Population structure of Wolbachia and cytoplasmic introgression in a complex of mosquito species

Abstract

Background: The maternally inherited bacterium Wolbachia often acts as a subtle parasite that manipulates insect reproduction, resulting potentially in reproductive isolation between host populations. Whilst distinct Wolbachia strains are documented in a group of evolutionarily closely related mosquitoes known as the Culex pipiens complex, their impact on mosquito population genetics remains unclear. To this aim, we developed a PCR-RFLP test that discriminates the five known Wolbachia groups found in this host complex. We further examined the Wolbachia genetic diversity, the variability in the coinherited host mitochondria and their partitioning among members of the Cx. pipiens complex, in order to assess the impact of Wolbachia on host population structure.

Results: There was a strong association between Wolbachia and mitochondrial haplotypes indicating a stable co-transmission in mosquito populations. Despite evidence that members of the Cx. pipiens complex are genetically distinct on the basis of nuclear DNA, the association of Wolbachia and mtDNA with members of the Cx. pipiens complex were limited. The Wolbachia wPip-I group, by far the most common, was associated with divergent Cx. pipiens members, including Cx. quinquefasciatus, Cx. pipiens pipiens form pipiens and Cx. pipiens pipiens form molestus. Four other wPip groups were also found in mosquito populations and all were shared between diverse Cx. pipiens members.

Conclusion: This data overall supports the hypothesis that wPip infections, and their allied mitochondria, are associated with regular transfers between Cx. pipiens members rather than specific host associations. Overall, this is suggestive of a recent and likely ongoing cytoplasmic introgression through hybridization events across the Cx. pipiens complex.

Figure 1

wPip haplotypes phylogeny constructed using Maximum Parsimony method based on concatenated sequences of ank2, pk1, MutL, GP15 and RepA genes. Known wPip strains haplotypes are marked by full circles. Arrows show the two non-assigned haplotypes. Numbers on branches indicate percentage bootstrap support (500 replicates). Only bootstrap values > 70 were shown. The scale bar indicates the number of substitutions. Numbers in brackets correspond to locality numbers in Additional file 1: Table S1.

Figure 2

Geographic distribution of wPip groups in the World (A) and in Europe (B). Numbers in the maps correspond to locality numbers in Additional file 1: Table S1. Bars and dots represent natural populations and laboratory strains, respectively. The bars show the prevalence of wPip group: blue, wPip-I infection; green, wPip-II; yellow, wPip-III; pink, wPip-IV; orange, wPip-V; black, undetermined group. On Figure  2A, Culex pipiens pipiens form pipiens and Culex pipiens pipiens form molestus = light blue; Cx. p. pallens = light green and Cx. quinquefasciatus = light pink (modified after Farajollahi et al. 2011). On Figure  2B, both Cx. p. pipiens form pipiens and Cx. p. pipiens form molestus are widespread in Europe. Details on sample size, prevalence and Cx. pipiens complex taxa are given in Additional file 1: Table S1.

Figure 3

Comparison of wPip groups (A) with Cx. pipiens genetic clusters revealed by Bayesian analysis using microsatellites loci (B). Each of the 113 individuals included in the analysis is represented by a vertical line, partitioned into five squares assigned different colours (blue, wPip-I infection; green, wPip-II; yellow, wPip-III; pink, wPip-IV; orange, wPip-V) (A) and segments of different colours that represent the individual’s probability of belonging to one of the four genetic clusters (black, pallens; medium and dark grey, quinquefasciatus; soft grey, molestus; white, pipiens) (B). Specimens were grouped by location (bracketed), and the indicated population numbers are the same as in Additional file 1: Table S1.