-
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?
!i1connect' ZOOLOGY
Enhance your study of this chapter with study tools and practice
tests. Also ask your instructor ahout the resources availahle
through Connect, including a media-rich eBook, interactive learning
tools, and animations.