Evolution and species-specific conservation of toll-like receptors in terrestrial vertebrates

Toll-like receptors (TLRs) are known for their essential roles in promotion of innate immunity and induction of adaptive immunity through recognition of pathogen-associated molecular patterns (PAMPs) or danger associated molecular patterns (DAMPs). TLR genes are excellent models for the study of the selective pressure enforced by microorganisms on the host genome. Phylogenetic analyses have shown that interactions between pathogens and immune systems have changed during evolution. Selective pressure for maintenance of specific pathogen recognition has led to evolution of TLRs under both positive and purifying selection. However, intracellular and cell-surface TLRs have been affected differently due to their variation in conservational constraints. In this review, we summarize some of the main studies on the influence of selection on shaping the evolution of several TLR gene families in different terrestrial vertebrate species. We also describe the effect of evolution on the function of different TLRs and their specific conservations in these species and show similarities and differences in evolutionary patterns of TLR orthologs among species as well as among TLR gene families.