Homology in the Alternation of Generations

The Alternation of Generations is homologous across plant taxa. The sporophyte, gametophyte, and associated structures are derived from a common ancestor (and are homologous), even though the plants and structures themselves may look very different from each other.

Gametophytes of the Non-Vascular Plants
(liverworts on the left, moss on the right)

Sporophytes of the Non-vascular Plants
(liverworts on the left, moss on the right)


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.


What do the sporophytes of these gametophytes look like?
In vascular plants, the situation is reversed: the sporophyte is conspicuous and long-lived, compared to the tiny gametophytes shown above.
As above, from left to right, the sporophytes of fern, horsetail and club moss:

They don't look much like the tiny sporophyte of the liverwort or moss, but they are all homologous.


Gametophytes of the Vascular Plants II: The Seed Plants
The sporophytes of the Seed Plants, such as conifers (left) and flowering plants (right) , are even larger and longer-lived.

These plants are homologous to the liverwort sporophyte above, which remained attached to the archegoniophote all its life, as well as to the moss sporophyte that sprouted briefly from the gametophyte to release its spores.

What about the gametophytes?
As the Vascular Plants became more derived, the gametophyte generation became more and more reduced. To see it, we have to look very close--at the microscopic level.
Let's start with the most familiar of the non-flowering seed plants, the Pine.

Unlike the frondy sporophyll of a typical fern, the sporophylls of a pine are spirally arranged around a central stem to form a structure called a strobilus, commonly called a "cone".

Pines produce separate male and female strobili on the same plant. Hence, pines are monoecious

The male cone lasts only a few weeks. But it takes the female a full two years to go from pollination (far left) to maturity and seed release (far right).

Each fleshy bract of the strobilus bears a sporophyll:
The Gametophytes of the Conifers
Each spore will develop into a haploid gametophyte, homologous to the gametophytes of the liverwort, moss, and fern. But these are highly reduced, compared to the liverwort, moss, and even the fern gametophyte.

The Male Gametophyte of the Pine
Each microspore inside the microsporangium develops into an individual, haploid male gametophyte that's little more than a container for two sperm. It is called a pollen grain, and here's what pine pollen (male gametophytes) look like:

  • Each pollen in the pictures above is homologous to the little green liverwort bearing antheridiophores, as well as to the little green fern, club moss, and horsetail gametophytes.

  • Pines are wind-pollinated, and they release huge numbers of pollen in the spring.

    The Female Gametophyte of the Pine

  • Meanwhile, back at the female, the megaspore inside its megasporangium develops into a female gametophyte.

  • The gametophyte in the picture above is homologous to the little green liverwort bearing archegoniophores, as well as to the little green fern, club moss, and horsetail gametophytes.

  • The female gametophyte remains inside the female megasporangium, which is surrounded by sporophyte integuments. This structure is known as an ovule.

  • Eventually, after the ovum is fertilized and becomes a zygote, the ovule will become a seed. The outer covering, the seed coat is derived from sporophyte tissue. Nutritive nucellus surrounds the embryo to give it a head start upon germination.

  • When mature, the seeds are released by the female strobilus, and are scattered by various dispersers (such as seed-eating animals).


    The Gametophyte of the Anthophyte
    You saw the cherry tree above. To see the gametophytes, we need to examine that structure unique to the Anthophytes, the flower.

  • The microsporophylls are the stamens, each tipped by the anther, which contains the diploid cells that will divide to become microspores.
  • As in the pines, each microspore will develop into the male gametophyte, pollen.
  • As in the pine pollen, the antheridia of the anthophyte pollen have been lost.

  • The megasporophylls are the carpels, each consisting of a stigma, style, and ovulary. Inside the ovulary, diploid cells will divide to become megaspores.
  • As in the pines, each megaspore will develop into the female gametophyte.
  • Unlike the pine female gametophyte, this one has lost her archegonia. She is nothing but a sac of cytoplasm containing eight identical nuclei, only three of which will fuse with a sperm.

  • the ovum will fuse with one sperm to produce the zygote.
  • the two polar nuclei will fuse with another sperm to produce the nutritive endosperm, analogous to the pine's nucelllus.