THE LIVING WORLD

Unit Four. The Evolution and Diversity of Life

 

17. Protists: Advent of the Eukaryotes

 

17.9. "Not Yet Located on the Protist Phylogenetic Tree"

 

Five major phyla of protists cannot yet be located on the protist phylogenetic tree. Amoebas, forams, radiolarians, and both cellular and plas- modial slime molds have no permanent locomotor apparatus, and instead use their cytoplasm to aid movement.

 

Amoebas

Amoebas, members of the phylum Rhizopoda, lack flagella and cell walls. There are several hundred species. They move from place to place by pseudopodia (Greek, pseudo, false, and podium, foot), flowing projections of cytoplasm that extend outward. In figure 17.20a, you can see an amoeba putting a pseudopod forward and then flowing into it. Amoebas are abundant in soil, and many are parasites of animals. Reproduction in amoebas occurs by simple fission (figure 17.20b). They lack meiosis and any form of sexual reproduction.

 

 

Figure 17.20 Amoebas.

(a) Amoeba proteus is a relatively large amoeba (x45). The projections are pseudopodia; an amoeba moves simply by flowing cytoplasm into them. The nucleus is plainly visible. (b) An amoeba divides by simply splitting into two, as shown here.

 

Forams

Forams, members of the phylum Foraminifera, possess rigid shells and move by cytoplasmic streaming. They are marine protists with pore-studded shells called tests that may be as big as several centimeters in diameter. There are several hundred species of forams. Their shells, built largely of calcium carbonate, are often brilliantly colored—vivid yellow, bright red, or salmon pink—and may have many chambers arrayed in a spiral shape resembling a tiny snail. Their multicham- bered body can be seen in figure 17.21.

 

 

Figure 17.21. A foram.

In this living foram, a representative of phylum Foraminifera, the podia—thin cytoplasmic projections—extend through pores in the calcareous test, or shell, of the organism.

 

Most forams live in sand, but a few are free-floating organisms, part of the ocean’s plankton community. Long, thin cytoplasmic projections called podia radiate out through the pores in the shells of these protists and are used for swimming and capturing prey. The life cycle of forams is complex, involving alternation between haploid and diploid generations. Forams have deposited massive accumulations of their shells for more than 200 million years. Limestone is often rich in forams’s remains—the White Cliffs of Dover, the famous landmark on the southern England seacoast shown in figure 17.22, is made almost entirely of foram shells.

 

 

Figure 17.22. White cliffs of Dover.

The limestone that forms these cliffs is composed almost entirely of fossil shells of protists, including foraminifera.

 

Radiolarians

Radiolarians are unusual amoeboid protists that belong to another phylum, Actinopoda. While most amoeboid cells have an amorphous shape, radiolarians secrete a glassy exoskeleton made of silica that gives their body a distinctive shape. Either radially or bilaterally symmetrical, the shells of different species form elaborate shapes. The pseudopods of Actinosphaerium, seen in figure 17.23, extrude outward along spiky projections of the glassy exoskeleton like thorns radiating out from the cell body.

 

 

Figure 17.23. A radiolarian.

This amoeba-like radiolarian Actinosphaerium (x300), of the phylum Actinopoda, has striking needlelike pseudopods.

 

 

Cellular Slime Molds

Slime molds are heterotrophic protists that are sometimes confused with fungi, although they do not resemble them in any significant respect. For example, slime molds have cell walls made of cellulose, whereas fungal walls are made of chitin. Also, slime molds carry out normal mitosis, while fungal mitosis is unusual and will be discussed in chapter 18.

The two major kinds of slime molds that originated at different times are only distantly related. In the cellular slime molds, phylum Acrasiomycota, individual cells aggregate and differentiate into complex associations called slugs. Slugs are mobile and while not truly multicellular, they are far along that evolutionary path.

Cellular slime molds are more closely related to amoebas than to any other phylum. There are 70 named species, the best known of which is Dictyostelium discoideum. Dictyostelium is basically a unicellular scavenger with the interesting life cycle shown in figure 17.24. When deprived of food, thousands of individual Dictyostelium amoebas 1 come together forming a slug (2 through 4) that moves to a new habitat. There, the colony differentiates into a base, a stalk, and a swollen tip 5 that develops spores. Each of these spores, when released 6, becomes a new amoeba, which begins to feed and so restarts the life cycle.

 

 

Figure 17.24. Slime mold.

The life cycle of the cellular slime mold Dictyostelium discoideum (phylum Acrasiomycota). 1 Germinating spores form amoebas. 2 The amoebas aggregate and move toward a fixed center. 3 They form a multicellular slug 2 to 3 millimeters long that migrates toward light. 4 The slug stops moving and begins to differentiate into a spore-forming body, called a sorocarp 5. Within the heads of the sorocarps, the amoebas become encysted as spores 6.

 

Plasmodial Slime Molds

Plasmodial slime molds, phylum Myxomycota, comprise a group of about 500 species that stream along as a plasmodium, a nonwalled multinucleate mass of cytoplasm. The yellow mass you see in figure 17.25 is composed of a group of cells without cell walls separating the individual cells. They move together as a single unit. Plasmodia can flow around obstacles and even pass through a mesh cloth. Extending pseudopodia as they move, they engulf and digest bacteria and other organic material. If the plasmodium begins to dry or starve, it migrates away rapidly and then stops and often divides into many small mounds, each of which produces a sporeladen structure. Spores germinate when favorable conditions return.

 

 

Figure 17.25. Plasmodial slime mold.

This multinucleate plasmodium (phylum Myxomycota) moves about in search of the bacteria and other organic particles that it ingests.

 

Key Learning Outcome 17.9. Amoebas, forams, radiolarians, and molds are heterotrophs with restricted mobility, many of which form aggregates. Their position on the protist phylogenetic tree is not yet firmly established.

 

Inquiry & Analysis

Defining a Treatment Window for Malaria

While malaria kills more people each year than any other infectious disease, the combination of mosquito control and effective treatment has virtually eliminated this disease from the United States. In 1941, more than 4,000 Americans died of malaria; in the year 2006, by contrast, fewer than five people died of malaria.

The key to controlling malaria has come from understanding its life cycle. The first critical advance came in 1897 in a remote field hospital in Secunderabad, India, when English physician Ronald Ross observed that hospital patients who did not have malaria were more likely to develop the disease in the open wards (those without screens or netting) than in wards with closed windows or screens. Observing closely, he saw that patients in the open wards were being bitten by mosquitoes of the genus Anopheles. Dissecting mosquitoes who had bitten malaria patients, he found the plasmodium parasite. Newly hatched mosquitoes who had not yet fed, when allowed to feed on malaria-free blood, did not acquire the parasite. Ross reached the conclusion that mosquitoes were spreading the disease from one person to another, passing along the parasite while feeding. In every country where it has been possible to eliminate the Anopheles mosquitoes, the incidence of the disease malaria has plummeted.

The second critical advance came with the development of drugs to treat malaria victims. The British had discovered in India in the mid-1800s that a bitter substance called quinine taken from the bark of cinchona trees was useful in suppressing attacks of malaria. The boys in the photograph are being treated with an intravenous solution of quinine. Quinine also reduces the fever during attacks, but does not cure the disease. Today physicians instead use the synthetic drugs chloroquine and primaquine, which are much more effective than quinine, with fewer side effects.

 

 

Unlike quinine, these two drugs can cure patients completely because they attack and destroy one of the phases of the plasmodium life cycle, the merozoites released into the bloodstream several days after infection—but only if the drugs are administered soon enough after the bite that starts the infection.

In order to determine the time frame for successful treatment, doctors have carefully studied the time course of a malarial infection.

The graph above presents what they have found. Numbers of merozoites are presented on the y axis on a log scale—each step reflects a 10-fold increase in numbers. The infection becomes life-threatening if 1% of red blood cells become infected, and death is almost inevitable if 20% of red blood cells are infected.

 

 

1. Applying Concepts. What is the dependent variable?

2. Making Inferences

a. How long after infection is it before the liver releases merozoites into the bloodstream (that is, initial infection by merozoites)? Before the disease becomes life-threatening? Before death is inevitable?

b. How long does it take merozoites to multiply 10-fold?

3. Drawing Conclusions. After the first appearance of clinical illness symptoms, how many days can the disease be treated before it becomes life-threatening? Before treatment has little or no chance of saving the patient's life?

 

Test Your Understanding

1. One piece of supporting evidence for the endosymbiotic theory for the origin of eukaryotic cells is that

a. eukaryotic cells have internal membranes.

b. mitochondria and chloroplasts have their own DNA.

c. Golgi bodies and endoplasmic reticulum were present in ancestral cells.

d. the nuclear membrane could only have come from another cell.

2. Many protists survive unfavorable environmental conditions by forming

a. gametes.

b. zygotes.

c. cysts.

d. aggregations.

3. Protists do not include

a. algae.

b. amoebas.

c. multicellular organisms.

d. mushrooms.

4. Some members of the phylum Euglenozoa

a. do not have chloroplasts.

b. conduct photosynthesis.

c. are human pathogens.

d. All of the above.

5. Kelp, which sometimes forms large, underwater forests, are actually protists called

a. diatoms.

b. brown algae.

c. dinoflagellates.

d. red algae.

6. Sporozoans have life cycles that

a. have both a sexual and an asexual phase.

b. have only an asexual phase.

c. have only a sexual phase.

d. only require fragmentation to produce new individuals.

7. The ancestor of plants was a member of what group of protists?

a. Brown algae

b. Alveolata

c. Green algae

d. Both b and c

8. Scientists believe that the ancestral animal cell comes from the protist group

a. ciliates.

b. choanoflagellates.

c. dinoflagellates.

d. euglenoids.

9. Amoebas, foraminifera, and radiolarians move using their

a. cytoplasm.

b. flagella.

c. cilia.

d. setae.

10. Protists that form aggregates, have cellulose cell walls, and are heterotrophic are probably

a. ciliates.

b. radiolarians.

c. slime molds.

d. euglenoids.