INTRODUCTION TO THE CONOSA

This large and variable taxon includes two large groups that we interpret as subphyla: Archamoebae and Mycetozoa (Protostelids, Cellular Slime Molds, and Acellular Slime Molds). Though some taxa are nonmotile, many in this class produce flagellated cells. Indeed, the nature of the flagellum and its flagellar root with a cone-shaped set of microtubules associated with the nucleus, is the structure that Cavalier-Smith (1998) referenced in giving this group its name, Conosa.

The archamoebae contain strange unicells that may be uninucleate or multinucleate. Typically, they do not have mitochondria and were, therefore, considered to be pre-mitochondriate eukaryotes (Margulis and Schwartz 1992 and 1998). Indeed, they seemed to be perfect examples of the Archaezoan cell form. But, to paraphrase T. H. Huxley, it was a beautiful theory that was destroyed by an ugly, nasty little fact: the amitochondriate nuclei contain mitochondrial genes. Thus, they must have had mitochondria in their ancestry. Most members of this group live in anaerobic or microaerophilic environments, and must have lost the mitochondria. The following examples are cells that live in anaerobic mud (Pelomyxa), and the digestive tract (Entamoeba).

Pelomyxa (Figure 1) is a giant amoeba that can be flagellate and can undergo a lifecycle in which it changes form and physiology. The cell has three endosymbiotic bacteria: a perinuclear bacterium (a methanogen), and two cytoplasmic bacteria (a methanogen and an uncharacterized G+ form) all of which seem to function as mitochondria. Indeed, the uncharacterized G+ bacterium may be a degenerate mitochondrion. They become O2 tolerant in the summer when the G+ endosymbionts dominate and O2 intolerant the rest of the year when the methanogens and other large endosymbionts dominate.

Entamoeba (Figure 2), the causative agent of amebic dysentery, is a parasite of the gut and, thus, has no mitochondria. Cysts are ingested and germinate in the lower bowel where they feed on the intestinal mucosa. When they are in large numbers, Entamoeba coli can induce large ulcerations leading to bloody stools. They can also enter the circulatory system from which they can begin feeding on other internal organs, including the brain. Most of the time, they cause a chronic diarrhea that leads to dehydration. Amebic dysentery and other forms of amebiasis are among the leading causes of death worldwide. As parasites, they infect approximately 50 million people each year, resulting in nearly 100,000 deaths, a rate surpassed only by malaria and schistosomiasis (CDC). Many taxa of related amoebae live as commensals, and, like Entamoeba, can reproduce and spread by means of cysts (text with tooltip) A cyst is a resting stage that is covered by a resistant outer covering. Usually, cysts are able to allow the cell to survive environmental extremes. They serve as the infective forms of parasitic protists. Usually in sexual microbial eukaryotic groups, the cyst is produced by the zygote. .

FIGURE 1. Pelomyxa is an amitochondriate amoeba with an internal flagellum that may emerge. The cell has a trailing uroid and a large pseudopodium (text with tooltip) A pseudopod (pseudopodium, sing; pseudopodia, pl) is an extension of a naked cell (no wall, pellicle, etc.) that is ephemeral and used for feeding or locomotion. . Image from http://microscope.mbl.edu/scripts/	FIGURE 2. Entamoeba causes amebic dysentery. It can produce cysts and, like Pelomyxa, lacks mitochondria. Image from http://www.dpd.cdc.gov/dpdx/

CELLULAR SLIME MOLDS
The dictyostelid cellular slime molds are small free-living amoebae that feed on bacteria associated with rotten wood and other organic matter. That is where their similarities with the plasmodial slime molds end. In their asexual cycle, one or more cells begin to release acrasin (cyclic AMP) in response to a depletion in food (Figure 3). As feeding cells are attracted to the source of acrasin, they begin to release more of it causing up to 100,000 amoebae to aggregate into a multicellular pseudoplasmodium. This new structure looks like and behaves like a small slug with a head, a tail, and a ventral region. The new multicellular slug moves to an appropriate site where it differentiates into a sporangium (see Figure 4) and encapsulates certain cells as spores. The spores then disperse, germinate as feeding amoebae, and begin to form their own population through mitosis, etc.

The sexual cycle is equally unusual. An amoeba will begin to attract other amoebae and engulf them. The resulting cell is called a macrocyst (or giant cell). Presumably, karyogamy occurs in the giant cell and haploid amoebae emerge from the macrocyst.

FIGURE 3. LIFE HISTORY OF DICTYOSTELIUM Dictyostelium feeds as a unicellular amoeba until starvation forces the cells to aggregrate to form a multicellular slug that crawls toward light and lower humidity (generally to a higher position). Culmination then ensues and a fruiting body (Figure 4) is formed and spores released.

FIGURE 4. Mature sporangia of Dictyostelium. Image by Bruno in Columbus, released to the Public Domain

OTHER MYCETOZOA

The plasmodial slime molds usually grow on rotting vegetation where they feed on bacteria as small haploid amoebae. During this trophic phase, they are cryptic, but they can grow into multinucleate plasmodia that can be very obvious and brightly-colored. In this phase, they also make sporangia that are diagnostic of the taxon (e.g. Figures 5-7).

Plasmodial slime molds are of three major types: the myxomycetes, the protostelids, and the hyperamoebae. The myxomycetes are the most commonly encountered members of this phylum. They make large brightly-colored plasmodia that can measure more than 10 cm across. The common laboratory slime mold, Physarum (Figure 8), typically is bright yellow, but other colors are common (e.g. oranges, browns, purples, etc.) There is Lycogala (Figure 6) that grows on decaying silver maple logs in my yard, often causing the exposed ends of the logs to turn purple. The life history of the plasmodial slime mold is fairly simple (see Figure 9). The trophic amoebae are uninucleate and haploid. They can become functional gametes and fuse. Following karyogamy, the cell enlarges as a diploid plasmodium with thousands of nuclei. Then, as the plasmodium begins to differentiate to form sporangia, nuclei in the sporangia form resistant spore walls. Meiosis occurs in the spores, but three of the resulting nuclei abort to form a uninucleate haploid spore, which, upon germination, releases a haploid trophic amoeba. The spores can over winter. Also, as the plasmodium dries out, parts of it can form a protective covering, called a sclerotium. This can survive to grow into another plasmodium when conditions improve.

The protostelids are highly reduced slime molds. Their life histories are similar to those of the myxomycetes, but the plasmodia are very small and the sporangia each release just a few spores (see Figure 10).

If enough moisture is present during the trophic phase, the amoebae may become flagellated. In this case, they usually have two flagella, one anteriorly-directed and the other recurrent. That is, the motile cells are biflagellated in a group that Cavalier-Smith (2003) called the unikonts (one flagellum). Hyperamoeba (Figure 11) is an amoeboflagellate that appears to be a degenerate plasmodial slime mold, but it does not form plasmodia. In the motile phase, it resembles the motile cells of the protostelids.

FIGURE 5. Sporangia of Ceratiomyxa. Image from http://www.plant.uga.edu/mycology-herbarium/myxogal/Cfruct2.jpg	FIGURE 6. Sporangia of Lycogala. Image from The Systematic Biology Biodiversity Archive	FIGURE 7. Sporangium of Echinostelium, though only distantly related resembles Protostelium. Image from http://www.uatx.mx/posgrados/pctbc/pmcb/estrada01.php	FIGURE 8. The anastomosing plasmodium of Physarum. Image from The Systematic Biology Biodiversity Archive

FIGURE 9. LIFE HISTORY OF PHYSARUM Uninucleate haploid cells emerge from spores and may be flagellated or not depending on available water. These cells function as gametes. The zygote then enlarges and becomes a large (>10 cm across) multinucleate plasmodium (see also Figure 8). As the plasmodium ages, sporangia are produced and meiosis occurs within the spores, which may overwinter.

FIGURE 10. Sporangia of Protostelium, a taxon that makes a small plasmodium. Image from http://comp.uark.edu/~fspiegel/	FIGURE 11. A flagellated stage of Hyperamoeba. Image from http://microscope.mbl.edu/baypaul/microscope/images/t_imgAZ/hyperamoeba2_atw.jpg

SYSTEMATICS OF THE CONOSA

As it is defined, the Phylum Conosa is a modification of Cavalier-Smith (2003), Cavalier-Smith et al. (2004), Ptackova et al. (2013), and Lahr et al. (2011) and the topologies of Figure 12 are generally supported. The relative positions of the protostelids are problematic and may represent a form rather than a taxon.

The true slime molds are treated as fungi by Alexopoulos and Mims (1979) and Bold et al. (1987). The same organisms are classified as members of the amoeboid taxa by Grell (1973), Kudo (1966), Hunter (1985) and Sleigh et al. (1984). The proposed system is based on Frederick (1990), Spiegel (1990) and Margulis and Schwartz (1988, Pr-23; and 1998, Pr-6) in which the true slime molds are given phylum-level status. I have included the protostelids in this phylum because they can form small plasmodia.

Frederick (1990) believes that the Myxomycota evolved from an orthogonally flagellated protist with a similar type of flagellar root system. Spiegel (1990) suggests that the Myxomycetea evolved from a line of protostelids. However, he acknowledges that the protostelids may represent many lines, and, therefore, their taxonomy requires much work. Walochnik et al. (2004) in a molecular-biological study of the amoebae concluded that the true slime molds (a group that they call the Myxogastrea) is monophyletic, separated from the cellular slime molds (Dictyostela). Furthermore, they included an unusual amoeboflagellate called Hyperamoeba within the myxomycote clade. Thus, I have included Hyperamoeba tentatively as a class within this otherwise traditional taxonomic treatment.

Adl et al. (2005 and 2012) have the Protostelia, Myxogastria (=Myxomycetes), and Dictyostelia of equal rank within a first rank taxon called Eumycetozoa within their Supergroup Amoebozoa. We have taken the more conservative approach of Cavalier-Smith et al. (2004).

FIGURE 12. A cladogram showing the relationships between the groups of the Conosa (taxa in the shaded box). The topology is supported by Cavalier-Smith et al. (2004) Ptackova et al. (2013), and Lahr et al. (2011).