Lecture 13 - Parasitism

Question 1

What is the introductory example of parasitism?

Answer 1

A horse hair worm gets into a cricket when it is small and consumes it from the inside out. For the worm to complete its life cycle it needs to get to the water and so manipulates the cricket to jump into the water. Once the worm exists the host the host dies.

Question 2

What is an example of a parasite interaction with clownfish?

Answer 2

A parasite swims into the mouth of the clownfish and bites its tongue, sucking it dry until it disappears. This parasite becomes the tongue and eats whatever the clownfish eats.

Question 3

What are symbionts?

Answer 3

Organisms that live in or on other organisms. (we are such hosts to many different organisms). Not all symbionts are parasites, some can be beneficial, some can have no effect

Question 4

What is a parasite?

Answer 4

An organism that consumes that tissues or body fluids of the organism on which it lives “(the host)

Question 5

What is a pathogen?

Answer 5

Parasites that cause diseases (an abnormal condition affecting the body of an organism)

Question 6

What is the range of harm a parasite can inflict on its host?

Answer 6

The fungus that causes athletes foot is relatively harmless, while Yersinia pestis is a bacterium that can cause the plague

Question 7

How are herbivores involved in the life history of parasites?

Answer 7

Herbivores are hosts for intermediate life stages of parasites

Question 8

What range of host individuals do parasites typically feed on?

Answer 8

Only one or a few host individuals. This includes herbivores such as aphids or nematodes that feed on one or a few host plants.

Question 9

What are parasitoids?

Answer 9

Insects whose larvae feed on a single host and almost always kill it

Question 10

What are macro- and micro parasites?

Answer 10

Large species such as arthropods and worms, or microscopic such as bacteria (can classify parasites on their size)

Question 11

How many parasites can attack a species?

Answer 11

Many different kinds of parasites can attack a species, and these parasites may even have their own parasites.

Question 12

Why are there so many species of parasites?

Answer 12

Because many parasites are closely adapted to particular host species

Question 13

What are ecto- and endoparasites?

Answer 13

Ectoparasites live on the outer body surface of the host, while endoparasites live inside their hosts within cells or tissues, or in the alimentary canal (digestive system). They do not feed on the host directly, but on the food intended for the host

Question 14

What are examples of ectoparasites?

Answer 14

Fungi. Mildews, rusts, and smuts grow on the surface and extend their hyphae (fungal filaments) into he plant to extract nutrients from its tissues. Plants are very susceptible to fungal infections.

Question 15

What are examples of animal ectoparasites?

Answer 15

Aphids, whiteflies, scale insects, nematodes, beetles and juvenile cicadas. They can be though of as both herbivores and parasites.

Question 16

What are examples of animals having many ectoparasites?

Answer 16

Athlete’s foot fungus, fleas, mites, lice and ticks. Some of these parasites also transmit disease organisms.

Question 17

Are disease organisms ecto or endoparasites?

Answer 17

Many disease organisms are endoparasites. The alimentary canal is an excellent habitat for many parasites. Most do not eat host tissue, but rob the host of nutrients. Tapeworms attach to the host’s intestinal wall and absorb digested food.

Question 18

Where do most endoparasites live?

Answer 18

In the host’s tissues/cells

Question 19

What is an example of plant’s having endoparasites?

Answer 19

Bacterial pathogens cause soft rot, fungi can rot various plant parts from the inside out.

Question 20

What is the difference in dispersal ability between ecto and endoparasites?

Answer 20

Ectoparasites can disperse more easily than endoparasites. Endoparasites have evolved various mechanisms for dispersal, including complex life cycles and enslaver parasites. Some parasites of the alimentary canal are dispersed in feces (diarrhea).

Question 21

Which of ecto and endoparasites are more exposed and vulnerable?

Answer 21

Ectoparasites are more exposed to predators, parasites, and parasitoids. For example, aphids are eaten by many birds and insects, and attacked by parasitoids and parasites.

Question 22

Even though endoparasites are less vulnerable, how can they still be attacked?

Answer 22

Endoparasites are protected from the external environment, and have easy access to food but can be attacked by the host’s immune system.

Question 23

What are selective pressures involved in host parasite interactions?

Answer 23

Hosts have adaptations for defending themselves against parasites, and parasites have adaptations for overcoming host defences. Strong selection pressure exerted by parasites on their host organisms and vice versa.

Question 24

What are examples of obvious host defence mechanisms?

Answer 24

Protective outer covering including skin and exoskeletons. Parasites that end up gaining entry are then killed by the hosts immune system.

Question 25

What unique defence is seen in vertebrate immune systems?

Answer 25

They have memory cells that can recognize micro parasites from previous exposures.

Question 26

What is a feature of other immune cells?

Answer 26

They engulf and destroy parasites or mark them with chemical that target them for later destruction.

Question 27

How can hosts regulate their biochemistry to deter parasites?

Answer 27

Vertebrate hosts have a protein called transferrin that removes iron from blood serum and stores it so bacterial and fungal endoparasites can’t get it. But some parasites can steal iron from transferrin.

Question 28

What are some examples of plant defence systems?

Answer 28

Resistance genes to specific parasitic genotypes, nonspecific immune responses such as antimicrobial and anti fungal compounds, chemicals that stimulate deposition of lignin, which makes a barrier to stop an invader’s spread, and chemical signals that warm nearby cells of imminent attack

Question 29

What is the function of secondary compounds released by plants?

Answer 29

Some animals eat specific plants to treat or prevent parasite infections. For example, a wooly bear caterpillar will switch from its usual food source to a poison hemlock when parasitic flies lay eggs on their bodies - the secondary compounds may kill the eggs

Question 30

What is another example of animals seeking out secondary compounds to help with parasite infection?

Answer 30

Chimpanzees infected with nematodes seek out and eat a bitter plant that contains chemicals that kill or paralyze the nematodes

Question 31

What happens when a parasite is too large to be encapsulated by a single host defence cell?

Answer 31

Some hosts can encapsulate endoparasites, or their eggs to make them harmless. Some insects have lamellocytes - blood cells that can form multicellular capsules around large objects such as nematodes

Question 32

How are parasites under selection pressure to develop counter defences?

Answer 32

Ectoparasites must penetrate external defences and toxic compounds produced by plants, endoparasites must cope with defences found inside the host

Question 33

What is an example of parasitoid wasps avoiding encapsulation? (look over slide 33 to 42)

Answer 33

By injecting virus-like particles that infect the lamellocytes and cause them to self-destruct. Other wasps lay eggs covered with filaments that become embedded in the host’s fat cells where they are not detected by circulating lamellocytes.

Question 34

How does parasite-host co-evolution occur?

Answer 34

When parasite and host each possess specific adaptations, it suggests that the strong selection pressure they impose on each other has caused both populations to co-evolve. The counter balance selection pressure allows them to evolve in synchrony.

Question 35

What is an example of co-evolution of parasites and hosts?

Answer 35

Rabbit population was introduced, was infected by virus and many members of the population died. But those that survived showed resistance to the virus. The virus evolved to become less lethal as a result. This happens partly due to the evolution of the virus itself, as well as the evolution of resistance by the rabbit.

Question 36

What is an example of host and parasite frequencies changing?

Answer 36

Trematode worm and its snail host

Question 37

What are some characteristics of this relationship?

Answer 37

Parasites infected snails from their home lake more effectively than snails from other lakes. Parasite genotypes in each lake had evolved rapidly enough to overcome the defences of the snail genotypes found in that lake.

Question 38

How do the snails evolve in response to the parasites?

Answer 38

Genotypes less common end up being more common in the following years.

Question 39

What type of snail genotype was more commonly affected by parasites?

Answer 39

Parasites infect snails with a common genotype more often than snails with a rare genotype. Because common genotypes are attacked by many parasites, driving down their numbers in a few years, the genotype frequencies vary year to year.

Question 40

What is the effect of these host parasite genotype frequencies?

Answer 40

These genotypes halt arms races. Common host genotypes typically decrease in frequency because they are attacked by many, which leads to an increase in previously rare genotypes. Species can persist without the requirement of constant co-evolutionary adjustments.

Question 41

Why do arm races stop because of genotype frequencies?

Answer 41

May stop because of trade-offs: a trait that improves host defences or parasite counter defences may reduce some other aspect of growth, survival or reproduction (if there are low numbers of parasites, a host may incur a cost if they are putting up defences)

Question 42

What is an example of a trade-off between host defences and their growth, etc.?

Answer 42

Wild flax and rust pathogen. Virulent rust genotypes produce fewer spores than other genotypes.

Question 43

What are some ecological effects of parasites?

Answer 43

By reducing survival or reproduction of their host, parasites can reduce the sizes of host populations and alter the outcomes of species interactions, thereby causing communities to change

Question 44

What is an example of ecological effects of a parasite?

Answer 44

Experiments with a beetle and a sexually transmitted mite showed a decrease in egg production by infected females. Hatching success is low in infected populations, which affects the reproductive success of these beetles

Question 45

How can parasites affect populations?

Answer 45

Parasites cause a reduction of population growth rates. Parasites can drive local host populations extinct and reduce their geographic ranges

Question 46

What is an example of parasites affecting host populations?

Answer 46

A fungal pathogen wiped out most chestnut populations and greatly reduced the geographic range of these species, which changed forest structure

Question 47

How do parasites impact host population cycles?

Answer 47

A parasitic trematode was known to decrease survival and reproductive success of red grouse populations, which tended to crash every 4 years.

Question 48

How was the red grouse population crash by trematodes tested?

Answer 48

When grouse populations were expected to crash, two populations were treated with a drug to kill the parasite. Control populations crashed as predicted, but the removal in the populations reduced fluctuations, reducing the population cycling. This implies that parasites are influential in population cycles.

Question 49

What is the widespread effect of parasites by their reduction of host performance and growth?

Answer 49

Parasites can change the outcome of species interactions, community composition, and even the physical environment

Question 50

What is an example of parasites affecting species interactions?

Answer 50

Two flour beetles and a protozoan parasite. When the parasite was absent, one beetle outcompeted the other driving it to extinction in most cases. When the parasite was present the opposite occurred, because the parasite had a large negative effect on the beetle that previously outcompeted the other, and no effect on the outcompeted beetle. The parasite was specific to one species so the outcome of competition switches depending on the presence of the parasite

Question 51

How do parasites alter the outcome of predator-prey interactions?

Answer 51

They decrease the physical condition of infected individuals. Predators may be less able to catch their prey, or prey less able to escape predation

Question 52

How else do parasites have effects on their host?

Answer 52

Parasites can change the behaviour of their host

Question 53

What are some examples of parasites affecting the behaviour of their hosts?

Answer 53

Protozoan makes rats less wary of cats, and some worm parasites cause amphipods to move from sheltered areas to areas of bright light, where they are more likely to be seen and eaten by fish or bird predators (enslaver parasite, controls the host and manipulates it to allow the parasite to reach its next life stage successfully)

Question 54

What is an ecosystem engineer?

Answer 54

This type of species can change the physical character of the environment, as when a beaver builds a dam.

Question 55

What is an example of an ecosystem engineer?

Answer 55

An amphipod is an ecosystem engineer in tidal mudflats. This makes burrows where it glues the sand together, preventing erosion. When the parasite drives this amphipod to extinction, erosion increases and the islands disappear