Widespread positive selection on cetacean TLR extracellular domain

Area of Science:

• Evolutionary biology
• Immunology
• Marine biology

Background:

• Toll-like receptors (TLRs) are crucial for innate immunity in vertebrates, recognizing diverse pathogens.
• Cetaceans evolved from terrestrial to aquatic environments, facing novel pathogen challenges.

Purpose of the Study:

• To investigate the evolutionary adaptation of Toll-like receptors (TLRs) in cetaceans.
• To identify sites under positive selection in the extracellular domain (ECD) of cetacean TLRs.

Main Methods:

• Sequencing of the extracellular domain (ECD) of 10 TLRs in cetacean lineages.
• Analysis of positive selection and amino acid changes in TLR ECDs.

Main Results:

• 148 sites in the ECD of 10 TLRs showed strong evidence of positive selection.
• The majority of selected sites (90.54%) exhibited radical amino acid changes.
• Increased positive selection was observed during the transition to aquatic habitats, suggesting adaptation to new pathogens.
• Viral and non-viral TLRs in cetaceans showed similar selective intensity, unlike in primates and birds.

Conclusions:

• Cetacean TLRs underwent significant adaptive evolution, particularly during their transition to aquatic life.
• The observed selection patterns may reflect adaptation to a broader range of pathogens encountered in marine environments compared to terrestrial mammals.