Cannibalism and predation as paths for horizontal passage of Wolbachia between terrestrial isopods

Abstract

The alpha-proteobacteria Wolbachia are the most widespread endosymbionts in arthropods and nematodes. Mainly maternally inherited, these so-called sex parasites have selected several strategies that increase their vertical dispersion in host populations. However, the lack of congruence between the Wolbachia and their host phylogenies suggests frequent horizontal transfers. One way that could be used for horizontal Wolbachia transfers between individuals is predation. The aim of this study was to test whether horizontal passage of Wolbachia is possible when an uninfected terrestrial isopod eats an infected one. After having eaten Armadillidium vulgare harbouring Wolbachia, the predator-recipients (the two woodlice A. vulgare and Porcellio dilatatus dilatatus) that were initially Wolbachia-free were tested positive for the presence of Wolbachia both by quantitative PCR and Fluorescence in situ Hybridization (FISH). Even if the titers were low compared to vertically infected individuals, this constitutes the first demonstration of Wolbachia occurrence in various organs of an initially uninfected host after eating an infected one.

Figure 1. Wolbachia loads in CNS, ovaries and hemocytes of the “predators” Porcellio d. dilatatus and Armadillidium vulgare.

The Wolbachia quantifications performed at 90 and 180 days post-ingestion (PI) revealed that the symbiont colonizes all tissues of the asymbiotic “predators” after ingestion of infected “preys”. Comparison of the Wolbachia titers between the two predator species (i.e. P. d. dilatatus or A. vulgare) revealed that bacterial loads were higher in A. vulgare, the native host of wVulC strain, at 90 days PI. Comparison of Wolbachia titers in the different tissues of P. d. dilatatus between 90 and 180 days PI showed that the infection was stable in ovaries and CNS but decreased in hemocytes. (Nat.titer: The Wolbachia titer in the “prey” hemolymph; #: No A. vulgare were infected with Wolbachia among the two tested animals).

Figure 2. Fluorescence in situ Hybridization (FISH) detection of Wolbachia in circulating hemocytes of the “predators” Porcellio d. dilatatus and Armadillidium vulgare 180 days after having ingested of an infected prey.

Some hemocytes of P. d. dilatatus (A and B) and of A. vulgare (C and D) were infected with Wolbachia (3 and 8% of hemocytes, respectively). A-D: Wolbachia in red, Actin in green, nuclei in blue.