Parasitism and Mutualism

Parasitism and Mutualism Notes

Symbiosis

• Definition: Symbiosis refers to the intimate and enduring relationship between two or more organisms of different species.
• Implications: The relationship determines the fate of individuals of one species based on their interaction with another species.
• Types of Outcomes:
  • Mutualism (++): Both species benefit from the relationship.
  • Parasitism (+-): One species benefits at the expense of the other.
  • Commensalism (+0): One species benefits while the other is unaffected.
  • Amensalism (-0): One species is harmed while the other is unaffected.

Parasitoids vs. Parasites

• Parasites: Typically do not kill their hosts; they draw resources from them.
• Parasitoids: Lay eggs in a host; larvae consume the host, eventually leading to the host's death.

Characteristics of Parasites

Microparasites

• Traits: Small size, short generation time, rapid multiplication within the host.
• Associated with: Disease.
• Infection Duration: Short in relation to host lifespan, typically transmitted directly from host to host or via a carrier (e.g., viruses, bacteria).

Macroparasites

• Traits: Larger size, longer generation time, usually do not complete life cycle in a single host.
• Transmission: Can occur directly between hosts or via carriers/intermediate hosts (e.g., flatworms, roundworms, lice).

Parasitic Plants

• Over 4000 species of parasitic plants exist.
• Haustorium: A modified root that penetrates host tissues to connect with vascular tissue.
• Types:
  • Hemiparasites: Photosynthetic, drawing water and nutrients from the host's xylem.
  • Holoparasites: Non-photosynthetic, obtaining all resources from the host's phloem and xylem.

Host Habitats and Parasite Life Cycles

• Ectoparasites: Live on the surface of hosts (e.g., skin, feathers).
• Endoparasites: Live within hosts, potentially affecting blood and organ systems.
• Life cycles can involve multiple hosts and stages; definitive hosts are where parasites achieve sexual maturity, and intermediate hosts support lifecycle stages.

Transmission of Parasites

• Direct Transmission: Occurs between hosts without intermediate organisms (e.g., through air or water).
• Indirect Transmission: Involves an intermediate vector that transmits the parasite from one host to another.

Host Responses to Parasitism

• Behavioral Defenses: Hosts can groom to remove ectoparasites, relocate to areas with fewer parasites (e.g., deer using dense, shaded spots).
• Immune Defenses:
  • Inflammatory Response: Triggers release of histamines, increased blood flow, and white blood cell activity.
  • Cysts: Hard encapsulated structures around parasites to isolate them.

Impact on Host Survival/Reproduction

• Parasitism may lead to decreased fitness (e.g., smaller egg sizes in infected females).
• Altered behaviors in hosts can increase vulnerability to predators (e.g., rabbits with tularemia, killifish with trematodes).

Emerging Diseases and Human Health

• Emerging Infectious Diseases: Diseases that have recently increased in incidence or range.
• Lyme Disease Example: Transmission through ticks that obtain bacteria from infected animals, affected by human-induced environmental changes.

Mutualism

Evolution from Parasitism to Mutualism

• In cases where parasites are less harmful and actually beneficial, relationships can shift from parasitism to mutualism (e.g., tapeworms aiding in host growth).
• Type of Mutualists:
  • Obligate: Cannot survive without the interaction.
  • Facultative: Can survive without the interaction.

Specific Mutualistic Interactions

• Coral-Algae Symbiosis: Corals obtain most of their energy from symbiotic algae (zooxanthellae), while algae receive shelter.
• Plant-Bacteria Relationships: Legumes associate with nitrogen-fixing bacteria (Rhizobium) to receive nitrogen, while bacteria gain carbon from plants.
• Mycorrhizal Fungi: Assist plants in nutrient uptake while receiving carbon, with endomycorrhizae and ectomycorrhizae being types of interactions.

Defense and Reproduction Mutualisms

• Defense: Examples where one species helps defend another, like alkaloids from fungi in grasses that deter grazers.
• Reproduction: Pollinators like insects and birds that not only gain food but also help fertilize plants by transferring pollen; might incur energy costs to the plant.

Seed Dispersal Mutualisms

• Ants and Myrmecochore Plants: Seeds that ants collect for their elaiosome (food body) to consume, helping disperse the seeds.
• Fruit-Eating Animals: Consume fruit while ensuring seeds are not eaten, promoting germination away from the parent plant.