Convergent balancing selection on an antimicrobial peptide in Drosophila

Unckless, R. L., Howick, V. M. and Lazzaro, B. P. (2016) Convergent balancing selection on an antimicrobial peptide in Drosophila. Current Biology, 26(2), pp. 257-262. (doi: 10.1016/j.cub.2015.11.063) (PMID:26776733) (PMCID:PMC4729654)

Abstract

Genes of the immune system often evolve rapidly and adaptively, presumably driven by antagonistic interactions with pathogens [1–4]. Those genes encoding secreted antimicrobial peptides (AMPs), however, have failed to exhibit conventional signatures of strong adaptive evolution, especially in arthropods (e.g., [5, 6]) and often segregate for null alleles and gene deletions [3, 4, 7, 8]. Furthermore, quantitative genetic studies have failed to associate naturally occurring polymorphismin AMP genes with variation in resistance to infection [9–11]. Both the lack of signatures of positive selection in AMPs and lack of association between genotype and immune phenotypes have yielded an interpretation that AMP genes evolve under relaxed evolutionary constraint, with enough functional redundancy that variation in, or even loss of, any particular peptide would have little effect on overall resistance [12, 13]. In stark contrast to the current paradigm, we identified a naturally occurring amino acid polymorphism in the AMP Diptericin that is highly predictive of resistance to bacterial infection in Drosophila melanogaster [13]. The identical amino acid polymorphism arose in parallel in the sister species D. simulans, by independent mutation with equivalent phenotypic effect. Convergent substitutions at the same amino acid residue have evolved at least five times across the Drosophila genus. We hypothesize that the alternative alleles are maintained by balancing selection through context-dependent or fluctuating selection. This pattern of evolution appears to be common in AMPs but is invisible to conventional screens for adaptive evolution that are predicated on elevated rates of amino acid divergence.

• Item Type: Articles
• Additional Information: Funding for this work was provided by National Institute of Allergy and Infectious Disease grants (R01 AI083932 and R01 AI064950) to B.P.L. and NIH National Research Service Award (F32-HD071703) and NIH Pathway to Independence Award (K99-GM114714) to R.L.U.
• Status: Published
• Refereed: Yes
• Glasgow Author(s) Enlighten ID: Howick, Dr Virginia
• Authors: Unckless, R. L., Howick, V. M., and Lazzaro, B. P.
• College/School: College of Medical Veterinary and Life Sciences > School of Biodiversity, One Health & Veterinary Medicine
• Journal Name: Current Biology
• Publisher: Elsevier
• ISSN: 0960-9822
• ISSN (Online): 1879-0445