Polymorphism at the defensin gene in the Anopheles gambiae complex: testing different selection hypotheses

Area of Science:

• * Molecular evolution and population genetics of insect immune response genes.
• * Analysis of genetic variation in defensin, a key immune effector in insects.

Background:

• * The Anopheles gambiae complex includes major vectors of malaria.
• * Defensins are crucial antimicrobial peptides involved in insect immunity.

Purpose of the Study:

• * To investigate genetic variation within and between populations of three Anopheles species.
• * To determine the evolutionary pressures acting on the defensin gene, particularly the mature peptide region.

Main Methods:

• * Analysis of genetic variation in defensin gene coding and non-coding regions.
• * Comparison of polymorphism and divergence patterns across three Anopheles species (An. gambiae, An. arabiensis, An. quadriannulatus).
• * Assessment of synonymous and non-synonymous variation to infer selective pressures.

Main Results:

• * Reduced polymorphism in defensin's mature peptide and coding regions, with fewer rare mutations.
• * Significantly lower non-synonymous diversity compared to synonymous variation in the mature peptide.
• * Conserved mature peptide sequence across the three species, with increased divergence in non-coding regions.

Conclusions:

• * Strong purifying selection acts on the defensin mature peptide and likely the entire coding region.
• * The conservation of the mature peptide suggests its function is critical and not solely driven by adaptation to human pathogens.
• * Human pathogens do not appear to be the primary selective agents shaping the evolution of this insect immune molecule.