Table 1.
Key features of the three model host–pathogen systems discussed throughout this review
| Plantago lanceolata–Podosphaera plantaginis (plant–powdery mildew) | Daphnia dentifera–Metschnikowia bicuspidata (zooplankton–yeast) | Bacteria–phage from horse chestnut trees (Aesculus hippocastanum) | |
|---|---|---|---|
| Host | |||
| Size (longest axis) | 10–20 cm | 1.5 mm | 0.5–5 μm |
| Lifespan | Perennial, up to 7 years | Up to 2 months | Unknown |
| Reproduction | Sexual (outcrossing) and asexual (side rosettes) | Cyclically parthenogenetic (sexual resting eggs) | Asexual (binary fission) |
| Generation time | 3 months (sexual) | 1 week (asexual) | Typically <1 day |
| Dispersal mode | Wind‐dispersed pollen | Swimming, currents, via resting eggs (e.g. on bird feet or via wind) | Water cycle, wind, rain, insect vectors |
| Offseason survival | Seed bank | Resting egg bank | Dormancy in soil or within tree host |
| Pathogen | |||
| Size (longest axis) | 30 μm (transmission spore) | 35–60 μm | 30–200 nm |
| Reproduction | Asexual transmission spores, possibly sexual resting spores | Parasexual | Asexual virions |
| Generation time | 7–12 days (asexual) | 10–20 days | Typically < 1 h |
| Transmission | Environmental, via wind | Environmental, host ingests free‐living spores in water | Environmental (passive) |
| Propagule release | Spores shed from live leaf | Obligate killer, spores released from dead host | Obligate killer, virions released from lysed cell |
| Dispersal range | 1 m | Unknown | Unknown |
| Offseason survival | Resting spores on dead leaves | Unknown, but likely in sediment | Unknown, but possibly within bacterial genome |
| Host × pathogen | |||
| Genetic specificity | Highly specific (gene for gene): recognition of pathogen avirulence allele by host resistance allele triggers defence responses. Also quantitative resistance | Genetic variation in host rate of parasite encounter and susceptibility given encounter, but no genetic variation in pathogen infectivity | Many known mechanisms of resistance/infectivity that vary from general to specific; local adaptation and infection network analyses often suggest high level of specificity |
| Environment | |||
| Habitat | Dry meadows in Åland archipelago, Finland | Lakes in temperate North America | Horse chestnut trees in the United Kingdom |
| Growing season | July–September | July–November | May–September |
| Abiotic factors | Temperature, rainfall, humidity, wind | Temperature, light, UV and nutrients | Temperature, rainfall, nutrient availability |
| Biotic factors | Hyperparasites | Resources, predators and diluter species | Bacterial competition, tree defences |