Cell Division and Multicellular Life Cycles.

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Introduction:

Typically we learn that meiosis is for producing gametes and mitosis for producing cells for growth repair and asexual reproduction. In truth the situation is more complex even among many very familiar kind of organisms. First of all, many one celled organisms spend much of their lives as haploid, rarely or never producing diploid cells and undergoing meiosis. Here I show you three basic multicellular life cycles that illustrate how mitosis and meiosis fit in to the life cycles of different major groups of organisms.

Animals.

In the animal life cycle the only multicellular stage is typically diploid. For example, humans have 46 chromosomes in their diploid cells. We produce gametes from specialized diploid cells called germ cells and the gametes, being haploid are produced by meiosis. Two gametes fuse to form the diploid zygote which then undergoes mitosis to produce daughter cells which also differentiate into different cell types.

Even within the animals there are a number of biologically interesting wrinkles. For instance in bees ants and wasps the male is haploid and the females diploid. Thus the male gametes are produced by mitosis(actually a highly modified form of meiosis) and the female gametes by regular meiosis!

Eventually an immature animal is produced and this in turn develops into an adult animal.

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Plants.

Plants have a radically different life cycle called alternation of generations. The mature fern plant for example is a diploid multicellular stage called a sporophyte. Special structures in the sporophyte(in the fern these are called Sori) have cells that undergo mitosis to produce haploid spores.

The spores are not gametes because they are carried away by the wind, land in a moist area and germinate. By mitosis the resulting cell develops into a multicellular gametophyte. Special structures in the gametophyte produce gametes by mitosis. One gamete is smaller and capable of swimming(at least in ferns) and swims to the other gamete to form a zygote. The zygote undergoes mitosis and development to form a new diploid sporophyte.

In flowering plants the same basic alternation of generations happens but it is highly modified. For instance the flowers and trees you see every day are sporophytes. The sore production and the female gametophyte are hidden from view inside the plant. The only obvious hint of a gametophyte is pollen, which is believed to correspond to a male gametophyte.

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Fungi.

Fungi can have very complicated life cycles but the standard fungus life cycle includes a multicellular or colonial haploid stage. This stage is in the form of a fibrous mass called a mycelium and each of the fibers is called a hypha.

Fungus life cycles also include the presence of genetically different mating strains. These look alike but the cells of the hypha. can tell them apart. What's critical for mating is that unlike mating strains (- and+) get together and their nuclei fuse to form the diploid zygote.

The zygote often develops into a resistant thick walled structure often called a zygospore. When conditions are ripe the cell in the zygospore produces sexual sores by meiosis. These are haploid and are dispersed through the air. When they land in a suitable spot such as bread in your bread box, the spores germinated and develop into new hyphae.

Also, the hyphae. will develop specialized structures that produce asexual spores by mitosis.

The key thing is that there is no true diploid multicellular stage in fungi. Even in mushrooms, the fruiting body that we see above ground is not diploid. The mushroom forms when hyphae. of different mating strains fuse to form a special type of cell with two separate nuclei one from each of the original mating strains. These cells are called not diploid by dikaryotic since the nuclei don't fuse.

Mushroom spores are produced by specialized dikaryotic cell in which the nuclei now fuse, produce a standard diploid zygote which then undergoes meiosis to produce spores.

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pgd 8/1/99 revised 5/2/02