Variability and evolution of bovine beta-defensin genes

Defensins comprise an important family of antimicrobial peptides. Among vertebrates numerous defensin genes have been detected, but their evolutionary background is still discussed. We investigated the molecular evolution of bovine defensins via screening of different bovine species including the extinct ancestor of domestic cattle (Bos primigenius) for beta-defensin encoding genes. We detected a large variability of new defensin encoding sequences similar to previously published bovine neutrophil beta-defensin (bnbd), neutrophil beta-defensin 12 (nbd12), enteric beta-defensin (ebd), lingual antimicrobial peptide (lap), and tracheal antimicrobial peptide (tap). Our data suggest that variants of the same so-called subfamily (tap, lap, ebd, and nbd12) each share a common ancestry independent of their species origin, implicating several duplication events of tap, lap, ebd, and nbd12 before the different bovine lineages diverged. Variants of bovine neutrophil beta-defensins bnbd5 and bnbd9 were detected exclusively in domestic cattle and aurochs. Values of synonymous and nonsynonymous substitutions demonstrated lap, bnbd5, bnbd9 and nbd12 evolving under positive selection, whereas amino-acid altering substitutions among variants of ebd and tap are purified. Comparison of the amino-acid sequences with available structures of human and murine defensins suggested conservation of the typical secondary elements of defensins in the absence of high sequence similarity.