Protista

Slime Molds

The slime molds warrant their own classification within Protista because of their unusual morphology. At some stages in their life cycle, they show clearly protozoan characteristics, but at other stages they become almost fungus-like. Slime molds fall into two distinct groups that are not closely related: the cellular slime molds and the true, or acellular, slime molds. A distinguishing morphological difference between the two groups is the vegetative state of cellular slime molds in a haploid amebiod cell, whereas the vegetative state of acellular slime molds is a multinucleate diploid ameboid mass called a plasmodium. Both groups grow in moist soil or decaying plant matter and are white, yellow, or red in color.

Cellular Slime Molds

As mentioned above, the vegetative phase of the cellular slime molds is an ameboid cell. Depending on environmental conditions, these cells will enter either the sexual or the asexual reproductive phase. For sexual reproduction, two cells of opposite mating types fuse to create a diploid cell. As other ameboid cells reach this cell, they are phagocytized and a giant cell forms. This cell undergoes meiosis and becomes a macrocyst. Spore formation takes place inside the macrocyst, from where the resulting haploid spores will be released.

Sexual reproduction is most likely to take place in very damp conditions. In drier conditions, cellular slime molds enter an asexual reproductive phase. Haploid ameboid cells cease feeding and clump together to form a slug-like pseudoplasmodium. From this forms a stalked fruiting body. In this fruiting body spores will be produced and released.

Acellular Slime Molds

The plasmodium phase of the acellular slime molds differs from the pseudoplasmodium of the cellular slime molds in that it is diploid. This mass gives rise to a fruiting body in which meiosis occurs and haploid spores are produced. The spores germinate to produce flagellated gametes. These gametes fuse to form a diploid zygote. The zygote grows and its nucleus divides mitotically, but the cytoplasm does not divide, resulting in another plasmodium.