parasite lec 9

Parasite-Host Evolution

Introduction to Parasite-Host Evolution

Parasite-host evolution explores the dynamic relationships between parasites and their hosts, focusing on how both parties adapt and co-evolve over time. This field is crucial for understanding biodiversity, disease transmission, and evolutionary biology.

Coevolution and Host-Switching

Parasite-Host Co-Adaptation

Coevolution refers to the reciprocal evolutionary changes that occur between interacting species, such as parasites and their hosts. Host-switching occurs when a parasite shifts from its typical host species to a new one, which can lead to new evolutionary trajectories.

• Ophiocordyceps (fungi): Exhibits host specificity, with single species containing multiple host-specific strains. Occasionally infects other ant species, often resulting in poor parasite reproduction due to premature host death.

• Spilopsyllus cuniculi (rabbit flea): Demonstrates physiological dependence on the hormones of the female rabbit host, leading to high host specificity. Fleas reproduce in synchrony with rabbit reproductive cycles and occasionally infest other mammals and birds.

Specificity in Parasite-Host Relationships

Specificity describes how narrowly a parasite targets its host(s). It can range from broad (multiple species or genera) to narrow (one-to-one species or even population-level specificity).

• Phylogenetic concept: Specificity is often analyzed by comparing the evolutionary trees (phylogenies) of hosts and parasites.

• Specificity hypothesis: Greater dependence on a host is expected to result in narrower specificity, as specialization increases.

Co-speciation

Definition and Patterns

Co-speciation occurs when a parasite speciates in tandem with its host, resulting in parallel evolutionary trees. This pattern is most common in permanent parasites with direct life cycles and poor dispersal abilities.

• Geomydoecus lice and pocket gophers: Classic example of co-speciation, though host-switching and phylogenetic uncertainty can complicate the pattern.

• Sarcoptid mites: Associated with mammals for a long evolutionary period, showing evidence of co-speciation.

Factors Affecting Co-speciation

• Direct life cycles (single host)

• Permanency on host

• Poor dispersal capabilities

• Arthropod parasites are common candidates

Parasite Life Cycles and Host Specificity

Enterobius (Pinworms)

Pinworms are nematodes with direct life cycles, infecting hosts through ingestion of eggs. They exhibit high host specificity and rapid life cycles.

• Life cycle: Eggs are ingested, larvae hatch in the intestine, and adults reproduce in the colon. Females lay thousands of eggs, completing the cycle in about one month.

Phylogenetic Patterns and Host Associations

Phylogenetic Matching and Divergence

Comparing host and parasite phylogenies reveals patterns of co-speciation, host switching, duplication (parasite speciation without host speciation), and extinction.

• Perfect matches indicate co-speciation.

• Mismatches may result from host switching, parasite duplication, or extinction events.

Host Switching

Mechanisms and Consequences

Host switching is common in parasites with non-permanent associations or indirect life cycles. It can lead to new host-parasite associations, sometimes resulting in speciation.

• Micro-predators: Mosquitoes and ticks often switch hosts, sometimes preferring certain species but capable of feeding on a range of hosts.

• Parasitoids: Generalist parasitoids can adapt to new hosts, though initial reproductive success may be reduced.

Outcomes of Host Switching

• Most host switches result in parasite death.

• Some may cause host death or failed reproduction.

• Rarely, successful reproduction in a new host leads to new host associations and potential speciation.

Case Studies in Host-Parasite Evolution

Corallorhiza wisteriana and Fungal Hosts

The orchid Corallorhiza wisteriana associates with different fungal families in eastern (Russulaceae) and western (Thelephoraceae) populations, illustrating host shifts and geographic variation in host associations.

Summary Table: Types of Host-Parasite Evolutionary Events

Event Type	Description	Example
Co-speciation	Parallel speciation of host and parasite	Geomydoecus lice and pocket gophers
Host Switching	Parasite shifts to a new host species	Ornithodoros turicata tick
Duplication	Parasite speciates without host speciation	Chirodiscid mites on beavers
Extinction	Loss of parasite or host lineage	Not specified

Conclusion

Parasite-host evolution is shaped by coevolution, specificity, co-speciation, and host switching. These processes contribute to the diversity of life and have significant implications for ecology, evolution, and disease dynamics.