11.1 Eukaryotes: An Overview

SECTION OBJECTIVES

  • Explain the processes of asexual (vegetative) reproduction and sexual reproduction.
  • Distinguish the key traits of fungi, algae, amebas, ciliates, and hemoflagellates.
  • Distinguish between microbial and invertebrate parasites.

Microbial eukaryotes—including protozoa, fungi, and algae—play key roles in all ecosystems. In the oceans, algae and protozoa (collectively called protists) provide the base of a vast food web that feeds fish and marine mammals. In forests, many fungi break down wood for nutrients, while others connect tree roots in a vast network of nutrient sharing. This chapter explores the diversity of microbial eukaryotes. In addition, we will introduce certain differentiated animals (helminths and arthropods) that act as infectious agents.

As presented in Chapter 5, the cells of all eukaryotes, whether multicellular or microbial, possess a nucleus and other key membranous organelles. Membranous organelles enable some microbial eukaryotes to grow to a million times larger than a bacterial cell. Yet others, particularly marine algae, have been downsized by evolution to less than 2 μm, comparable in size to Escherichia coli. Even so, tiny eukaryotes such as the alga Ostreococcus tauri (Figure 11.1) possess all the key organelles of a eukaryote. These organelles include the nucleus, containing linear chromosomes; mitochondria, the powerhouses of respiration; the endomembrane system and Golgi, for intracellular transport; and a chloroplast, for photosynthesis.

Figure 11.1 Even a Tiny Eukaryote Has Organelles
A micrograph and a 3 D tomograph of the eukaryote Ostreococcus Tauri and its organelles.
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A micrograph and a 3 D tomograph of the eukaryote Ostreococcus Tauri and its organelles. The first part is the micrograph and the second part is the tomograph. Both show the same cell. In the micrograph, the organelle membranes are visible. The organelles have grainy interiors. In the tomograph, the organelles are clearly defined and labeled. From largest to smallest, the organelles inside the cell are the chloroplast, nucleus, mitochondrion, granules and ribosomes. The Golgi apparatus has a structure similar to stacked sheets and is only partially visible behind the chloroplast and the nucleus.

Ostreococcus tauri, visualized by cryo-EM (A) and in 3D by tomography, colorized (B).

Some eukaryotes are parasites that cause diseases such as malaria and trypanosomiasis (sleeping sickness). Leishmania is a protozoan that causes leishmaniasis, described in the chapter-opening case history. Parasitic protozoa often have complex life cycles, sometimes involving multiple hosts. In the life cycle of Leishmania, the promastigote, or flagellated form of the protozoan, is carried by a sand fly (Figure 11.2, step 1), which injects it into the skin of the host. In the bloodstream, the promastigotes are phagocytosed by macrophages (white blood cells), which attempt to destroy them (step 2). But the promastigotes survive within the macrophage and develop into amastigotes (unflagellated forms) that multiply intracellularly (step 3). The macrophage bursts, releasing amastigotes that multiply within various host tissues, causing sores (step 4). When another sand fly takes a blood meal (step 5), it ingests infected macrophages full of amastigotes. The amastigotes develop into promastigotes within the sand fly’s midgut (step 6). The promastigotes multiply and migrate to the proboscis, where they can be transmitted to the next host that the sand fly bites. Complex reproductive cycles are typical of infectious parasites and pose challenges for therapy.

Figure 11.2 Life Cycle of Leishmania
A diagram of the stages in the lifecycle of the Leishmania parasite.
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A diagram of the stages in the lifecycle of the Leishmania parasite. The first stage shows a sand fly feeding on blood from a human hand, injecting the Leishmania parasites into the tubular blood vessels of the arm of the host. The caption reads, Sand fly takes a blood meal, injecting Leishmania promastigotes into the bloodstream. Next, the promastigotes are phagocytosed by macrophages. The text for the third stage reads, Within the macrophage, promastigotes transform into amastigotes and multiply until the host cell lyses. Note that each amastigote has a nucleus. A light micrograph shows the large oval shaped structure of a macrophage. It has two large purple circles within it, and many small circle shaped Leishmania parasites throughout its cytoplasm. The fourth stage shows a human hand with a wound that has irregular puffy and ridged borders. The caption reads, Amastigotes multiply in and damage local tissue, causing sores. The fifth stage shows a sand fly feeding on the blood from the tubular blood vessels of the host’s fingers, and indicates the Prescence of circular cells in the bloodstream. The caption reads, Sand fly takes a blood meal and ingests infected macrophages. The sixth stage shows a light micrograph. The micrograph shows promastigotes, each with elongated oval shaped body structures, one or two flagella, and the nucleus, a smaller darker structure near the center. An inset shows a sand fly that is the size of a dot compared to the human hand but carries numerous Leishmania parasites. The caption reads, Amastigotes transform into promastigotes, which multiply in the midgut and migrate to the proboscis. An arrowhead from the sixth stage moves to the first stage shows the continuation of the cycle.

Reproduction of Eukaryotic Cells

All eukaryotes have linear chromosomes that must divide by mitosis. The “ends” of linear chromosomes require special means of replication not required for most bacteria with their circular chromosomes. DNA replication occurs during a portion of interphase, the active state of the cell. Once DNA replication is complete, the cell halts most of its enzymatic activities and prepares for mitosis (Figure 11.3A). Mitosis is the process of segregating the two copies of all chromosomes evenly into the daughter cells. The process ensures that each daughter cell receives a full set of daughter chromosomes.

Figure 11.3 Mitosis and Meiosis
A diagram of mitosis.
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A diagram of mitosis. There are four phases in mitosis, prophase, metaphase, anaphase and telophase. Mitosis results in two identical daughter cells. A caption reads, before mitosis, chromosomes are replicated to form identical D N A pairs. Step 1, nuclear membrane falls apart. Chromosomes condense. Centrioles move to poles. There is an illustration of a cell during prophase. Chromosomes within the nucleus are condensing into x shapes. The membrane around the nucleus has fragmented. In the cytoplasm, centrioles are seen drifting to opposite poles. A caption reads, step 2, chromosomes, or paired copies of D N A, align at the cell equator. There is an illustration of a cell during metaphase. 4 X shaped chromosomes are lined up horizontally across the cell. The centromeres are located in opposite poles of the cell. Spindles from the centromeres extend toward the line of chromosomes. A caption reads, step 3, D N A copies separate and move toward poles. There is an illustration of a cell during anaphase. The four chromosomes have been split into eight halves. A set of four is pulled toward either centromere by spindle fibers. A caption reads, step 4, nuclear membranes and centriole re form, making two identical daughter cells. There is an illustration of two cells during telophase. Each cell has a nucleus containing four chromosomes and a centromere. The nucleus is enclosed within a complete nuclear membrane. There is an equal mix of the chromosome types in each cell.

A. Mitosis: Chromosomes replicate, and the pairs of sister chromatids line up. One round of cell division yields two identical diploid (2n) daughter cells.
A diagram of meiosis.
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A diagram of meiosis. There are two phases in meiosis, meiosis 1 and meiosis 2. Meiosis results in four unique gamete cells. A caption reads, before meiosis, chromosomes are replicated to form identical D N A pairs. A cell is shown during meiosis 1. The cell contains a nucleus with condensing chromosomes and a fragmenting nuclear membrane. In the cytoplasm, centromeres are drifting to opposite poles of the cell. In the next step, chromosome tetrads are organized in a horizontal line through the center of the cell. Spindle fibers extend toward the tetrads from the centromeres at both poles. In the next step, tetrads are split and chromosomes are pulled to opposite centromeres. The cell begins to divide into two cells. The captions for meiosis 1 read, step 1, nuclear membrane falls apart and centrioles move to poles. Duplicated chromosomes pair with homologs, forming tetrads. Arms cross over. Step 2, tetrads, or pairs of homologs, align at cell equator. Step 3, homologs separate and move toward poles, with D N A copies still paired. There is an illustration of two daughter cells at the start of meiosis 2. In each cell, four chromosome arms are aligned in the middle of the cell. Two arms are pulled toward one centromere and two arms are pulled toward the other centromere. In the next step, each daughter cell has generated two more cells. There are four cells total. Each cell contains a nucleus with unique chromosomes enclosed in a nuclear membrane. Each cell also has a centromere. The captions for meiosis 2 read, step 4, daughter cells form but enter a second round of division similar to mitosis. Step 5, four unique gamete cells form with one copy of each chromosome, recombined from the parent.

B. Meiosis: Homologous chromosomes associate in pairs. The chromosomes replicate, and all four DNA helices exchange arms in the process of crossover (recombination). Two rounds of cell division yield four haploid (n) gamete cells.

In most cases, mitosis involves four phases:

  • Prophase. Paired chromosomes (postreplication) condense into short rods. In most species the nuclear membrane dissolves.
  • Metaphase. The chromosomes arrange themselves along a plane across the equator of the cell.
  • Anaphase. The pairs separate, pulled by spindle fibers composed of microtubules that contract. The separated chromosomes are pulled toward opposite poles of the cell.
  • Telophase. The chromosomes decondense (become long and thin again), and a nuclear membrane forms around each set. The cell completes division of its cytoplasm, forming two daughter cells.

Many eukaryotic microbes, such as yeasts, can proliferate indefinitely by mitosis, a process called asexual reproduction or vegetative reproduction. But most eukaryotes, single-celled or multicellular, also have the option of sexual reproduction. Sexual reproduction requires the reassortment of genetic material from different chromosomes. A sexual life cycle alternates between cells that are diploid (2n, containing two copies of each chromosome) and sex cells that are haploid (n, containing a single copy of each chromosome). Two of the haploid sex cells (called gametes) can join each other by fertilization to regenerate a diploid cell (called a zygote). The diploid thus possesses two homologs of each chromosome—that is, two versions of the same chromosome from two different parents.

The process of gamete formation requires a special modification of mitotic cell division called meiosis (Figure 11.3B). Meiosis includes two cell divisions: meiosis I and meiosis II. Like mitosis, meiosis I must be preceded by replication of all chromosomes (during interphase). So a diploid (2n) cell temporarily has four copies of each DNA homolog. In meiosis, unlike mitosis, prophase I requires that each replicated pair line up with its homolog, the homologous chromosome inherited from the other parent. Now the aligned homologs exchange portions of their DNA. This genetic exchange reassorts the gene versions from the two parents and increases genetic diversity in the next generation.

In meiosis I, the paired homologs separate during metaphase I and anaphase I. A short telophase occurs, in which each daughter cell now has a total of 2n DNA helices, but as n pairs containing a chromosome from each parent. The chromosome pairs then separate during meiosis II, which includes prophase II, metaphase II, anaphase II, and telophase II. The result is four haploid (n) cells, as seen in Figure 11.3B.

For many microbes, the haploid forms may undergo asexual (vegetative) reproduction. At some point, the haploid forms may develop into specialized cells that reunite through fertilization, restoring a diploid form (the zygote). The asexual reproduction of both haploid and diploid forms leads to a life cycle known as alternation of generations.

What is the advantage of alternation between haploid (n) and diploid (2n) forms? Consider a protozoan parasite that grows within a human host. The haploid form of the parasite requires fewer resources and generates fewer varieties of coat proteins that might activate the host’s immune system. But the diploid form generates novel combinations of genes that may provide an advantage when the environment changes or when the parasite enters a new host. Thus, most eukaryotic microbes maintain the option of a sexual cycle that alternates between haploid and diploid. In some cases, one form exists only briefly; for example, the malarial parasite proliferates mainly as a haploid but undergoes a brief cycle of fertilization and meiosis within the insect vector.

Diversity of Microbial Eukaryotes

Microbial eukaryotes are classified traditionally as fungi, protozoa, and algae. For most of human history, life was understood in terms of multicellular eukaryotes. There were animals (creatures that move to obtain food) and plants (rooted organisms that grow in sunlight). Fungi (singular, fungus) lack photosynthesis yet were considered a form of plant because they grow on the soil or other substrate. As a result, mycology, the study of fungi, was often included with botany, the study of plants. Later, microscopists came to recognize microscopic forms of fungi such as hyphae (filaments of cells) and yeasts (single cells); thus, fungi were studied also by microbiologists. Surprisingly, however, fungi show close genetic relatedness to multicellular animals.

Other microscopic life forms, such as amebas and paramecia, are motile and appear more like microscopic animals. Motile organisms were called protozoa (singular, protozoan), meaning “first animals,” although they are more distantly related to animals than fungi are. Microscopic life forms containing green chloroplasts were called algae (singular, alga) and were thought of as primitive plants. But some protozoa turned out to have chloroplasts, and some algae turned out to be motile with flagella. Microbiologists refer to algae and protozoa collectively as protists.

How do microbial eukaryotes relate to multicellular animals and plants? Plants show close genetic relationship with the “primary algae,” green algae that evolved from a common chloroplast-bearing ancestor. Animals, however, are most closely related to fungi, based on comparing their DNA sequences. DNA sequence analysis shows that several different groups of protists are more different from each other—and from animals—than animals are from fungi.

Major kinds of microbial eukaryotes are summarized in Table 11.1. Although all these groups include microbial forms, they also include species that can be observed by the unaided eye (such as giant amebas) as well as multicellular forms (such as mushrooms and kelps). Thus, the term “microbe” is a working description but does not define a strict category of life.

Table 11.1

Eukaryotic Microbes and Multicellular Infectious Agents

Major Category

Example

Major Category

Example

Fungi and microsporidians

Filaments or yeasts that usually lack motility; tough cell walls made of polysaccharides; heterotrophs by absorptive nutrition; include decomposers and parasites

A scanning electron micrograph of Aspergillus, an example of a fungi.
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A scanning electron micrograph of Aspergillus, an example of a fungi. Aspergillus consists of a stalk like structure with a bunch of small spherical shaped structures at the tip. The tip is about 15 micrometers long and 15 micrometers wide. Each spherical component has a diameter of about 1 micrometer.

Aspergillus

Amebas (amoebas)

Heterotrophic protists with amorphous shape; motile with pseudopods

A light micrograph of Chaos carolinensis, an example of an amoeba.
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A light micrograph of Chaos carolinensis, an example of an amoeba. It is an irregularly shaped, long translucent organism with finger like projections. There is a green splotch on one side near the center and the body appears to have a white and light blue bubble like texture. It is about 1500 micrometers long.

Chaos carolinensis

Ciliates

Protists with cilia to capture prey; most are free-living, but some are parasites

A light micrograph of Balantidium coli, an example of a ciliate.
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A light micrograph of Balantidium coli, an example of a ciliate. It has the structure of a translucent oval with two little bumps at one end. It has a red bean shaped structure inside, and its interior texture is cloudy purple and white. The membrane appears thick and white. It is about 50 micrometers long.

Balantidium coli

Apicomplexans

Intracellular parasites with complex life cycles; apical complex enables attachment and penetration of host cell
A light micrograph of Plasmodium falciparum, an example of an apicomplexan.
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A light micrograph of Plasmodium falciparum, an example of an apicomplexan. The micrograph shows a blood smear. The red blood cells are pale red discs with lighter centers. P falciparum is a darkly staining organism seen in and around the red blood cells. Within the red blood cells, it is shaped similar to a ring, with a dark point where the gem would be. Outside of the blood cells, it is oval shaped. One such oval shaped organisms is about 7 micrometers long. Each blood cell has a diameter of about 5 micrometers.

Plasmodium falciparum

Hemoflagellates and metamonads

Extracellular parasites with complex life cycles
A light micrograph of Trypanosoma cruzi, an example of a hemoflagellate or metamonad.
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A light micrograph of Trypanosoma cruzi, an example of a hemoflagellate or metamonad. The micrograph shows a blood smear. The red blood cells are pale red discs with lighter centers. Outside of the blood cells, the elongated thin C shaped structure of Trypanosoma cruzi can be seen. It has a long tapering tail and a dark oval spot in the center. The Trypanosoma cruzi cell is about 5 micrometers long. Each blood cell has a diameter of about 4 micrometers.

Trypanosoma cruzi

Algae

Chloroplast-containing protists that conduct photosynthesis; dinoflagellates produce neurotoxins
A light micrograph of Chlamydomonas, an example of algae.
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A light micrograph of Chlamydomonas, an example of algae. Chlamydomonas has a spherical structure with numerous green bead shaped structures inside. Two long transparent thin structures extend out in different directions from one side. They are labeled as flagella. The algae’s body is about 10 micrometers in diameter. The flagella extend beyond the field of view, but are at least 40 micrometers long.

Chlamydomonas

Helminths

Multicellular worms, including nematodes (roundworms), cestodes (tapeworms), and trematodes (flukes); include parasites, as well as free-living members of soil ecosystems
A light micrograph of Taenia solium, an example of a helminth.
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A light micrograph of Taenia solium, an example of a helminth. The light micrograph is focused on the scolex of the tapeworm, Taenia solium. The tapeworm has a tubular shape. Closer to the tip of the tubular structure, there are four circular structures of suckers with indents in their centers, two on the left and two on the right. The tubular structure ends with a circular tip. This tip is filled with numerous layers of hook like fibers. The scolex is about 0.4 millimeters wide, and the circular tip is about 0.15 millimeters in diameter.

Taenia solium (scolex of tapeworm)

Arthropods

Multicellular insects and related microscopic organisms; include parasites and free-living members of soil ecosystems

A light micrograph of a body louse, an example of an arthropod.
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A light micrograph of a body louse, an example of an arthropod. The louse consists of a small oval like structure with legs. It has an ovoid head with two antennae structures pointing out from it, a thorax containing three pairs of legs, and a larger oval shaped and broadly segmented abdomen. It is about 2 millimeters in length.

Body louse

Major groups of microbial eukaryotes include the following:

  • Fungi grow in chains called hyphae or as single-celled yeasts. They have absorptive nutrition, digesting many complex plant polymers. Most are nonmotile. Microsporidians are spore-forming organisms closely related to fungi.
  • Amebas are single-celled protists of highly variable shape that form pseudopods, locomotor extensions of cytoplasm enclosed by the cell membrane.
  • Ciliates are single-celled protists with short, whiplike motility organelles called cilia. Most are free-living, but some are human parasites.
  • Apicomplexans are intracellular parasites with an apical complex for attachment and penetration of the host cell.
  • Hemoflagellates (such as trypanosomes) and metamonads are flagellated protists with complex parasitic life cycles involving developmental stages within hosts.
  • Algae are protists containing chloroplasts that conduct photosynthesis, related to those of plants. Some algae are single-celled, whereas others grow as filaments or sheets. Dinoflagellates are flagellated algae that possess chloroplasts obtained by their ancestral cells via endosymbiosis of a green alga.

These groups are considered “true” microbes, although they include large macroscopic forms, such as mushrooms (fungal fruiting bodies) and sheets of kelp (algae). Certain other multicellular parasites are not considered microbes even though they may be too small to see, such as mites and worms. These invertebrate animals have fully differentiated organ systems, sometimes comparable in complexity to those of vertebrates. Nevertheless, the dynamics of transmission and infection of invertebrate parasites parallel those of microbial pathogens. Therefore, invertebrate parasites are often covered by health professionals under the category of “eukaryotic microbiology.” In particular:

  • Helminths are multicellular worms, including the nematodes (roundworms), cestodes (tapeworms), and trematodes (flukes).
  • Arthropods include insects such as fleas and lice, as well as noninsects such as mites.

Worms and arthropods are a major source of morbidity and mortality worldwide. A quarter of the world’s population is parasitized by worms, including at least 10% of people in the United States. The presence of worms causes nutritional impairment and developmental delays. The horror of infection by worms and arthropods inspired the classic science fiction film Alien, which depicts an intelligent extraterrestrial parasite.

SECTION SUMMARY

  • Microbial eukaryotes may undergo asexual (vegetative) reproduction. Alternatively, they may undergo a sexual cycle involving meiosis and fertilization.
  • Fungi grow by absorptive nutrition, as single cells or by extending multicellular hyphae. Microsporidians are related to fungi.
  • Amebas are single-celled protists with pseudopod motility.
  • Ciliates are single-celled protists covered with hairlike microtubular organelles.
  • Hemoflagellates and metamonads are flagellated single-celled protists with complex parasitic cycles.
  • Apicomplexans are intracellular parasites with an apical complex.
  • Algae are protists that contain chloroplasts and conduct photosynthesis. Dinoflagellates are flagellated protozoa with chloroplasts.
  • Invertebrate parasites include helminths and arthropods.

Glossary

mitosis
The orderly replication and segregation of eukaryotic chromosomes, usually prior to cell division.
asexual reproduction
Also called vegetative reproduction. Reproduction of a cell by fission or mitosis to form identical daughter cells. Compare with sexual reproduction.
vegetative reproduction
Reproduction of a cell by fission or mitosis to form identical daughter cells. Compare with sexual reproduction.
sexual reproduction
Reproduction involving the joining of gametes generated by meiosis. Compare with asexual reproduction.
diploid
Describing an organism (or cell) that contains two copies of each chromosome in its cells. Compare with haploid.
haploid
Describing an organism (or cell) that contains one copy of each chromosome in its cells. Compare with diploid.
zygote
The diploid cell produced by fusion of a male gamete and a female gamete.
meiosis
A form of cell division by which a diploid eukaryotic cell generates haploid sex cells that contain recombinant chromosomes.
alternation of generations
Alternation of generations of haploid (n) asexual reproduction and gamete production with generations of fertilization and diploid (2n) asexual reproduction, followed by meiosis to form n cells.
fungus ( pl. fungi)
A heterotrophic eukaryote with chitinous cell walls; nutrition is by absorption. Fungi include Eumycota.
mycology
The study of fungi.
protozoan ( pl. protozoa)
A heterotrophic eukaryotic microbe, usually motile, that is not a fungus.
alga ( pl. algae)
A microbial eukaryote that contains chloroplasts and conducts photosynthesis.
protist
A single-celled eukaryotic microbe, usually heterotrophic and motile; not a fungus.
microsporidian
Any of a large phylum of obligate intracellular fungal parasites that produce spores.
pseudopod
A flexible locomotor extension of cytoplasm bounded by the cell membrane.
ciliate
A motile protozoan, typically identified by having many cilia on the cell surface and two different-sized nuclei.
apicomplexan
Any of the eukaryotic phylum Apicomplexa, which contains parasitic alveolates that possess an apical complex used for entry into a host cell.
hemoflagellate
A flagellated protist that lives in the bloodstream. Includes the genera Trypanosoma and Leishmania.
trypanosome
A parasitic protozoan that has a cortical skeleton of microtubules culminating in a long flagellum. Trypanosomes are the cause of diseases such as Chagas disease (Trypanosoma cruzi) and African trypanosomiasis (T. brucei).
metamonad
Any of a phylum of flagellated protozoa that have no mitochondria. Several metamonads, such as Giardia intestinalis, parasitize humans.
dinoflagellate
Any of a group of secondary endosymbiont algae that conduct photosynthesis and have two flagella, one of which is wrapped distinctively around the cell equator.
helminth
A multicellular worm that is a parasite.
arthropod
Any member of the phylum Arthropoda, characterized by an exoskeleton, segmented body, and appendages with joints. Many are parasites.
Fungi
A heterotrophic eukaryote with chitinous cell walls; nutrition is by absorption. Fungi include Eumycota.
Fungi
A heterotrophic eukaryote with chitinous cell walls; nutrition is by absorption. Fungi include Eumycota.
protozoa
A heterotrophic eukaryotic microbe, usually motile, that is not a fungus.
Algae
A microbial eukaryote that contains chloroplasts and conducts photosynthesis.
Algae
A microbial eukaryote that contains chloroplasts and conducts photosynthesis.
Amebas
A protist that moves via pseudopods.