page · 2006-04-12 · page ‹#› mutualism in one environment may be parasitism in a different...

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End of mutualism lecture…

Cheaters in mutualisms highlight theclose evolutionary relationshipbetween mutualism and parasitism

For example lycaenid butterflymutualists may evolve into parasites

But also, mutualism in one environmentmay be parasitism in another

B. Mutualism in one environment may beparasitism in a different environment

An example with a yucca moth relative, Greyaand a wildflower in the Northwest,Lithophragma


Greya, like yucca moths, lay eggs in someovules and pollinate others


Unlike yucca, however, Lithophragma may bepollinated by other pollinators, such as bee-flies

Bee-flies areefficient pollinatorsand do not layeggs in any flowers

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Mutualism in one environment may beparasitism in a different environmentLithophragma occurs in several mountain

valleys and at different elevations in OregonAt high elevation sites, Greya is the only

pollinator, and here it serves as a mutualist -the plant benefits by its presence

At lower elevation sites, the beeflies arepresent and the plant sets more seed whenGreya is excluded.

Here Greya isa parasite,reducingplantfitness.

So whether the interaction between Greya andLithophragma should be termed a mutualismor parasitism of the plant by Greya dependson the presence of a third species!

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IV. Mutualism/parasitism - closely relatedinteractionsMutualism may evolve into parasitismParasitism may evolve into mutualism

And some relationships may be difficult to classify

IV. Mutualism/parasitism - closely relatedinteractions

C. A story of parasitism leading to mutualdependency

1. Parasitic wasps may be infected with abacterium called Wolbachia

The bacteria are transmitted only in theeggs

Infected egg Uninfected egg

1. Parasitic wasps may be infected with abacteria called Wolbachia

It is not in the evolutionary interests of thebacteria to end up in a male wasp. Whynot?

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The bacteria ‘do not want’ to end up in amale.

So.… they make all male eggs female!

How do they do that?

They manipulate the haplodiploid sexdetermination of their wasp hosts

Haplodiploidy

nn

n

2n

Female Male

How does it do that?

So in haplodiploid systemsFemales develop from fertilized, diploid eggsMales develop from haploid, unfertilized eggs

In Wolbachia infected wasps, thechromosomes in the male eggsdouble and the egg develops as afemale that cantransmit the bacteria...

When Wolbachia invades a populationof parasitic wasps, eventually themales disappear

In this species,the only way tosee a male is totreat the femaleswith antibiotics!

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And because there’s no natural selectionon male function, mutations accumulateand the males produced when bacteria isremoved are sterile

When you treatthese femaleswith antibiotics,you get maleswith no maturesperm

So the wasp is completely dependent onthe bacterium for reproduction and thestrain of bacteria is completely dependenton the wasp for a home.Is this a mutualism?

It may be difficult to categorize aninteraction if you cannot comparespecies with and without the interactingspecies

Pop quiz

Grading: 5 points for name (being present inlecture), and 5 points for correct answer tothe question (10 total pts.)

On one side of card, write your 4-digit code(you will use this for picking up your quiz)

On the other side write your name, and answerthis question:

Define competitive displacement

Today: Parasitism and disease

I. Introduction to parasites anddisease

a worm’s eye view

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A. Parasitism - the most commonanimal lifestyle?

Estimates suggest that as many as halfof all species are parasites

A. Parasitism - the most commonlifestyle?

Why so many parasites?

Because most free-living specieshave parasites associated with them

Parasite load (worms only) of NorthAmerican mammals(76 mammal spp. sampled)

Mean no. of No. ofparasites per species ofindividual parasite perhost host population

Platyhelminthes Trematodes 108 1.8 Cestodes 140 2.8Nematodes 117 5.3Acanthocephalans 1 0.3

Mean no. ofparasite species 3 10

Why so many parasites?

Because most free-living specieshave parasites associated withthem

AND, most parasites are specialists(they attack few or maybe just onespecies)

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Remember how two species obligate mutualismscan lead to co-speciation? The same is true forspecialist parasites and their hosts...

Coevolution of primates and parasitic nematodes

Nematodes Primates

Why are most parasites specialists?

Parasites must1) establish on/in host - may need to cross skin,

gut wall2) evade host immune system, or if

ectoparasite, behavioral defense (grooming,swatting)

3) grow and reproduce in host4) disperse from and find new host

Adaptations for one species unlikely to beeffective on another

B. Types of parasitesSimply grouped by size...

Microparasites - viruses or single cellsincludes viruses, bacteria, protists, andsimple (unicellular) fungi

B. Types of parasitesSimply grouped by size...

Microparasites - viruses or single cellsincludes viruses, bacteria, protists, andsimple fungi

Macroparasites - multicellularincludes multicellular fungi, arthropods,and parasitic worms (e.g. nematodes andflatworms)

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B. Types of parasitesSize influences the way the host is used...

Microparasites generally infect cells, whilemacroparasites are usually external or inthe gut

B. Types of parasitesHuman diseases caused by microparasites

Viruses - influenza, HIV

Bacteria- tuberculosis, plague

Protists - malaria (trypanosome), amoebicdysentery (amoeba)

B. Types of parasitesHuman macroparasites

Arthropods - lice, mosquitoes, ticks

Nematodes - Onchocerca (River blindness),intestinal roundworms

Platyhelminthes - Schistosoma, tapeworms

III. Parasite transmission

How are parasites transmitted?

Direct or indirect transmission

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1. Direct transmission - from one host toanother without vectors or intermediatehosts




1. Direct transmission - from one host toanother without vectors or intermediatehosts

2. Indirect transmission - transmission viaanother species

1. Direct transmission may be vertical orhorizontal or both

Vertical transmission - from parent tooffspring in early development. Generallymother-offspring. Why?

•through infected gametes (most ofteneggs)•through birth process: mother-offspring


Examples of vertically-transmitted humandiseases -

rubella, syphilis, hepatitis B, HIV

- these are also horizontally transmitted

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b. Horizontal transmission - transfer viacontact with infected individuals orcontaminated products

2. Indirect transmission (horizontal only)

When parasite is transmitted from one hostto the other host via another species.

The other species may be vectors orintermediate hosts.

Vectors - ectoparasites of the host, serveas a hypodermic of pathogen

Mosquitobitinghuman

Mosquitoesvector …?

Intermediate hostshouse the parasite for part of its life cycle,Definitive hostwhere reproduction occurs

Intermediate hosts may live in proximity todefinitive hosts (e.g. Schistosoma), or maybe prey of the final hosts

Video clip: trematodes, snails and birds

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The snail-trematode example

Which was the intermediate host?Which was the definitive host?

Example also illustrates host behaviormodification, whereby the parasiteincreases its transmission by changingthe behavior of its host

IV. Selection on parasites to maximizetransmission

The problem - how to get you or yourprogeny off the host ‘island?’


A. For direct, horizontally-transmittedparasites, exit routes may be feces, bodyfluids, lesions


A. For direct, horizontally-transmittedparasites transmission dependent on:

density of hosts and

frequency of encounters between infected and uninfected individuals

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A. For indirect parasites transmissiondependent on:

a whole bunch of factors concerned with biology and ecology of both hosts


D. Virulence and transmission - more

What’s virulence?

What’s virulence?

How sick the host gets...X X

What’s virulence?

How sick the host gets...

Generally - virulence is related to the speedat which host is converted to parasites…

For example - 48 -72 h cycle of fever andchills of malaria

X X

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D. Virulence and transmission

1. The conventional (and not necessarilycorrect) wisdom: parasites become lessvirulent over evolutionary time

Why is this sometimes true?

1. The conventional (and not necessarilycorrect) wisdom: parasites become lessvirulent over evolutionary time

Why is this sometimes true?

a. Hosts evolve resistance

1. The conventional (and NOT alwayscorrect) wisdom: parasites become lessvirulent over evolutionary time

But if more virulent parasites reproducemore, why doesn’t selection on parasiteslead to greater virulence?

So if more virulent parasites reproducemore, why doesn’t selection always lead togreater virulence?

Generally, dead hosts don’t transmit theparasite…

Natural selection acts to maximizetransmission of the parasite

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Natural selection acts to maximizetransmission of the parasite

An example of evolutionary change invirulence of a virus

Rabbits from North Africaaccidentally introduced toAustralia in 1800s

Example Rabbits from NorthAfrica accidentally introducedto Australia in 1800s

spread andbecamemajor pests,endangeringnativeplants andanimals

Rabbits from North Africaaccidentally introduced toAustralia in 1800s

spread and becamemajor pests,endangering nativeplants and animals

Biological control program started: myxoma virus from S. America released in 1950

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Initially, virus was highly virulent

Then, variation in virulence appeared

Highly Intermed. LowVirulent virulence

0

numbers0

100

100

Prop

ortio

n

High virulence, low transmission Myxoma virus is indirect, horizontally -

transmitted virus, spread by mosquitovectors

Why might high virulence lead to lowtransmission?

Infective forms of virus causes lesions -areas where mosquitoes can bite

Why might low virulence lead to lowtransmission?

Initially, virus was highly virulent

Highly Intermed. LowVirulent virulence virulence

0

0100

100

Prop

ortio

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Low virulence,low transmission

100

0

Eventually, the dominant formhad intermediate virulence

Moderate virulence,high transmission

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What does the rabbit/myxoma virusexample illustrate?

What does the rabbit-myxoma virusexample illustrate?

Natural selection acts to maximizetransmission of the parasite - notnecessarily to reduce its virulence!

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