8

Animal-Like Protists:

The Protozoa

This ciliated protist (Paramecium spp.) is not an animal but is traditionally

studied in zoology courses. It is a member of one of four protist lineages in Eukarya that are studied as animal-like protists or protozoa.

8. 1. EVOLUTIONARY PERSPECTIVE

OF THE PROTISTS

LEARNING OUTCOMES

1. Describe why protists are considered to be polyphyletic.2. Describe how some protists are plantlike whereas others are animal-like.

Where are your "roots"? Although most people are content to go back into their family

tree several hundred years, scientists look back billions and millions of years to the origin of all life-forms. The first evidence of what appears to be a protist is found in tiny fossils in rock 1.5 billion years old. These fossils are much larger than bacteria and

contain small membrane-bound structures. The fossil record indicates that virtually all protist and animal phyla living today were present during the Cambrian period, about 550 million years ago. Unfortunately, fossil evidence of the evolutionary pathways that gave rise to these phyla is scant. Instead, scientists gather evidence by examining the structure and function of living species. The "Evolutionary Perspective" sec-

tions in chapters 8 to 22 present hypotheses regarding the origin Animation of protist and animal phyla. These hypotheses seem reasonable to Three

Domains most zoologists; however, alternative interpretations are in the sci-entific literature.

As indicated in chapter 7 (see figure 7.2) and the phylogenetic tree on pages xvi-xvii, members of all three domains (Eubacteria, Archaea, and Eukarya) arose from a common ancestor. The Eubacteria and Archaea diverged from a

common ancestor about 1.5 billion years ago. Ancient members of the Archaea were the first living organisms on this planet. The Archaea and Eubacteria probably contributed to the origin of the protists about 1.5 billion years ago. The endosymbiont hypothesis is one of a number of explanations of how this could have occurred (see Evolutionary Insights, page 33). Most scientists agree that the protists probably arose from more than one ancestral Archaean

group. According to the most recent classification scheme (based on morphological, biochemical, and physiological analysis), the International Society of Protistologists recognizes six phylogenetically coherent protist clusters called supergroups. The protists as a whole represent a polyphyletic assemblage, and

the monophyly of each supergroup lineage is being evaluated by ongoing research. Some protists are plantlike because they are primarily autotrophic

(they produce their own food). Others are animal-like because they are primarily heterotrophic (they feed on other organisms). As a result, this chapter will

Chapter Outline 8.1 Evolutionary Perspective of the Protists 8.2 Life within a Single Plasma Membrane

Maintaining Homeostasis Reproduction

8.3 Symbiotic Lifestyles 8.4 Protists and Protozoan Taxonomy

Supergroup Excavata Supergroup Amoebozoa Supergroup Rhizaria Supergroup Chromalveolata

8.5 Further Phylogenetic Considerations

130 CHAPTER EIGHT

(a)

FIGURE 8.1

The Challenge of Protist Classification. Our understanding of the evolutiona1y relationships among the protists is currently in flux. The most recent data support six, possibly monophyletic, supergroups within the protists. Four (represented by the lineages shaded in lavendar) of the six supergroups contain the protozoa. As is the case with all phylogenies, this is a working hypothesis. Many questions concerning how to classify the protozoa are being addressed with new molecular methods, and as new information shapes our understanding of the phylogeny of protists. Representative examples of the four supergroups include: (a) E:xcavata (the flagellated protozoan Giardia intestinalis), (b) Amoebozoa (the amoeba Amoeba proteus), (c) Rhizaria (the foraminiferan Cibicides labatulus), and (d) Chromalveolata (the dinoflagellate Gymnodinium).

use the terms protozoa and protozoan informally, presenting these organisms in a single chapter for convenience and not implying that they form a monophyletic group. Within four of these six protist supergroups (figure 8.1) are found the protozoa. Certain protozoans have had, and continue to have, important influences on human health and welfare. It is these protozoans (figures 8.1 and 8.2) that are emphasized in this chapter.

SECTION REVIEW 8.1

The protists comprise a polyphyletic assemblage comprised of six, possibly monophyletic, lineages. Some protists are plantlike because they are primarily autotrophic (they produce their own food), whereas others are animal-like because they are primarily heterotrophic (they feed on other organisms).

What are protists?

Nucleolus Plasma membrane

/ Pelllcle

Y Free ribosomes

d'

\ Contract lievacuole

Collecl1ng tubules of contractile

vacuoles Lysosorne Cytopyge vacuole

FIGURE 8.2

A Protozoan. This drawing of a stylized protozoan with a flagellum illustrates the basic protozoan morphology. From: "A LIFE OF INVERTEBRATES" © 1979 w. D. Rmsell-H,mrer

p

.2 LI •E

MEM

II

LEARNING OUTCOMES

1. Describe a protozoan.

A

NE

IN ii -;,

2. Classify protozoan organelles involved with feedingand digestion.

3. Explain how protozoans reproduce.

The term protozoa has traditionally referred to chemoorganotrophic protists. (The term "chemoorganotrophic" refers to those organisms that use organic compounds as a source of energy, electrons, and carbon for biosynthesis.) Zoologists who specialize in the study of protozoa are called protozoologists, and the study of all protists, regardless of their metabolic type, is called protistology.

By definition, a protozoan (Gr. pmto, first + zoa, animal) is a complete organism in which all life activities are carried on within a single plasma membrane. Protozoans lack collagen and chitinous cell walls. Protozoa display unicellular (cytoplasmic) euka1yote organization, which does not necessarily imply that they are simple organisms. Often, they are more complex than any particular cell in higher organisms. In some protozoans, individuals group to form colonies, associations of individuals that are not dependent on one another for most functions. Protozoan colonies, however, can become complex, with some individuals becoming so specialized that differentiating between a colony and a multicellubr organism becomes difficult.

Anirn:tl-Like Protists: The Protozoa 131

Maintaining Homeostasis

Organelles that are similar to the organelles of other eukaryotic cells carry out specific functions in protozoa (figure 8.2; see also figure 2.2). Some protozoan organelles, however,reflect specializations for unicellular lifestyles.

A regular arrangement of microtubules, called the pellicle, underlies the plasma membrane of many protozoa. The pellicle is rigid enough to maintain the shape of the protozoan, but it is also flexible.

The cytoplasm of a protozoan is differentiated into two regions. The portion of the cytoplasm just beneath the pellicle is called ectoplasm ( Gr. ectos, outside + plasma, toform). It is relatively clear and firm. The inner cytoplasm, called endoplasm (Gr. endon, within), is usually granularand more fluid. The conversion of cytoplasm between these two states is important in one kind of protozoan locomotion and is discussed later in the chapter.

Most marine protozoa have solute concentrations similar to that of their environments. Freshwater protozoa, however, must regulate the water and solute concentrations of their cytoplasm. Water enters freshwater protozoa by osmosis because of higher solute concentrations in the protozoan

than in the environment. Contractile vacuoles or water expulsion vacuoles remove this excess water (figure 8.2). In some protozoa, contractile vacuoles form by the coalescence of smaller vacuoles. In others, the vacuoles are permanent organelles that collecting tubules radiating into the cytoplasm fill. Contracting microfilaments (see figure 2.20) have been implicated in the emptying of contractile vacuoles.

Most protozoa absorb dissolved nutrients either by active transport or by ingesting whole or particulate food through endocytosis (see figure 2. 14). Some protozoa ingestfood in a specialized region analogous to a mouth, called the cytopharynx. Digestion and transpo1t of food occurs in food vacuoles that form during endocytosis. Enzymes and acidity changes mediate digestion. Food vacuoles fuse with enzymecontaining lysosomes and circulate through the cytoplasm, distributing the products of digestion. After digestion is complete, the vacuoles are called egestion vacuoles. They release their waste contents hy exocytosis, sometimes at a specialized region of the plasma membrane or pellicle called the cytopyge.

Because protozoa are small, they have a large surface

area in propo1tion to their volume (see figure 23). This highsurface-area-to-volume ratio facilitates two other maintenance functions: gas exchange and excretion. Gas exchange involves acquiring oxygen for cellular respiration and eliminating the carbon dioxide produced as a by-product. Excretion is the elimination of the nitrogenous by-products of protein metabolism. The prima1y by-product in protozoa is ammonia. Both gas exchange and excretion occur by diffusion across the plasma membrane.

Reproduction

Both asexual and sexual reproduction occur among the protozoa. One of the simplest and most common forms of

132 CHAPTER EIGHT

asexual reproduction is binary fission. In binary fission,

mitosis produces two nuclei that are distributed into two

similar-sized individuals when the cytoplasm divides. During

cytokinesis, some organelles duplicate to ensure that each

new protozoan has the needed organelles to continue life.

Depending on the group of protozoa, cyto- d''" Animation kinesis may be longitudinal or transverse ·uo Binary

(figures 8.3 and 8.4). Fission

Other forms of asexual reproduction are common. Dur

ing budding, mitosis is followed by the incorporation of one

nucleus into a cytoplasmic mass that is much smaller than the parent cell. Multiple fission or schizogony (Gr. schizein,

to split) occurs when a large number of daughter cells form from the division of a single protozoan. Schizogony begins with multiple mitotic nuclear divisions in a mature individual.

When a certain number of nuclei have been produced, cyto

plasmic division results in the separation of each nucleus into a new cell.

Sexual reproduction requires gamete formation and the subsequent fusion of gametes to form a zygote. In most pro

tozoa, the sexually mature individual is haploid. Gametes are

produced by mitosis, and meiosis follows the union of the

gametes. Ciliated protozoa are an exception to this pattern. Specialized forms of sexual reproduction are covered as indi

vidual protozoan groups are discussed.

SECTION REVIEW 8.2

Protozoans are unicellular chemoheterotrophs. Some move by flagella, pseudopods, or cilia. Most are free living but some

are pathogens in humans and animals. Many have complex life cycles. Protozoan homeostasis is maintained by special

ized structures. A region analogous to a mouth is called a

cytopharynx; digestion can occur within food vacuoles; wastes

are removed by egestion vacuoles or a cytopyge. Protozoans can reproduce by bmary f1ss1on, budding, multiple fission or schizogony, and by sexual methods.

What physiological processes in protists are analogous

to excretory and digestive functions of animals?

(a)

FIGURE 8.4

(a)

(b)

FIGURE 8.3

Asexual Reproduction in Protozoa. Binary fission begins with mitosis. Cytoplasmic division (cytokinesis) divides the organelles between the two cells and results in two similarly sized protozoa. Bina1y fission is (a) longitudinal in some protozoa (e.g., euglenoids) and (b) transverse in other protozoa (e.g., ciliates)

(b)

Binary Fission of the Amoebozoan Amoeba proteus. (a) Light microscopy of Amoeba proteus. The cleavage is almost complete in this image. (b) This image shows complete cell division with two daughter cells (LM X50).

8. .. YMBI< TIC LI ESTYLE

LEARNING OUTCOME

1. Compare the different types of symbiosis that can existwithin the protozoa.

Many protozoa have symbiotic lifestyles. Symbiosis (Gr. syn, with + bias, life) is an intimate association between two organisms. For many protozoa, these interactions involve a form of symbiosis called parasitism, in which one organism lives in or on a second organism, called a host. The host is harmed but usually survives, at least long enough for the parasite to complete one or more life cycles.

The relationships between a parasite and its host(s) are often complex. Some parasites have life cycles involving multiple hosts. The definitive host harbors the sexual stages of the parasite. The sexual stages may produce offspring that enter another host, called an intermediate host, where they reproduce asexually. Some life cycles require more than one intermediate host and more than one immature stage. For the life cycle to be complete, the final, asexual stage must have access to a definitive host.

Other kinds of symbiosis involve relationships that do not harm the host. Commensalism is a symbiotic relationship in which one member of the relationship benefits, and the second member is neither benefited nor harmed. Mutualism is a symbiotic relationship in which both species benefit.

SECTION REVIEW 8.3

In parasitism one organism lives in or on another known as the host. Definitive hosts harbor the sexual stage of the protozoan. Intermediate hosts harbor the asexually reproducing stages of the protozoan. In commensalism, one member ben

efits while the other is neither harmed nor is benefited. In mutualism, both species benefit.

Control strategies for combating parasitic protists

often target intermediate host organisms. Why are these strategies effective?

8.4 P1 Tl T ND

P It O 'I ) J'. O \. r\l TA :'\ 0 "' ) M '\

LEARNING OcJTCOMES

1. Differentiate the Fornicata from the Amoebozoa.2. Identify the different stages in the life cycle of

Plasmodium.

Ever since Antony van Leeuwenhoek described the first protozoan "animalcule" in 1674, the taxonomic classification of these protists has remained in flux. For many years the protozoa were classified into four major groups based on their means of locomotion: flagellates (Mastigophora), ciliates (Ciliophora or Infusoria), amoebae (Sarcodina),

Animal-Like Protists: The Protozoa 133

and stationary forms (Sporozoa). Although some zoologists and protozoologists still use these terms, these divisions have no bearing on evolutionary relationships and should be avoided. It is now agreed that the old classification system is best abandoned, but for many years there was little agreement on what should take its place. Recent morphological, biochemical, and phylogenetic analyses have resulted in the development of a higher-level classification system for the protists, including the protozoa. This scheme, as proposed by the International Society of Protistologists in 2005, is followed in this chapter. It should be noted that this scheme (table 8.1) does not use formal hierarchical rank designations such as class and order, reflecting the fact that protists and protozoan taxonomy remain active areas of research.

Supergroup Excavata

The supergroup Excavata includes some of the oldest eukaryotes. Most possess a cytostome characterized by a suspension-feeding groove ("excavated" groove, hence the name) with a posterior-directed flagellum that is used to generate a feeding current. This enables the capture of small food pa11icles. Those that lack this feature are presumed to have had it at some time during their evolution.

Fornicata

Members of the Fornicata have flagella, a feeding groove, and are uninucleate. They have modified mitochondria called mitosomes. These organelles lack functional electron transport chains and hence cannot use m,7gen to help extract energy from carbohydrates. Instead, the Fornicata get the energy they need from anaerobic pathways, such as glycolysis. These protozoa use their flagella for locomotion. Flagella produce two-dimensional whiplike or helical movements and push or pull the protozoan through its aquatic medium. These protozoa also possess a pellicle that gives the body a definitive shape reproduced only by binary fission. The most important member of this group is Giardia intestinalis, which causes the disease giardiasis (figure 8.5). Giardiasis is a waterborne disease. In the United States, this protist is the most common cause of epidemic waterborne diarrhea, affecting children more so than adults.

Parabasalia

Members of the Parabasalia are flagellated (in fact, they may have thousands of flagella) and endosymbionts of animals. They have a parabasal body (a Golgi body located near the kinetosome) and striated parabasal fibers that connect the Golgi to the flagella. Since they do not have a distinct cytostome, they use phagocytosis to engulf food items. The parabasalids have reduced mitochondria called hydrogenosomes that can generate some energy anaerobically, releasing hydrogen gas as a by-product. One member of this group is Trichomonas vaginalis (figure 8.6), which causes the

134 CHAPTER EIGHT

TABLE 8.1

CLASSIFICATION OF THE ANIMAL-Lll.ERV/lN PEATURES

Suspension feeding groove (cytostome) present or presumed to have been lost; feed by a flagella-generated current

Amoeboid motility with lobopodia; naked or testate; mitochondria with tubular cristae; uninucleate or multinucleate; cysts common

Possess thin pseudopodia (filopodia)

Plastid from secondary endosymbiosis with an ancestral archaeplastid; plastid then lost in some; required in others

EIRST RANK

Fornicata Parabasalia Euglenozoa

Tubulinea Acantbamoebidae Entamoebidae

Foraminifera Radio/aria

Cryptopbyceae Haptopbyta- ----·-·-Stramenopbiles Alveolata

EXAMPLES

Giardia Tricbomonas, Histomonas Euglena, Leisbmania, Trypanosoma

Amoeba Acantbamoeba, Naegleria Entamoeba

Globigerina, Difflugia Acantbometra

Cryptomonas, Coccoliths, Diatoms Aptcomplexa· (e;g�Plasmodium, · ·-·�--Toxoplasma, Eimeria, Cryptosporida) Ciliopbora (e.g., Paramecium) Dinojlagellata (e.g., Ceratium, Gymnodinium)

'A1,J�J')r"'1 rmin l'.111·,, ·, L W. !!l JI LllJ1 I) Fl�d\ • .... m111!cJ mu!tJJ,1u111: 11111dy,,._, \'luld ., wcll-(,..,n!Vt.'

protozoans is the presence of a spiral or crystalline rod of unknown function inside one of their flagella.

The phytoflagellated protozoa The phytoflagellated protozoa possess one or two flagella and produce a large portion of the food in marine food webs. Much of the oxygen used in aquatic habitats comes from photosynthesis by these marine protozoa.

Euglena is a freshwater phytoflagellated protozoan (figure 8.7). Each chloroplast has a pyrenoid, which synthesizes and stores polysaccharides. If cultured in the dark, euglenoids feed by absorption and lose their green color. Some euglenoids (e.g., Peranema) lack chloroplasts and are always heterotrophic.

Euglena orients toward light of certain intensities. A pigment shield (stigma) covers a photoreceptor at the base of the flagellum. The stigma permits light to strike the photoreceptor from only one direction, allowing Euglena to orient and move in relation to a light source.

Photoreceptor

Mitochondrion

FIGURE 8.7

Locomotor flagellum

Rudimentary flagellum

Contractile vacuole

·'\,'5;,.-...;,----,-- Golgi apparatus

'---- Pellicle

The Structure of Euglena. Note the large, well-organized chloroplasts. The photoreceptor allows the organism to swim toward light. The organism is about 50 pm long.

Animal-Like Protists: The Protozoa 135

Euglenoid flagellates are haploid and reproduce by longitudinal bina1y fission (see figure 8.]a). Sexual reproduction in these species is unknown.

The zooflagellated protozoa Members of the zooflagellated protozoa lack chloroplasts and are heterotrophic. These protists also have a single, large mitochondrion that contains an organized mass of DNA called a kinetoplast. Some members of this group are important parasites of humans.

One of the most important species is Trypanosoma bntcei. This species is divided into three subspecies: T. b. brucei, T. b. gambiense, and T. b. rhodesiense, often referred to asthe Trypanosoma brucei complex. The first of these threesubspecies is a parasite of nonhuman mammals of Africa. Thelatter two cause African sleeping sickness in humans. Tsetse flies (Glossina spp.) are intermediate hosts and vectorsof all three subspecies. When a tsetse fly bites an infectedhuman or mammal, it picks up parasites in addition to itsmeal of blood. Trypanosomes multiply asexually in the gut ofthe fly for about 10 days, then migrate to the salivary glands.While in the fly, tiypanosomes transform, in 15 to 35 days,through a number of body forms. When the infected tsetse flybites another vertebrate host, the parasites travel with salivarysecretions into the blood of a new definitive host. The parasites multiply asexually in the new host and again transformthrough a number of body forms. Parasites may live in theblood, lymph, spleen, central nervous system, and cerebraspinal fluid (figure 8.8).

When trypanosomes enter the central nervous system, they cause general apathy, mental dullness, and lack of coor

dination. "Sleepiness" develops, and the infected individual may fall asleep during normal daytime activities. Death results from the pathology occurring in the nervous system, as well as from heart failure, malnutrition, and other weakened conditions. If detected early, sleeping sickness is curable. However, if an infection has advanced to the central nervous system, recovery is unlikely.

Supergroup Amoebozoa

Members of the Amoebozoa are the amoebae (sing., amoeba). When feeding and moving, they form tempora1y cell extensions called pseudopodia (sing., pseudopodium) (Gr. pseudes, false + podion, little foot). Pseudopodia exist

in a variety of forms. Lobopodia (sing., lobopodium) (Gr. !obos, lobe) are broad cell processes containing ectoplasmand endoplasm and are used for locomotion and engulfingfood (figure 8.9a). Filopodia (sing., filopodium) (L. Jilum,thread) contain ectoplasm only and provide a constant twoway streaming that delivers food in a conveyor-belt fashion(figure 8.9/J). Reticulopodia (sing., reticulopodium) (L. reticulatus, netlike) are similar to filopodia, except that they branchand rejoin to form a netlike series of cell extensions (figure

8.9c). Axopodia (sing., axopodium) (L. axis, axle) are thin,filamentous, and supported by a central axis of microtubules.

136 CHAPTER EIGHT

(a)

(b)

Undulating {Fold of pellicle membrane Attached flagellum \

Endoplasmic reticulum------

Free flagellum

FIGURE 8.8

Ribosomes

£Golgi apparatus

. Basal body of flagellum

Kinetoplast

The Life Cycle of Trypanosoma brucei. (a) When a tsetse fly feeds on a vertebrate host, trypanosomes enter the vertebrate's circulat01y system (first arrow on right) with the fly's saliva. Tiypanosomes multiply in the vertebrate ·s circulat01y and lymphatic systems by binaiy fission. When another tsetse flv bites this vertebrate host again, t1ypanosomes move into the gut of the fly and undergo bina1y fission. Trypanosomes then migrate to the fly's saliva1y glands, where they are available to infect a new host. (/J) Structure of the flagellate Trvpanosoma lmtcei rhodesiense. This flagellate is about 25 pm long.

The cytoplasm covering the central axis is adhesive and mov

able. Food caught on axopodia can be delivered to the central

cytoplasm of the amoeba (figure 8.9cl).

Some members in this supergroup lack a test, cell wall,

or other supporting structures. These amoebae are naked and

are normally found on shallow-water substrates of freshwater

ponds, lakes, and slow-moving streams, where they feed on

other protists and bacteria. They engulf food by phagocytosis, a process that involves the cytoplasmic changes described

(a) Ectoplasm Endoplasm

I

) --- ·.,, Lobopodium _...-,,,....-....

Axopodium

FIGURE 8.9

Variations in Pseudopodia. (a) Lobopodia of Amoeba contain both ectoplasm and endoplasm and are used for locomotion and engulfing food. (h) Filopoclia of a shelled amoeba contain ectoplasm only and provide constant two-way streaming that delivers food particles to this protozoan in a conveyor-belt fashion. (c) Reticulopodia are similar to filopoclia except that they branch and rejoin to form a netlike series of cell extensions. They occur in protozoa such as G/ohigerina. (d) Axopoclia on the surface of a heliozoan such as Actinosphaerium deliver food to the central cytoplasm.

earlier for amoeboid locomotion (see figure 2.14). In the process, food is incorporated into food vacuoles. Bina1y fission

occurs when an amoeba reaches a certain size limit. As ,vith

other amoebae, no sexuai reproduction 1s known to occur.

Other members possess a test (shelD. Tests are pro

tective structures that the cytoplasm secretes. They may be calcareous (made of calcium carbonate), proteinaceous

(made of protein), siliceous (made of silica [SiO2]), or chitin

ous (made of chitin-a polysaccharide). Other tests may be

composed of sand or other debris cemented into a secreted

matrix. Usually, one or more openings in the test allow pseu

dopodia to be extruded. Arcella is a common freshwater,

shelled amoeba. It has a brown, proteinaceous test that is flattened on one side and domed on the other. Pseudopodia

project from an opening on the flattened side. Dijflugia is

another common freshwater, shelled amoeba

FIGURE 8.10

Another Amoebozoan. Dif.flugia oblongata, a common freshwater, shelled amoeba. The test consists of cemented mineral particles.

invertebrates, fishes, and mammals. The popular introduct01y laboratory protozoan Amoeba proteus is included in this group (figure 8.11).

Acanthamoebida There are two important members of the Acanthamoebida:

Naegleria Jowleri and Acanthamoeba spp. These protozoans are aerobic inhabitants of soil and water and posses both a flagellated stage and an amoeboid form. Both of these members can become facultative parasites of humans when

humans come into contact with water harboring these freeliving amoeba. Acanthamoebida causes inflammation of brain tissue known as meningoencephalitis, and Naegleria infects the cornea of the eye, leading to inflammation and opacity.

Entamoebida Members of this first rank have no flagella or centrioles and lack mitochondria. All free-living amoebae are particle feeders, using their pseudopodia to capture food; a few are pathogenic. For example, Entamoeba histolytica causes one form of dysentery in humans. Inflammation and ulceration of the lower intestinal tract and a debilitating diarrhea that includes

blood and mucus characterize dysente1y. Amoebic dysente1y is a worldwide problem that plagues humans in crowded,

unsanita1y conditions.

A significant problem in the control of Entamoeba histolytica is that an individual can be infected and contagious

without experiencing symptoms of the disease. Amoebae live in the folds of the intestinal wall, feeding on starch and mucoid secretions. They pass from one host to another in

the form of cysts transmitted by fecal contamination of food or water. After ingestion by a new host, amoebae leave their

cysts and take up residence in the host's intestinal wall, caus

ing a multitude of problems.

Animal-Like Protists: The Protozoa 137

(a)

Pseudopadlurn Nucleus Endoplasm

Ectoplasm vacuole

(b)

FIGURE 8.11

An Amoebozoan. (a) Amoeba proteus, showing blunt pseudopodia (lobopodia) (LM X 160). (b) Anatomy of Amoeba proteus.

Supergroup Rhizaria

•'l•l• MP3

Ameboid locomotion

These protozoans are amoeboid in morphology; however, molecular phylogenetic analysis makes it clear that the Amoeboza and Rhizaria are not sister taxa. Some members of the

Rhizaria have fine pseudopodia (filopodia; see figure 8.9b). Filopodia supp01ted by microtubules are known as axopo

dia. Axopodia protmde from a central region of the protozoan called the axoplast and are used primarily in feeding.

Foraminifera Members of this first rank have filopodia with a granular cytoplasm that forms a complex network of reticulopodia. Fora

maniferans (from the Latin joramen, little hole, and fera, to bear) (commonly called forams) are primarily a marine group

138 CHAPTER EIGHT

How Do We Know That the Amoeboid Protozoa

Probably Appeared Early in Eukaryote

Evolutionary History?

S ome structural characters used to suggest ancient evolutionary relationships

include permanent cytostomes and both flagellated and amoeboid

stages during certain parts of the protozoologists to have diverged from the eukaryotic line prior to the latter's acquisition of mitochondria and subsequent diversification.

life cycle. At least one important parasite, Entamoeba histolytica, lacks mitochondria, and on the basis of its RNA, is thought by some

of protozoa. Foraminiferans possess filopodia arranged in a branching network called reticulopodia and secrete a test that is primarily calcium carbonate. As foraminiferans grow, they secrete new, larger chambers that remain attached to the older chambers (figure 8.12). Test enlargement follows a symmetrical pattern that may result in a straight chain of chambers or a spiral arrangement that resembles a snail shell. Many of these tests become relatively large; for example, "Mermaid's pennies," found in Australia, may be several centimeters in diameter.

Foram tests are abundant in the fossil record since the Cambrian period (543 million years ago). They make up a large component of marine sediments, and their accumulation on the floor of primeval oceans resulted in limestone and chalk deposits. The white cliffs of Dover in England and the great Egyptian pyramids are examples of foraminiferan-chalk deposits. Oil geologists use fossilized forams to identify geologic strata when taking exploratory cores.

Heliozoans are aquatic protozoa that are either planktonic or live ,itt,iched by 8 stalk to some substr,ite (The pfankton of a body of water consists of those organisms that float freely in the water.) Heliozoans are either naked or enclosed within a test that contains openings for axopodia (figure 8.13a).

Radio/aria

Members of this first rank exhibit radial symmetry, from which the name (radiolarian) is derived. All have a porous capsular wall through which axopodia project. Morphology can be simple to complex. The mitochondria have tubular cristae. Radiolarians are planktonic marine and freshwater protozoa. They are relatively large; some colonial forms may reach several centimeters in diameter. They possess a test (usually siliceous) of long, movable spines and needles or of a highly sculptured and ornamented lattice (figure 8.13b). When radiolarians die, their tests drift to the ocean floor. Some of the oldest known fossils of eukaryotic organisms are radiolarians.

Supergroup Chromalveolata

The chromalveolates are a very diverse supergroup of protozoans. Members can be either autotrophic, mixotrophic,

or heterotrophic. They are all united, however, in the common feature of plastid origin. Based on current data, the plastid appears to have been acquired by endosymbiosis with an ancestral archaeplastid by some and then lost in

FIGURE 8.12

Foraminiferan Test (Cibicides labatulus). As this foraminiferan grows, it secretes new, larger chambers that remain attached to older chambers, making this protozoan look like a tiny snail. The chambers are penetrated by pores through which cellular contents are extruded (SEM X 120).

(a)

(b)

FIGURE 8.13

Heliozoan and Radiolarian Tests. (a) Actinosphaerium sol has a spherical body covered with fine, long axopodia made of numerous microtubules and surrounded by streaming cytoplasm. Following phagocytosis by an axopodium, waves of cytoplasmic movement cany trapped food particles into the main body of this protozoan (LM X 200). ( b) The radiolarian Spaerostylus is typically spherical with a highly sculptured test (SEM Xl35).

others. Although there are three first-rank subgroups within

this supergroup, only one, the Alveolata, .....will be considered since it contains the Animation

only protozoan protists. Endosymbiosis

Subgroup Alveolata The Alveolata (alveolates) is a large subgroup that includes the

Dinoflagellata (dinoflagellates), Apicomplexa, and Ciliophora

FIGURE 8.14

Animal-Like Protists: The Protozoa 139

L...____J

0.83 µ,m

A Dinoflagellate. Although this protozoan (Gymnodinium) is small in size, large numbers of them can color the sea reel and produce toxins that result in large fish kills along continental shelves.

(ciliates). One common trait is the presence of flattened vesi

cles called alveoli (hence the name Alveolata) that are stacked

in a continuous layer below the plasma membrane. The alve

oli function in membrane transport, similar to Golgi bodies.

In addition, the alveolates comprise what is believed to be a

monophyletic subgroup of protozoa with varied forms of locomotion, reproduction, and characteristic submembrane vesicles.

Dinoflagellates are marine-flagellated protozoa (figure 8.14) that contain various pigments such as chlorophyll.

They have one flagellum that wraps around the protozoan in a

transverse groove called the girdle. The primary action of this

flagellum causes the protozoan to spin on its axis. (The name

dinoflagellate is derived from the Greek dinein, "to whirl.") A

second flagellum is a trailing flagellum that pushes the proto

zoan forward. In addition to chlorophyll, many dinoflagellates

contain xanthophyll pigments, which gives them a golden

brown color. At times, dinoflagellates become so numerous

that they color the water. Several members, such as Gym

nodinium, have representatives that produce toxins. Periodic

"blooms" of these protozoa are called "red tides" and result in

fish kills along the continental shelves. Humans who consume

tainted molluscs or fish may die. The Bible reports that the

first plague Moses visited upon the Egyptians was a blood-red

tide that killed fish and fouled water. Indeed, the Red Sea is

probably named after these toxic dinoflagellate blooms.

140 CHAF!rn ElGHT

Members of the Apicomplexa (a"pi-kom-plex'ah) (L. apex, point + com, together, + plexus, interweaving) are

all parasites. Characteristics of apicomplexans include:

1. Apical complex for penetrating host cells (an apicalcomplex is a dense ring and conelike structure at the

anterior end of the organism).2. Single type of nucleus.3. No cilia and flagella, except in certain reproductive stages.4. Life cycles that typically include asexual (schizogony,

sporogony) and sexual (gametogony) phases.

Nearly all apicomplexans are parasites of animals, and some cause serious disease. These parasites spread through their hosts as tiny infectious cells called sporozoites. Apicomplexans are so named because one end (the apex) of the sporozoite contains a complex of organelles specialized for penetrating host cells and tissues. Ce1tain members, such as Ciyptosporidium, Toxoplasma, Cyclospora, Babesia, and Plasmodium, cause a variety of diseases in domestic animals and humans.

Although the life cycles of these protozoa vary considerably, certain generalizations are possible. Many are intracellular parasites, and their life cycles have three phases. Schizogony is multiple fission of an asexual stage in host cells to form many more (usually asexual) individuals, called merozoites, that leave the host cell and infect many other cells. (Schizogony to produce merozoites is also called merogony.)

Some of the merozoites undergo gametogony, which begins the sexual phase of the life cycle. The parasite forms either microgametocytes or macrogametocytes. Microgametocytes undergo multiple fission to produce biflagellate microgametes that emerge from the infected host cell. The macrogametocyte develops directly into a single macrogamete. A microgamete fertilizes a macrogamete to produce a zygote that becomes enclosed in a membranous cyst called an oocyst.

The zygote undergoes meiosis, and the resulting cells divide repeatedly by mitosis. This process, called sporogony, produces many rodlike sporozoites in the oocyst. Sporozoites infect the cells of a new host after the new host ingests and digests the oocyst, or sporozoites are otherwise introduced (e.g., by a mosquito bite).

One genus, Plasmodium, causes malaria and has a long recorded histmy of devastating effects on humans. Accounts of the disease go back as far as 1550 B.c. Malaria contributed significantly to the failure of the Crusades during the medieval era and, along with typhus, has devastated more armies than has actual combat. Recently (since the early 1970s), malaria has resurged throughout the world. More than 300-500 million humans are estimated to annually contract the disease, and more than one million people die from these infections

each year. The Plasmodium life cycle involves vertebrate and mos

quito hosts (figure 8.15). Schizogony occurs first in liver cells and later in red blood cells, and gametogony also occurs in red blood cells. A mosquito takes in gametocytes during a meal of blood, and the gametocytes subsequently fuse. The zygote penetrates the gut of the mosquito and transforms into

an oocyst. Sporogony forms haploid sporozoites that may enter a new host when the mosquito bites the host.

The symptoms of malaria recur periodically and are called paroxysms. Chills and fever correlate with the maturation of parasites, the rupture of reel blood cells, and the release of toxic metabolites.

Four species of Plasmodium are the most important human malarial species. P. vivax causes malaria in which the paroxysms recur eve1y 48 h. This species occurs in temperate regions and has been nearly eradicated in many parts of the world. P. Jalciparnm causes the most virulent form of malaria in humans. Paroxysms are more irregular than in the other species. P.jalciparum was once worldwide, but is now mainly tropical and subtropical in distribution. It remains one of the greatest killers of humanity, especially in Africa. P. malariae is worldwide in distribution and causes malariawith paroxysms that recur eve1y 72 h. P. ovate is the rarest ofthe four human malarial species and is primarily tropical indistribution.

Other Apicomplexans also cause important diseases. Coccidiosis is primarily a disease of poult1y, sheep, cattle, and rabbits. Two genera, Jsospora and Eimeria, are particularly important parasites of poultry. Yearly losses to the global agricultural indust1y are estimated to be in the hundreds of millions of dollars. Another coccidian, Cryptosporidium, which has become more well known with the advent of AIDS since it causes chronic diarrhea in AIDS patients, is the only known protozoan to resist chlorination, and is most virulent in immunosuppressed individuals. Toxoplasmosis is a disease of mammals, including humans, and birds. Sexual reproduction of Toxoplasma occurs primarily in cats. Infections occur when oocysts are ingested with food contaminated by cat feces, or when meat containing encysted merozoites is eaten raw or poorly cooked. Most infections in humans are asymptomatic, and once infection occurs, an effective immunity develops. However, if a woman is infected near the time of pregnancy, or during pregnancy, congenital toxoplasmosis may develop in a fetus. Congenital toxoplasmosis is a major cause of stillbirths and spontaneous abo1tions. Fetuses that survive frequently show signs of mental retardation and epileptic seizures. Congenital toxoplasmosis has no cure. Toxoplasmosis also ranks high among the opportunistic diseases afflicting AIDS patients. Steps to avoid infections by Toxoplasma include keeping stray and pet cats away from children's sandboxes; using sandbox covers; and awareness, on the part of couples considering having children, of the potential dangers of eating raw or ve1y rare pork, lamb, and beef.

The ciliates (Ciliophora) (sil"i-of' or-ah) include some of the most complex protozoa. Ciliates are widely distributed in freshwater and marine environments. A few ciliates are symbiotic. Characteristics of the ciliates include:

1. Cilia for locomotion and for the generation of feeding

currents in water.2. Relatively rigid pellicle and more or less fixed shape.3. Distinct cytostome (mouth) structure.

Animal-Like Protists: The Protozoa 141

Mosquito StagesHuman Liver Stages

D Infected Mosquito takes liver cell a blood meal Liver cell �. .

� (injects � ,_ sporozoltes)

,N-· ....... 111""" 1--==-� Exoerythr�ytic cycle

' - ' ' \ \ \ ' 'Release of

a sporozoltes

� Sporogonic cycle

Ookloete � Macrogametocyte

Microgamete entering macrogamete

FIGURE 8.15

Exflagellated microgametocyte

Mosquito takes a blood meal (ingests gametocytes)

II = Infective stagem = Diagnostic stage

Ruptured ·'·i, t Schizontsch.iz:;/

nt ,;: �,,

• I 1•�·'' .. . :

Human Blood Stages

,� Immature trophozoite

"" (ring stage)

Mature

t Erythrocytic cycle \� trophozolte ,mt

' .

. . ' -- ,--- :} ... -.. :�� ·,.- . ' ' .

Ruptured schizont

�

am

..... � ti m

Gametocytes Schizont

/Gametocytes

P. vlvax

P. ovale

P. malaria

The Life Cycle of Plasmodium. Schizogony (merogony) occurs in liver cells and, later, in the reel blood cells (RBCs) of humans. Gametogony occurs in RBCs. During a blood meal, the mosquito takes in micro- and macrogarnetes, which fuse to form zygotes. Zygotespenetrate the gut of the mosquito and form oocysts. Meiosis and sporogony form many haploid sporozoites that may Animationenter a new host when the mosquito bites the host. Malaria Life Cycle of

P/asmodlum

4. Dimorphic nuclei, typically a larger macronucleus andone or more smaller micronuclei.

Cilia are generally similar to flagella, except that they are much shorter, more numerous, and widely distributed over the surface of the protozoan (figure 8.16). Cilia1y movements are coordinated so that cilia1y waves pass over the surface of the ciliate. Many ciliates can reverse the direction of ciliaiy beating and the direction of cell movement.

Basal bodies (kinetosomes) of adjacent cilia are interconnected with an elaborate network of fibers believed to anchor the cilia and give shape to the organism.

Some ciliates have evolved specialized cilia. Cilia may cover the outer surface of the protozoan. They may join to form cirri, which are used in movement. Alternatively, cilia may be lost from large regions of a ciliate.

Trichocysts are pellicular structures primarily used for protection. They are rodlike or oval organelles oriented perpendicular to the plasma membrane. In Paramecium, they

have a "golf tee" appearance. The pellicle can discharge trichocysts, which then remain connected to the body by a sticky thread (figure 8.17).

Some ciliates, such as Parameciu1n, have a ciliated oral groove along one side of the body (seejxgure 8.16). Cilia of the oral groove sweep small food particles toward the end of the cytophaiynx, where a food vacuole forms. When the food vacuole reaches an upper size limit, it breaks free and circulates through the encloplasm. Indigestible material is voided either through a tempora1y opening or through a permanent cytopyge which is found in many ciliates.

Some free-living ciliates prey upon other protists or small animals. Prey capture is usually a case of fortuitous contact. The ciliate Didinium feeds principally on Paramecium, a prey that is bigger than itself. Didinium forms a tempora1y opening that can greatly enlarge to consume its prey (figure 8.18).

Suctorians are ciliates that live attached to their substrate (figure 8.19). They possess tentacles whose secretions

142 CHAPTER EIGHT

(a)

Anterior end

Cytopharynx wfth rows ot cllla used in feeding

(b)

FIGURE 8.16

Pellicle

Ht--- Anterior contractile vacuole

Cilia

Phagocytic vacuole

-=--�'-----'!r- Cytostorne

1----...;..- Food vacuole

Posterior end

Cytopyge

Posterior contractile vacuole

Ciliophora. (a) The ciliate Paramecium sonneborn. This paramecium is 40 pm in length. Note the oral groove near the middle of the body that leads into the cytopharynx (LM XZ,500). (b) The structure of a typical ciliate such as Paramecium.

paralyze prey, often ciliates or amoebae. The tentacles ensnare and manipulate prey the prey, and prey cytoplasm is drawn into the suctorian through the tentacles and encoporated into a food vacule within the protist. The mechanism for this probably involves tentacular microtubules.

Ciliates have two kinds of nuclei. A large, polyploid macronucleus regulates daily metabolic activities. One or more smaller micronuclei are the genetic reserve of the cell.

Ciliates reproduce asexually by transverse binary fission and, occasionally, by budding. Budding occurs in suctorians and results in the formation of ciliated, free�swimming organisms that attach to the substrate and take the form of the adult.

Ciliates reproduce sexually by conjugation (figure 8.20). The partners involved are called conjugants. Many species of ciliates have numerous mating· types, not -all· of which --are

FIGURE 8.17

Discharged Trichocysts of Paramecium. Each trichocyst transforms itself into a long, sticky, proteinaceous thread when discharged (LM X 150).

FIGURE 8.18

A Single-Celled Hunter and Its Prey. The juglike DidiniumCleft) swallowing a slipper-shaped Paramecium (right) (SEM X550).

mutually compatible. Initial contact between individuals is apparently random, and sticky secretions of the pellicle facilitate adhesion. Ciliate plasma membranes then fuse and remain that way for several hours.

The macronucleus does not participate in the genetic exchange that follows. Instead, the macronucleus breaks up during or after micronuclear events, and re-forms from micronuclei of the daughter ciliates.

After separation, the exconjugants undergo a series of nuclear divisions to restore the nuclear characteristics of the particular species, including the formation of a macronucleus from one or more micronuclei. Cytoplasmic divisions that form daughter cells accompany these events.

Most ciliates are free living; however, some are commensalistic- or mutualistic, and a fi::-. are parasitic. Balantidium

FIGURE 8.19

Two Suctorians. Suctorians are stalked ciliate protozoa, seen here attached to a filamentous algae. Larval suctorians possess cilia but mature adults lack them and use their tentacles to capture prey (LM X20).

coli is an important parasitic ciliate that lives in the large intestines of humans, pigs, and other mammals. At times, it is a ciliaty feeder; at other times, it produces proteolytic enzymes that digest host epithelium, causing a flask-shaped ulcer. (Its pathogenicity resembles that of Entamoeba histo

(vtica.) B. coli is passed from one host to another in cysts that form as feces begin to dehydrate in the large intestine. Fecal contamination of food or water is the most common form of transmission. Its distribution is potentially worldwide, but it is most common in the Philippines.

Large numbers of different species of ciliates also inhabit the rumen of many ungulates (hoofed animals). These ciliates contribute to the digestive processes of their hosts.

Animal-Like Protists: The Protozoa 143

SECTION REVIEW 8.4

According to the most recent classification of protists, there are six supergroups. The four protozoaon supergroups and several common examples within each are discussed in this chapter. These include the Excavata that possess a cytostome and a posterior flagellum. Examples include Giardia, Trichomonas, Euglena, and Trypanosomes. Members of the Amoebozoa possess pseudopodia and examples include Amoeba, Naegleria, and Entamoeba. Forminiferans and radiolarians are common marine Rhizaria that possess filopodia. Difflugia is a representative example of the Rhizaria. The Chromalveolata are a vety diverse supergroup of protists protozoans. They are all united in the common feature of having a plastid origin. The Alveolata is a large supergroup that includes the dinoflagellates and ciliates. Members of the Apicomplexa are all parasites and include the malaria causing Plasmodium. Many Apicomplexans have a three-part life cycle involving schizogony, gametogony, and sporogony.

Why would it be very difficult to find a poison to fight

the malaria-causing protists Plasmodium?

8. 5 Ft H 1111 R PH I< wl NE IC

LEARNING OUTCOME

1. Explain the tentative phylogeny of the protist euka1y-otes based on 18S rRNA sequence comparisons.

Protozoa probably originated about 1.5 billion years ago. Although known fossil species exceed 30,000, they are of little use in investigations of the origin and evolution of the various protozoan groups. Only protozoa with hard parts

(tests) have left much of a fossil record, and only the foraminiferans and radiolarians have well established fossil records in Precambrian rocks. Recent evidence from the study of base sequences in ribosomal RNA indicates that each of the four supergroups covered in this chapter probably had separate origins (figure 8.21). Additional modifications to the present scheme of protozoan classification are continually being proposed as the results of new ultrastructural and molecular studies are published.

SECTION REVIEW 8.5

Recent molecular phylogeny of nuclear rRNA indicates that the protists known as the protozoa represent four distinct lineages that are probably monophyletic.

Why is the fossil record of little value in establishing

relationships within protozoan groups?

144 CHAPTER EIGHT

FIGURE 8.20

Conjugation

and nuclear segregation

Macronucleus

Macronuclear degeneration and meiosis of the micronuclei

Micronuclear multiplication

Beginning of nuclear modification

Protist separation and fusion of gamete nuclei

and nuclear segregation

Mitotic division of remaining pronuclei

Micronuclear migration and fertilization

Exconjugation

Development of other exconjugant

Conjugation in Paramecium. During conjugation, conjugants exchang� gene_tic material c:ontained ig_ micronuclei and micronuclei from separate individuals then fuse. After conjugants separate micronuclei multiply and reorganize to form the nuclear characteristic of the species and cell division occurs. Eight new protists result from each conjugation. (Events occurring in a single exconjugant are shown here.)

Chromalveolata

FIGURE 8.21

Archaeplastida

1 Dinoflagellates 2 Apicomplexans 3 Ciliates 4 Oomycetes 5 Diatoms 6 Brown algae 7 Radiolara 8 Foraminifera

Animal-Like Protists: The Protozoa 145

9 Cercozoa 10 Charophytes 11 Land plants 12 Parabasalid 13 Amoebozoa 14 Fungi 15 Choanoflagellates 16 Animals

Tentative Phylogeny of the Eukaryotic Tree of Life Based on 18S rRNA Sequence Comparisons. Recent molecular phylogeny of the nuclear rRNA indicates that proka1yotes are polyphyletic and separated into six supergroups shown in this illustration. The taxon "Protozoa" should not be used in classification schemes that seek to represent true molecular evolutionary histories. The word "protozoa" can still be used (as it is in this chapter) to denote a polyphyletic group of protist organisms that share some morphological, reproductive, ecological, and biochemical characteristics.

EVOLUTIONARY INSIGHTS

TI1e Animal-Like Protists May Lie at the Crossroads between the Simpler and the Complex

B ctween the un,c:ellular mi rrn>rg,u,i...111,; CF11hal·tl·rla and Arclt.ae�i) and the 111ulticdlul.1r •ukaryulc. lie lh · protbts, The protists ma • rcprust·m a bridg, from �impk• 10 com

plex life-forms. As noted in this chapter, most protists are single

eukaryotic cells that provide some insight as to what the earliest eukaryotes might have been like.

Along these lines, protists are of interest to evolutionary biologists because extant organisms may retain clues to important

( Continued)

146 CHAPTER EIGHT

Collar

(a) (b) (c)

BOX FIGURE 8.1 Zooflagellate Diversity. Choanoflagellates: (a) Stephanoeca. (b) Codosiga, a colonial species. (c) Proterospongia, another colonial species, with individuals embedded in a thick, gelatinous matrix.

milestones in euka1yotic evolution. For example, a group of protists called jakobids have mitochondria that resemble bacteria more than those of any other type of eukaryote. Therefore, jakobids may resemble those microorganisms that lived shortly after cells acquired aerobic bacteria as endosymbionts (see box in chapter 2). At the other end of the evolutionary spectrum are the choanoflagellates (a group of free-living zooflagellates found primarily in freshwater). Many choanoflagellate species are sessile, being permanently attached to a substrate (box figure 8.1). Each individual has a single flagellum that bears an uncanny resemblance to the "collar cells" of sponges (see figure 9.4). Commonly, individuals are stalked and/or embedded in a gelatinous secretion. Most species are colonial and immobile. Members of the genus Proterospongia form (planktonic) colonies of up to several hundred cells and bear a striking resemblance to primitive sponges. Whether this simple relationship reflects a true phylogenetic relationship and crossroads between the unicellular flagellates and the more complex multicellular sponges or whether the similarities are a product of independent, convergent evolution is not certain. Definitive answers will have to await nucleic acid sequencing, which provides a more objective measure of relatedness than comparing possible superficial appearances.

SUMMARY

8.1 Evolutionary Perspective of the Protists

The protists are a polyphyletic group that arose about 1.5 billion years ago when the Archaea and Euka1ya diverged. The protists are divided into six supergroups, four of which contain the protozoa. The evolutionaiy pathways leading to modem protozoa are uncertain.

The animal and fungal kingdoms, and one group related to the protists, are found within the Eukarya (see figure 8.21) in the supergroup Opistokonta. Evolutionary biologists are interested in the Opisthokonta because it holds molecular clues to the origin of animals. Its protist members include the choanoflagellates, whose ancestors may also be the ancestors of all animals containing the choanoflagellates. Currently, those evolutionary biologists who

are interested in the origin of animals are studying these choanoflagellates for molecular clues. Recently, a genome sequence for the choanoflagellate Monosiga brevicol/is has been accomplished, and several genes that are present only in choanoflagellates and animals have been identified. Some of these shared genes encode cell adhesion and extracellular matrix proteins that help choanoflagellates attach to surfaces and were also essential to the multicellularity in animals. The close relationship of choanoflagellates to animals has been further demonstrated by the strong homology between a surface receptor (a tyrosine kinase receptor) found in both choanoflagellates and sponges. This surface receptor initiates a common signaling pathway involving phosphorylation-a major source of control for common protein functions found in both sponges and choanoflagellates.

8.2 Life within a Single Plasma Membrane

Protozoa occur as both single cells and entire organism.s. Organelles specialized for the unicellular lifestyle cany out many protozoan functions.

8.3 Symbiotic Lifestyles

Many protozoa live in symbiotic relationships with other organisms, often in a host-parasite relationship.

8.4 Protists and Protozoan Taxonomy

Most members of the Excava.ta possess a cytostome and a posteriorly directed flagellum. Examples include Giaradia, Trichomonas, Euglena, and the zooflagellate Trypanosoma, which causes sleeping sickness.

Members of the Amoebozoa possess pseudopodia. Amoebozoans use pseudopodia for feeding and locomotion. Examples include Amoeba, Naegleria, and Entamoeba.

Foraminiferans and radiolarians are common marine Rhizaria that possess thin pseudopodia (filopodia). Dif.flugia is a typical example of this supergroup.

The Chromalveolata are a ve1y diverse supergroup of protists protozoans. Members can be either autotrophic, mixotrophic, or heterotrophic. They are all united in the common feature of a plastid origin. The Alveolata is a large subgroup that includes the dinoflagellates, Apicomplexa, and Ciliophora. Apicomplexans are all parasites and include Plasmodium and Toxoplasma, which cause malaria and toxoplasmosis, respectively. Many apicomplexans have a three-pait life cycle involving schizogony, gametogony, and sporogony. The ciliates represent some of the most complex protozoa. Ciliates possess cilia, a macronucleus, and one or more micronuclei.

8.5 Further Phylogenetic Considerations

Precise evolutiona1y relationships are difficult to determine for the protozoa. The fossil record is sparse, and what does exist is not particularly helpful in deducing relationships. However, ribosomal RNA sequence comparisons indicate that each of the four protist supergroups probably had separate origins.

CONCEPT REVIEW QUESTIONS

1. Which of the following moves by flagella?

a. Amoeba

b. Euglena

c. Paramecium

d. Both a and h are correct.

e. None of the choices are correct.

2. Ciliates

a. can move by pseuclopocls.

b. are not as varied as other protists.

c. have a gullet for food procurement.

cl. are closely related to the radiolarians.

e. are mostly parasites.

3. Dinoflagellates

a. reproduce sexually.

h. have protective cellulose plates.

c. clo not produce much food and oxygen

cl. have cilia instead of flagella.

e. are the largest protozoans.

Animal-Like Protists: The Protozoa 147

4. Which of the following groups of protozoans has no locomotor organelles?

a. Apicomplexans

b. Euglenoids

c. Amoeba

d. Dinoflagellates

e. T1ypanosomes

5. Which of the following protozoans possesses an eyespot fordetecting light needed for photosynthesis?

a. Apicomplexans

b. Euglenoicls

c. Amoeba

cl. Dinoflagellates

e. T1ypanosomes

ANALYSIS AND APPLICATION

QUESTIONS

1. If it is impossible to know for certain the evolutiona1y pathways that gave rise to protozoa and animal phyla, is it worthconstructing hypotheses about those relationships? Why orwhy not?

2. In what ways are protozoa similar to animal cells? In whatways are they different?

3. If sexual reproduction is unknown in Euglena, how do youthink this lineage of organisms has smvived through evolutiona1ytime? (Recall that sexual reproduction provides the genetic variability that allows species to adapt to environmental changes.)

4. The use of DDT has been greatly curtailed for ecologicalreasons. In the past, it has proved to be an effective malariadeterrent. Many organizations would like to see this form of mosquito control resumed. Do you agree or disagree? Explainyour reasoning.

5. If you were traveling out of the countly and were concernedabout contracting amoebic dysente1y, what steps could youtake to avoid contracting the disease? How would the precautions differ if you were going to a counuy where malaria is aproblem?

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