Abstract
Wolbachia is an intracellular α-proteobacterium which is transmitted vertically from mother to offspring but also frequently switches horizontally from one host to another. Our hypothesis is based on the role of immune cells and the organs that produce them, the hematopoietic organs (HOs), as primordial niches for the propagation of Wolbachia via hemocytes both (i) within hosts: to initiate and maintain the systemic infection and (ii) between hosts: to promote both vertical and horizontal transmission of Wolbachia. Therefore, we review some fundamental ideas underlying this hypothesis and go further with new empirical data that lead to a first close-up analysis of the potential role of HOs in Wolbachia propagation. The monitoring of the first steps of Wolbachia infection in horizontally infected host organs by transmission electron microscopy and qPCR suggests that (i) HOs are colonized early and extensively as soon as they are in contact with Wolbachia which find in these cells a favorable niche to multiply and (ii) infected HOs which expel hemocytes all lifelong can generate and maintain a systemic infection that could contribute to increase both vertical and horizontal propagation of these symbionts.
Highlights
The kind of symbiotic interactions formed between a symbiont and its host depends on the virulence of the first and the resistance of the latter (Combes, 2001)
We present in this paper empirical elements that led to a first close-up analysis of the potential role of Hematopoietic Organs (HOs), involved in the hemocyte production, in Wolbachia propagation
Armadillidium vulgare individuals used in these experiments were either infected with a feminizing Wolbachia strain [lines originating from Helsingör (Denmark) or Celles-sur-Belle (France) (Rigaud et al, 1991; Cordaux et al, 2004)] or uninfected animals [lines originating from Helsingör (Denmark) or Nice (France) (Bouchon et al, 1998; Sicard et al, 2010)]
Summary
The kind of symbiotic interactions formed between a symbiont and its host depends on the virulence of the first and the resistance of the latter (Combes, 2001). The HOs which are localized between the sixth and seventh pereion segment and the first telson segment, along the dorsal vessel, were dissected from animals (either infected with Wolbachia or not) They were sucked up with a thin glass Pasteur pipette and grafted in the recipient host (either infected with Wolbachia or not) for which a small square of 2 mm × 2 mm of cuticle was cut off to allow the introduction of the Pasteur pipette containing the graft. The quantification of Wolbachia was performed by qPCR on DNA samples from ovaries, nervous chain, hemocyte and HO for each treatment collected 15 days after transinfection The quantification of Wolbachia by qPCR in 12 asymbiotic recipient animals 15 days after their transinfection with hemolymph from symbiotic animals revealed that all tissues collected (ovaries, nervous chain, hemocytes and HOs) in the recipient animals were infected with Wolbachia (Figure 2A). The highest densities of Wolbachia were registered in HOs (some individuals showed more than 2000 Wolbachia/ng DNA in HOs while other tissues never exhibit more than 200 Wolbachia/ng DNA in any recipient individuals)
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