The recognition of pathogens by host receptors is a fundamental process of immunity. The peptidoglycan recognition protein SD (PGRP-SD), a pattern recognition receptor (PRR) of Drosophila innate immune system plays an important role in detecting Gram-positive bacteria. Here we used functional assay to explore the evolutionary and immune significance of changes appearing in the Drosophila melanogaster PGRP-SD gene. The results of clean injury on the transgenic lines’ adults indicated that the target of one major PGRP-SD allele has surprisingly shifted target from bacterial peptidoglycan (PGN) to self abnormally exposed connatural molecule in adults. One amino acid mutation on the allele 1 achieved a functional turnover from recognition of pathogen-accociated molecular pattern (PAMP) to that of damage-associated molecular pattern (DAMP). By checking the exposure of self molecule, the allele 1 could trigger the immune system to react to invaders which may pass through parietal hurt after the injury. Thus one amino acid substitution has resulted in shift of recognition strategy in the fruit fly, and allow the host to fight against a much broader spectrum of microorganisms. Combined reported anti-immune strategies of microbial to PAMP and DAMP, strong dimorphic haplotype and functional differentiation of the two major PGRP-SD alleles suggested that they are under balancing selection. As effector of innate immunity, antimicrobial peptides play an important role in killing invading microorganisms. Numerous antimicrobial peptides have been identified from amphibians which are mainly secreted by skin. From two skin cDNA libraries of two individuals of the Chinese red belly toad (Bombina maxima), we identified 56 different antimicrobial peptide cDNA sequences, each of which encodes a precursor peptide that can give rise to two kinds of antimicrobial peptides, maximin and maximin H. Among these cDNA, we found that the mean number of nucleotide substitution per non-synonymous site in both the maximin and maximin H domains significantly exceed the mean number of nucleotide substitution per synonymous site, whereas the same pattern was not observed in other structural regions, such as the signal and propiece peptide regions, suggesting that these antimicrobial peptide genes have been experiencing rapid diversification driven by overdominant selection. We cloned and sequenced seven genes amplified from skin or liver genomic DNA. These genes have three exons and share the same gene structure, in which both maximin and maximin H are encoded by the third exon. This suggests that alternative splicing and somatic recombination are less likely to play a role in creating the diversity of maximins and maximin Hs. The gene trees based on different domain regions revealed that domain shuffling and/or gene conversion among these genes might have happened frequently.
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