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A short class with Sarah and her students. The other ways are under More.
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In humans, meiosis makes the sperm and eggs directly. Do you think a lily does the same?
I assumed so. Isn't meiosis always how gametes are made?
Not in plants. In the anther and the ovule, meiosis makes haploid spores, and each functional spore grows into a small gametophyte.
So what actually makes the gametes?
The gametophyte does, by mitosis. Because it is already haploid, mitosis gives it haploid gametes.
Then the pollen grain is a whole tiny generation, not just a gamete?
Exactly. The pollen grain is the male gametophyte and the embryo sac is the female gametophyte.
A microspore's nucleus divides once by mitosis. What is unusual about that division?
It is unequal. We get a big vegetative cell and a small generative cell.
Right. The vegetative cell stores food and will form the pollen tube. What does the generative cell do?
It divides again to make two male gametes.
In most flowering plants that second division happens inside the pollen tube, because the pollen is shed at the 2-celled stage.
So whether pollen is shed 2-celled or 3-celled just depends on when the generative cell divides.
Start from one functional megaspore. How many mitotic divisions give eight nuclei?
One gives two, the second gives four, the third gives eight. So three divisions.
Good. Walls form only after the 8-nucleate stage. How many cells do we end up with?
Seven: three antipodals, two synergids, one egg and the central cell with two polar nuclei.
And which of these is the female gamete?
Only the egg. The polar nuclei fuse with the second male gamete to form the endosperm nucleus.
meiosis makes haploid spores, and the gametophytes that grow from them make gametes by mitosis.
the pollen grain, with a vegetative cell and a generative cell that divides into two male gametes.
the embryo sac, formed by three free-nuclear mitotic divisions of the functional megaspore.
the egg, two synergids, three antipodals and a central cell with two polar nuclei.
most flowering plants shed pollen before the generative cell has divided.
One pollen mother cell → meiosis → 4 microspores → 4 pollen grains, each with 2 male gametes, so one meiosis in the anther finally gives 8 male gametes.
One functional megaspore → 3 free-nuclear mitoses → 2, then 4, then 8 nuclei → an embryo sac of 7 cells containing a single egg.
To produce 100 pollen grains a plant needs 25 meiotic divisions (25 pollen mother cells), but to produce 100 embryo sacs it needs 100 (one megaspore mother cell for each).
First the spore by meiosis made, then by mitosis the gametes laid.
A microspore split into a big vegetative cell, who packed the food and promised to drive the tube, and a small generative cell, who later split into twin male gametes for the journey to the egg.
1-2-4-8 - one megaspore nucleus doubles three times to give the eight nuclei of the embryo sac.
E.S.A.P. - Egg and Synergids at the micropyle, Antipodals at the chalaza, Polar nuclei in the centre.
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Which cell of the pollen grain gives rise to the male gametes?
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How many nuclei and cells does a typical mature embryo sac have?
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What is a free nuclear division?
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At what stage is pollen shed in most flowering plants?
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What guides the pollen tube into the embryo sac?
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The haploid generation that produces gametes by mitosis; in flowering plants, the pollen grain and the embryo sac.
A haploid spore formed by meiosis of a pollen mother cell; it develops into a pollen grain.
The larger cell of the pollen grain, rich in food reserves, which forms the pollen tube.
The small cell of the pollen grain that divides by mitosis to form two male gametes.
The one megaspore out of four that survives and develops into the embryo sac.
Division of nuclei without the immediate formation of cell walls, as in the developing embryo sac.
The egg cell and the two synergids at the micropylar end of the embryo sac.
The two nuclei of the central cell that fuse with a male gamete to form the primary endosperm nucleus.
Gametogenesis in flowering plants is the formation of the male gametes inside the pollen grain and of the egg inside the embryo sac. Unlike animals, plants make their gametes by mitosis, in haploid gametophytes that grow from spores produced by meiosis.
In a flowering plant, gametes are not formed directly by meiosis. Meiosis first produces haploid spores: microspores in the anther and megaspores in the ovule. Each functional spore then grows by mitosis into a gametophyte. A microspore becomes the pollen grain, whose generative cell divides to give two male gametes; a functional megaspore goes through three free-nuclear mitotic divisions to form the 8-nucleate, 7-celled embryo sac that contains the egg. This lesson traces both pathways, compares them and shows how they prepare the flower for double fertilisation.
The plant you see is the diploid sporophyte. In reproduction its job is to make spores, and it does so by meiosis. The spores are haploid, and each one develops into a tiny gametophyte, which makes gametes by mitosis. So in flowering plants:
The lessons on Microsporogenesis and Megasporogenesis describe the meiotic step. This lesson follows what happens after it.
The mature pollen grain, with its vegetative cell and two male gametes, is the male gametophyte.
The mature embryo sac, the female gametophyte, therefore has 8 nuclei in 7 cells:
Gametogenesis sets the stage for double fertilisation. One male gamete fuses with the egg (syngamy) to form the diploid zygote, and the other fuses with the two polar nuclei (triple fusion) to form the triploid primary endosperm nucleus. Since one pollen grain carries two male gametes, a single pollen grain is enough for one embryo sac.