Adaptation and Cryptic Pseudogenization in Penguin Toll-Like Receptors

Area of Science:

• Comparative immunogenetics
• Evolutionary biology
• Avian immunology

Background:

• Penguins inhabit diverse Southern Hemisphere regions, facing varied pathogen pressures.
• Toll-like receptors (TLRs) are crucial for innate immunity, detecting conserved microbial patterns.
• TLRs often exhibit positive selection due to microbial challenges, reflecting host-pathogen co-evolution.

Purpose of the Study:

• To investigate adaptive evolution in penguin Toll-like receptors (TLRs) across all species.
• To identify specific TLRs under positive selection and their functional implications.
• To explore the pseudogenization of TLR15 in Eudyptes penguins and its potential link to fungal infections.

Main Methods:

• Comparative immunogenetic analysis of TLRs across all penguin species.
• Identification of positive selection acting on TLR genes, particularly at agonist-binding sites.
• In vitro functional analysis of the Toll-like receptor 15 (TLR15) gene in Eudyptes penguins.

Main Results:

• Evidence of adaptive evolution and positive selection in cell surface-expressed TLRs (TLR1B, TLR4, TLR5) at key functional sites.
• TLR15 in Eudyptes penguins showed multiple pseudogenization events, with a rare nonfunctional haplotype found.
• Even full-length TLR15 in Eudyptes was nonfunctional, suggesting an earlier, cryptic pseudogenization event.

Conclusions:

• Penguin TLRs have undergone adaptive evolution, particularly surface receptors involved in pathogen recognition.
• The pseudogenization of TLR15 in Eudyptes penguins suggests a specialized immune adaptation, potentially related to fungal pathogen resistance.
• Understanding these evolutionary changes in TLRs provides insights into avian immune defense strategies against specific pathogens like Aspergillus.