Parasites of Arthropods and Helminths

 A number of disease-causing multicellular organisms are also studied using the same microscopic and immunological techniques that are used to study microorganisms and viruses. As a result, they are included here. Most of the medically important multicellular parasites fall into one of two groups: arthropods and helminths.  

The arthropods are more highly advanced on the evolutionary scale and include the insects, ticks, lice, and mites.

 Their main medical importance is that they serve as vectors that may transmit microorganisms and viruses to humans. The helminths, which include the nematodes (roundworms), cestodes (tapeworms), and the trematodes (flukes), are more primitive animals. In only a few instances do they transmit microbial infections to their host animal. Instead, they cause disease by invading the host’s tissues or robbing it of nutrients.

Most multicellular parasites have been well controlled in the industrialized nations, but they still cause death and misery to many millions in the economically underdeveloped areas of the world.

 Our need to know about these problems has come about because more people are traveling farther, more people are moving from one place to another, and more goods are being exchanged worldwide. A clear example of this occurred in New York City in the summer of 1999 when West Nile fever was contracted by a number of people.

 At least 61 persons suffered serious disease and seven people died. A significant number of crows died at the same time and were found to be carrying the disease. In addition to birds and people, horses, cats, and dogs were also found to carry the virus. It is not clear how the virus arrived in New York City, but it perhaps could have been carried by a traveler from Africa, West Asia, or the Middle East, where it is commonly found. It could possibly have been brought by an imported bird from the same areas. Worldwide travel makes us more vulnerable to diseases from other parts of the world

In addition, worldwide climatic conditions are changing and bringing increases in certain insect populations to areas that were previously free of them. As a result, more cases of multi- cellular parasitic infections are being seen by physicians in the United States than previously.

ascomycota characteristics,ascomycota life cycle,ascomycota classification,ascomycota reproduction

 Members of Ascomycota, or ascomycetes, commonly known as sac fungi, are named for their characteristic reproductive structure, the saclikeascus (pl., asci; Greek askos, sac). 

Ascomycetes are  ecologically  important  in  freshwater, marine,and terrestrial habitats because they degrade many chemically stable organic compounds, including lignin, cellulose, and col­lagen.

 Many species are quite familiar and economically im­portant. For example, most of the red, brown, and blue-green molds that cause food spoilage are ascomycetes.

 The powdery  mildews  that  attack  plant  leaves  and  the  fungi  that cause chestnut blight and Dutchelm disease are ascomycetes. Many yeasts as well as edible morels and truffles are also asco­ mycetes.

 The pink bread mold Neurospora crassa is an impor­tant research tool in genetics and biochemistry.  A  new ascomycete genus, Archaeorhizomyces, was described in 2011.

 The  group  is  globally  distributed and is  commonly  found in  as­sociation with the roots of pine trees but does not appear to be mycorrhizal, as it can be grown in pure culture. Like Neuros­ pora and Aspergillus, Archaeorhizomyces spp. are filamentous, producing slender hyphae.

Some ascomycetes are yeasts, while others have a life cycle that alternates between yeast and filamentous forms. The life cycle of the yeast Saccharomyces cerevisiae, commonly known ashas been invaluable for understanding the loss of cell cycle con­trol that occurs in cancerous cells.

Filamentous ascomycetes form septate hyphae. Asexual re­ production is common and is associated with the production  of conidia . 

Sexual reproduction involves ascus for­mation, with each ascus usually bearing eight haploid asco­ spores, although some species can produce over 1,000. Such hyphae are said to be ascogenous.

 Mating starts  when  two strains  of  opposite  mating  types  form  ascogenous  hyphae into which  pairs of  nuclei migrate.

  One nucleus of each pair originates from a "male" mycelium (antheridium) or cell and the other from a "female" organ or cell (ascogonium) that has fused with it. As the ascogenous hyphae grow, the paired nuclei divide so that there is one pair of nuclei in each cell.

After the ascogenous hyphae have matured, nuclear fusion occurs at the hyphal tips in the ascus mother cells. The diploid zygote nucleus then undergoes meiosis, and the resulting four haploid nuclei divide mitotically again to produce a row of eight brewer's or baker's yeast, is well understood . S. ceretfisiae alternates between haploid and diploid states. As long as nutrients remain plentiful, haploid and diploid cells undergo mitosis to produce haploid and diploid daughter cells, respec­ tively. 

Each daughter cell leaves a scar on the mother cell as it separates, and daughter cells bud only from unscarred regions of the cell wall. When a mother cell has no more unscarred cell wall re­ maining, it can no  longer  repro­ duce and will senesce (die).

 When nutrients are limited, diploid S. ceretfisiae cells undergo  meio- sis to produce four haploid cells that remain bound within a com­ mon cell wall, the ascus. Upon theaddition of nutrients, two haploid cells of opposite mating types  come into contact and fuse to cre­ ate a diploid. 

Typically only cells of opposite mating types can fuse; this process is tightly regulated by the action of pheromones.

S. ceretfisiae is a valuable model   organism.   Research  on this  organism  has  revealed the importance of many cellular pro­ cesses. For instance, it is a favorite model system for studying cell cycling and the events during mi­ tosis. 

This research is critical not only for our understanding of normal cell division, but it alsonuclei in each developing ascus. These nuclei are walled off from one another. Thousands of asci may be packed together in a cup- or flask-shaped fruiting body called an ascocarp.  

When the ascospores mature, they often are released from the asci with great force. If the mature ascocarp is jarred, it may appear to belch puffs of "smoke" consisting of thousands of ascospores. Upon reaching a suitable environment, the ascospores germi­ nate and start the cycle anew.

Several Aspergillus species are noteworthy. A. fumigatus is ubiquitous in the environment, commonly found in homes and the workplace.

 It is known to trigger allergic responses and is im­ plicated in the increased incidence in severe asthma and sinusitis. 

It is also pathogenic, infecting immunocompromised individuals with a mortality rate of nearly 50%. A. nidulans is a model organ­ ism used to study questions of eukaryotic cell and developmental biology. 

A. oryzae is used in the production of traditional fer­ mented foods and beverages in Japan, including saki and soy sauce. Because A. oryzae secretes many industrially useful pro­ teins and can be genetically manipulated, it has become an impor­tant organism in biotechnology.   

Many ascomycetes are parasites of higher plants. Claviceps purpurea parasitizes rye and other grasses, causing the plant dis­ ease ergot.

 Ergotism, the toxic condition in hu­mans and animals that eat grain infected with the fungus, is often accompanied by gangrene, psychotic delusions, nervous spasms, abortion, and convulsions.

 During the Middle Ages, ergotism, then known as St. Anthony's fire, killed thousands of people. For example, over 40,000 deaths from ergot poisoning were recorded in France in the year 943.

 It has been suggested that the widespread accusations of witchcraft in Salem Village and other New England communities in the 1690s may have resulted from outbreaks of ergotism. The pharmacological activities are due to an active ingredient, lysergic acid diethyl­ amide (LSD).

 In controlled dosages, other active compounds can be used to induce labor, lower blood pressure, and ease migraine headaches.

Although conidia are the major form of dissemination, some filamentous fungi also produce sclerotia. Sclerotia are compact masses of hyphae that can survive the winter.

 In the spring they germinate to produce more hyphae or conidia. These structures confer a competitive advantage to the fungi that produce them.

 For instance, some species of Aspergillus that infect plants form sclerotia to remain viable in the soil, where they can take advan­ tage of nutrient resources when the temperature rises.

 Most fungal pathogens that infect animals, including hu­ mans, are ascomycetes . Many are opportunistic pathogens such as those in the genera Candida, Blastomyces,

and Histoplasma. In addition, the cause of"sick building syndrome;'

Stachybotrys chartarum is also an ascomycete .

 Finally, the Aspergillus toxins known as aflatoxins are an important cause of  food  contamination.  Exposure to aflatoxins can result in liver cancer. 

 

Zygomycota,zygomycota reproduction cycle,zygomycota reproduction,zygomycota definition

Zygomycota contains fungi informally called zygomycetes.

Most live on decaying plant and animal matter in the soil; a few are parasites of plants, insects, and animals, including humans.

 The hyphae of zygomycetes are coenocytic, with many haploid nuclei. Asexual spores develop in sporangia at the tips of aerial hyphae and are usually wind dispersed. Sexual reproduction produces tough, thick-walled zygotes called zygospores that can remain dormant when the environment is too harsh for growth of the fungus.

The mold Rhizopus stolonifer is a common member of this division. This fungus grows on the surface of moist, carbohydrate-rich foods, such as breads, fruits, and vegetables. 

Hyphae called rhizoids extend into the bread and absorb nutri­ents. Other hyphae (stolons) become erect, then arch back into the substratum, forming new rhizoids.

Still others remain erect and produce at their tips asexual sporangia filled with black spores, giving the mold its characteristic color. Each spore, when liberated, can germinate to start a new mycelium.

Rhizopus spp.  usually  reproduce asexually,  but if  food be­comes scarce or environmental conditions unfavorable, sexual reproduction occurs . Sexual reproduction requires compatible strains of opposite mating types. When the two mat­ ing strains are close, each produces a different hormone, called a pheromone, that causes their hyphae to form projections called progametangia; these mature into gametangia.

 After fusion of the gametangia, the nuclei of the two gametes fuse, forming a zygote.

 The zygote devel­ops a thick, rough, black coat and be­ comes a dormant zygospore.

Meiosis often occurs at the time of germination; the zygospore then splits open and pro­duces a hypha that bears an asexual spo­rangium to begin the cycle again.

One member of the genus Rhizopus is important because it is involved in the rice disease known as seedling blight. 

If one considers that rice feeds more people on Earth than any other crop, the impli­ cations of this disease are obvious. it was thought that the fungus secreted a toxin that kills rice seedlings, so scientists set about isolating the toxin and the genes that produce it. 

Much to everyone's sur­prise, an a-proteobacterium, Burkholde­ ria sp. found growing within the fungus produces the toxin.

Zygomycetes also contribute to hu­man welfare. 

 For example, one species of Rhizopus is used in Indonesia to produce a food called tempeh from boiled, skinless soybeans. 

Another zygomycete (Mucor spp.) is used with soybeans in Asia to make a curd called sufu.

Others are em­ployed in the commercial preparation of some anesthetics, birth control agents, in- dustrial alcohols, meat tenderizers,  and the yellow coloring used in margarine and butter substitutes.

 

Lysosomes, lysosomes function,lysosomes function in animal cell,lysosomes structure

 Lysosomes are found in animal cells. 

They are roughly spherical, are enclosed in a single membrane, and average about 500 nm in diameter but range from 50 nm to several 11m in size. 

 They are involved in intracellular digestion and contain the enzymes needed to digest all types of macromolecules. 

These enzymes, called hydrolases, catalyze the hydrolysis of molecules and func­ tionbest under slightly acidic conditions (usually around pH 3.5to 5.0). 

 Lysosomes maintain an acidic environment by pumping protons into their interior.

 

Lysosome-like organelles are found in fungal and protist cells, where they are usually called vacuoles, phagocytic vacu­ oles, or food vacuoles.

They function in intracellular digestion, but many have other functions, including storage of calciumions, phosphate, and amino acids. These organelles are compo­nents of the endocytic pathways observed in protists and fungi, just as lysosomes are part of the endocytic pathways observed in animal cells.

Fungal Distribution and Importance ,Fungal Structure,Fungal Reproduction

   

  Unlike protists, fungi are primarily terrestrial organisms. They have a global distribution from polar to tropical regions. Fungi are saprophytes, securing nutrients from dead organic material by re­ leasing degradative enzymes into the environment.  

     This enables their absorption of the soluble products-a process sometimes called osmotrophy. Fungi are important decomposers. They degrade complex organic materials in the environment to simple organic compounds and inorganic molecules. In this way, carbon, nitrogen, phosphorus, and other critical constituents of dead organisms are released and made available for living organisms. Many fungi are pathogenic, with over 5,000 species known to attack economically valuable crops and many other plants.

 About20 new human fungal pathogens are documented each year. Conversely, fungi also form beneficial relationships with other or­ ganisms. For example, the vast majority of vascular plant roots form  important  associations  with  fungi  called mycorrhizae.

Mycorrhizae 

Fungi, especially yeasts (single-celled fungi), are essential to many industrial processes involving fermentation. Examples in­ clude the making of bread, wine, beer, cheeses, and soy sauce. They are also important in the commercial production of many organic acids (citric, gallic) and certain drugs (ergometrine, cor­ tisone), and in the manufacture of many antibiotics (penicillin, griseofulvin) and the immunosuppressive drug cyclosporine. 

 In addition, fungi are important research tools in the study of fun­damental biological processes. Cytologists, geneticists, bio­ chemists, biophysicists, and microbiologists regularly use fungi in their research. The yeast Saccharomyces cerevisiae  is  the best understood eukaryotic cell. It has been a valuable model organism in the study of cell biology, genetics, and cancer.

Fungal Structure

The body or vegetative structure of a fungus is called a thallus (pl., thalli). It varies in complexity and size. Single-cell microscopic fungi are referred to as yeasts, while multicellular masses are called molds.

 Fungi also include macroscopic puffballs and mushrooms. Like most bacteria, fungi possess cell walls; however,fungal cell walls are usually made of chitin. Chitin is a strong but flexible nitrogen containing polysaccharide consisting of N-acetylglucosamine residues. Instead of chitin, some fungal cell walls are composed of other polysaccharides such as man­ nans, galactosans, or cellulose.

A yeast is a unicellular fungus with a single nucleus that reproduces either asexually by budding and transverse division or sexually through spore formation. Each bud that separates can grow into a new cell, and some group together to form colo­ nies. 

 Generally yeast cells are larger than bacteria and are com­monly spherical to egg-shaped. They lack flagella and cilia but have most other eukaryotic organelles. The thallus of a mold consists of long, branched, threadlike filaments of cells called hyphae (s., hypha; Greek hyphe, web) that form a tangled mass called a mycelium (pl., mycelia). In some fungi, protoplasm streams through hyphae,uninterrupted by cross walls.

 These hyphae are called coenocytic or aseptate hy­phae. The hyphae of other fungi  have cross walls called septa {s., septum) with either a single pore  or multiple pores  that enable cyto­plasmic streaming. These hyphae are termed septate hyphae.

Hyphae are composed of an outer cell wall and an inner lumen, which contains the cytosol and organelles. A plasma membrane sur­ rounds the cytoplasm and lies next to the cell wall. The filamentous nature of hyphae results in a large surface area relative to the volume of cytoplasm. This makes adequate nutrient absorption possible.

Fungal Reproduction

Reproduction in fungi can be either asexual or sexual.

 Asexual reproduction is accomplished in several ways: 

               1) a parent cell undergoes mitosis and divides into two daughter cells by a cen­ tral constriction and formation of a new cell  wall    

               2) mitosis in vegetative cells may be concurrent with budding to produce a daughter cell. 

                This is very common in yeasts. The formation of asexual spores often accompanies asexual reproduction and is usually used as a means of dispersal. There are many types of asexual spores, each with its own name. Arthroconidia (arthrospores) are formed when hyphae frag­ ment through splitting of the cell wall or septum. Sporangiospores develop within a sac (sporangium; pl., sporangia) at a hyphal tip. CONIDIOSPORES are spores that are not enclosed in a sac but produced at the tips or sides of the hy­pha. BLASTOSPORES are  produced  from a  vegeta­tive mother cell budding.

Sexual reproduction in fungi involves the fusion of compat­ible nuclei. Homothallic fungal species are self-fertilizing and produce sexually compatible gametes on the same mycelium. Heterothallic species require outcrossing between different but sexually compatible mycelia. Depending on the species, sexualfusion may occur between haploid gametes, gamete-producing bodies called gametangia, or hyphae. Sometimes both the cyto­plasm and haploid nuclei fuse immediately to produce the dip­loid zygote,  as seen in higher  eukaryotes.  Usually,   however,there is a delay between cytoplasmic and  nuclear  fusion.  

  Thisproduces a dikaryotic STAGE in which cells contain two separate haploid  nuclei  (N  +  N), one  from  each  parent.After a period of dikaryotic existence, the two nuclei fuse and undergo meiosis to yield haploid spores. This is seen in both ascomycetes and basidiomycetes, so these are sometimes re­ ferred to as dikaryotic fungi.

Fungal spores, both asexual and sexual, are important for fusionHaploid stage (N)several reasons. They enable fungi to survive environmental stresses such as desiccation, nutrient limitation, and extreme temperatures, although they are not as stress resistant as bacterial endospores. 

They aid in fungal dissemination, which helps ex­ plain their wide distribution. Because spores are often small and light, they can remain suspended in air for long periods  and  are often spread by adhering to the bodies of insects and other animals.  

     The bright colors and fluffy textures of many molds often are due to their aerial hyphae and spores. Finally, the size, shape, color, and number of spores are useful in the identification of fungal species.

Eumycetozoa,slime molds,

First described in the 1880s, members of Eumycetozoa or "slime molds" have been classified as plants, animals, and fungi. 

 As we examine their morphology and behavior, the source of this con­ fusion should become apparent.

 Recent analysis of certain pro­ teins (e.g., elongation factor EF-1, a-tubulin, and actin) as well as physiological, behavioral, biochemical, and developmental data point to a monophyletic group.

 Eumycetozoa includes Myxogas­ tria and Dictyostelia. 

The acellular slime  mold  (Myxogastria) life cycle includes a distinctive stage when the organisms exist as streaming masses of colorful protoplasm. 

The protoplasm creeps along in amoeboid fashion over moist, rotting logs, leaves, and other organic matter, which it degrades .

 Acellular slime molds derive their name from the presence of a large, mul­ tinucleate mass called a plasmodium; there can be as many as 10,000 synchronously dividing nuclei within a single plasmo­ dium.

 Feeding is by endocytosis.

 When starved or dried, the plasmodium develops ornate fruiting bodies.

 As these mature, they form stalks with cellulose walls that are resistant to environmental stressors .

 When conditions im­ prove, spores germinate and release haploid amoeboflagellates. These fuse, and as the resulting zygotes feed, nuclear divisions give rise to the multinucleate plasmodium.

Cellular slime molds (Dictyostelia) are strictly amoeboid and use endocytosis to feed on bacteria and yeasts. 

Their com­ plex life cycle involves true multicellularity, despite their primi­ tive evolutionary status .

 The species Dictyostelium discoideum is an attractive model organism.

 During its life cy­ cle, a pseudoplasmodium is formed. 

This consists of many, many individual vegetative cells moving together as a mass. Thus it differs from the acellular slime mold's true plasmodium. 

The pseudoplasmodium forms when starved cells release cyclic AMP and a specific glycoprotein, which serve as molecular sig­ nals.

 Other cells sense these compounds and respond by forming an aggregate around the signal-producing cells. 

In this way, large, motile, multicellular slugs develop and serve asprecursors to fruiting body formation.

 Fruiting body morpho­ genesis commences when the slug stops and cells pile on top of each other.

 Cells at the bottom of this vertically oriented struc­ ture form a stalk by secreting cellulose, while cells at the tip differ­ entiate into spores .

 Germinated spores become vegetative amoebae to start this asexual cycle anew.

Dictyostelium spp. display complex behaviors. 

In addition to farming, described in the chapter opening story, they also differ­ entiate to resemble primitive immune cells.

 During slug forma­ tion, some cells become "sentinel cells" and vanquish harmful bacteria.

 Sentinel cells accomplish this by producing proteins that are similar to those involved in immune responses in higher or­ ganisms.

 These cells roam within the slug as if patrolling for pathogenic bacteria such as Legionella pneumophila, which are known to infect Dictyostelium spp.

 Sentinel cells have been found in several species related to Dictyostelium discoideum; immunolo­ gists are not too surprised, noting that all multicellular organisms need protection against bacterial pathogens.

Sexual reproduction in D. discoideum involves the formation of special spores call macrocysts.

 These arise by a form of conju­ gation that has some unusual features. 

First, a group of amoebae become enclosed within a wall of cellulose.Conjugation occurs between members of different mating types, of which there are three, as well as those capable of self-fertilization. 

Mating type (denoted as Type I, II, or III) is controlled by the nucleotide se­ quence of a single gene. Following conjugation, a large amoeba forms and cannibalizes the remaining amoebae. 

The now-giant amoeba matures into a macrocyst. Macrocysts can remain dor­ mant within their cellulose walls for extended periods. Vegetative growth resumes after the diploid nucleus undergoes meiosis to generate haploid amoebae.

 

Overview of Protists,Protist Morpholog,Encystment and Excystment,Protist Reproductive Cells and Structures,Protist Taxonomy

Most eukaryotes are microbes. 

 It is therefore not surprising that the vast diversity of protists is a function of their capacity to thrive in a wide variety of habitats.

 Their one common require­ ment is moisture because all are susceptible to desiccation.

 Most protists are free living and inhabit freshwater or marine environ­ ments.

 Many terrestrial chemoorganotrophic forms can be found in decaying organic matter and soil. 

Whether terrestrial or aquatic, protists play an important role in nutrient cycling.

Protozoa, or chemoorganoheterotrophic protists, may be saprophytes, securing nutrients from dead organic material by releasing degradative enzymes into the environment. 

 They then absorb the soluble products-a process sometimes called osmotrophy.

 Other protozoa employ holozoic nutrition, in which solid nutrients are acquired by phagocytosis.

 Photo­ autotrophic protists are strict aerobes and, like cyanobacteria, use photosystems I and II to perform oxygenic photosynthesis.

 It is difficult to classify the nutritional strategies of some protists be­ cause they simultaneously use both reduced organic molecules and C02 as carbon sources. 

This strategy is called mixotrophy.

Protist Morphology

Despite their diversity, protists share many common features. 

 In many respects, their morphology and physiology are the same as the cells of multicellular plants and animals.

 However, because many protists are unicellular, all of life's various functions must be performed within a single cell.

 Those that are multicellular lack highly differentiated tissues. Therefore the structural complexity observed in protists arises at the level of specialized organelles.

The protist cell membrane is called the plasmalemma and is identical to that of multicellular organisms.

 In some protists, the cytoplasm immediately under the plasmalemma is divided into an outer gelatinous region called the ectoplasm and an  inner  fluid  region,  the  endoplasm.  

The  ectoplasm  imparts rigidity to the cell body.

 Many protists also have a supportive mechanism called the pellicle.

 The pellicle consists of the plasma­ lemma and a relatively rigid layer just beneath it. The pellicle may

be simple in structure. 

For example, Euglena spp. are protists with a series of overlapping strips with a ridge at the edge of each strip fitting into a groove on the adjacent one, much like how the "tongue-and-groove" boards of a hardwood floor fit together.

 In contrast, the pellicles of ciliate protists are exceptionally complex with two membranes and a variety of associated structures. 

Al­ though pellicles are not as strong and rigid as cell walls, for those that possess them, pellicles impart the characteristic shape asso­ ciated with that particular species.

One or more vacuoles are usually present in the cytoplasm of protozoa.

 These are differentiated into contractile, secretory, and food or phagocytic vacuoles.

 Contractile vacuoles function as osmoregulatory organelles in those protists that live in hypotonic environments, such as freshwater lakes. 

Osmotic balance is maintained by continuous water expulsion.

 Phagocytic vacuoles are conspicuous in protists that ingest food by phagocytosis ( ho­ lozoic protists) and in parasitic species. Phagocytic vacuoles are the sites of food digestion.

 In some organisms, they may occur anywhere on the cell surface, while others have a specialized structure for phagocytosis called the cytostome (cell mouth). 

When digestion commences, the phagocytic vacuole is acidic, and as digestion proceeds, the vacuole membrane forms small blebs.

 These pinch off and carry nutrients throughout the cyto­plasm. 

The undigested contents of the original phagocytic vacu­ ole are expelled from the cell either at a random site on the cell membrane or at a designated position called the cytoproct.

Several energy-conserving organelles are observed in protists.

 Most aerobic chemoorganotrophic protists have mitochondria, while photosynthetic forms have chloroplasts.

 A dense protein­ aceous area, the pyrenoid, which is associated with the synthesis and storage of starch, may be present in chloroplasts. 

The majority of anaerobic chemoorganotrophic protists lack mitochondria; some of these organisms have hydrogenosomes.

Many protists feature cilia or flagella at some point in their life cycle. Their formation is associated with a basal bodylike or­ ganelle called the kinetosome. In addition to aiding in motility, these organelles may be used to generate water currents for feed­ ing and respiration.

Encystment and Excystment

Many protists are capable of encystment. During encystment, the organism becomes simpler in morphology and develops into a resting stage called a cyst. The cyst is a dormant form marked by the presence of a cell wall and very low metabolic activity. Cyst formation is particularly common among aquatic, free-living protists and parasitic forms. Cysts serve three major functions:

(1) they protect against adverse changes in the environment, such as nutrient deficiency, desiccation, adverse pH, and low levels of 02

 (2) they are sites for nuclear reorganization and cell division (reproductive cysts) and

 (3) they serve as a means of transfer between hosts in parasitic species (i.e., they are the infective stage).

 Protists escape from cysts by a process called excystment . Although the exact stimulus for excystment is un­ known for most protists, it is generally triggered by a return to favorable environmental conditions. For example, cysts of para­ sitic species excyst after ingestion by the host.

Most protists have both asexual and sexual reproductive phases in their life cycles. The most common method of asexual repro­ duction is binary fission. During this process, the nucleus first undergoes mitosis and then the cytoplasm divides by cytokinesis to form two identical individuals . Multiple fission is also common, as is budding. Some filamentous, photosynthetic protists undergo fragmentation so that each piece of the broken filament grows independently.

Sexual reproduction involves the formation of gametes. Protist cells that produce gametes are termed gamonts. The fusion of haploid gametes is called syngamy. Among protists, syngamy can involve the fusion of two morphologically simi­ lar gametes (isogamy) or two morphologically different types . Meiosis may occur before the formation and union of gametes, as in most animals, or just after fertilization, as is the case with lower plants. Furthermore, the exchange of nuclear material may occur in the familiar fashion-between two different individuals (conjugation)-or by the develop­ ment of a genetically distinct nucleus within a single individ­ ual (autogamy).

With this level of reproductive complexity, perhaps it is not surprising that the nuclei among protists show considerable di­ versity. Most commonly, a vesicular nucleus is present. This is 1 to  10  1-1m in diameter,  spherical,  and has a distinct nucleolus and uncondensed chromosomes . Ovular nuclei are up to 10 times this size and possess many peripheral nucleoli. Still others have chromosomal nuclei, in which the chromo­ somes remain condensed throughout the cell cycle. Finally, many ciliated forms have two types of nuclei: a large macronu­ cleus with distinct nucleoli and condensed chromatin, and a smaller, diploid micronucleus with dispersed chromatin but lacking nucleoli .

Protist Reproductive Cells and Structures

Most protists have both asexual and sexual reproductive phases in their life cycles. The most common method of asexual repro­ duction is binary fission. During this process, the nucleus first undergoes mitosis and then the cytoplasm divides by cytokinesis to form two identical individuals . Multiple fission is also common, as is budding. Some filamentous, photosynthetic protists undergo fragmentation so that each piece of the broken filament grows independently.

Protist Taxonomy

Ever since Antony van Leeuwenhoek described the first proto­ zoan "animalcule" in 1674, the taxonomic classification of the protists has remained in flux. 

 During the twentieth century, classification schemes were based on morphology rather than evolutionary relationships.

 Protists were often classified into four major groups based on their means of locomotion: flagel­ lates (Mastigophora), ciliates (Infusoria or Ciliophora), amoebae (Sarcodina), and stationary forms (Sporozoa).

 Although these terms may still be encountered, they are without evolutionary context and should be avoided.

 While it is now agreed that the old classification system is best abandoned, little agreement exists on what should take its place. Here we use the higher-level classification system for the eukaryotes based on morphologi­ cal, biochemical, and phylogenetic analyses proposed by the International Society of Protistologists in 2005. This scheme does not use formal hierarchical rank designations such as class and order, reflecting the fact that protist taxonomy remains an area of active research. 

The Classification of the Protists  as proposed by the International Society of Protistologists is presented in a table on the text website.