First described in the 1880s, members of Eumycetozoa or "slime molds" have been classified as plants, animals, and fungi.
As we examine their morphology and behavior, the source of this con fusion should become apparent.
Recent analysis of certain pro teins (e.g., elongation factor EF-1, a-tubulin, and actin) as well as physiological, behavioral, biochemical, and developmental data point to a monophyletic group.
Eumycetozoa includes Myxogas tria and Dictyostelia.
The acellular slime mold (Myxogastria) life cycle includes a distinctive stage when the organisms exist as streaming masses of colorful protoplasm.
The protoplasm creeps along in amoeboid fashion over moist, rotting logs, leaves, and other organic matter, which it degrades .
Acellular slime molds derive their name from the presence of a large, mul tinucleate mass called a plasmodium; there can be as many as 10,000 synchronously dividing nuclei within a single plasmo dium.
Feeding is by endocytosis.
When starved or dried, the plasmodium develops ornate fruiting bodies.
As these mature, they form stalks with cellulose walls that are resistant to environmental stressors .
When conditions im prove, spores germinate and release haploid amoeboflagellates. These fuse, and as the resulting zygotes feed, nuclear divisions give rise to the multinucleate plasmodium.
Cellular slime molds (Dictyostelia) are strictly amoeboid and use endocytosis to feed on bacteria and yeasts.
Their com plex life cycle involves true multicellularity, despite their primi tive evolutionary status .
The species Dictyostelium discoideum is an attractive model organism.
During its life cy cle, a pseudoplasmodium is formed.
This consists of many, many individual vegetative cells moving together as a mass. Thus it differs from the acellular slime mold's true plasmodium.
The pseudoplasmodium forms when starved cells release cyclic AMP and a specific glycoprotein, which serve as molecular sig nals.
Other cells sense these compounds and respond by forming an aggregate around the signal-producing cells.
In this way, large, motile, multicellular slugs develop and serve asprecursors to fruiting body formation.
Fruiting body morpho genesis commences when the slug stops and cells pile on top of each other.
Cells at the bottom of this vertically oriented struc ture form a stalk by secreting cellulose, while cells at the tip differ entiate into spores .
Germinated spores become vegetative amoebae to start this asexual cycle anew.
Dictyostelium spp. display complex behaviors.
In addition to farming, described in the chapter opening story, they also differ entiate to resemble primitive immune cells.
During slug forma tion, some cells become "sentinel cells" and vanquish harmful bacteria.
Sentinel cells accomplish this by producing proteins that are similar to those involved in immune responses in higher or ganisms.
These cells roam within the slug as if patrolling for pathogenic bacteria such as Legionella pneumophila, which are known to infect Dictyostelium spp.
Sentinel cells have been found in several species related to Dictyostelium discoideum; immunolo gists are not too surprised, noting that all multicellular organisms need protection against bacterial pathogens.
Sexual reproduction in D. discoideum involves the formation of special spores call macrocysts.
These arise by a form of conju gation that has some unusual features.
First, a group of amoebae become enclosed within a wall of cellulose.Conjugation occurs between members of different mating types, of which there are three, as well as those capable of self-fertilization.
Mating type (denoted as Type I, II, or III) is controlled by the nucleotide se quence of a single gene. Following conjugation, a large amoeba forms and cannibalizes the remaining amoebae.
The now-giant amoeba matures into a macrocyst. Macrocysts can remain dor mant within their cellulose walls for extended periods. Vegetative growth resumes after the diploid nucleus undergoes meiosis to generate haploid amoebae.
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