Alternation of Generations
All Embryophytes and some algae undergo an Alternation of Generations.
Each generation of offspring has a different ploidy than the 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...
1. a haploid (n) gametophyte generates gametes (n) via mitosis
2. gametes fuse to form a diploid (2n) zygote
3. zygote grows mitotically into the sporophyte (2n)
4. the diploid (2n) sporophyte generates spores (n) via meiosis
5. released spores (n) grow into mature gametophytes (n)
...and the cycle continues, generation after generation.
The haploid (gametophyte) generation "gives birth" to the diploid (sporophyte) generation, and that diploid sporophyte generation "gives birth" to the next haploid gametophyte generation, and so on.
All plants go through this life cycle, though the (homologous) stages may look very different across taxa.
The generalized cycle of the Alternation of Generations:
I. TERMS RELATED TO THE (HAPLOID) GAMETOPHYTE GENERATION:
- gametophyte - haploid plant that produces gametes via mitosis
- gamete - haploid ovum (female) or sperm (male)
- gametangium - generic term for a multicellular plant sex organ. It can be either...
- archegonium - the female sex organ (analogous to the animal ovary)
- antheridium - the male sex organ (analogous to the animal testis)
- zygote - single, diploid cell produced by the union of sperm and egg inside the
archegonium. This will grow and develop into the sporophyte.
II. TERMS RELATED TO THE (DIPLOID) SPOROPHYTE GENERATION:
- sporophyte - a diploid plant that produces spores via meiosis
- spore - haploid cell that will grow into the haploid gametophyte
- megaspore - a spore that develops into a female gametophyte
- microspore - a spore that develops into a male gametophyte
- sporangium - a compartment in the sporophyte where diploid cells undergo meiosis to become haploid spores
- megasporangium - a sporangium that produces (female) megaspores
- microsporangium - a sporangium that produces (male) microspores
- sporophyll - a specialized leaf bearing sporangia on its leaf blade.
- megasporophyll - a sporophyll bearing megaspores
- microsporophyll - a sporophyll bearing microspores
III. GENERAL TERMS:
- dioecious - each individual is either male or female (separate
- monoecious - each individual has both male and female reproductive
To fully understand the cycle it might be helpful to use an animal-based analogy.
What doGAMETOPHYTES 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.
II. Tracheophyte (vascular plant) Gametophytes
- 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.
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 a liverwort gametophyte, don't they? For good reason. Recall the link between similar ontogenies and common ancestry.
These are often bisexual: each gametophyte has both antheridia and archegonia.
2. Gametophytes of the Vascular Plants II: The Gymnosperms
In the gymnosperms ("naked seed") plants, 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
Below, from left to right:
- remains 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.
- The small red ones are brand new and ready for pollination.
- The slightly larger brown ones are one year old, and fertilization is taking place inside them.
- The large, woody cone is two years old. The bracts have opened up to release the mature seeds.
- 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.
3. Gametophytes of the Vascular Plants III: The Angiosperms
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's just a mass of cytoplasm containing eight identical, haploid nuclei.
The male gametophyte
- is a pollen grain (right).
- It has no antheridia, and produces only two sperm.
- The pollen develop from microspores in microsporangia known as anthers (left)
- 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
- lacks 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.
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 goes straight to gamete!
What doSPOROPHYTES 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.