Wolbachia transmission dynamics in Formica wood ants

Lumi Viljakainen,Max Reuter,Pekka Pamilo

doi:10.1186/1471-2148-8-55

Lumi Viljakainen, Max Reuter + Show 1 more

Open Access

https://doi.org/10.1186/1471-2148-8-55

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Abstract

BackgroundThe role of Wolbachia endosymbionts in shaping the mitochondrial diversity of their arthropod host depends on the effects they have on host reproduction and on the mode of transmission of the bacteria. We have compared the sequence diversity of wsp (Wolbachia surface protein gene) and the host mtDNA in a group of Formica ant species that have diverged approximately 0.5 million years ago (MYA). The aim was to study the relationship of Wolbachia and its ant hosts in terms of vertical and horizontal transmission of the bacteria.ResultsAll studied ant species were doubly infected with two Wolbachia strains (wFex1 and wFex4) all over their geographical distribution area in Eurasia. The most common haplotypes of these strains were identical with strains previously described from a more distantly related Formica ant, with an estimated divergence time of 3.5 – 4 MYA. Some strain haplotypes were associated to the same or closely related mtDNA haplotypes as expected under vertical transmission. However, in several cases the wsp haplotypes coexisted with distant mtDNA haplotypes, a pattern which is more compatible with horizontal transmission of the bacteria.ConclusionTwo lines of evidence suggest that the sharing of Wolbachia strains by all F. rufa species is rather due to horizontal than vertical transmission. First, the fact that endosymbiont strains identical to those of F. rufa ants have been found in another species that diverged 3.5–4 MYA strongly suggests that horizontal transfer can and does occur between Formica ants. Second, the frequent sharing of identical Wolbachia strains by distant mitochondrial lineages within the F. rufa group further shows that horizontal transmission has occurred repeatedly. Nevertheless, our dataset also provides some evidence for longer-term persistence of infection, indicating that Wolbachia infection within this host clade has been shaped by both horizontal and vertical transmission of symbionts. The fact that all the ants were infected irrespective of the family structure of their societies gives no support to the proposed hypotheses that the spreading of Wolbachia in ants might be associated to the types of their societies.

Highlights

The role of Wolbachia endosymbionts in shaping the mitochondrial diversity of their arthropod host depends on the effects they have on host reproduction and on the mode of transmission of the bacteria
Two lines of evidence suggest that the sharing of Wolbachia strains by all F. rufa species is rather due to horizontal than vertical transmission
The fact that endosymbiont strains identical to those of F. rufa ants have been found in another species that diverged 3.5–4 million years ago (MYA) strongly suggests that horizontal transfer can and does occur between Formica ants

Summary

Introduction

The role of Wolbachia endosymbionts in shaping the mitochondrial diversity of their arthropod host depends on the effects they have on host reproduction and on the mode of transmission of the bacteria. Defined by using a polymerase chain reaction (PCR) assay and the transmission dynamics investigated by comparing Wolbachia strains with associated mtDNA haplotypes. For this purpose we used the mitochondrial phylogeny of F. rufa group ants [20] based on samples from a large geographical scale covering most of the Eurasian distribution of the wood ants, and we screened Wolbachia in ants from the same nests used in the mtDNA study. The effect of Wolbachia on host genetic variation depends on its transmission dynamics, i.e. whether it is strictly vertically transmitted from mother to offspring, or whether horizontal transmission occurs between individuals or even between species. Horizontal transmission of Wolbachia will weaken the association between Wolbachia and the mitochondrial haplotype, and the effects on host mitochondrial DNA variation are presumed to be negligible

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