The observation that levels of Wolbachia in the somatic tissues of workers are lower than their queen counterparts suggests that either they regulate infection differently or else that Wolbachia growth varies with caste. It has previously been reported that adult workers of this species and of the ant Formica truncorum, can have lower infection prevalences than queens and males, suggesting that infection may be lost over a worker’s development. Whilst we did not investigate the infection intensity over time, our results suggest that adult workers maintain infection at a lower level and may therefore have a greater potential to lose infection than queens. Social insect castes are known to differ in immune and hormonal profiles. The variation in these factors could all provide cues for a parasite-mediated regulation of infection intensity or else affect the way in which the host interacts with different Wolbachia strains. It must however be noted that a study of Acromyrmex octospinosus has shown that Wolbachia infection intensity of workers appears to increase with age. It may be that the increase with age occurs Mechlorethamine hydrochloride faster in the reproductive queens and this is why higher infection intensity is seen here. Alternatively, it may be that an initial suppression in workers is either absent or at a lower strength in the reproductive females. The relatively low infection intensity in the ovaries of queens is not consistent with specialisation for reproductive parasitism that has long been thought to be the main transmission strategy of Wolbachia. The considerable intensities of Wolbachia in somatic tissues of queens and sterile castes found here add to the growing body of evidence that there is far more to Wolbachia infections than simply vertical transmission. Here, the distribution of Wolbachia seen in the somatic tissues of the queens shows tropism, with higher densities of Wolbachia being found in the fat body and haemolymph compared to the gut and ovary, suggesting that the distribution of Wolbachia in somatic tissues is not simply a side-effect of infection of the reproductive tissues. There are three non-mutually exclusive hypotheses for this interesting result. First, it could be that horizontal transmission of Wolbachia is not an evolutionary ��accident’ and is instead an important transmission route for Wolbachia, as supported by phylogenetic studies. Second, infection of the somatic tissues could be maintained if transfer of Wolbachia between somatic and germ tissue is required for vertical transmission. In the ovaries of Drosophila melanogaster and Zyginidia pullula, for example, Wolbachia have been found in high densities in bacteriocyte-like cells that are possibly of somatic Capromorelin tartrate origin. The authors of this study note that the vertically transmitted symbionts that infect the bacteriocytes of the cockroach Blattella germanica, migrate from somatic tissues within their bacteriocytes which are subsequently incorporated into the ovary, from which the bacteria can go on to infect developing oocytes, suggesting that this may also applicable to some Wolbachia-insect systems. Third, Wolbachia may have specialised on 
particular somatic tissues to produce effects other than those associated with sex ratio distortion. A similar pattern of high infection intensity in fat body and other somatic tissues was seen in the mosquito Anopheles gambiae after transfection with the virulent popcorn Wolbachia strain wMelPop.