Chap 15 parasitism and mutualism

symbiosis

Intimate and long-term association between 2 or more organisms of different species

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- does not specify whether the result of association is +, - or benign

fate of individuals of one species

depends on their association with individuals of another

Parasitic relationship

one species (parasite) benefits and the other species (host) is harmed

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HOW CAN PARASITES INCREASE THEIR FITNESS?

- Parasites increase their fitness by exploiting host organisms for food, habitat, and dispersal.

How can the host fitness be decreased (parasitic relationship)

host may die from secondary infection or suffer reduced fitness as a result of stunted growth, emaciation, modification of behavior, or sterility.

parasites are generally

- smaller than their hosts

- highly specialized for their mode of life

- reproduce more quickly and in greater # than their hosts

parasites are found in

many groups

- can be categorized by size

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EXAMPLE:

- viruses

- bacteria

- protists

- fungi

- plants

- invertebrates

infection

heavy load of parasites

disease

outcome of an infection

Parasites can be categorized by size

- Microparasites

- Macroparasites

microparasites

characterized by small size and a short generation time (r-strategist)

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FEATURES:

- can develop and multiply rapidly within the host

- are associated with the term "disease"

- transmission from host to host is most often direct, or through a carrier

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EXAMPLES:

- viruses, bacteria, protists

macroparasites

characterized by a large size and a longer generation time (K-strategist)

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FEATURES:

- typically do not complete an entire life cycle in a single host organism

- can spread by direct transmission from host to host

- can spread through indirect transmission though a carrier or intermediate host

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EXAMPLES:

- flatworms, flukes, roundworms, lice, fleas, ticks, fungi

Holoparasites

nonphotosynthetic plants that function as heterotrophs

- lack chlorophyll and are nonphotosynthetic

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FUNCTION:

- they use the host's phloem and xylem to supply carbon, water, nutrients

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EXAMPLE:

- squawroot on the roots of oak

Hemiparasites

photosynthetic plants that obtain water and nutrients from host xylem

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FEATURES:

- they contain chlorophyll when mature

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EXAMPLE:

- mistletoe

hosts are the habitats of parasites

and the diverse arrays of parasites that have evolved exploit conceivable habitat on and within their hosts

ectoparasites

parasites that live on the host's skin within the protective cover of feathers and hair

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Ectoparasites of insects live on the legs, upper and lower body surfaces, the mouthparts

endoparasites

live within the host

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FEATURES:

- some burrow beneath the skin

- some live in the bloodstream

- some live in organs, or within lining tissues, such as the nasal tract

Parasites enter and exit pathways

- mouth

- nasal passages

- skin

- rectum

- urogenital system

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they can travel to their point of infection through the pulmonary, circulatory or digestive systems

host organism is like an island for parasites (ANALOGY)

Because the host serves as a habitat enabling their survival and reproduction, parasites must escape from one host and locate another, which is something that they cannot do at will

endo-macroparasites

can escape only during the larval stage of their development (infective stage) and must come in contact with the next host

process of transmission can occur

directly or indirectly

- can involve adaptations by parasites to virtually all aspects of feeding, social, and mating behaviors in host species

Direct transmission

a parasite is transferred from one host to another without the involvement of an intermediate organism

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POSSIBLE WAYS OF TRANSMISSION

- can occur by direct contact with a carrier

- parasite can be dispersed from one host to another through air, water, or other substrate

microparasites are transmitted directly

- influenza virus (airborne)

- smallpox virus (direct contact)

- variety of bacterial and viral parasites associated with sexually transmitted diseases (STD's)

macroparasites can be transmitted

through direct transmission from infected to uninfected hosts

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EXAMPLES: (internal parasite)

- roundworms live in the digestive tract of mammals

External macroparasites of birds and mammals are transmitted

by direct contact

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EXAMPLES INCLUDE:

- lice, teas, fleas, botfly larvae, mites that cause mange

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FEATURES OF THESE PARASITES:

- some lay their eggs directly on the host

- some many lay there eggs in the environemnt and after the eggs hatch they jump onto nearby hosts

Parasitic plants are transmitted

by direct transmission

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EXAMPLE

- holoparasites

- squawroot: parasitizes the roots of oaks

- beechdrops: parasitized mostly the roots of beech trees

Parasitic seed plants

seeds of these plants are dispersed locally; upon germination, their roots extend through the soil and attach to the roots of the host plant

fungal parasites of plants

spread through root grafts

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EXAMPLE:

- when the roots of one tree grow onto the roots of a neighboring tree and attach

indirect transmission

transmission between hosts caused by an intermediate vector

- often an arthropod

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EXAMPLE:

- Lyme disease

Lyme disease

major arthropod disease caused by a bacterial spirochete Borrelia Burgdorferi

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FEATURES:

- lives in the bloodstream of vertebrates, from birds to mice to deer and humans

- is transmited through the vector called "black-legged tick"

Malaria

disease caused by protist parasites

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FEATURES:

- different species infect specific host animals (EX: in humans, there are four species of protists parasites that cause malaria)

- vector is an anopheles mosquito through bite of a female

mosquitos

are known to transmite more than 50% of the about 102 arboviruses (a contraction of "arthropod-born viruses") that can produce disease in humans

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INCLUDES

- dengue

- yellow fever

insect vectors

involved in the transmission of parasites among plants

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EXAMPLE:

- dutch elm disease

Dutch elm disease

a fungal disease of elm trees that is spread by elm bark beetles

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HOW IS IT CARRIED?

- spores of fungi ulmi are carried and are spread from tree to tree

mistletoe

plant parasite that draws water and nutrients from their host plant

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VECTOR FOR MISTLETOE?

- birds are the vector, dispersing mistletoe seeds

Life cycle of an (parasite) organism

includes distinct stages

- juvenile (pre-reproductive)

- adult (reproductive. and postreproductive)

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Some species of parasites cannot complete their entire life cycle in a single host species.

definitive host

host species in which the parasite becomes an adult and reaches maturity

intermediate host

hosts during a stage in the parasite life cycle

each stage can develop only

if the parasite can be transmited to the appropriate intermediate host

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WHAT DOES THIS MEAN?

- dynamics of a parasite population are closely tied to the population dynamics, movement patterns, and interactions of the various host species

Meningeal worm

parasite that has a complex life cycle

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REQUIREMENTS:

- requires indirect transmission and multiple hosts

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HOSTS:

- definitive host = white-tailed deer

- intermediate host = snails and slugs in grass

movement of meningeal worm

1) moves into of digestive system into the nervous system (spine)

2) will move towards the brain where it will mature

3) will move into the respiratory system then digestive tract (will move through many tissue types

behavioral defenses

may help a host to avoid infection

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EXAMPLE:

- grooming

- location

grooming (behavioral defence)

birds and mammals rid themselves of ectoparasites

- can remove both adult and juvenile parasites.

Location (behavioral defence)

mammels can move to an area with fewer parasites

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EXAMPLE:

- deer use dense, shaded places to avoid deerfly vectors

Host defence on parasites

- Innate immune responce

- internal cysts

innate immune response

immune cells that respond to pathogen-associated molecular patterns on parasites

invertebrates (innate immune responce)

hemocytes aggregate around parasite, release cytotoxic molecules, and/or encapsulate parasite

vertebrates (innate immune response)

blood flow to the site increases, WBC and other cells to attack infection

- leads to the formation of a scab to reduce further entry

internal cysts

reaction can produce a hard cyst in the muscle or skin that encases the parasite

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EXAMPLE:

- pigs form cysts around roundworms that infect their muscles

Immune responce

second line of defence for fighting a parasitic infection

- formation of an antigen

antigen

a foreign object such as a virus or bacteria enters the body via bloodstream, it elicits an immune response

Lymphocytes

WBC producd by lymph glands that produce antibodies

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WHAT DO THESE ANTIBODIES DO?

- target specific chemicals on or released by the parasite that act as an antigen

- can affect the ability of the parasite to feed, spread, and reproduce, but do not have to kill it

immune system is extremely

specific, and it has a remarkable "memory"

- it can remember antigens it has encountered in the past and react more quickly and vigorously to them in subsequent exposure

how can the immune response can be breached

- some parasites vary their antigens more or less continuously

- parasites can keep one jump ahead of the host's response

antibodies are made mostly of

protein

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If the animal suffers from poor nutrition and its protein deficiency is severe, normal production of antibodies is inhibited

HIV (human immunodeficiency virus)

attacks immune system cells

- human host is susceptible to parasitic infections

Plant responses to parasites include

- cysts or scab formation in roots and fruits (formed in response to bacterial and fungal infections)

- insects attacks on leaves, stems, fruits, seeds by forming abnormal structures (galls) unique to the particular type of insect

isolates parasite

no contact with healthy tissue

formation of abnormal galls

may expose the insect larvae to predation

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EXAMPLE:

- Downy woodpeckers excavate goldenrod ball galls containing insect larvae

mechanisms used by hosts to defend against parasites require

resources which can in turn, affect physiology, growth, and reproduction of host

individuals in a community of snakes

vary in their infection of intracellular protists

- infected snakes had reduced immune function and increased dehydration

Western Fence Lizard

can be infected by malaria parasite

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FEATURES:

- infected males have fewer courtship and territorial behaviors, altered coloration, and smaller testes

- infected females store less fat during summer so have less energy for egg production in spring

parasitic infection can reduce reproductive success

Can impact males by impacting their ability to attract mates

(which lowers their fitness)

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HOW?

- coloration in secondary sexual characteristics may be affected resulting in female species choosing another mate

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EXAMPLE:

- color of zebra finch beak depends on the level of carotenoid pigments derived from diet

(only male finches with few parasites and disease have enough pigment to make a red beak)

Parasites infection altering behavior of the host

can increase the susceptibility to predation

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EXAMPLE:

- killifish (host) with a parasitic trematode infection display abnormal behaviors such as surfacing and jerking

what might be a significance of behaviors of a killifish

- fish eating birds are the definitive hosts for the trematode parasite that infects killifish

How can parasites and a host coexist?

- A host needs to resist invasion by eliminating the parasites or minimizing their effects

- a parasite needs to have an effect that allows it to survive but does not kill the host

natural selection does not favor

peaceful coexistence between host and parasites

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HOW DO THEY MAXIMIZE FITNESS:

- parasite should balance the trade-off between virulence and other componets of fitness such as transmissibility

virulence vs transmission success

- High virulence = parasites are often deadly

- low virulence = parasites may be benign and unable to transmit from host to host

vertical trasmission

the transmission of parasites from mother of offspring just before or just after birth

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PARASITES WITH THIS MODE:

- are less virulent than those passed directly though adults

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REQUIREMENTS:

- recipient host (offspring) must survive to reproduce to pass on the parasite

Parasites being density-dependent regulators

Density-dependent regulation of host populations typically occurs with directly transmitted endemic (native) parasites that are maintained in the population by a small reservoir of infected carrier individuals.

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DISEASE AFFECT:

- outbreaks of these diseases appear to occur when the host population density is high

- tend to reduce host populations sharply

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RESULT

- population of host and parasite would be similar as predator and prey cycles

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EXAMPLE:

distemper in raccoons and rabies in fox (both viral parasites)

parasites functioning as a selective agent of mortality

parasites would infect only a subset of the population

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EXAMPLE:

- distribution of macroparasites through indirect transmission will tend to have a high clumped distribution

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EFFECTS:

- some individuals in the host population will carry a higher load of parasites than others do

- are more likely to have a reduced reproduction and increased morality as a result of the parasite

how is parasitism a symbiotic relationship

- parasite befits at the expence of the host

- hosts will evolve defences to reduce the parasite's negative effect

commensalism (parasite example)

situation in which adaptations have countered negative impacts

- relationship between two species in which one species benefits without significantly affecting the other

mutualism (parasite example)

parasitic relationship in which there is an apparent benefit to the host organism

- the relationship becomes beneficial to both the host and parasite

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EXAMPLE:

- rats and tapeworm

rats and tapeworm (mutualism example)

rats infected with the intermediate stages of the tapeworm

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OVERALL SUMMARY

- Tapeworms will grow large because tapeworm larva produce an analogue of vertebrate growth hormone

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HOW DOES THIS BENEFIT THE RAT

- rat will develop thicker, and heavier shells that could be deemed an advantage

clear examples of evolution from parasites to mutualists involve

parasites that are transmitted vertically from mother to offspring

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WHAT DOES THIS MEAN?

- its a theory that produces that vertically transmitted parasites are selected to increase the host survival and reproduction

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WHY?

- maximization of host reproductive success benefits both the parasite and host.

Wolbachia bacteria (vertically transmitted parasites theory)

DEFINITION

- common group of bacteria that infect the reproductive tissues of arthropods

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FEATURES:

- they can infect the reproductive tissues of 40% of insects

- effects can bee seen in wasps (nasonia)

nasonia wasps (vertically transmitted parasites theory)

infection increases host fitness and that infected females produce more offspring than uninfected females

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similar increase in fitness has been seen in fruit flies

mutualist relationships

may be more reciprical exploitation than cooperation

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BENEFITS:

- interaction for one or both species may depend on the environment

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EXAMPLE:

- trees and their mycorrhizal fungi

Trees and their mycorrhizal fungi (Mutualistic relationship example)

the trees benefit in nutrient-poor soils (fungus is a mutualist) but incur a cost in nutrient-rich soils (fungus appears to be more parasitic)