4. Mutualism, Parasitism, and Disease

Mutualism, Parasitism and Disease  

• Complex Interactions  

• Behavioural Modification by Parasites  

• Ecology of Disease  

• Mutualist-Exploiter Continuum  

• Evolution of Mutualism  

Mutualist-Exploiter Continuum  

• Symbiotic relationships: organisms live in close proximity to each other.  

• Mutualism and parasitism are two points on a continuum of possible ecological interactions.  

• Bear in mind that mutualisms are exploitive interactions that happen to be reciprocal 

Complex Interactions  

• Types of parasitic and mutualistic interactions that occur in nature are very diverse. • Defy easy generalization.  

• Instead we consider some representative examples that show diversity 

Control: Behaviour Modification by Parasites  

• Many parasites alter behaviour of host in ways that benefit transmission and reproduction of parasite.  

• e.g. Spiny-headed worms (Acanthocephalans).  

-Starling parasite that uses amphipods as intermediate host.  

-Infected amphipods spend less time in sheltered areas and on light-coloured substrates where more obvious to birds.  

-This changed behaviour of infected hosts increases rate at which parasite infects Starlings.  

Diagram info  Adult female Plagiorhynchus

Diagram info 

Adult female Plagiorhynchus lays eggs within the intestines of infected birds. The eggs are shed with feces. -> A terrestrial isopod eats the feces of an infected burd. The eggs of Plagiorhynchus hatch within a few hours; they develop into a mature larva in 60-65 days.-> The mature larvae of Plagiorhynchis alter isopod behavior; infected isopods leave sheltered areas and wander in the open. -> Leaving shelter makes the isopods more conspicuous and vulnerable to predation by birds. When eaten by a bird. The mature Plagiorhynchus attaches to the bird's intestinal wall. 

Infestation: Ghost Moose and Winter Ticks  

• Winter ticks Demacentor albipictus feed on variety of hosts, such as moose, elk, deer. • Unique among ticks in that have single host species for all stages.  

• In study of moose hides from Western Canada, average moose had 33,000 winter ticks, while 3% had >100,000 ticks.  

• Winter ticks densities on moose 25 times greater than on alternative hosts.  

• Moose heavily infested with ticks groom extensively to point where outer fur layer lost.  

Figure caption: (a) With few ticks, moose retain a brown coat; (b) at high numbers of ticks, moose self-groom, destroying the winter coat of hair and giving the whitish image of a “ghost” moose; (c) winter ticks can occur at extremely high densities on the skin of a moose; (d) the ranges of winter ticks and moose overlap over much of Canada and the northern United States 

Moose population

• Number of ticks changes in sync with change in moose population.  

• Similar to predator-prey cycles. 

Number of ticks are tightly linked with the number of moose 

Figure 15.4 Relationship between average number of ticks on a moose and moose population sizes in Elk Island National Park, Alberta   

Review diagram on slide 9 

Moose will see a decrease in reproduction and maybe other things because spend so much energy being a host to ticks. Female ticks are draining their blood 

Slide 11  mite reproduction

Slide 11 

What happens with the lifecycle would be  

Hige lag in mite reproduction, it takes them some time to rebuild their population by the time september comes less bees and more mites. If the mite population goes un 

 

Mites used to only reproduce using male___ only 10%  and then evolved to be able to infect the european honeybee which has 100% availability 

Some honeybees are able to control the amount of parasitic infestation 

Honey bees and Varroa  

• Behaviour modifications based on parasites can also occur in honey bees  

• ‘virulent’ mites (i.e. mites with a high potential to induce fatal DWV infections in parasitised pupae) were removed significantly more often  

• brood parasitised by ‘less virulent’ mites (i.e. mites with a very low potential to induce overt DWV infections) or non-parasitised 

 

interactions: Predation, Parasitism, and Competition 

• Adelina tribolii protozoan parasite of Tribolium (flour beetles).  

• Reduces density of T. castaneum but has little effect on T. confusum.  

• Presence of parasite reverses outcome of competition between species 

 

Parasites can not just transmit viruses and reduce your chances of survival, can also change the outcomes of competition that would normally occur 

In absence of Adenline, T. castaneum outcompetes T.confusum all the time. However,in the presence of Adelina, T.confusum is usally the better competitor. 

III. Mutualisms 

 • Mutualism: Interactions between individuals of different species that benefit both partners.  

-Facultative mutualism: species does not require mutualistic partner for survival.  

-Obligate mutualism: species is dependent on mutualistic relationship.  

• Eukaryotes thought to have originated as mutualistic associations 

Protection: Ants and Bullhorn Acacia  

• Obligate mutualism between ants (Pseudomyrmex) and acacias in Central America. 

Figure 15.6 Split thorn of a bullhorn acacia, revealing a nest of its ant mutualists. 

• Ants:  

-fast, agile runners, good vision, forage independently  

-aggressive toward vegetation and animals that contact home tree  

-maintain large colony sizes  

-show 24-hour activity outside nests  

• Acacia:  

-have enlarged thorns with soft, easily excavated pith  

-year-round leaf production  

-enlarged foliar nectaries  

-leaflet tips modified into concentrated food source 

• Plants benefit from presences of ants, with increased growth and survival of shoots 

Diagram info: Acacia shoots occupied by ants grew much faster 

Survival of acacia suckers was much higher where they were occupied by ants 

• Plants without ants have more herbivorous insects. 

Acacia shoots without ants have much larger numbers of herbivorous insects 

Construction: Zooxanthellae and Corals  

• Zooxanthellae (Dinoflagellates) live at very high densities within coral tissues.  

-Zooxanthellae receive nutrients from coral, coral receives organic compounds synthesized by zooxanthellae during photosynthesis.  

-Corals control release of organic compounds from zooxanthellae with compounds that alter permeability of zooxanthellae cell membrane.  

-Corals also control rate of zooxanthellae population growth and population density 

• Benefit to zooxanthellae appears to be nutrients obtained from coral, especially nitrogen and phosphorous.  

Diagram info: Corals without zooxanthella excrete ammonium into the environment. In contrast, corals with zooxanthella absorb ammonium from the environment 

Figure 15.10 a. Tubastrea does not harbor zooxanthella, while b. Pocillopora damicornis does. C. Harbouring zooxanthella by corals reduces ammonium flux 

IV. Ecology of Disease  

• Disease: atypical condition in living organism that cause some sort of physiological impairment.  

• Can be caused by variety of factors, e.g. genetic abnormalities, exposure to toxins, other organisms.  

• Basic ecological principles are useful in understanding how diseases are transmitted and cured.  

• Pathogen population size affected by processes within host (e.g. immune response, competition) and outside host (e.g. transmission rates).  

• Efficiency of transmission has important implications for population dynamics of host.

• Transmission may be direct (e.g. contact with infected host) or indirect (e.g. contact with surface that has infection).  

• Horizontal transmission: transfer of disease among individuals of same generation. • Vertical transmission: transfer of disease from parent to offspring.  

Compartmental Models  

• Useful to model disease as “compartments” representing subpopulations of host.  

-3 subpopulations: susceptible (uninfected) individuals, infected individuals, immune individuals  

• For each subpopulation, birth and death rates (of host).  

• Also some probability of individuals moving from one subpopulation to another (change in disease status). 

• In a compartmental model, the population is separated into multiple sub-population based on specific features (e.g., whether a person is susceptible, infectious, or has recovered) 

DIagram slide 26 

hosts for diseases and immunity

• All host individuals have background mortality rate, d.  

• Infected individuals may have altered mortality rate, indicated by (d + α)  

• α is change in per-capita mortality associated with death, referred to as virulence  

• very high virulence may cause host death before pathogen can be spread; often natural selection for decreased virulence 

Natural selection will go toward decreased virulence because if all of your hosts die the pathogen will die too 

Immunity is relative. Recovered is probably a more encompassing approach 

Subpopulations new potential host categories: susceptible (uninfected), infected, immune and all of these could lead to death but could also go though the cycle and start again  

Figure 15.11 A compartmental model of the population growth of a disease-causing organism 

Quarantine During a SARS Outbreak  

• SARS (Severe Acute Respiratory Syndrome) appeared first in China, 2002.  

• Mortality rate ~15% of infected.  

• SARS outbreak in Toronto in 2003 resulting in 44 deaths.  

• Quarantine (separating individuals exposed to disease) and isolation (separating sick individuals) is disruptive.  

• Is it worthwhile? 

• Compartmental model used to assess effectiveness of quarantine and isolation during SARS outbreak. 

Susceptible, asymptomatic, symptomatic, recovered 

Asymptomatic -> quarantined-> isolated-> recovered 

from diagram:Delays in isolation and quarantine increase mortality within the population 

Using Vaccination to Provide Herd Immunity

 • Herd immunity: negative pathogen population growth rates lead pathogen to go extinct.  

• Vaccination moves individuals from susceptible to immune subpopulation.  

-This reduces available hosts for pathogen.  

• If large enough proportion of the total population is successfully vaccinated, herd immunity achieved. 

 

Diagram on slide 34 don’t think need to learn this but maybe 

Mutualist-Exploiter Continuum  

• Most mutualisms are facultative and involve sets of species rather than single pair.  

-e.g. Most flowers are pollinated by many species 

Figure 15.18 Mutualisms, such as those that occur among plants and pollinators, generally involve large numbers of species. Here are three pollinators of Camas quamash, a plant found throughout southwestern Canada 

Pollination: Optimal Foraging vs. Pollen Movement Plant’s Perspective  

• Plants require pollen vectors to move pollen from anther to stigma.  

• For animal-pollinated species, traits will improve pollination rates if they:  

-Increase visitation rates by pollinators  

-Enhance pollen deposition  

-Ensure pollen moves to another individual of same species with minimum energetic cost. 

Pollinator’s Perspective  

• Pollinator needs to acquire food for minimum cost.  

• Traits that increase efficiency of locating food and reduce handling time are favoured.

• For pollinator, selection should favour  

-Visit multiple flowers on one plant to reduce travel time.  

-Spend little time on flower, taking only nectar 

conflict between best for plant and best for pollinator

• Conflict between what is best for plant and best for pollinator. How do these conflicts of interest drive evolution of species involved?  

• Plants species vary in length flowers stay open. Can model of plant-pollinator interactions explain variance?  

-Flower must be open to allow pollination. But after first pollinator, benefit of subsequent pollinators has diminishing returns.  

-Costs to keeping flower open (increased evaporation, respiration, etc.) 

 

 Benefirs from maintaining flowers open will be accrued at different rates for different species. This will be influenced by the pollinators involved 

The model predicts flowers will stay open for a duration that maximizes the difference between cost and benefit 

The model assumes a constant cost of maintaining a flower 

• Yucca plant and Yucca moth

• Yucca plant and Yucca moth have obligate mutualism 

(a) Yucca baccata in full flower; (b) a Yucca moth, Tegeticula planella, ovipositing on a Yucca flower. 

Yucca plant and Yucca moth  

• Complex association:  

-Moths emerge from soil in spring.  

-Female moth enters flower, forms pollen ball.  

-Female travels to new flower, lays eggs in ovary and places pollen ball in stigma.  

-Yucca moth eggs hatch; larvae feed on seeds in Yucca fruit.  

-Fruits split open, releasing seeds and moth larvae to ground. Moth overwinter as pupae in soil. 

Studies by Addicott and students show:  

-Despite similarity in process, costs and rewards of mutualism vary substantially among Yucca spp.  

-Ratio of seeds: ovules varied depending on probability of fruit being occupied by larvae and larval feeding rate.  

-A number of Yucca moth females ‘cheat’ – lay eggs but do not form pollen ball or deposit it. 

Cheating in plant-pollinator mutualism occurs in both pollinator and plant.  

• Nectar robbers: pollinators that exploit energy-rich nectar but do not move pollen.  

• Many orchard species produce flowers that mimic shape, size, and odour of female wasps, inducing pseudocopulation by male wasps, which pick up and deposit pollen in the process. 

Mycorrhizae: Nutrient Gain vs. Carbon Gain  

• Long evolutionary history of relationship between plants and mycorrhizal fungi.  

-Mycorrhizae provide plant with inorganic nutrients and feed on root exudates of plants.  

• Arbuscular mycorrhizal fungi (AMF)  

-Produce arbuscules, site of exchange between plants and fungi; hyphae, fungal filaments; and vesicles, energy storage organs.  

• Ectomycorrhizae (ECM)  

-Form mantle around roots and net like structure around root cells 

Mycorrhizae and Plant Water Balance

• Plants with mycorrhizae appear better able to extract soil water 

A gropyron with mycorrhizae maintained higher leaf water potential throughout a hot summer day. 

• Removing mycorrhizae hyphae decreases rates of transpiration.  

• Indicates direct role of mycorrhizae in plant-water relations. 

Removing mycorrhizae reduces rate of transpiration by red clover 

Mycorrhizae Nutrient Availability and Mutualistic Balance Sheet  

• Variation among mycorrhizae in rate supply nutrients to plant host.  

• Johnson (1993) investigated whether fertilization can select for less mutualistic mycorrhizal fungi.  

• i.e., fungus receives more from plant for less nutrients 

• Experimental design used by Johnson (1993) to determine if fertilizing soil selects for less mutualistic mycorrhizal fungi 

DIagram slide 49 

Nutrient Availability 

 • Results suggested mycorrhizal fungi from unfertilized soils supplied plants with more nutrients.  

-Plants able to invest more energy in above-ground photosynthetic material.  

• Found Andropogon produced inflorescens only in nitrogen-supplemented treatments. 

Mycorrhizae Nutrient Availability and Mutualistic Balance Sheet  

• Both added fertilization and mycorrhizae led to increased shoot mass and decreased shoot:root ratios.  

• BUT, faster growth rate of young plants in unfertilized plants with mycorrhizae. (Data not shown. 

Andropohon with mycorrhizae grew to a larger size at all four nutrient levels 

Adding nutrients and inoculating with mycorrhizal fungi reduced root: shoot ratios 

Mycorrhizae  

• Are AMF generally mutualists or parasites?  

• Study of 64 species grown with and without AMF show relationship generally neutral! 

There were 64 species found in old fields around the University of Guelph 

Micorrhizae increased growth of some plants...had relatively little impact on most plant species...and decreased growth in others 

Evolution of Mutualism 

 • Theory predicts mutualism will evolve where the benefits of mutualism exceed the costs.  

• Cost-benefit analyses used by Keeler (1981, 1985) to model mutualistic interactions. -Consider population polymorphic for mutualism with three kinds of individuals:  

-Successful mutualists: Give and receive measurable benefits. 

 -Unsuccessful mutualists: Give, but do not receive benefit.  

-Nonmutualists: Do not give or receive benefits 

• For a population to be mutualistic,  

-Fitness of successful mutualists must be greater than either unsuccessful or nonmutualists.  

-Combined fitness of successful and unsuccessful mutualists must exceed fitness of nonmutualists. 

Evolution of Mutualism Start by defining variables:  

• wnm = fitness of nonmutualists  

• wms = fitness of successful mutualists  

• wmu = fitness of unsuccessful mutualists  

• Fitness of mutualists,  

• wm = pwms + qwmu  

• Condition for evolution and persistence of mutualism: wm > wnm  

• or, pwms + qwmu > wnm 

Evolution of Mutualism: Model of Ant-Plant Protection  

• Convenient to think of relationships in terms of selection coefficient (s): relative selective costs associated with being successful mutualist, unsuccessful mutualist, or non-mutualist.  

• Selective costs:  

-sms = (H)(1 – A)(1 – D) + IA + ID  

-smu = (H)(1 – D) + IA + ID  

-snm = (H)(1 – D) + ID  

• H = proportion of plant tissue damaged in unprotected plants  

• Selective costs:  

• sms = (H)(1 – A)(1 – D) + IA + ID  

• smu = (H)(1 – D) + IA + ID  

• snm = (H)(1 – D) + ID  

• H = proportion of plant tissue damaged in unprotected plants  

• (1 – D) = amount of tissue damage that would occur in spite of ants  

• (1 – A) = amount of herbivory in spite of ants  

• IA = investment by plant in benefits to ants  

• ID = investment in defences other than ants 

Keeler’s conditions for evolution and persistence of ant-plant mutualism

• Keeler’s conditions for evolution and persistence of ant-plant mutualism is: 

 • p(1 – sms) – q(1 – smu) > 1 – snm  

• Substituting selective coefficient equations into above and simplifying result gives expression of benefits relative to costs:  

• p[H(1-D)A] > IA 

• Applying this to facultative mutualism between plants with extrafloral nectaries and ants that feed at nectaries and provide protection to plant.  

• For mutualism to evolve, proportion of plant’s energy budget that ants save from herbivores must exceed proportion of energy budget invested in extrafloral nectaries

Evolution of Mutualism: Model of Facultative Ant-Plant Mutualisms  

• Conditions that may produce higher benefits than costs:  

-Low proportion of plant’s energy budget invested in extrafloral nectaries.  

-High probability of attracting ants (p).  

-High potential for herbivory (H).  

-Low effectiveness of alternate defences (D).  

-Highly effective ant defence (A) 

Lecture Concepts  

• There is great diversity in the types of parasitic and mutualistic interactions that exist, defying easy generalization.  

• Basic ecological principles can be applied to our understanding of disease, and the population dynamics of pathogens can be predicted using a compartmental model.  

• Many forms of interactions can switch from parasitic to mutualistic, depending upon the specific conditions of the local environment.  

• Theory predicts that mutualism will evolve where the benefits of mutualism exceed the cost