LAUSR

Swamp eel, Monopterus albus, is an important aquaculture species in Asia (mainly China) whose production has seriously suffered from infectious diseases. In spite of the critical requirement for aquaculture practices, to date there is scanty information on its immune defence. Here, genetic characteristics of Toll-like receptor 9 (TLR9) that plays crucial roles in the initiation of host defence against microbial invasion were analyzed. It exhibits a striking lack of genetic variation resulting from recent demographic bottleneck. A comparison with the homologue of M. javanensis revealed that replacement but not silent differences have nonrandomly accumulated in the coding sequences at the early stage following their split from a common ancestor. Further, the replacements relevant to the type II functional divergence have mainly occurred in structural motifs mediating ligand recognition and receptor homodimerization. These results provide hints to comprehend the diversity-based strategy of TLR9 in arms race against pathogens. Further, the findings reported here give credence to the importance of basic immunology knowledge, especially for the key elements, in genetic engineering and breeding for disease resistance of the eel and other fishes.