Abstract
BackgroundMaternally inherited Wolbachia bacteria infect many insect species. They can also be transferred horizontally into uninfected host lineages. A Wolbachia spillover from an infected source population must occur prior to the establishment of heritable infections, but this spillover may be transient. In a previous study of tephritid fruit fly species of tropical Australia we detected a high incidence of identical Wolbachia strains in several species as well as Wolbachia pseudogenes. Here, we have investigated this further by analysing field specimens of 24 species collected along a 3,000 km climate gradient of eastern Australia.ResultsWolbachia sequences were detected in individuals of nine of the 24 (37 %) species. Seven (29 %) species displayed four distinct Wolbachia strains based on characterisation of full multi locus sequencing (MLST) profiles; the strains occurred as single and double infections in a small number of individuals (2–17 %). For the two remaining species all individuals had incomplete MLST profiles and Wolbachia pseudogenes that may be indicative of lateral gene transfer into host genomes. The detection of Wolbachia was restricted to northern Australia, including in five species that only occur in the tropics. Within the more widely distributed Bactrocera tryoni and Bactrocera neohumeralis, Wolbachia also only occurred in the north, and was not linked to any particular mitochondrial haplotypes.ConclusionsThe presence of Wolbachia pseudogenes at high prevalence in two species in absence of complete MLST profiles may represent footprints of historic infections that have been lost. The detection of identical low prevalence strains in a small number of individuals of seven species may question their role as reproductive manipulator and their vertical inheritance. Instead, the findings may be indicative of transient infections that result from spillover events from a yet unknown source. These spillover events appear to be restricted to northern Australia, without proliferation in host lineages further south. Our study highlights that tropical fruit fly communities contain Wolbachia pseudogenes and may be exposed to frequent horizontal Wolbachia transfer. It also emphasises that global estimates of Wolbachia frequencies may need to consider lateral gene transfer and Wolbachia spillover that may be regionally restricted, transient and not inherited.Electronic supplementary materialThe online version of this article (doi:10.1186/s12862-015-0474-2) contains supplementary material, which is available to authorized users.
Highlights
Inherited Wolbachia bacteria infect many insect species
We considered these four individuals infected with low titre Wolbachia because we had previously characterised complete multi locus sequence typing (MLST) profiles in other individuals of these host species
In this study of Australian tephritids we detected several Wolbachia pseudogenes that may be host-genomic footprints of historic infections that have since been lost from host species
Summary
Inherited Wolbachia bacteria infect many insect species They can be transferred horizontally into uninfected host lineages. It can be expected that the frequency of spillover is larger than the actual establishment of newly inherited infections, and this difference may inflate global estimates of actual infection frequencies Another overestimation of infection frequencies (in particular in studies that rely on a single or a few marker genes) may stem from lateral transfer of Wolbachia genes into host genomes that can be seen as footprints of historic infections [14] that may occur without the presence of current infections, yet this has so far rarely been addressed in Wolbachia surveys [15]
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