Exploring diversity in avian immune defence: Insights from cathelicidin clusters

Cathelicidins are a family of proteins from which a class of Host Defence Peptides (HDPs) is derived. They are components of the innate immune system of most vertebrates, including birds. Despite their promising activities, the genomic organisation and interspecies diversity of avian cathelicidins has been less studied than in other animal groups. In this research, we investigated the cathelicidin cluster in 72 avian species from 26 different orders by mining the avian genome assemblies available in NCBI database, using bioinformatics tools to analyse the cluster composition, gene structure and phylogenetic relationships. Cathelicidin clusters were found principally on chromosomes 1 and 2, usually located at the ends of the chromosomes, except in Falconiformes and Psittaciformes. The Galloanserae cluster diverged from the rest of avian groups by having cath1 in the Galliformes and a putative pseudogene of cathB1 in Anseriformes. In contrast, the remaining avian species displayed a predominantly cathelicidin cluster comprising cathB1, cath3, and cath2. However, Passeriformes lacked cath3 while Falconiformes exhibited the pseudogenisation of cath3. In addition, we found kelch like family member 18 and transforming growth factor beta 4 (zinc finger protein 777 in Passeriformes) as flanking genes. We identified 190 putative cathelicidins genes, of which 103 were undescribed, that displayed a high percentage of identity across cathelicidin type. Phylogenetic analysis revealed that cathelicidin genes are highly conserved supporting the hypothesis that cathelicidins play a crucial role in avian immunity. This work highlights the use of bioinformatic tools to improve our understanding of avian cathelicidins and the evolution of this important family protein.