:



Paris's megaspores will develop into female gametophytes.

 The haploid male (Smurf) gametophytes have male sex organs (analogous to testes) called antheridia.  Antheridia produce sperm.  The haploid female (Smurfette) gametophytes have female sex organs called archegonia.

 Archegonia produce ova (eggs). In their damp little glade, the small forest of Smurfs and Smurfettes wait for that fateful event they need for reproduction: A GOOD RAIN STORM.

At last it rained! (Use your imagination.) (But not too much.)

Fortunately, the picture is blurry. Because with the rain, the males release their sperm into the storm. The sperm swim valiantly, searching for the females. Following a chemical trail sent out by the Smurfette gametophytes, the sperm wiggle up the legs of the Smurfettes and...BEYOND. Deep in the BEYOND, the sperm encounter the female sex organ, the archegonium. Inside the archegonium, one sperm and one egg fuse to form a diploid zygote.

The stage is set for tragedy. (At least if you're Smurfette.)

Life Cycle: Return of the Sporophyte

Without a muscular, innervated uterus, Smurfette cannot give birth to the diploid offspring that will develop from the zygote. As her child grows, it gradually crushes her out of existence, erupting from her quivering shell. Thus begins the life of the new SPOROPHYTE.

The diploid sporophyte will grow to adult size. When it is sexually mature, sporangia will erupt from his epithelium. And the Life Cycle spins on.

And That's How Plants Do.

To keep track of this complex cycle, we need to use a complex terminology. I. TERMS RELATED TO THE (HAPLOID) GAMETOPHYTE GENERATION: o gametophyte - a haploid plant which produces gametes o gamete - haploid ovum (female) or sperm (male), produced via mitosis in the sex organs of the haploid gametophyte. o archegonium - the multicellular sex organ of the female gametophyte, analogous to an ovary. o antheridium - the multicellular sex organ of the male gametophyte, analogous to a testis. o archegoniophore - a structure bearing archegonia o antheridiophore - a structure bearing antheridia o zygote - single, diploid cell produced by the union of sperm and egg inside the archegonium. It will develop into the sporophyte. II. TERMS RELATED TO THE (DIPLOID) SPOROPHYTE GENERATION: o sporophyte - a diploid plant which produces spores. o spore - haploid cell produced via meiosis in the sporangium of the sporophyte. It will develop into the gametophyte.

o sporangium - structure inside which diploid sporophyte cells undergo meiosis to become spores. o megaspore - a spore which develops into a female gametophyte o microspore - a spore which develops into a male gametophyte o sporophyll - a specialized leaf bearing sporangia on its leaf blade. o megasporophyll - a sporophyll bearing megaspores o microsporophyll - a sporophyll bearing microspores III. GENERAL TERMS:  

dioecious - each individual is either male or female (separate sexes) monoecious - each individual has both male and female reproductive structures (bisexual)

What do GAMETOPHYTES look like? It depends on the plant taxon. Let's have a look. I. Bryophyte (non-vascular plant) Gametophytes In non-vascular plants (mosses, liverworts, hornworts), the gametophyte is the dominant generation. When you see a typical moss or liverwort, you are seeing a haploid plant that can live for many years. 

Liverwort gametophytes, not sexually reproducing (at the moment). In the photo on the right, tiny gemmae cups contain asexually grown plantlets (gemmae) that will splash out in the rain and grow to become new liverworts, genetically identical to the parent thallus.



Moss gametophytes:

They can be very comfy.

The Liverwort will show us the generalized life cycle of a bryophyte.

II. Tracheophyte (vascular plant) Gametophytes In vascular plants (tracheophytes), the gametophyte is reduced, and does not live more than a season. The more derived the plant group, the more reduced the gametophyte. 1. Gametophytes of the Vascular Plants I: The Seedless Vascular Plants (from right to left: fern, horsetail, club moss)

They look amazingly like liverwort gametophytes, don't they? For good reason. Recall the link between similar ontogenies and common ancestry. Seedless Tracheophyte gametophytes are often bisexual: each gametophyte has both antheridia and archegonia. The fern will represent a generalized life cycle of a seedless tracheophyte:

2. Gametophytes of the Vascular Plants II: The Gymnosperms In the gymnosperms (from the Greek gymnos meaning "naked" and sperm meaning "seed"), the gametophytes are drastically reduced, and are no longer recognizable as plants (though, technically, they are). Gymnosperms include such plants as cycads, ginkgoes, and conifers. Our representative, typical gymnosperm will be the pine. The male gametophyte

   

is a pollen grain (left). The antheridia have been lost; each pollen produces only two sperm. The male pine cones grow on the ends of pine tree branches in the spring (center) The pollen develop from microspores in microsporangia on male pine cones (longitudinal section, right)

 Each pollen in the pictures above is homologous to the liverwort thallus, and the fern and horsetail gametophytes.  Pines are wind-pollinated, and they release huge numbers of pollen gametophytes in the spring.

Click on the picture to see a video of spectacular pollen release. The female gametophyte 

permanently resides inside the female pine cone megasporangium (left).

   

Each gametophyte produces 1-6 archegonia (depending on species), but only one produces a viable ovum. The female pine cones grow on the ends of pine tree branches in the spring (center) The female gametophytes develop from megaspores in megasporangia on female pine cones (longitudinal section, right) In the center picture, three different years' female pine cones are shown. o The small red ones are brand new and ready for pollination. o The slightly larger brown ones are one year old, and fertilization is taking place inside them. o The large, woody cone is two years old. The bracts have opened up to release the mature seeds.

Below, from left to right:   

female gametophyte (microscopic view) female strobilus, longitudinal section, showing location of gametophytes (inside ovule) female strobili: three different years

 The female gametophyte is surrounded by sporophyte integuments. This entire structure is known as an ovule.  After the gametophyte's ovum is fertilized to become a zygote, the ovule becomes a seed.  The seed coat is made of sporophyte tissue. Nutritive, fatty nucellus surrounds the embryo to give it a head start upon germination. (This is why pine nuts are so

delicious!)

 Mature seeds are released by the female strobilus to be dispersed by wind or animals. The pine tree will represent the general life cycle of a conifer:

3. Gametophytes of the Vascular Plants III: The Angiosperms The angiosperm sporophyte produces a unique structure for sexual reproduction: the flower.

All the parts of the flower are modified leaves. From outermost to innermost, these are    

sepals (collectively called the calyx) petals (collectively called the calyx) anthers (collectively called the androecium), the male sporophylls (microsporophylls) carpels (collectively called the gynoecium), the female sporophylls (megasporophylls)

The gametophytes of flowering plants are even more reduced than those of the gymnosperms. The male is still a pollen grain, and fairly similar (in function) to pine pollen. But the female gametophyte no longer even develops archegonia. She is reduced to a mass of cytoplasm containing eight identical, haploid nuclei. The male gametophyte 

is a pollen grain (right) o has no antheridia; produces only two sperm

o The pollen develop from microspores in microsporangia known as anthers (left) o The anthers are located just inside the petals in a complete flower (i.e., one that has sepals, petals, stamens, and pistils).

The female gametophyte      

is a mass of cytoplasm containing eight identical, haploid nuclei has no archegonia (she is even more reduced than the pine gametophyte) remains inside the female megasporangium inside the flower's ovary. The female gametophyte is also known as the 8-nucleate embryo sac. She develops from a single megaspore inside the megasporangium inside the ovule. Like the pine ovule, the angiosperm ovule's outer coat is made from mother sporophyte tissue.

Last stop, the angiosperm life cycle as represented by the Lily:

The archegonial symplesiomorphy: It looks similar across species. The archegonium is the sex organ (analogous to an ovary) of the female gametophyte. It looks remarkably similar across taxa, though its location becomes less obvious as the gametophyte becomes smaller in the more derived plants.

Female liverwort: archegonia are on the underside of the female gametophyte archegoniophore :

Female moss: archegonia are at the tip of a female gametophyte's shoot

Bisexual Fern: archegonia are clustered around the notch of the little heart-shaped gametophyte

Female gymnosperm: archegonia are inside the female gametophyte inside the ovule

Female angiosperm: archegonia have been lost. The 8-nucleate embryo sac directly produces a single ovum.

What do SPOROPHYTES look like? This is the diploid generation. In bryophytes, the sporophyte is ephemeral and small. In tracheophytes, the sporophyte is the dominant, long-lived generation.

Liverwort sporophyte: These are little teardrop-shaped capsules attached to the bottom of the female archegoniophore. They release their spores, then dry up and die.

Moss sporophytes: As in the liverwort, these sprout out of the archegonium. They're little more than a stalk with a sporangium at the tip.

Fern sporophyte: The fern plant you know and love is a sporophyte.

Gymnosperm sporophyte: The large, (often) cone-bearing plant is the sporophyte

Angiosperm sporophyte: The flowering plants you see around you are all sporophyte generation individuals.

Notice the reversal in size and lifespan from bryophytes to tracheophytes. In tracheophytes, the sporophyte is the dominant, long-lived generation. Made it all the way to the end? Here's a little reward. A video of fern alternation of generations. (Can you find the factual boo boo in this video? Two points extra credit if you can!)...