However, some other internal factors may influence maximum horizontal transfers and maximum infection rates in the same individuals. These factors include competition for space and resources among two or more symbionts [22, 43], or on the contrary, positive interaction between the symbionts may contribute to maximum infection in one individual . Another important factor is the host response to the presence of these symbionts which in most cases will influence the bacterial community residing within the host. The occurrence
of mixed infections in both species also suggests that these secondary symbionts are non-essential for these whiteflies, allowing their presence to be variable. In one report, Hamiltonella was found in 40% of B. tabaci populations , signaling pathway and 0 to 40% of pea aphid populations have been found to harbor Rickettsia [45–50]. Only Hamiltonella was highly prevalent in B. tabaci populations and sometimes reached fixation, an indication of a mutualistic or obligatory
interaction with the insect. Such interactions can occur via complementation of the primary symbiont’s function with regard to completing the host’s dietary needs or enhancing host fitness. All of the symbionts detected in both whitefly species were located together with the primary symbiont Portiera in the bacteriocytes at one or more stages of development. However, some were strictly localized to the bacteriocytes during all developmental stages–Hamiltonella selleck chemicals and Wolbachia in B. tabaci, and Hamiltonella and Arsenophonus in T. vaporariorum, while others were located inside and outside the bacteriocyte–Rickettsia and Cardinium in B. tabaci. Symbionts that are strictly localized to the bacteriocytes are vertically transmitted and thus they may contribute to their host’s fitness . However, they are less likely
to be able to manipulate their host’s reproduction since this requires invading reproductive organs outside the bacteriocyte. Thus, the restricted Cyclin-dependent kinase 3 localization of Hamiltonella in both B. tabaci and T. vaporariorum, Wolbachia in B. tabaci and Arsenophonus in T. vaporariorum suggests their involvement in providing the host with a functional advantage rather than in manipulating its reproduction. Interestingly, Wolbachia was localized to the bacteriocyte and was not observed outside it, invading other organs. Wolbachia can be found in all major insect orders at various different frequencies, and it has been associated with reproductive disorders . However, the localization pattern in B. tabaci observed here suggests that Wolbachia does not manipulate reproduction in this whitefly, but rather performs other unknown functions.