BSC 314 Lecture Notes - Lecture 51: Phytophthora Infestans, Terrestrial Animal, Avocado
27 Jun 2018
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Fungi Like Protista‐
The Kingdom Protista was established in the 1860s as a place for the slime molds that
are plant like in forming spores in multinucleate, erect, sporangia and having cellulose ‐
in their cell walls, animal like in having an amoeboid stage in their life cycle during which‐
they creep about ingesting their food, and fungal like in general appearance and habits.‐
Like fungi these slime molds grow in damp, organic rich sites. Rotting logs or decaying ‐
plants on the forest floor are favorite habitats. Their amoeboid form is frequently a
brightly colored orange or yellow blob of viscous, slippery protoplasm that streams
slowly into a network of branching, anastomosing projections that move the whole mass
forward. This is the feeding stage and bacteria, yeasts, fungi, or bits of vegetation are
incorporated into the mass as it moves. Two principal groups of slime molds are
recognized, with a third unrelated group closely associated:
Dictyosteliomycota form a motile mass of protoplasm—a “slug”—by
aggregating individual amoeboid cells that retain their identity in the slug, hence
their common name, cellular slime molds.
Myxomycota, the plasmodial slime molds, lose their cell membranes when
they come together and the nuclei float freely in the combined, membrane bound
mass of cytoplasm, which is called a plasmodium. (The slug of the Dictyostelio‐
mycota is a false plasmodium or a pseudoplasmodium.)
Labyrinthulomycota, the slime nets, are basically unicellular, but live together
in colonies. They secrete a membrane outside of their cells, which forms a
network of filaments through which the cells travel. The net, sometimes several
centimeters in diameter, resembles the plasmodium of the slime molds, but if
examined microscopically the cells in their tracks can be seen as distinctively
different.
The three groups are neither related to the fungi nor to each other, but they have in common heterotrophy
and a sporangia form in their life cycle.
The myxomycetes are reproductively more advanced than the cellular slime molds and form a true
plasmodium that can be several centimeters in diameter and look to the untrained eye like a patch of
yellow vomitus spewed over a decaying log on the forest floor. The plasmodium is multinucleate and
diploid. When food supplies are limited or the environment dries, the plasmodia cease their streaming and
may form thick-walled structures called sclerotia (singular, sclerotium) in which the protoplasm can
withstand adverse environmental conditions. Growth is resumed with the return of more favorable
circumstances.
Both asexual and sexual reproduction occurs, but the initiation clues to each process remain obscure.
Under some conditions, the plasmodium produces erect sporangiophores with sporangia on their tips.
Meiosis takes place and haploid spores result. Like the sclerotia, the spores are resistant and able to
sustain the slime mold over adverse growth periods. When conditions again are suitable for growth, the
spores germinate. Some develop into amoebas that move about, feeding. Others become flagellated
gametes. After a period of time, the cytoplasm of a pair of genetically different amoeboid or flagellated
cells fuse, but individuality of the nuclei is retained, a process known as plasmogamy. Karyogamy, the
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Document Summary
Like fungi these slime molds grow in damp, organic rich sites. Rotting logs or decaying plants on the forest floor are favorite habitats. Their amoeboid form is frequently a brightly colored orange or yellow blob of viscous, slippery protoplasm that streams slowly into a network of branching, anastomosing projections that move the whole mass forward. This is the feeding stage and bacteria, yeasts, fungi, or bits of vegetation are incorporated into the mass as it moves. Two principal groups of slime molds are recognized, with a third unrelated group closely associated: Dictyosteliomycota form a motile mass of protoplasm a slug by aggregating individual amoeboid cells that retain their identity in the slug, hence their common name, cellular slime molds. Labyrinthulomycota, the slime nets, are basically unicellular, but live together in colonies. They secrete a membrane outside of their cells, which forms a network of filaments through which the cells travel.
