Major histocompatibility complex selection dynamics in pathogen-infected túngara frog (Physalaemus pustulosus)...

Area of Science:

• Evolutionary biology
• Immunogenetics
• Amphibian disease ecology

Background:

• Pathogen-driven selection can favor specific immune system alleles.
• Strong selection may deplete necessary genetic variation for robust immunity.
• The amphibian chytrid fungus (Batrachochytrium dendrobatidis, or Bd) causes significant declines in amphibian populations.

Purpose of the Study:

• To examine selection dynamics at a major histocompatibility complex class II (MHC-II) locus in Panamanian túngara frogs (Physalaemus pustulosus).
• To compare MHC-II diversity in highland populations experiencing Bd-related declines with lowland populations showing no decline.
• To understand how environmental conditions and pathogen pressure influence MHC-II evolution.

Main Methods:

• Studied MHC-II locus diversity in túngara frog populations from highland and lowland regions of Panama.
• Compared genetic variation and allele frequencies between populations with differing Bd prevalence and impact.
• Assessed evidence for directional selection versus inbreeding in shaping MHC-II variation.

Main Results:

• Highland frogs possessed MHC-II variants conferring resistance to Bd.
• Directional selection, not inbreeding, appeared to drive variant fixation in highland populations, maintaining overall genetic variation.
• In Bd-infected lowland sites, individuals with single Bd-resistance alleles were more frequent, suggesting a different selective advantage.
• Lower environmental favorability for Bd in lowlands may reduce selection for specific resistance.

Conclusions:

• MHC selection dynamics in túngara frogs are influenced by habitat suitability for pathogen spread and host vulnerability.
• Environmental conditions and pathogen pressure create fluctuating selective pressures on immune genes.
• Understanding these dynamics is crucial for predicting amphibian responses to disease.