Mutualism

Question 1

How do life histories of most mutualists differ from those of parasites?

Answer 1

• The life histories of most symbiotic mutualists are remarkably simple

Question 2

How does the expression of sexuality of most endosymbionts differ from that of parasites?

Answer 2

• Sexuality appears to be suppressed in endosymbiotic mutualists

Question 3

Compare the importance of a dispersal phase between mutualists and parasites?

Answer 3

• There is rarely a conspicuous dispersal phase in endosymbionts
• Dispersal rules the population dynamics of most parasites

Question 4

How does the stability of mutualist populations compare with those of parasites?

Answer 4

• Populations of mutualists seem to have great stability

Question 5

What does the fact that the number of endosymbionts per host seems to be fairly constant suggest about their population dynamics?

Answer 5

• This suggests that their population dynamics must have elements of density dependence

Question 6

How does the range and niche breadth of organisms in mutualistic relationships compare to those of either species living alone? How to parasites affect the range and niche breadth of their hosts?

Answer 6

• The ecological range and niche breadth of organisms in mutualistic relationships appears to be greater than that of either species living alone.
• Parasites usually reduce the host’s ecological range and niche breadth

Question 7

How does host specificity compare between most mutualists compare to parasites?

Answer 7

• Species specificity of both partners in mutualisms is often quite flexible which contrasts with the extreme host specificity of so many parasites

Question 8

Describe the relationship between the Central American acacia trees and their ant mutualists.

Answer 8

• Acacias provide ants with safe place to live and plenty to eat
  o Acacias have large, hollow thorns in which the vicious, stinging ants live.
  o The ants feed on nutritious protein nodules formed on the tips of the leaves and on sugar-rich, extra floral nectarines at the base of the leaves
• Ants provide protection to tree from crowding and other competing plants, fire, keeping the base of the tree clean
  o Ants patrol tree and attack herbivores or plants that land on the tree or tries to grow around its base

Question 9

Describe the mutualistic relationship between the leaf cutting ants and the fungus they culture.

Answer 9

• Ants cut leaves from neighboring vegetation, transport them to underground colonies and culture a fungus on the leaf cuttings
• Fungus fed to larval members of colony that rely on this as sole food source
• Fungus gains from being fed and dispersed by ants
• Ants harvest 17% of total leaf production, ecologically dominant herbivores in community
• Another factor
  o Streptomyces bacteria helps to maintain mutualism, carried in ants’ bodies, produces antibiotic that targets virulent parasite that invades fungal gardens, protecting fungus that sustains the ants

Question 10

Describe the mutualistic relationship between cleaner fish and shrimp and their customers.

Answer 10

• In coral reefs, feed on ectoparasites, bacterial/necrotic tissue
• Set up “cleaning stations”
• When removed from a patch of reef, customer fish developed skin diseases and pops decline within two weeks

Question 11

Describe the mutualistic relationship between the honey guide and the honey badger.

Answer 11

• Honey guide (bird) locates bees nest and leads badger to it
• Badger tears open nest and feeds on honey and bee larvae
• Honey guide eats beeswax and larvae later (can locate nests but can’t break them open)

Question 12

In general terms, describe the mutualistic relationship between the flowering plants and their animal pollinators.

Answer 12

• Animal-pollinated flowers offer nectar/pollen to reward visitors
• Plants derive benefits of out-crossing/avoiding dangers of inbreeding
• Ensures less pollen is wasted, much pollen transferred by wind never reaches intended flowers
• Most pollinators are insects

Question 13

What does the female yucca moth do to discourage other females from depositing their eggs on a plant that she has deposited eggs on?

Answer 13

• When a female lays her eggs on a particular flower, she drags her abdomen about the surface of the flower in order to leave her own scent as a warning to future visitors

Question 14

What is the principal ecological role of the mutualistic microbes inhabiting the herbivore gut?

Answer 14

• It’s crucial for the digestion of cellulose and also for the synthesis of vitamins

Question 15

Describe the mutualistic relationship between the obligate anaerobes and their ruminant hosts.

Answer 15

• Cellulose and other fibers need to broken down by enzymes that ruminant’s don’t produce
• The microbes aid in digestion and gain a continuous supply of food in a stable environment
• The ruminant gains digestible resources from a died which its own enzymes can’t handle

Question 16

Describe the mutualistic relationships found in the gut of some primitive termites.

Answer 16

• Termites rely on anaerobic flagellates and other microbes to digest cellulose and lignins they take in while feeding on wood
• Two mutualisms
  o Spirochaetes and rod-shaped bacteria cover surfaces of some protozoans, propel them through medium in synchronized movements, provide mobility
   Bacteria receive nutrients from the protozoans
  o Bacteria in termite gut capable of fixing gaseous nitrogen, only example of a nitrogen-fixing symbiont in insects

Question 17

Describe the mutualistic relationship between the rhizobia bacteria and leguminous plants.

Answer 17

• Legumes (during development) develop nodules on roots containing nitrogen-fixing colonies of rhizobia bacteria
• Bacteria receive photosynthetic products from plant
• Plant receives steady supply of fixed-nitrogen compounds
• Plant provides tightly packed boundary layer of cells in nodules as barrier to oxygen diffusion because oxygen inactivates enzyme required for nitrogen fixation

Question 18

Describe the mutualistic relationship between the mycorrhizae and the higher plants.

Answer 18

• Plants lack sufficient surface area for efficient absorption of water and nutrients
• Association provides a protected environment with an adequate food source for the fungus (sugars produced by plant)
• Increased surface area for nutrient and water uptake by the plant
• Two main types of mycorrhizae

Question 19

Distinguish between the ectomycorrhizae and endomycorrhizae.

Answer 19

• Ectomycorrhizae—the fungus that forms a sheath that surrounds but doen’t penetrate living cells in the root
  o Root surrounded by mantle of hyphae from which mycelia strands extend into surrounding soil
  o Restricted to temperate trees and shrubs
• Endomycorrhizae—found in about 80% of all vascular plants, penetrate cortex of plant root, form specialized structures for exchange
  o Hyphae extend into surrounding soil for several centimeters to increase surface area

Question 20

Where and on what kind of plants are the ectomycorrhizae most commonly found?

Answer 20

• Temperate trees and shrubs

Question 21

List the benefits high plants derive from their association with mycorrhizae.

Answer 21

• Increasing uptake of essential nutrients (phosphates and water) allowing plants to grow better
• Less fertilizer needed, withstand heavy metal and acid rain better
• Grow better on infertile soils of marginal lands, on mine spoils and other areas needing revegetation
• Survive transplant shock better, more resistant to soil-borne diseases, withstand higher soil temps and salinities and a wider range of soil pH