7.37: Diversity of Protists IV

Amoebozoa represent a diverse group of terrestrial and aquatic protists that utilize lobe-shaped pseudopodia for locomotion and feeding. This characteristic differentiates them from the Rhizaria, which possess threadlike pseudopodia. The primary classifications within Amoebozoa include gymnamoebas, entamoebas, and the plasmodial and cellular slime molds. Phylogenetic evidence indicates that Amoebozoa diverged from a lineage that ultimately gave rise to fungi and animals.

Gymnamoebas and Entamoebas

Gymnamoebas are free-living protists that inhabit aquatic and soil environments. Their movement is facilitated by pseudopodia through a process termed amoeboid movement, which involves cytoplasmic streaming. In this mechanism, the cytoplasm advances in a less contracted and more fluid region at the leading edge of the cell, following the path of least resistance. This streaming process is supported by microfilaments located just beneath the cytoplasmic membrane. Gymnamoebas vary significantly in size, ranging from as small as 15 micrometers in diameter to over 750 micrometers, the latter of which is visible to the naked eye.

In contrast to free living gymnamoebas, entamoebas are parasitic protists that primarily inhabit the oral cavity or intestinal tract of vertebrates and invertebrates. Notably, Entamoeba histolytica is an anaerobic species that acts as a human pathogen, causing amebic dysentery. This condition leads to ulceration of the intestinal tract and results in bloody diarrhea. The transmission of this parasite occurs via cysts, which spread through the fecal contamination of water, food, and eating utensils.

Slime Molds

Slime molds were historically classified with fungi due to their similar life cycles and the production of fruiting bodies containing spores for dispersal. However, unlike fungi, slime molds exhibit motility, allowing them to move rapidly across solid surfaces. These organisms are classified into two major groups: plasmodial slime molds, also known as acellular slime molds, and cellular slime molds.

Plasmodial slime molds exist in their vegetative stage as an amorphous mass of protoplasm known as a plasmodium (NOT same as the pathogenic genus Plasmodium that causes malaria), which contains multiple diploid nuclei. This plasmodium of slime molds is actively motile and propagates through amoeboid movement. Under favorable conditions, it forms sporangia, structures that contain haploid spores. These spores germinate and develop into flagellated swarm cells, which subsequently fuse to regenerate a diploid plasmodium. A well-known and studied genus includes Physarum.

Cellular slime molds, on the other hand, remain as individual haploid cells in their vegetative state and only form diploid structures under specific environmental conditions. The well-studied species Dictyostelium discoideum undergoes an asexual life cycle characterized by aggregation, migration, and fruiting body formation. In response to starvation, individual Dictyostelium cells aggregate to form a pseudoplasmodium, in which cells lose their individuality without undergoing fusion. The aggregation process is triggered by the release of cyclic adenosine monophosphate (cAMP), a signaling molecule that attracts neighboring cells. The aggregated mass of cells, known as a slug, exhibits motility before becoming stationary and vertically oriented, at which point fruiting body formation is initiated.

The fruiting body consists of a stalk and a head. Stalk cells synthesize cellulose, providing structural support, while head cells differentiate into spores. Upon dispersal, these spores germinate and develop into new amoeboid cells, thus perpetuating the life cycle. In addition to asexual reproduction, Dictyostelium can also undergo sexual reproduction, in which two amoebae fuse to form a single giant amoeba. This structure develops a thick cellulose wall, forming a macrocyst that can remain dormant for extended periods. Eventually, meiosis occurs within the macrocyst, producing haploid nuclei that integrate into new amoebae, thereby reinitiating the asexual cycle.