Characterization and comprehensive analysis of the miiuy croaker TLR2 reveals a direct evidence for intron insert and loss

Toll-like receptor 2 (TLR2) is a member of an ancient pattern recognition receptor family, conserved from insects to mammals and it is best known as a receptor for recognizing conserved components of Gram-positive bacteria. In present study, the genomic structure of TLR2 gene from miiuy croaker was identified and characterized. It comprises twelve exons and eleven introns. The lengths of exons 3 to 10 of miiuy croaker TLR2 and exons 2 to 9 of fugu and pufferfish TLR2 are exactly the same, but most importantly, both of fugu and pufferfish have only eleven exons and ten introns. An intron insert event probably happened on exon 1 of miiuy croaker TLR2 after its divergence from ancestor of zebrafish, and an intron loss event probably happened on those of Tetraodontiformes TLR2 after the divergence with ancestor of miiuy croaker. Our study showed the direct evidence and strongly supported the intron insert and loss on fish TLR2. The pathogen injection experiments indicated that TLR2 might not be an important responder to Gram-negative bacteria in miiuy croaker. Molecular evolutionary analyses indicated TLR2 genes were under strong purifying selection pressure, showing a quite strong functional constraint in both of fish and mammals, despite of their distinct living environment conditions.

► Genomic structure of miiuy croaker TLR2 gene was obtained.
► It has one more exon and intron comparing to fugu and pufferfish, showing direct evidence for intron insertion on fish TLR2.
► High levels of TLR2 were detected in organs in direct contact with the environment.
► TLR2 genes are under strong purifying selection pressure in both of fish and mammals.