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Lecture 13

Lecture 13: "Parasitism"

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Biology 2483A
Hugh Henry

Ecology Lecture No. 13: Parasitism rd Tuesday October 23 , 2012 Introduction: -Symbionts are organisms that live in or on other organisms and comprise of more than half of Earth’s total species. Our own bodies can be a home to many other species. A parasite consumes the tissues or bodily fluids of the organism on which it lives (the host). Pathogens are parasites that cause disease (an abnormal condition affecting the body of an organism). Parasites typically harm, but don’t immediately kill, the organisms they eat (unlike predators). This degree of harm varies widely as witnessed between the fungus that causes athlete’s foot, and Yersinia pestis, the bacterium that causes the plague. Parasite Natural History: -Parasites typically feed on only one or a few host individuals, which include herbivores such as aphids or nematodes that feed on one or a few host plants. Parasitoids are insects whose larvae feed on a single host and almost always kill it. Macroparasites are large species such as arthropods and worms, while microparasites are microscopic parasitic species, such as bacteria. Most species are attacked by more than one kind of parasite (even parasites have parasites). Many parasites are closely adapted to particular host species. This specialization helps explain why there are so many species of parasites. Ectoparasites: -Ectoparasites live on the outer body surface of the host. Many fungi are ectoparasites and more than 5,000 species of fungi attack crop plants. Mildews, rusts, and smuts grow on the surface and extend their hyphae (fungal filaments) into the plant to extract nutrients from its tissues. Plants are also attacked by animal ectoparasites such as aphids, whiteflies, scale insects, nematodes, beetles, and juvenile cicadas. They can be thought of as both herbivores and parasites. -Animals also have many ectoparasites such as athlete’s foot fungus, fleas, mites, lice, and ticks. Some of these parasites also transmit disease organisms. Ectoparasites are more exposed to predators, parasites, and parasitoids. For example, aphids are eaten by many birds and insects, and attacked by other parasites and parasitoids. Endoparasites: -Endoparasites live inside their hosts, within cells or tissues, or in the alimentary canal. Many disease organisms are endoparasites. The alimentary canal is excellent habitat for many parasites. Most do not eat host tissue, but rob the host of nutrients. Tapeworms will attach to the host’s intestinal wall and absorb digested food. Many endoparasites live in the host’s tissues/cells such as Yersinia pestis (the bacterium that causes the plague) and Mycobacterium tuberculosis (the bacterium that causes tuberculosis). -Plants also have endoparasites. Bacterial pathogens as well as fungi can rot various plant parts from the inside out. Some bacteria invade vascular tissues, disrupting the flow of water and nutrients, causing wilting and often death. Endoparasites have evolved various mechanisms for dispersal, including complex life cycles, enslaver parasites and dispersal in the feces. Endoparasites are protected from the external environment, and have easy access to food. But they can also be attacked by the host’s immune system. Defenses & Counter-Defenses: -Hosts have adaptations for defending themselves against parasites, and parasites have adaptations for overcoming host defenses. Adaptations of host organisms include protective outer coverings (skin and exoskeletons) and immune systems which kill any entering parasites. Vertebrate immune systems have “memory cells” that can recognize microparasites from previous exposures. Other immune system cells engulf and destroy parasites or mark them with chemicals that target them for later destruction. Hosts can regulate biochemistry to deter parasites. For example, vertebrate hosts have a protein called transferrin that removes iron from blood serum and keeps it from bacterial and fungal endoparasitic possession. -Plants have many chemical weapons called secondary compounds. Some animals eat specific plants to treat or prevent parasite infections. For example, woolly bear caterpillars switch from their usual food plants to poison hemlock when parasitic flies lay eggs on their bodies. Chimpanzees infected with nematodes also seek out and eat a bitter plant that contains chemicals that kill or paralyze the nematodes (Huffman 1997). Some hosts can encapsulate
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