Origin of Prokaryotes and Eukaryotes: Slime Molds
Slime molds are interesting because they have a dual identity, each with separate characteristics. During part of their life cycle, they appear as small blobs of protoplasm that are similar in movement to an amoeba as they surround and engulf bac-teria and other available organic nutrients. When harsh conditions threaten, they form into a completely different-looking moldlike mass that produces funguslike reproductive spores. There are three types of slime molds:
Oomycota slime molds are all parasitic and are usually aquatic; however, some are also found in the soil. These molds are responsible for the white, threadlike growths seen on diseased tropical fish in home aquariums. They also caused the great Irish potato famine that led to the deaths of more than 400,000 people. The Irish potato famine was actually a series of crop failures that occurred in Ireland during the years of 1845, 1846, 1848, and 1851. The Irish economy was heavily based on the production and sale of potatoes. When the blight attacked the young plants, the tubers (potatoes) rotted so no crops were available to generate income. As a result, widespread starvation ensued with an estimated one million people emigrating to the United States or Great Britain.
Acellular slime molds, Myxomycota, are unique in producing structures called plasmodia, which appear to be a mass of cells, but in reality are one large cell that contains many nuclei free to float unhindered by internal membrane barriers. The plasmodium slime mold remains in this blob stage feeding on bacteria, fungi, and other organic nutrients, typically found on the floor of a deciduous forest, because of its mobility. In its blob form, it also has greater surface area to transfer materials to and from the environment. However, when food or water is scarce or the environment becomes too dry, the plasmodium transfigures into the moldlike mass and produces the characteristic fungilike fruiting bodies. The fruiting bodies create haploid spores via meiosis, which then combine to become diploid and begin growing into amoebalike blobs when conditions again become favorable.
Acrasiomycota are cellular slime molds that are haploid blobs that live independently in much the same fashion as other slime molds until food or water becomes unavailable. Interestingly, they then leave a chemical trail that acts like a pheromone to attract other cellular slime mold cells to a central location, where they form a pseudoplasmodium. During this stage, the individual cells, by still unknown means, communicate and coordinate their movements to act as one large blob. As the pseudoplasmodium moves, it can divide and (in unfavorable conditions, such as high heat or excessive dryness) form fungilike fruiting bodies that develop haploid spores. The haploid spores remain dormant until favorable conditions return, at which time they grow into individual haploid slime molds.
Excerpted from The Complete Idiot's Guide to Biology 2004 by Glen E. Moulton, Ed.D.. All rights reserved including the right of reproduction in whole or in part in any form. Used by arrangement with Alpha Books, a member of Penguin Group (USA) Inc.