Evolution and diversification in toll-like receptors of fundulus heteroclitus and teleosts based on whole genomic data.

Abstract

Toll-like receptors (TLRs) are a major component of the innate immune system and are pivotal for organismal survival by recognizing a diverse group of pathogens. Compared to mammals, TLRs are found to be more diverse in teleost species, which suggests the essential role of innate immunity in teleosts due to the lack of their diversification in adaptive immunity.The purpose of this study was, 1) to estimate an evolutionary tree of teleosts TLRs, 2) to verify and clarify the number of TLR gene copies in Fundulus heteroclitus, and 3) to test for evidence of positive selection acting on the various TLRs. A gene tree of the TLR family in teleosts was constructed based on the whole genome data from Ensembl database, and it allowed for a clarification in teleost TLRs’ nomenclature and the description of the full complement of TLRs in F. heteroclitus.Positively selected sites were detected in all TLRs with TLR22 found to be under the highest positive selection rate (4.63% of TLR gene sites) among all TLRs. Copy number variation of teleost-specific TLR22 gene moreover suggested gene duplication and diversification potentially to compensate for the lack of other cell surface TLRs. These results suggested that various TLR22 paralogues have likely arisen from positive selection pressure as well as gene expansion and functional diversification in teleost species.