Slime molds and water mold resemble fungi in appearance and lifestyle, but the similarities are the result of convergence. In their cellular organization, reproduction, and life cycles, slime molds depart from the true fungi and probably have their closest relatives among the amoeboid protists. Although water molds lack chloroplasts, molecular comparisons suggest they are related to certain algae. The superficial similarity of the funguslike protists to true fungi is partly due to convergent evolution of filamentous body structure, a morphological adaptation that increases exposure to the environment and enhances the ecological role of these organisms as decomposers. The complex life cycles of funguslike protists are adaptations that contribute to survival in changing habitats and facilitate dispersal to new sources.
 
 

Myxomycota (Plasmodial Slime Molds) The phylum Myxomycota consists of plasmodia slime molds, which are more attractive than their name implies. Many species are brightly pigmented, usually yellow or orange, but slime molds are not photosynthetic; all are heterotrophs. The feeding stage of the life cycle is an amoeboid mass called a plasmodium, which may grow to a diameter of several centimeters. Large as it is, the plasmodium is not multicellular; it is known as a coenocytic mass, a multinucleate continuum of cytoplasm undivided by membranes or walls. In most species, the nuclei of the plasmodium are diploid and division are synchronous, with each of thousands of nuclei going through each phase of mitosis at the same time. Because of this characteristic, plasmodia slime molds have been used to study the molecular details of mitosis. Within the fine channels of the plasmodium, cytoplasm streams first one way, then the other; in pulsing flows that are beautiful to watch through a microscope The cytoplasmic streaming apparently helps distribute nutrients and oxygen. The plasmodium engulfs food particles by phagocytosis as it grows by extending pseudopodia through moist soil, leaf mulch, or rotting logs. If the habitat of a slime mold beings to dry up or there is no food left, the plasmodium ceases growth and differentiates into a stage of the life cycle that functions in sexual reproduction.

 

Acrasiomycota (Cellular Slime Molds) Members of Acrasiomycota, the cellular slime molds, pose a semantic question about what it means to be an individual organism. The feeding stage of the life cycle consists of solitary cells that function individually. When there is no more food, the cells form an aggregate that functions as a unit. Although the mass of cells resembles a plasmodial slime mold, the important distinction is that the cells of a cellular slime mold maintain their identity and remain separated by their membranes.

 

Oomycota (Water Molds) This phylum includes water molds, white rusts, and downy mildews. These organisms consist of coenocytic hyphae (fine, branching filaments), an adaptation analogous to the morphology of true fungi. However, water molds and their relatives have cell walls most commonly made of cellulose, while the walls of true fungi are made of another polysaccharide, chitin. The diploid condition prevails in the life cycles of most members of Oomycota but is reduced in true fungi. Biflagellated cells occur in the life cycles of oomycotes, while almost all true fungi lack flagella. Oomycota means "egg fungi," a reference to the mode of sexual reproduction in water molds. A relatively large egg cell is fertilized by a smaller "sperm nucleus," forming a resistant zygote.