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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 wall, have at least one motile stage in their life cycle and most ingest their food.
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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 wall, have at least one motile stage in their life cycle and most ingest their food. • Protozoan cell is much larger and more complex than prokaryotic cell and contains a variety of organelles (e.g. Golgi apparatus, mitochondria, ribosomes, etc).
Protozoans • Eukaryotic cell was developed through endosymbiosis. • In distant past aerobic bacteria appear to have been engulfed by anaerobic bacteria, but not digested. Ultimately, the two developed a symbiotic relationship with the engulfed aerobic bacteria becoming mitochondria and eukaryotic cells developed. • In a similar fashion, ancestors of chloroplasts formed symbiotic union with other prokaryotes.
Protozoans • Protozoans include both autotrophs and heterotrophs. They include free-living and parasitic forms. • Reproduction can be asexual by fission or budding or sexual by conjugation or syngamy (fusion of gametes).
Protozoans • The protozoa were once considered a single phylum, now at least 7 phyla are recognized. • Were also once grouped with unicellular algae into the Protista, an even larger paraphyletic group.
Movement in Protozoa • Protozoa move mainly using cilia or flagella and by using pseudopodia • Cilia also used for feeding in many small metazoans.
Cilia and flagella • No real morphological distinction between the two structures, but cilia are usually shorter and more abundant and flagella fewer and longer. • Each flagellum or cilium is composed of 9 pairs of longitudinal microtubules arranged in a circle around a central pair.
Cilia and flagella • The collection of tubules is referred to as the axoneme and it is covered with a membrane continuous with the rest of the organism’s cell membrane. • Axoneme anchors where it inserts into the main body of the cell with a basal body.
Figure 11.09a Protein spoke Dynein motor Basal body
Cilia and flagella • The outer microtubules are connected to the central pair by protein spokes. • Neighboring pairs of outer microtubules (doublets) are connected to each other by an elastic protein.
Figure 11.09a Protein spoke Dynein motor
Cilia and flagella • Cilium is powered by dynein motors on the outer doublets. As these motors crawl up the adjacent doublet (movement is powered by ATP) they cause the entire axoneme to bend. • The dynein motors do not cause the doublets to slide past each other because the doublets are attached to each other by the elastic proteins and the radial spokes and have little freedom of movement up and down. Instead the walking motion causes the doublets to bend.
Flagella, “intelligent design” and irreducible complexity • Oddly, the humble flagellum has been dragged into the evolution culture wars!
Flagella, “intelligent design” and irreducible complexity • The U.S. Supreme Court has prohibited the teaching of creationism in public schools as a violation of the “establishment of religion” clause of the constitution. • Latest attempt to insert creationism into schools is the idea of “Intelligent Design.”
Flagella, “intelligent design” and irreducible complexity • The concept of “intelligent design” is outlined most clearly in Michael Behe’s book “Darwin’s Black Box.” • The central idea in “intelligent design” is that some structures in the body are so complex that they could not possibly have evolved by a gradual process of natural selection. These structures are said to “irreducibly complex.”
Flagella, “intelligent design” and irreducible complexity • By “irreducibly complex” Behe means that a complex structure cannot be broken down into components that are themselves functional and that the structure must have come into existence in its complete form.
Flagella, “intelligent design” and irreducible complexity • If structures are “irreducibly complex” Behe claims that they cannot have evolved. • Thus, their existence implies they must have been created by a designer (i.e. God, although the designer is not explicitly referred to as such).
Flagella, “intelligent design” and irreducible complexity • One of Behe’s main examples is flagella/cilia. • Behe claims that because cilia are composed of at least half a dozen proteins, which combine to perform one task, and that all of the proteins must be present for a cilium to work and that cilia could not have evolved in a step-by step process of gradual improvement.
Flagella, “intelligent design” and irreducible complexity • The flagellum is not, in fact, irreducibly complex. • For example, the flagellum in eel sperm lacks several of the components found in other flagella (including the central pair of microtubules, radial spokes, and outer row of dynein motors), yet the flagellum functions well.
Flagella, “intelligent design” and irreducible complexity • The whole “irreducible complexity” argument could in reality be recast as an argument of “personal incredulity.” • “I personally cannot imagine a sequence of steps by which this complex structure could have evolved. Therefore, it must have been created.”
Movement in Protozoa: Pseudopodia • Pseudopodia are chief means of locomotion of amoebas but are also formed by other protozoa and amoeboid cells of many invertebrates. • In amoeboid movement the organism extends a pseudopodium in the direction it wishes to travel and then flows into it.
Pseudopodia • Amoeboid movement involves endoplasm and ectoplasm. Endoplasm is more fluid than ectoplasm which is gel-like. • When a pseudopodium forms, an extension of ectoplasm (the hyaline cap) appears and endoplasm flows into it and fountains to the periphery where it becomes ectoplasm. Thus, a tube of ectoplasm forms that the endoplasm flows through. The pseudopodium anchors to the substrate and the organism moves forward.
Feeding in amebas • Feeding in amoebas involves using pseudpodia to surround and engulf a particle in the process of phagocytosis. • The particle is surrounded and a food vacuole forms into which digestive enzymes are poured and the digested remains are absorbed across the cell membrane.
Reproduction in protozoa • The commonest form of reproduction is binary fission in which two essentially identical individuals result. • In some ciliates budding occurs in which a smaller progeny cell is budded off which later grows to adult size.
Binary fission in various taxa
Sexual reproduction in protozoa • All protozoa reproduce asexually, but sex is widespread in the protozoa too. • In ciliates such as Paramecium, a type of sexual reproduction called conjugation takes place in which two Paramecia join together and exchange genetic material
Diseases caused by protozoa • Many diseases are caused by protozaon parasites • These include: • Malaria (caused by a sporozaon) • Giardia, Sleeping sickness (caused by flagellates) • Amoebic dysentry (caused by amoebae)
Malaria • Malaria is one of the most important diseases in the world. • About 500 million cases and an estimated 700,000 to 2.7 million deaths occur worldwide each year (CDC). • Malaria was well known to the Ancient Greeks and Romans. The Romans thought the disease was caused by bad air (in Latin mal-aria) from swamps, which they drained to prevent the disease.
Malaria symptoms • The severity of an infection may range from asymptomatic (no apparent sign of illness) to the classic symptoms of malaria (fever, chills, sweating, headaches, muscle pains), to severe complications (cerebral malaria, anemia, kidney failure) that can result in death. • Factors such as the species of Plasmodium and the victims genetic background and acquired immunity affect the severity of symptoms.
Malaria • Despite humans long history with malaria its cause, a sporozoan parasite called Plasmodium, was not discovered until 1889 when Charles Louis Alphonse Laveran a French army physician identified it, a discovery for which he won the Nobel Prize in 1907.
Malaria • In 1897 an equally important discovery, the mode of transmission of malaria, was made by Ronald Ross. • His identification of the Anopheles mosquito as the transmitting agent earned him the 1902 Nobel Prize and a knighthood in 1911.
Plasmodium • There are four species of Plasmodium: P. falciparum, P. vivax, P.ovale and P. malariae. • P. falciparum causes severe often fatal malaria and is responsible for most deaths, with most victims being children.
Plasmodium • Both Plasmodium vivax and P. ovale can go dormant, hiding out in the liver. The parasites can reactivate and cause malaria months or years after the initial infection. • P. malariae causes a long-lasting infection. If the infection is untreated it can persist asymptomatically for the lifetime of the host.
Life cycle of malaria • Plasmodium has two hosts: mosquitoes and humans. • Sexual reproduction takes place in the mosquito and the parasite is transmitted to humans when the mosquito takes a blood meal.
Life cycle of malaria: humans • The mosquito injects Plasmodium into a human in the form of sporozoites. • The sporozoites first invade liver cells and asexually reproduce to produce huge numbers of merozoites which spread to red blood cells where more merozoites are produced through more asexual reproduction. • Some parasites transform into sexually reproducing gametocytes and these if ingested by a mosquito continue the cycle.
Life cycle of malaria: mosquitoes • Gametocytes ingested by a mosquito combine in the mosquito’s stomach to produce zygotes. • These zygotes develop into motile elongated ookinites. • The ookinites invade the mosquito’s midgut wall where they ultimately produce sporozoites, which make their way to the salivary glands where they can be injected into a new human host.
How Plasmodium enhances transmission rates • The Plasmodium parasite engages in a number of manipulative behaviors to enhance its chances of being transmitted between hosts. • Such manipulations are a common feature of parasite behavior, in general, as we will see throughout the semester.
How Plasmodium enhances transmission rates • Mosquitoes risk death when feeding and attempt to minimize risk and maximize reward when doing so. • To obtain blood a mosquito must insert its proboscis through the skin and then locate a blood vessel. The longer this takes, the greater the risk.
How Plasmodium enhances transmission rates • As soon as the mosquito hits a blood vessel the host’s body responds by clotting the wound. • Platelets clump around the proboscis and release chemicals which cause the platelets to clot together.
How Plasmodium enhances transmission rates • To slow clotting and speed feeding, mosquitoes inject anticoagulants including one called apyrase that unglues the platelets. They also inject other chemicals that expand the blood vessels. • Plasmodium in the host helps the mosquito feed by releasing chemicals that also slow clotting. The parasite’s help increases the chances of the mosquito feeding successfully and sucking up the parasite.
How Plasmodium enhances transmission rates • Once in the mosquito Plasmodium needs about 10 days to produce sporozoites that are ready to be injected into a human. • During this time, to reduce the chances of the mosquito dying, Plasmodium apparently discourages its host from eating. Although how the parasite does this is not clear, mosquitoes containing ookinites abandon feeding attempts sooner than parasite-free mosquitoes.
