Evolution of Immune Proteins in Insects

Abstract

Insect immune proteins evolve more rapidly than nonimmune proteins. Based largely on genetic studies in Drosophila species, this elevated rate appears to be due to adaptive evolution in only a small subset of immune proteins. The genetic signature of adaptive evolution is apparently a response by the host to interference or evasion of the immune system by pathogens, which are usually unknown. In contrast, many insect immune proteins appear to be constrained not to change because they interact with conserved molecular targets in a pathogen's cell wall or plasma membrane. Adaptive immune protein evolution may be attributable to a sustained arms race between host and pathogen or to rapid environmental change that exposes insects to new pathogen suites. Evidence of a correlation between the strength of selection and functional divergence in antimicrobial peptides provides confirmation that the inferred genetic signatures of adaptive evolution reflect a response to pathogen selective pressure.

Key Concepts:

  • A small set of immune proteins in Drosophila evolve much more rapidly than other proteins.

  • Insect antiviral proteins of the RNA interference system are among the top 3% of rapidly evolving proteins in Drosophila.

  • There is surprisingly little evidence of adaptive evolution in Drosophila antimicrobial peptides.

  • Many receptors and effectors of the insect immune system appear to be evolutionary constrained due to their interaction with conserved target molecules.

  • Pathogens target certain vulnerable immune proteins for suppression and positive selection in these proteins appears to reflect a counter response to this suppression.

  • The selective pressure imposed by pathogens appears to be greater for social than nonsocial insects.

  • Rapid changes in the types of pathogen that hosts encounter as well as long‐term co‐evolutionary arms races may drive adaptive immune protein evolution.

  • Positive selection appears to drive the functional divergence of antimicrobial peptides in termites and other animals.

Keywords: innate immunity; positive selection; balancing selection; purifying selection; adaptive evolution; arms race; co‐evolution

Figure 1.

Termicins are composed of 37 amino acids forming an α helix and β sheet (shown as a tubular structure in black boxes) connected by three disulfide bridges (orange lines). Positive selection directed the change of three positively charged amino acids (shown in yellow) to neutrally or negatively charged amino acids.

Figure 2.

AMP1 is predicted to be equivalent to the reconstructed AMPa in its effectiveness at damaging or destroying pathogen P1 associated with the host that produces AMP1 because the protein has been constrained not to change by negative selection. However, AMP2 is predicted to be more effective than AMPa against P2 because the molecule has changed in response to divergence in P2. The two AMPs are also predicted to be best at damaging or destroying their pathogen that they have evolved to cope with. For P1 the efficacy of AMP1 is predicted to be greater than AMP2 and for P2 the efficacy of AMP2 is predicted to be greater than AMP1.

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Further Reading

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Bulmer, Mark S(Nov 2010) Evolution of Immune Proteins in Insects. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0022889]