Tian et al. BMC Genomics 2010, 11:187 http://www.biomedcentral.com/1471-2164/11/187 Page 12 of 19 Figure 9 AMPs from N. vitripennis and other insects. A. Number and feature comparison; B. Evolutionary divergence of N. vitripennis AMPs, in which gene duplication, terminal extension, tandem repeat and gene loss events are highlighted. further research. Finally, a typical amphipathic surface is present in the carboxyl-terminal a-helical region of the navidefensin1 subfamily members, suggesting a new antimicrobial unit has evolved on the classical defensin scaffold. Exon duplication In N. vitripennis, we identified two unusual defensin-like genes (i.e. nasonin-2 and nasonin-6) that encode proteins carrying 4-5 repeats of NLD without protease processing signals (Figure 11). Although we do not know how these genes originated, the gene structure of nasonin-6 gives a glimpse into its history. As shown in Figure 11, each NLD is encoded by a single exon, which is evidence for exon duplication [88]. From an evolutionary per- spective, exon duplication mediated internal repeats could be viewed in the same light as gene duplication, which is usually considered to be the principal path for complexity increase of a protein. For example, the leucin-rich repeats (LRRs) in ribonuclease inhibitor (RI) are displayed in 1-30 tandem copies within a pro- tein and such high LRR unit number could be critical for the evolution of both broad specificity and high affinity by RI [89]. The presence of multiple defensin repeats in nasonin-2 and -6 could have some structural significance if we consider the functional importance of forming oligomerization for membrane permeabiliza- tion of b-defensins [90]. Alternatively, new functional features were also evolved in some repeats, such as protease inhibitory activity [91] which may be useful in preventing from degradation of the whole molecule. The last domain of nasonin-2 has high sequence iden- tity (86%) and a conserved phase 1 intron after signal peptide to nasonin-1, suggesting it originated from one ancestral copy of nasonin-1 by exon duplication and subsequent intron loss (Figure 1). Exon-shuffling Exon-shuffling represents a major mechanism by which a limited set of modules can be shuffled by intronic recombination. Such mechanism has significantly con- tributed to the emergence of novel complex multido- main proteins, novel functions and increased organismic complexity. Gene structure conservation in multidomain immune molecules suggests that exon shuffling may be involved in the evolution of the immune system [92]. However, in most case tracing such event is rather diffi- cult due to accelerated sequence changes. Nabaecins, chimeric genes of abaecin and navitripenicin as men- tioned previously (Figure 5), provides some clues to see a history of immune-related genes, in which exon-shuf- fling is likely involved. At the sequence level, the NtAU