Chapter 15: Parasitism and Mutualism 12/1/15 9:33 AM

I. Coevolution

The change of a biological object triggered by the change of a

related object

o Change in at least two species’ genetic compositions

reciprocally affect each other’s evolution

This is evident in the interactions between parasites and their hosts

Symbiosis – intimate association between two different organism

Very interconnected relationship

Parasite really needs the host in order to survive or reproduce

Can be positive, negative, or benign

Parasites – usually regarded in term of their detrimental effects; but they’re

actually energy budget engineers

Draw resources from hosts:

can see stunted growth, sterility, etc

Parasites generally:

Usually much smaller than the host

Highly specialized

Reproduces more quickly

A heavy load of parasites is an infection

The outcome of an infection is disease

Parasites can be categorized by size: micro and macro parasites

Microparasites

Usually associated with disease

Short generation time and multiply rapidly within hosts; usually

complete life cycle with one host

Macroparasites

Grow slower, longer generation time, larger

Roundworms, flatworms

Infections tend to be chronic and accumulate slowly

Hemiparasites

Photosynthetic but steal the water

Example: mistletoe

o Actually makes an excellent habitat for a lot of different

animals

Holoparasites

Steal water and sugars from the host

Ectoparasites

Live on the outside of the host

Live in scales, hair, skin, feathers for vertebrates

Live on the legs, upper and lower body part, etc for insects

Endoparasites

Live on the inside of the host

How parasites get into their hosts

Can enter through mouth, nasal passages, skin, rectum

They then travel to preferred habitat

Direct Transmission

Direct contact is the easiest to understand

When it moves from one host to the next host but it’s the same

species

When people spread the cold or the flu

Can sometimes be spread airborne, water, or the environment

Transmission can occur between hosts

Many external macroparasites are spread directly

o Lice (crazy jumpers)

o Ticks

o Fleas

o Mange mites

Some lay their eggs directly on the host

Some parasitic plants use direct transmission: spread through root

grafts, when the roots of one tree grow onto the roots of a

neighboring tree and attach

Intermediate Vector

Some parasites are transmitted between hosts by an intermediate

vector often an arthropod

1) Lyme disease: chronic fatigue hosts are vertebrates, most commonly

birds, mice, deer, and humans

Larvae that hatch (free of bacteria), after they have a blood meal

they morph to a nymph, then they look for a second host where the

ideal group are rats/squirrels, but if they land on a human they also

infect the human

o After second nymph cycle they turn into adult and land on a

deer (deer ticks), where they mate and produce offspring, and

make eggs again

o The eggs are always free of the disease

o The true host is the mouse that is being the reservoir of the

disease

2) Malaria: disease caused by protist parasites mosquitoes

vector is a female mosquito

mosquitoes transmit more than 50% of the approx. 102 arboviruses,

including yellow fever

more than 1 million are killed each year by malaria

3) Dutch Elm disease – in trees

devastated the elm tree (host) populations in North America

parasite is an ascomycete fungus; vascular wilt disease

insect vectors are elm bark beetles that carry spores from one tree

to another

Multiple Hosts and Stages of Transmission

Definitive host – the host in which the parasite reaches maturity

Intermediate – harbors the host during some developmental phase

Dynamics of a parasite is tied to the population dynamics,

movement, and interaction of various hosts

Why would a parasite evolve to use more than one host to compete

its life cycle? Why would you need two hosts to complete your cycle

o Reasons for more complicated life cycle: a lot of

metamorphosis occurs in parasites, and need different

requirements in the stages; so need different types of hosts to

meet those requirements; need to maximize their fitness

Example: Meningeal worm

Two hosts, deer and snail

At different stages of life cycle, lives in different parts of the deer

itself

Goes through to its infective stage, deer accidentally consumes it,

goes into its stomach

After going into stomach, the larvae leave the snail and go through

the abdomen of the deer, then go into the spinal cord and travel to

the brain, where they mate and produce eggs

Then move through bloodstream into the lungs, causing deer to

cough and be reuptake into the stomach, exiting with feces

Hosts Respond to Parasitic Invasions

Behavioral defenses may help a host to avoid infection

1) grooming

2) moving location

If parasite has infected host, defenses include

1) inflammatory response

2) internal cysts

Plant responses to parasites include

Cyst or scab formation in roots and fruits in response to bacterial

and fungal infections

Can sometimes provide them as food source to other animals like

birds

A second line of defense is the immune response

Antigen – a toxin or other foreign substance that induces an

immune response in the body

Antibodies are produced

Parasites can circumvent the immune system

If animal is protein deficient, really struggle to get rid of parasites

Example: Western Fence Lizards can be infected by malaria

- infected males have fewer courtship and territorial behaviors,

altered correlations, etc

Effects on Host Survival and Reproduction

Sometimes parasites can change behavior of host organism

Killifish with a parasitic tematode – flat worm infection display

abnormal behaviors such as surfacing and jerking

o Affects its brain

Frequency of conspicuous behavior was positively correlated to the

intensity of parasitism

Parasites may regulate host populations

The hosts needs to resist infection or eliminate or minimize trade-

offs

Virulence vs transmissibility

Virulence – how deadly

Transmissibility – how it can spread

Ebola = highly virulent, kills host so quickly it can kill itself out

Vertical transmission: the transmission of parasites from mother to

offspring just before or just after birth

Parasites with this mode of transmission are generally less virulent

Horizontal Transmission: contagious and passes on to other organisms

Parasites may be density-dependent regulators of host populations

Clumped distribution of parasites

For the most part, parasites are reserved in a few individuals until

the population becomes dense enough

Outbreaks occur when host density population is high

Parasitism is a symbiotic relationship

If the parasite remains in the host for an indefinite time, hosts

evolve defenses to reduce the parasite’s negative effect

Changes from parasitism to mutualism

Most probable examples of evolution from parasite to mutualist are

parasites with vertical transmission (mother to offspring)

Want to induce host to produce more offspring

Many mutualistic relationships are reciprocal exploitation rather than

cooperation

Benefits of the interaction depends on the environment

Trees and their mycorhizzal fungi: the trees benefit in nutrient-poor

soils but fungus appears to be more parasitic in nutrient rich soil

Mutualistic relationships

Obligate – cannot survive or reproduce without each other

Facultative – can survive without each other but do better with each other

Specialist – only one partner

Generalist – diversity of partners

Corals are symbiotic with algae

- live in small cups with the skeleton of some animals in the reef

Warm water temperatures can result in coral bleaching

Corals will expel the algae (zooxanthellae) living in their tissues

causing the corals to turn completely white; coral will die and

fungus will grow on it

Lichen – can survive in some of the most harsh environments on the planet

have a fungus and an algae; symbiotic relationship

Non-symbiotic Mutualisms

Two organisms do not physically coexist

Pollination of flowers and seed dispersers

Generally facultative relationships

Oftentimes seeds will have to pass through digestive system of

animal to be able to break dormancy and be dispersed

Nitrogen fixing bacteria

Infection as well

Plant receives nitrogen from bacteria

The bacteria receive carbon and other resources from the plant

Too much fertilizer in fields will inhibit this process and make the

soil less fertile

II. Mutualism

Cleaning each other

Example: eel and crocodile, oxpecker feeds exclusively on ticks of large

mammals

Mutualism with a middle man

Example:

Truffle (Fruiting body) and a vole will go along and eat it

Then in the fecal pellets of the vole it will carry spores, these will

then land around some rootlets and colonize the roots of the tree

with mycorrhizae

Helps tree receive more nutrients and grow better

Mutualism can influence population dynamics

Mathematical models

Equilibrium above carrying capacity of both populations if they

existed alone

Ecological Issues

How do land-use changes affect the abundance and dispersal of pathogens

and parasites?

The impact of forest clearing on the spread of Lyme disease has

been well-documented

The number of reported cases in North America has dramatically

increased

About 300,000 people contract the disease annually

White tailed deer are the primary host species for adult blacklegged ticks

Forest clearing and fragmentation have increased the population of

both deer and nice, resulting in an increase in the population of

ticks and the transmission rate of the Lyme disease bacterium

11/5/15 9:31 AM

11/5/15 9:31 AM