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

Abstract

Genetic variation in defensin, a gene encoding a major effector molecule of insects immune response was analyzed within and between populations of three members of the Anopheles gambiae complex. The species selected included the two anthropophilic species, An. gambiae and An. arabiensis and the most zoophilic species of the complex, An. quadriannulatus. The first species was represented by four populations spanning its extreme genetic and geographical ranges, whereas each of the other two species was represented by a single population. We found (i) reduced overall polymorphism in the mature peptide region and in the total coding region, together with specific reductions in rare and moderately frequent mutations (sites) in the coding region compared with non-coding regions, (ii) markedly reduced rate of non-synonymous diversity compared with synonymous variation in the mature peptide and virtually identical mature peptide across the three species, and (iii) increased divergence between species in the mature peptide together with reduced differentiation between populations of An. gambiae in the same DNA region. These patterns suggest a strong purifying selection on the mature peptide and probably the whole coding region. Because An. quadriannulatus is not exposed to human pathogens, identical mature peptide and similar pattern of polymorphism across species implies that human pathogens played no role as selective agents on this peptide.

Figure 1

Variation of nucleotide diversity (π) along the defensin gene sequence within species and populations of the An. gambiae complex. Above: Total nucleotide diversity variation within An. arabiensis (black line, closed diamonds), An. quadriannulatus (black line, open diamonds) and the An. gambiae populations from West-Kenya (grey line, closed circles), East-Kenya (grey line, closed triangles), Nigeria (grey line, closed squares) and Senegal (grey line, closed diamonds). Graphs were generated using DnaSp 4.0 (Rozas & Rozas, 1999) using a sliding window of 100bp and a step length of 25 bp. Below: Schematics of the defensin gene showing different functional regions: the 5′ and 3′ non-transcribed non-coding flanking regions ( NoTran), the transcribed but non coding region including the intron (□ TrNoCo), and both exons of the defensin gene encoding the signal peptide ( Sig), the clipped pro-peptide ( Pro) and the mature peptide (■ Mature).