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
Figure%: Reproduction of 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
Figure%: Reproduction of 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.
