One major advantage of the confined localization of some symbionts with the primary symbiont in the bacteriocyte is that the host immune system is thus avoided, representing a bidirectional advantage for the host which invests fewer resources in maintaining the symbiont levels and for the symbiont, which is not recognized by the immune system of the host. This confined localization ensures low cell numbers of the bacterium because of the limited space in the bacteriosome, and thus for the host, a lower fitness cost is associated with maintaining the

symbiont. An additional advantage for the symbiont is the ease of vertical transmission from one generation to the next. “”Hitching a ride”" with the primary symbiont in the bacteriocyte exempts the secondary see more symbiont from invading and entering the egg alone

during oogenesis, and ensures its transmission during the transfer of the bacteriocyte to the egg [16]. The localization pattern of the secondary symbionts confined to the bacteriocyte Temsirolimus in vivo in both B. tabaci and T. vaporariorum showed some specific localization to patches. This localization pattern was consistent in all of the individuals tested, and suggests specific sharing inside the bacteriocyte, with each symbiont, primary and secondary, occupying its own niche. Interestingly, all of the symbionts detected in B. tabaci were found to co-exist in the same individual, in varying percentages, suggesting little or no competition for space, with the exception of Arsenophonus and Hamiltonella which were not found together in B. tabaci, although they were found together in T. vaporariorum. Interestingly, in this latter species, their localization pattern in the bacteriocyte looked exactly the same, suggesting localization in exactly the same places or one inside the other [52]. Future experiments using TEM and ultrastructural localization should shed more light on the exact location of these symbionts relative to one another. In contrast to the symbionts that were restricted

to the bacteriocytes, Rickettsia and Cardinium in B. tabaci showed a scattered localization pattern and were seen outside PIK3C2G the bacteriocyte. These two symbionts are known to manipulate host reproduction in many arthropods [53, 54], and this fits well with their localization pattern in B. tabaci. Previously, Rickettsia has been shown to exhibit two different localization phenotypes: scattered throughout the body and confined to the bacteriocyte [22]. These two phenotypes were never observed together in the same individuals. It is not clear whether these localization phenotypes are characteristic of the host or if they are due to different bacteria localizing differently in the host’s body. Our FISH results showed the presence of both scattered and confined phenotypes in the same individuals for Rickettsia (Figure 10), and Cardinium (Figure 8).