Molecular evolution of the three short PGRPs of the malaria vectors Anopheles gambiae and Anopheles arabiensis in East Africa

Abstract

Background: Immune responses to parasites, which start with pathogen recognition, play a decisive role in the control of the infection in mosquitoes. Peptidoglycan recognition proteins (PGRPs) are an important family of pattern recognition receptors that are involved in the activation of these immune reactions. Pathogen pressure can exert adaptive changes in host genes that are crucial components of the vector's defence. The aim of this study was to determine the molecular evolution of the three short PGRPs (PGRP-S1, PGRP-S2 and PGRP-S3) in the two main African malaria vectors - Anopheles gambiae and Anopheles arabiensis.

Results: Genetic diversity of An. gambiae and An. arabiensis PGRP-S1, PGRP-S2 and PGRP-S3 was investigated in samples collected from Mozambique and Tanzania. PGRP-S1 diversity was lower than for PGRP-S2 and PGRP-S3. PGRP-S1 was the only gene differentiated between the two species. All the comparisons made for PGRP-S1 showed significant P-values for Fst estimates and AMOVA confirming a clear separation between species. For PGRP-S2 and PGRP-S3 genes it was not possible to group populations either by species or by geographic region. Phylogenetic networks reinforced the results obtained by the AMOVA and Fst values. The ratio of nonsynonymous substitutions (Ka)/synonymous substitutions (Ks) for the duplicate pair PGRP-S2 and PGRP-S3 was very similar and lower than 1. The 3D model of the different proteins coded by these genes showed that amino acid substitutions were concentrated at the periphery of the protein rather than at the peptidoglycan recognition site.

Conclusions: PGRP-S1 is less diverse and showed higher divergence between An. gambiae and An. arabiensis regardless of geographic location. This probably relates to its location in the chromosome-X, while PGRP-S2 and PGRP-S3, located in chromosome-2L, showed signs of autosomal introgression. The two short PGRP genes located in the chromosome-2L were under purifying selection, which suggests functional constraints. Different types of selection acting on PGRP-S1 and PGRP-S2 and S3 might be related to their different function and catalytic activity.

Figure 1

Median-joining network for PGRP-S1, PGRP-S2 and PGRP-S3 genes. Network was based on 13 haplotypes for PGRP-S1 gene, 25 haplotypes for PGRP-S2 gene and 43 haplotypes for PGRP-S3. The area of circles is proportional to the frequency of the haplotypes. Black - An. arabiensis, Tanzania; Dark-grey - An. arabiensis, Mozambique; Light-grey - An. gambiae, Mozambique and White - An. gambiae, Tanzania.

Figure 2

Median-joining network for PGRP-S2 and PGRP-S3 coding regions of both An. arabiensis and An. gambiae. Network was based on thirty four haplotypes. The area of circles is proportional to the frequency of the haplotypes. A: Black - PGRP-S3, Grey - PGRP-S2; B: Black - An. gambiae, Grey - An. arabiensis.

Figure 3

Evolutionary relationships of the fourteen PGRP-S2/3 protein types. The evolutionary history was inferred using the Neighbor-Joining method. The bootstrap consensus tree was inferred from 1000 replicates. Tree was drawn to scale, with branch lengths in the same units as those of the evolutionary distances used to infer the phylogenetic tree. The evolutionary distances were computed using the Poisson correction method and are in the units of the number of amino acid substitutions per site. Red dots represent PGRP-S2 proteins, blue dots represent PGRP-S3 proteins and purple represent protein types shared by PGRP-S2 and PGRP-S3.

Figure 4

Structural model of PGRP-S2 and PGRP-S3 proteins. Three-dimensional (3D) structural localization of mutated amino acids represented as yellow and blue (Van de Walls spheres). The PGRP domain has three α helices (red), five β strands (green) and coils (grey); Arrow indicates the specificity-determining residues responsible for the muramyl pentapeptide - MPP-Dap recognition.