Natural selection on immune defense: A field experiment.

Area of Science:

• Evolutionary biology
• Immunology
• Ecology

Background:

• Understanding natural selection on host immune defense is crucial for predicting evolutionary trajectories.
• Theory predicts stabilizing selection on immune traits due to fitness trade-offs, but empirical studies often show directional selection.
• Discrepancies may arise from limited phenotypic variation or resource-level confounds in empirical studies.

Purpose of the Study:

• To investigate phenotypic selection on immune defense traits in Lymnaea stagnalis snails under controlled conditions.
• To examine the influence of resource availability on selection pressures acting on immune traits.
• To determine the relationship between immune activity, growth, survival, and fecundity.

Main Methods:

• A field experiment using Lymnaea stagnalis snails housed in cages in a lake.
• Measurement of phenoloxidase (PO)-like activity and antibacterial activity in snail hemolymph.
• Manipulation of snail resource levels and monitoring of immune activity, growth, survival, and fecundity over six weeks.

Main Results:

• Phenoloxidase (PO)-like activity and growth exhibited stabilizing selection.
• Antibacterial activity showed positive directional selection.
• Selection on immune traits was primarily influenced by variation in snail survival.

Conclusions:

• The form of natural selection on host immune defense traits is trait-specific.
• Phenoloxidase (PO)-like activity and growth are under stabilizing selection, suggesting optimal levels for fitness.
• Antibacterial activity is under directional selection, potentially reflecting adaptation to the local parasite community.