A coexistence story oat crown rust Puccinia coronata Emily Burns - - PowerPoint PPT Presentation

A coexistence story

• at crown rust

Puccinia coronata Emily Burns (UVa) & Georgiana May (UMN)

A coexistence story

• Multiple rust strains (genotypes)
• Each strain carries multiple (~10‐15)

p ( ) virulence alleles

• Different hosts (oat cultivers) bred

with varying degrees of resistance with varying degrees of resistance

• Cost of virulence: rust strains show

negative correlation between number f i l ll l d l i d

• at crown rust

Puccinia coronata

• f virulence alleles and latent period

(time between initial infection and later spore production) p p

• Competition‐colonization trade‐off?

Emily Burns (UVa) & Georgiana May (UMN)

Aphidius ervi, a parasitoid wasp

Micro‐ vs. macro‐parasites: A useful (?) distinction for animal parasites Microparasites (e g viruses bacteria protozoa): Microparasites (e.g., viruses, bacteria, protozoa):

• reproduce within host
• often small
• short generation time

short generation time

• hosts can develop immunity
• one does not need to account for the severity of infection within host
• and so, hosts can be classified by their infection status (susceptible,

infectious, recovered) Macroparasites (e.g., parasitic helminths [worms], arthropods [fleas, ticks, lice, mites], copepods)

• No (or slow) reproduction within host
• often larger in size
• longer generation time
• longer generation time
• short‐lived immunity of hosts
• aggregated distribution of parasites among hosts. Average parasite burden is

not necessarily representative not necessarily representative.

• riginally from R. M. Anderson & R. M. May; summary courtesy A. L. Lloyd

Aggregated distribution of macroparasites among hosts Ascaris lumbricoides (roundworm) in humans  = 2.18, 2 = 5.05 Ancylostoma duodenale (hookworm) in humans = 24.5

Host‐parasitoid models: A brief history

• Nicholson‐Bailey model: A discrete‐time predator‐prey
• model. Thought to be useful for host‐parasitoid systems.
• Problem: the Nicholson‐Bailey model predicts unstable
• scillations and rapid extinction of one or both species.
• 1980s, 90s: Long search for mechanisms that might stabilize

host‐parasitoid dynamics p y

• Result: Most commonly cited stabilizing mechanism is

aggregation of risk: host individuals vary in their aggregation of risk: host individuals vary in their susceptibility to parasitism