Protozoans Protozoans include a wide diversity of taxa that do not form a monophyletic group but all are unicellular eukaryotes. Protozoa lack a cell

ProtozoansProtozoans

Protozoans include a wide diversity of taxa that do Protozoans include a wide diversity of taxa that do not form a monophyletic group but all are not form a monophyletic group but all are unicellular eukaryotes.unicellular eukaryotes.

Protozoa lack a cell wall, have at least one motile Protozoa lack a cell wall, have at least one motile stage in their life cycle and most ingest their food.stage in their life cycle and most ingest their food.

Protozoan cell is much larger and more complex Protozoan cell is much larger and more complex than prokaryotic cell and contains a variety of than prokaryotic cell and contains a variety of organelles (e.g. Golgi apparatus, mitochondria, organelles (e.g. Golgi apparatus, mitochondria, ribosomes, etc).ribosomes, etc).


Eukaryotic cell was developed through endosymbiosis.Eukaryotic cell was developed through endosymbiosis.

In distant past aerobic bacteria appear to have been In distant past aerobic bacteria appear to have been engulfed by anaerobic bacteria, but not digested. engulfed by anaerobic bacteria, but not digested. Ultimately, the two developed a symbiotic relationship Ultimately, the two developed a symbiotic relationship with the engulfed aerobic bacteria becoming with the engulfed aerobic bacteria becoming mitochondria and eukaryotic cells developed. mitochondria and eukaryotic cells developed.

In a similar fashion, ancestors of chloroplasts formed In a similar fashion, ancestors of chloroplasts formed symbiotic union with other prokaryotes.symbiotic union with other prokaryotes.


Protozoans include both autotrophs and Protozoans include both autotrophs and heterotrophs. They include free-living and heterotrophs. They include free-living and parasitic forms.parasitic forms.

Reproduction can be asexual by fission or Reproduction can be asexual by fission or budding or sexual by conjugation or budding or sexual by conjugation or syngamy (fusion of gametes).syngamy (fusion of gametes).


The protozoa were once considered a The protozoa were once considered a single phylum, now at least 7 phyla are single phylum, now at least 7 phyla are recognized. recognized.

Were also once grouped with unicellular Were also once grouped with unicellular algae into the Protista, an even larger algae into the Protista, an even larger paraphyletic group.paraphyletic group.

Figure 11.01

Movement in ProtozoaMovement in Protozoa

Protozoa move mainly using cilia or Protozoa move mainly using cilia or flagella and by using pseudopodiaflagella and by using pseudopodia

Cilia also used for feeding in many small Cilia also used for feeding in many small metazoans.metazoans.

Cilia and flagellaCilia and flagella

No real morphological distinction between No real morphological distinction between the two structures, but cilia are usually the two structures, but cilia are usually shorter and more abundant and flagella shorter and more abundant and flagella fewer and longer.fewer and longer.

Each flagellum or cilium is composed of 9 Each flagellum or cilium is composed of 9 pairs of longitudinal microtubules arranged pairs of longitudinal microtubules arranged in a circle around a central pair.in a circle around a central pair.


The collection of tubules is referred to as The collection of tubules is referred to as the the axonemeaxoneme and it is covered with a and it is covered with a membrane continuous with the rest of the membrane continuous with the rest of the organism’s cell membrane.organism’s cell membrane.

Axoneme anchors where it inserts into the Axoneme anchors where it inserts into the main body of the cell with a basal body.main body of the cell with a basal body.

Figure 11.09aFigure 11.09a

Protein spoke

Dynein motor

Basal body


The outer microtubules are connected to The outer microtubules are connected to the central pair by protein spokes. the central pair by protein spokes.

Neighboring pairs of outer microtubules Neighboring pairs of outer microtubules (doublets) are connected to each other by (doublets) are connected to each other by an elastic protein.an elastic protein.

Figure 11.09aFigure 11.09a

Protein spoke

Dynein motor


Cilium is powered by dynein motors on the outer Cilium is powered by dynein motors on the outer doublets. As these motors crawl up the adjacent doublets. As these motors crawl up the adjacent doublet (movement is powered by ATP) they doublet (movement is powered by ATP) they cause the entire axoneme to bend.cause the entire axoneme to bend.

The dynein motors do not cause the doublets to The dynein motors do not cause the doublets to slide past each other because the doublets are slide past each other because the doublets are attached to each other by the elastic proteins attached to each other by the elastic proteins and the radial spokes and have little freedom of and the radial spokes and have little freedom of movement up and down. Instead the walking movement up and down. Instead the walking motion causes the doublets to bend. motion causes the doublets to bend.

Flagella, “intelligent design” and Flagella, “intelligent design” and irreducible complexityirreducible complexity

Oddly, the humble flagellum has been Oddly, the humble flagellum has been dragged into the evolution culture wars!dragged into the evolution culture wars!


The U.S. Supreme Court has prohibited The U.S. Supreme Court has prohibited the teaching of creationism in public the teaching of creationism in public schools as a violation of the schools as a violation of the “establishment of religion” clause of the “establishment of religion” clause of the constitution.constitution.

Latest attempt to insert creationism into Latest attempt to insert creationism into schools is the idea of “Intelligent Design.”schools is the idea of “Intelligent Design.”


The concept of “intelligent design” is outlined The concept of “intelligent design” is outlined most clearly in Michael Behe’s book “Darwin’s most clearly in Michael Behe’s book “Darwin’s Black Box.”Black Box.”

The central idea in “intelligent design” is that The central idea in “intelligent design” is that some structures in the body are so complex that some structures in the body are so complex that they could not possibly have evolved by a they could not possibly have evolved by a gradual process of natural selection. These gradual process of natural selection. These structures are said to “irreducibly complex.”structures are said to “irreducibly complex.”


By “irreducibly complex” Behe means that By “irreducibly complex” Behe means that a complex structure cannot be broken a complex structure cannot be broken down into components that are down into components that are themselves functional and that the themselves functional and that the structure must have come into existence in structure must have come into existence in its complete form.its complete form.


If structures are “irreducibly complex” If structures are “irreducibly complex” Behe claims that they cannot have Behe claims that they cannot have evolved. evolved.

Thus, their existence implies they must Thus, their existence implies they must have been created by a designer (i.e. God, have been created by a designer (i.e. God, although the designer is not explicitly although the designer is not explicitly referred to as such).referred to as such).


One of Behe’s main examples is flagella/cilia.One of Behe’s main examples is flagella/cilia.

Behe claims that because cilia are composed of Behe claims that because cilia are composed of at least half a dozen proteins, which combine to at least half a dozen proteins, which combine to perform one task, and that all of the proteins perform one task, and that all of the proteins must be present for a cilium to work and that must be present for a cilium to work and that cilia could not have evolved in a step-by step cilia could not have evolved in a step-by step process of gradual improvement.process of gradual improvement.


The flagellum is not, in fact, irreducibly complex. The flagellum is not, in fact, irreducibly complex.

For example, the flagellum in eel sperm lacks For example, the flagellum in eel sperm lacks several of the components found in other flagella several of the components found in other flagella (including the central pair of microtubules, radial (including the central pair of microtubules, radial spokes, and outer row of dynein motors), yet the spokes, and outer row of dynein motors), yet the flagellum functions well.flagellum functions well.


The whole “irreducible complexity” The whole “irreducible complexity” argument could in reality be recast as an argument could in reality be recast as an argument of “personal incredulity.” argument of “personal incredulity.”

““I personally cannot imagine a sequence I personally cannot imagine a sequence of steps by which this complex structure of steps by which this complex structure could have evolved. Therefore, it must could have evolved. Therefore, it must have been created.”have been created.”

Movement in Protozoa: Movement in Protozoa: PseudopodiaPseudopodia

Pseudopodia are chief means of Pseudopodia are chief means of locomotion of amoebas but are also locomotion of amoebas but are also formed by other protozoa and amoeboid formed by other protozoa and amoeboid cells of many invertebrates.cells of many invertebrates.

In amoeboid movement the organism In amoeboid movement the organism extends a pseudopodium in the direction it extends a pseudopodium in the direction it wishes to travel and then flows into it.wishes to travel and then flows into it.

PseudopodiaPseudopodia

Amoeboid movement involves endoplasm and Amoeboid movement involves endoplasm and ectoplasm. Endoplasm is more fluid than ectoplasm. Endoplasm is more fluid than ectoplasm which is gel-like.ectoplasm which is gel-like.

When a pseudopodium forms, an extension of When a pseudopodium forms, an extension of ectoplasm (the hyaline cap) appears and ectoplasm (the hyaline cap) appears and endoplasm flows into it and fountains to the endoplasm flows into it and fountains to the periphery where it becomes ectoplasm. Thus, a periphery where it becomes ectoplasm. Thus, a tube of ectoplasm forms that the endoplasm tube of ectoplasm forms that the endoplasm flows through. The pseudopodium anchors to flows through. The pseudopodium anchors to the substrate and the organism moves forward.the substrate and the organism moves forward.

Figure 11.10Figure 11.10

Feeding in amebasFeeding in amebas

Feeding in amoebas involves using Feeding in amoebas involves using pseudpodia to surround and engulf a pseudpodia to surround and engulf a particle in the process of particle in the process of phagocytosisphagocytosis..

The particle is surrounded and a food The particle is surrounded and a food vacuole forms into which digestive vacuole forms into which digestive enzymes are poured and the digested enzymes are poured and the digested remains are absorbed across the cell remains are absorbed across the cell membrane.membrane.

Phagocytosis

Reproduction in protozoaReproduction in protozoa

The commonest form of reproduction is The commonest form of reproduction is binary fissionbinary fission in which two essentially in which two essentially identical individuals result.identical individuals result.

In some ciliates In some ciliates buddingbudding occurs in which occurs in which a smaller progeny cell is budded off which a smaller progeny cell is budded off which later grows to adult size.later grows to adult size.

Binary fissionin various taxa

Sexual reproduction in protozoaSexual reproduction in protozoa

All protozoa reproduce asexually, but sex All protozoa reproduce asexually, but sex is widespread in the protozoa too.is widespread in the protozoa too.

In ciliates such as In ciliates such as Paramecium, Paramecium, a type of a type of sexual reproduction called conjugation sexual reproduction called conjugation takes place in which two Paramecia join takes place in which two Paramecia join together and exchange genetic materialtogether and exchange genetic material

Figure 11.28

Diseases caused by protozoaDiseases caused by protozoa

Many diseases are caused by protozaon Many diseases are caused by protozaon parasites parasites

These include:These include: Malaria (caused by a sporozaon)Malaria (caused by a sporozaon) Giardia, Sleeping sickness (caused by Giardia, Sleeping sickness (caused by

flagellates)flagellates) Amoebic dysentry (caused by amoebae)Amoebic dysentry (caused by amoebae)

Malaria Malaria

Malaria is one of the most important diseases in the Malaria is one of the most important diseases in the world.world.

About 500 million cases and an estimated 700,000 to About 500 million cases and an estimated 700,000 to 2.7 million deaths occur worldwide each year (CDC).2.7 million deaths occur worldwide each year (CDC).

Malaria was well known to the Ancient Greeks and Malaria was well known to the Ancient Greeks and Romans. The Romans thought the disease was caused Romans. The Romans thought the disease was caused by bad air (in Latin by bad air (in Latin mal-ariamal-aria) from swamps, which they ) from swamps, which they drained to prevent the disease.drained to prevent the disease.

Malaria symptomsMalaria symptoms

The severity of an infection may range from The severity of an infection may range from asymptomatic (no apparent sign of illness) to the asymptomatic (no apparent sign of illness) to the classic symptoms of malaria (fever, chills, classic symptoms of malaria (fever, chills, sweating, headaches, muscle pains), to severe sweating, headaches, muscle pains), to severe complications (cerebral malaria, anemia, kidney complications (cerebral malaria, anemia, kidney failure) that can result in death. failure) that can result in death.

Factors such as the species of Factors such as the species of PlasmodiumPlasmodium and and the victims genetic background and acquired the victims genetic background and acquired immunity affect the severity of symptoms.immunity affect the severity of symptoms.

MalariaMalaria

Despite humans long history with malaria Despite humans long history with malaria its cause, a sporozoan parasite called its cause, a sporozoan parasite called Plasmodium, Plasmodium, was not discovered until was not discovered until 1889 when Charles Louis Alphonse 1889 when Charles Louis Alphonse Laveran a French army physician Laveran a French army physician identified it, a discovery for which he won identified it, a discovery for which he won the Nobel Prize in 1907.the Nobel Prize in 1907.

MalariaMalaria

In 1897 an equally important discovery, In 1897 an equally important discovery, the mode of transmission of malaria, was the mode of transmission of malaria, was made by Ronald Ross.made by Ronald Ross.

His identification of the His identification of the AnophelesAnopheles mosquito as the transmitting agent earned mosquito as the transmitting agent earned him the 1902 Nobel Prize and a him the 1902 Nobel Prize and a knighthood in 1911.knighthood in 1911.

PlasmodiumPlasmodium

There are four species of There are four species of PlasmodiumPlasmodium: : P. P. falciparum, P. vivax, P.ovalefalciparum, P. vivax, P.ovale and and P. P. malariae.malariae.

P. falciparum P. falciparum causes severe often fatal causes severe often fatal malaria and is responsible for most malaria and is responsible for most deaths, with most victims being children.deaths, with most victims being children.

PlasmodiumPlasmodium

BothBoth Plasmodium vivax Plasmodium vivax and and P. ovaleP. ovale can go can go dormant, hiding out in the liver. The parasites dormant, hiding out in the liver. The parasites can reactivate and cause malaria months or can reactivate and cause malaria months or years after the initial infection. years after the initial infection.

P. malariaeP. malariae causes a long-lasting infection. If causes a long-lasting infection. If the infection is untreated it can persist the infection is untreated it can persist asymptomatically for the lifetime of the host.asymptomatically for the lifetime of the host.

Life cycle of malariaLife cycle of malaria

PlasmodiumPlasmodium has two hosts: mosquitoes has two hosts: mosquitoes and humans.and humans.

Sexual reproduction takes place in the Sexual reproduction takes place in the mosquito and the parasite is transmitted to mosquito and the parasite is transmitted to humans when the mosquito takes a blood humans when the mosquito takes a blood meal.meal.

Life cycle of malaria: humansLife cycle of malaria: humans

The mosquito injects The mosquito injects PlasmodiumPlasmodium into a human in the into a human in the form of form of sporozoitessporozoites. .

The sporozoites first invade liver cells and asexually The sporozoites first invade liver cells and asexually reproduce to produce huge numbers of reproduce to produce huge numbers of merozoitesmerozoites which spread to red blood cells where more merozoites which spread to red blood cells where more merozoites are produced through more asexual reproduction.are produced through more asexual reproduction.

Some parasites transform into sexually reproducing Some parasites transform into sexually reproducing gametocytesgametocytes and these if ingested by a mosquito and these if ingested by a mosquito continue the cycle.continue the cycle.

Plasmodium gametocyte

Life cycle of malaria: mosquitoesLife cycle of malaria: mosquitoes

Gametocytes ingested by a mosquito combine in Gametocytes ingested by a mosquito combine in the mosquito’s stomach to produce zygotes.the mosquito’s stomach to produce zygotes.

These zygotes develop into motile elongated These zygotes develop into motile elongated ookinitesookinites. .

The ookinites invade the mosquito’s midgut wall The ookinites invade the mosquito’s midgut wall where they ultimately produce sporozoites, where they ultimately produce sporozoites, which make their way to the salivary glands which make their way to the salivary glands where they can be injected into a new human where they can be injected into a new human host. host.

How How PlasmodiumPlasmodium enhances enhances transmission ratestransmission rates

The The PlasmodiumPlasmodium parasite engages in a parasite engages in a number of manipulative behaviors to number of manipulative behaviors to enhance its chances of being transmitted enhance its chances of being transmitted between hosts.between hosts.

Such manipulations are a common feature Such manipulations are a common feature of parasite behavior, in general, as we will of parasite behavior, in general, as we will see throughout the semester.see throughout the semester.


Mosquitoes risk death when feeding and Mosquitoes risk death when feeding and attempt to minimize risk and maximize attempt to minimize risk and maximize reward when doing so.reward when doing so.

To obtain blood a mosquito must insert its To obtain blood a mosquito must insert its proboscis through the skin and then locate proboscis through the skin and then locate a blood vessel. The longer this takes, the a blood vessel. The longer this takes, the greater the risk.greater the risk.


As soon as the mosquito hits a blood As soon as the mosquito hits a blood vessel the host’s body responds by vessel the host’s body responds by clotting the wound.clotting the wound.

Platelets clump around the proboscis and Platelets clump around the proboscis and release chemicals which cause the release chemicals which cause the platelets to clot together.platelets to clot together.


To slow clotting and speed feeding, mosquitoes To slow clotting and speed feeding, mosquitoes inject anticoagulants including one called inject anticoagulants including one called apyrase that unglues the platelets. They also apyrase that unglues the platelets. They also inject other chemicals that expand the blood inject other chemicals that expand the blood vessels.vessels.

PlasmodiumPlasmodium in the host helps the mosquito feed in the host helps the mosquito feed by releasing chemicals that also slow clotting. by releasing chemicals that also slow clotting. The parasite’s help increases the chances of the The parasite’s help increases the chances of the mosquito feeding successfully and sucking up mosquito feeding successfully and sucking up the parasite.the parasite.


Once in the mosquito Once in the mosquito PlasmodiumPlasmodium needs about needs about 10 days to produce sporozoites that are ready to 10 days to produce sporozoites that are ready to be injected into a human. be injected into a human.

During this time, to reduce the chances of the During this time, to reduce the chances of the mosquito dying, mosquito dying, PlasmodiumPlasmodium apparently apparently discourages its host from eating. Although how discourages its host from eating. Although how the parasite does this is not clear, mosquitoes the parasite does this is not clear, mosquitoes containing ookinites abandon feeding attempts containing ookinites abandon feeding attempts sooner than parasite-free mosquitoes. sooner than parasite-free mosquitoes.


Once sporozoites are in the salivary Once sporozoites are in the salivary glands, however, glands, however, PlasmodiumPlasmodium wants the wants the mosquito to bite and bite often.mosquito to bite and bite often.

In the salivary gland the parasite cuts off In the salivary gland the parasite cuts off the mosquito’s anticoagulant apyrase the mosquito’s anticoagulant apyrase supply. This makes it harder for the supply. This makes it harder for the mosquito to feed so it is hungrier and bites mosquito to feed so it is hungrier and bites more hosts.more hosts.


As a result, an infected mosquito is twice As a result, an infected mosquito is twice as likely to bite two people in a single night as likely to bite two people in a single night as an uninfected mosquito is. as an uninfected mosquito is.

As a result, the parasite is spread more As a result, the parasite is spread more widely.widely.

Behavior of Behavior of PlasmodiumPlasmodium in humans in humans

PlasmodiumPlasmodium enters the blood stream through a enters the blood stream through a mosquito bite.mosquito bite.

The parasite must avoid the host’s immune The parasite must avoid the host’s immune system. To do so while in the body it moves system. To do so while in the body it moves from one hiding place to another.from one hiding place to another.

The parasite moves first to the liver. Can get The parasite moves first to the liver. Can get there in about 30 minutes, which is usually fast there in about 30 minutes, which is usually fast enough to avoid triggering the immune system.enough to avoid triggering the immune system.


At the liverAt the liver Plasmodium Plasmodium enters a liver cell. enters a liver cell.

The cell responds by grabbing The cell responds by grabbing PlasmodiumPlasmodium proteins and displaying the proteins and displaying the antigens on its cell surface in a special cup antigens on its cell surface in a special cup the major histocompatibility complex or the major histocompatibility complex or MHC.MHC.


The immune system recognizes the The immune system recognizes the PlasmodiumPlasmodium antigens and mounts an antigens and mounts an immune response. immune response.

However, in a week before the immune However, in a week before the immune system has mounted its full response the system has mounted its full response the parasite has produced about 40,000 parasite has produced about 40,000 copies of itself and these burst out of the copies of itself and these burst out of the liver to seek red blood cells.liver to seek red blood cells.


The parasites leave the liver, reenter the The parasites leave the liver, reenter the bloodstream, and find a red blood cell to bloodstream, and find a red blood cell to enter. enter.

Each parasite spends two days in a red Each parasite spends two days in a red blood cell consuming the hemoglobin and blood cell consuming the hemoglobin and reproducing.reproducing.

Plasmodium in red blood cell

Red blood cellsRed blood cells

Red blood cells (strictly red blood Red blood cells (strictly red blood corpuscles) are a challenging environment corpuscles) are a challenging environment to live in. to live in.

They lack a nucleus and have little They lack a nucleus and have little metabolic activity. As a result, they have metabolic activity. As a result, they have few proteins for generating energy and few proteins for generating energy and also lack most of a normal cell’s channels also lack most of a normal cell’s channels for transporting fuel in and wastes out. for transporting fuel in and wastes out.


Red blood cells are specialized to Red blood cells are specialized to transport oxygen, which they carry by transport oxygen, which they carry by binding and wrapping in hemoglobin binding and wrapping in hemoglobin molecules. molecules.

A red blood cell is pumped around the A red blood cell is pumped around the body by the heart and travels about 300 body by the heart and travels about 300 miles over its lifetime.miles over its lifetime.


Red blood cells are squeezed through Red blood cells are squeezed through slender capillaries and compressed to one slender capillaries and compressed to one fifth of their normal diameter before fifth of their normal diameter before rebounding.rebounding.

To survive this squeezing, red blood cells To survive this squeezing, red blood cells have a network of proteins under their have a network of proteins under their membrane that can fold like a concertina membrane that can fold like a concertina and allow the cell to stretch and squeeze and allow the cell to stretch and squeeze as needed.as needed.


Old red blood cells eventually lose their Old red blood cells eventually lose their elasticity and become stiff.elasticity and become stiff.

Those that show signs of such aging are Those that show signs of such aging are filtered out as they pass through the filtered out as they pass through the spleen and destroyed.spleen and destroyed.


PlasmodiumPlasmodium cannot swim but uses hooks cannot swim but uses hooks to move along the blood vessels.to move along the blood vessels.

At the parasite’s tip are sensors that At the parasite’s tip are sensors that respond only to young red blood cells and respond only to young red blood cells and clasp on to proteins on the cell’s surface.clasp on to proteins on the cell’s surface.


The parasite uses a set of organelles The parasite uses a set of organelles concentrated at its apical end to gain concentrated at its apical end to gain entry. A suite of proteins are produced entry. A suite of proteins are produced that cause the red blood cell’s membrane that cause the red blood cell’s membrane to open and let the parasite squeeze in.to open and let the parasite squeeze in.

It takes only about 15 seconds for the It takes only about 15 seconds for the parasite to get in.parasite to get in.

Figure 11.30

Plasmodium Sporozoite


Inside in the red blood cell the Inside in the red blood cell the PlasmodiumPlasmodium consumes the hemoglobin. It consumes the hemoglobin. It takes in a small amount of hemoglobin, takes in a small amount of hemoglobin, slices it apart with enzymes and harvests slices it apart with enzymes and harvests the energy released. the energy released.

The toxic core of the hemoglobin molecule The toxic core of the hemoglobin molecule is processed into an inert molecule called is processed into an inert molecule called hemozoin.hemozoin.


In order to reproduce, In order to reproduce, PlasmodiumPlasmodium needs more needs more than hemoglobin. than hemoglobin.

It sets about modifying the red blood corpuscle It sets about modifying the red blood corpuscle so it can obtain amino acids and make proteins.so it can obtain amino acids and make proteins.

The parasite builds a series of tubes that The parasite builds a series of tubes that connect it to the surface of the cell and uses connect it to the surface of the cell and uses these to bring in materials from the blood steam these to bring in materials from the blood steam and to pump out wastes.and to pump out wastes.


The parasite also produces proteins that help to The parasite also produces proteins that help to maintain the red blood cell’s springiness for as maintain the red blood cell’s springiness for as long as possible so it is not eliminated by the long as possible so it is not eliminated by the spleen.spleen.

After a few hours, however, the red blood cell After a few hours, however, the red blood cell has been too modified by the parasite to fool the has been too modified by the parasite to fool the spleen. The parasite now produces sticky spleen. The parasite now produces sticky latchlatch proteinsproteins that glue the cell to blood vessel walls. that glue the cell to blood vessel walls.


Infected cells clump up in capillaries.Infected cells clump up in capillaries.

After another day the contents of the cell have After another day the contents of the cell have been used up. The cell ruptures and 16 new been used up. The cell ruptures and 16 new parasites burst out to infect other red blood cells.parasites burst out to infect other red blood cells.

Some of these parasites transform into sexually Some of these parasites transform into sexually reproducing gametocytes and, as mentioned reproducing gametocytes and, as mentioned previously, these if ingested by a mosquito will previously, these if ingested by a mosquito will continue the cycle.continue the cycle.


While in the red blood cells While in the red blood cells PlasmodiumPlasmodium is is invisible to the immune system because invisible to the immune system because the red blood cells have no MHC and the red blood cells have no MHC and cannot alert the immune system.cannot alert the immune system.

The latch proteins however do stimulate The latch proteins however do stimulate the immune system.the immune system.


The latch protein is made by a single The latch protein is made by a single gene, but gene, but PlasmodiumPlasmodium has over 100 such has over 100 such genes each of which produces a unique genes each of which produces a unique latch. latch.

In each generation some of the new In each generation some of the new parasites switch on a new latch gene and parasites switch on a new latch gene and so the immune system is always playing so the immune system is always playing catch up.catch up.

Effects of malaria on human Effects of malaria on human evolutionevolution

The intense selection pressure imposed The intense selection pressure imposed by malaria has resulted in a large number by malaria has resulted in a large number of mutations that provide protection of mutations that provide protection against the parasite being selected for in against the parasite being selected for in humans.humans.

The best known is sickle cell anemia.The best known is sickle cell anemia.

Anti-malaria mutations: Sickle cell Anti-malaria mutations: Sickle cell anemiaanemia

Sickle cell anemia is a condition commonSickle cell anemia is a condition commonin West Africans (and African Americans in West Africans (and African Americans of West African ancestry).of West African ancestry).

In sickle cell anemia red blood cells areIn sickle cell anemia red blood cells aresickle shaped as a result of a mutation sickle shaped as a result of a mutation which causes hemoglobin chains to stick which causes hemoglobin chains to stick together.together.

Anti-malaria mutations: Sickle cell Anti-malaria mutations: Sickle cell anemiaanemia

People with the sickle cell allele are protected People with the sickle cell allele are protected against against PlasmodiumPlasmodium because their hemoglobin because their hemoglobin under low oxygen conditions contracts into under low oxygen conditions contracts into needle-shaped clumps.needle-shaped clumps.

This contraction not only causes the sickling of This contraction not only causes the sickling of the cell, but harms the parasite. Parasites are the cell, but harms the parasite. Parasites are impaled on the clumps and the cell loses its impaled on the clumps and the cell loses its ability to pump potassium, which the parasite ability to pump potassium, which the parasite needs.needs.

Anti-malaria mutations: Sickle cell Anti-malaria mutations: Sickle cell alleleallele

People with two copies of the sickle cell People with two copies of the sickle cell allele usually die young, but heterozygotes allele usually die young, but heterozygotes are protected against malaria.are protected against malaria.

As a result the geographic distribution of As a result the geographic distribution of the allele and malaria in Africa match quite the allele and malaria in Africa match quite closely.closely.

Anti-malaria mutations: (G6PD) Anti-malaria mutations: (G6PD) deficiencydeficiency

Glucose-6-phosphate dehydrogenaseGlucose-6-phosphate dehydrogenase (G6PD)(G6PD) deficiency.deficiency. There are hundreds There are hundreds of alleles known and with more than 400 of alleles known and with more than 400 million people affected G6PD deficiency is million people affected G6PD deficiency is the commonest enzyme deficiency known.the commonest enzyme deficiency known.

Anti-malaria mutations: Anti-malaria mutations: ThalassemiaThalassemia

Geographic distribution suggests it Geographic distribution suggests it protects against malaria and protects against malaria and epidemiological evidence also supports epidemiological evidence also supports this. this.

People with G6PD-202A a reduced activity People with G6PD-202A a reduced activity variant common in Africa have a variant common in Africa have a significantly reduced risk of suffering significantly reduced risk of suffering severe malaria.severe malaria.

Anti-malaria mutations: Anti-malaria mutations: ThalassemiaThalassemia

Thalassemia: People with thalassemia Thalassemia: People with thalassemia make the ingredients of hemoglobin in the make the ingredients of hemoglobin in the wrong amounts.wrong amounts.

Too many or too few Too many or too few αα or ß or ß hemoglobin hemoglobin chains are produced and when they are chains are produced and when they are assembled into hemoglobin molecules assembled into hemoglobin molecules spare chains are left over.spare chains are left over.

Other anti-malaria mutations: Other anti-malaria mutations: ThalassemiaThalassemia

Extra chains clump together and cause major Extra chains clump together and cause major problems in the cell. These clumps grab oxygen, problems in the cell. These clumps grab oxygen, but don’t enclose it and the oxygen often but don’t enclose it and the oxygen often escapes and because it is strongly charged, the escapes and because it is strongly charged, the oxygen damages other molecules in the cell.oxygen damages other molecules in the cell.

Severe thalassemia is fatal, but mild forms Severe thalassemia is fatal, but mild forms protect against malaria because the loose protect against malaria because the loose oxygen severely damages the parasite and oxygen severely damages the parasite and renders it unable to invade new cells.renders it unable to invade new cells.

Anti-malaria mutations: Anti-malaria mutations: OvalocytosisOvalocytosis

OvalocytosisOvalocytosis: Occurs in South east Asia and : Occurs in South east Asia and has same genetic rules and consequences as has same genetic rules and consequences as sickle cell anemia.sickle cell anemia.

People with ovalocytosis have blood cell walls People with ovalocytosis have blood cell walls that are so rigid they can’t slip through that are so rigid they can’t slip through capillaries. The rigid cell walls make it hard for capillaries. The rigid cell walls make it hard for the parasite to enter the cell and the cell’s the parasite to enter the cell and the cell’s rigidity appears to prevent the parasite pumping rigidity appears to prevent the parasite pumping in phosphates and sulphates it needs to survive.in phosphates and sulphates it needs to survive.

Anti-malaria mutations:Anti-malaria mutations: One major advantage of these various anti-One major advantage of these various anti-

malarial mutations appears to be that they malarial mutations appears to be that they provide a natural vaccination program for provide a natural vaccination program for children.children.

By slowing the development of the parasite By slowing the development of the parasite these mutations give a child’s naïve immune these mutations give a child’s naïve immune system time to overcome system time to overcome Plasmodium’sPlasmodium’s attempts to elude the immune system and mount attempts to elude the immune system and mount an immune response. Mild cases of malaria an immune response. Mild cases of malaria thus immunize children to malaria and allow thus immunize children to malaria and allow them to survive to adulthood.them to survive to adulthood.

Mosquito nets save livesMosquito nets save lives

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Human African Trypanosomiasis Human African Trypanosomiasis (Sleeping sickness)(Sleeping sickness)

Sleeping sickness is a protozoan disease, which Sleeping sickness is a protozoan disease, which like malaria is spread by an insect vector, the like malaria is spread by an insect vector, the tsetse fly.tsetse fly.

The disease is endemic to sub-Saharan Africa The disease is endemic to sub-Saharan Africa and an estimated 300,000 people are infected and an estimated 300,000 people are infected annually with about 40,000 deaths.annually with about 40,000 deaths.

The disease organism is The disease organism is TrypanosomaTrypanosoma bruceibrucei..

Trypanosoma forms in blood smear from patient with African trypanosomiasis http://en.wikipedia.org/wiki/File:Trypanosoma_sp._PHIL_613_lores.jpg

Sleeping SicknessSleeping Sickness Symptoms:Symptoms:

Begins with fever, headaches, and joint pains. Begins with fever, headaches, and joint pains.

Lymph nodes may swell enormously and parasite numbers Lymph nodes may swell enormously and parasite numbers may be incredibly high. Greatly enlarged lymph nodes in may be incredibly high. Greatly enlarged lymph nodes in the back of the neck are tell-tale signs of the disease.the back of the neck are tell-tale signs of the disease.

If untreated the parasite may cross the blood-brain barrier, If untreated the parasite may cross the blood-brain barrier, which causes the characteristic symptoms the disease is which causes the characteristic symptoms the disease is named for. The patient becomes confused and the sleep named for. The patient becomes confused and the sleep cycle is disturbed with the patient alternating between cycle is disturbed with the patient alternating between manic periods and complete lethargy. Progressive mental manic periods and complete lethargy. Progressive mental deterioration is followed by coma and death.deterioration is followed by coma and death.

Sleeping SicknessSleeping Sickness

Trypanosome levels in infected patients show a Trypanosome levels in infected patients show a cycle of sharp peaks and valleys in parasite cycle of sharp peaks and valleys in parasite numbers of approximately a week in length.numbers of approximately a week in length.

The cycle occurs because the immune system The cycle occurs because the immune system recognizes the glycoprotein in the trypanosomes recognizes the glycoprotein in the trypanosomes coat and mounts an immune response to it, coat and mounts an immune response to it, which eliminates parasites with that glycoprotein.which eliminates parasites with that glycoprotein.


Trypanosomes, however, possess about 1,000 Trypanosomes, however, possess about 1,000 different coat-building genes and periodically a different coat-building genes and periodically a new one is turned on by a trypanosome that new one is turned on by a trypanosome that produces a different coat, which the immune produces a different coat, which the immune system doesn’t recognize.system doesn’t recognize.

Trypanosomes with this new coat reproduce Trypanosomes with this new coat reproduce undetected until the immune system can mount undetected until the immune system can mount a response to the new coat. a response to the new coat.


If the first generation of trypanosomes to infect a If the first generation of trypanosomes to infect a host turned on their coat genes at random the host turned on their coat genes at random the immune system could learn to recognize the immune system could learn to recognize the various possibilities quickly, remember them, various possibilities quickly, remember them, and eliminate the parasite.and eliminate the parasite.

Instead the coat-building genes are turned on in Instead the coat-building genes are turned on in pre-set sequence.pre-set sequence. This means that the immune This means that the immune system every week or so is faced with a new system every week or so is faced with a new coat that it has not seen before. coat that it has not seen before.


As a result of the As a result of the sequentialsequential coat-switching, the coat-switching, the immune system becomes chronically over-immune system becomes chronically over-stimulated and begins to attack the host’s body.stimulated and begins to attack the host’s body.

The overstimulation of the immune system and The overstimulation of the immune system and the movement of parasites into the central the movement of parasites into the central nervous, where they escape the immune system nervous, where they escape the immune system altogether, eventually kills the patient.altogether, eventually kills the patient.

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Protozoans Protozoans include a wide diversity of taxa that do not form a monophyletic group but all are unicellular eukaryotes. Protozoa lack a cell