Biology 104 Embryology Jan 24, 2005 Albert Harris
How do (& how did?) one celled organisms become many-celled organisms
Volvox and Dictyostelium/ become multicellular in very different ways.
Volvox is a genus of multicellular plants. Each individual volvox is a hollow sphere of cells.
Each cell has flagella, and the coordinated beating of these flagella cause the Volvox to swim.
The next generation of volvox (the 'babies' of the next generation) develop from special cells,
and grow as smaller, inside-out spheres inside each parent. They are inside-out in the sense that
the flagellar end of each cell points inward. When they are mature, they invert right-side-out and break through their mother's surface to be released out into the world.
Notice the similarity between Volvox and a genus of one-celled algae named Chlamydomonas.
These live as individuals, with each cell having two flagella at one and, which swim in a pattern resembling a "breast stroke". There are many different species of Chlamydomonas, and also many different species of Volvox, and there are also many species of some morphological intermediates (that look sort of like smaller Volvox) such as those of the genus Eudorina. DNA sequencing of parts of the genomes of many of these species seems to prove that certain species of Eudorina evolved from certain species of Chlamydomonas,
and they in turn begat Volvox (my use of the verb begat is to assuage Creationists), and that this evolutionary sequence has happened several times, separately. ("Convergent Evolution")
In other words, species X of Volvox has gene sequences that are more similar to species Q of Chlamydomonas, and sometimes to species B of Eudorina. Whereas species Y of Volvox has gene sequences that are more similar to species R of Chlamydomonas, and to species C of Eudorina.
That is strong evidence that the "Volvox" morphology, shape, appearance and way of life has evolved over and over again from Chlamydomonas (in the distant past, of course), and that the species classified as Eudorina and Pandorina are living examples of intermediate stages in this evolutionary achievement of multicellularity (this use of the word "achievement" may reflect multicellular chauvinism). Different species of Volvox evolved separately, three or more separate times, always from unicellular algae that would be classified as Chlamydomonas. Nobody watched this happen, any more than they made a time-lapse video of the erosion of the Grand Canyon, but the evidence from the genetic base sequences is clear.
Higher plants evolved from Green Algae (& not from Brown Algae like Kelp, or from Red Algae).
Chlamydomonas and Volvox are both classified as Green Algae (but they aren't the ancestors of higher plants)
But nobody knows whether higher plants evolved just once, from one kind of Green Algae.
Multicellularity among animals may also have evolved just once, or maybe two or more separate times. Many scientists thank sponges may have evolved multicellularity separately from Hydra, flat-worms and more complex phyla of animals. Some recent evidence indicates not. Personally, I liked that, because I discovered some interesting phenomena about sponges, and don't like it when people say "Oh, that's not important! Sponges evolved separately! They aren't cool like Roundworms."
Several kinds of protozoa live together in colonies of mitotic daughter cells that stay together;
That is probably the sequence by which multicellularity evolved in animals. However.....
A very different way to become multicellular occurs in a group of "animals" (?) called "Cellular Slime Molds". Are they fungus-like amoeboid animals? Or are they animal-like ameboid fungi?
The best-studied species in this group is Dictyostelium discoideum, which was discovered by a UNC graduate named Kenneth Raper in the 1930s. Hundreds of different research laboratories, all over the world, have concentrated on studying this one species.
They are small unicellular amoebae, that eat bacteria by phagocytosing them.
When they run out of food, these amoebae begin to attract one another by chemotaxis.
These amoebae aggregate into "slugs", which may contain any where from hundreds of thousands of cells per slug, down to as few as a hundred cells or less. (These are not the same as the snail-like
animals called slugs, which are molluscs). You can call one a "pseudoplasmodium" or a "grex".
These slugs elongate and crawl around for a few hours, or less, until they find a good spot, and then some of the cells differentiate into stalk cells, and the majority differentiate into spore cells.
This process of "fruiting" produces a tall stalk, with an approximately spherical mass of spores at its top end. "Grex" is Latin for herd (gregarious).
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