As argued above, secondary symbionts are very common in arthropods. What is not known is what proportion of secondary symbionts are reproductive parasites, how many propagate through horizontal transmission combined with vertical transmission, how many produce a direct benefit, and, of these, what proportion produce resistance to pathogens and parasites.
In this context, it is particularly interesting to examine the 'common bacteria': Wolbachia, Rickettsia, Spiroplasma, Cardinium, and Arsenophonus. For Wolbachia, there are a number of cases where reproductive parasitism phenotypes have been sought, but have proved either weak or absent (e.g. Hoffmann et al., 1996). Study of the frequency of these infections in natural populations, and comparison of infection frequency in male and female hosts, has indicated that reproductive parasitism is a poor explanation for infection presence in many cases (Duron et al., 2008b).
Without reproductive parasitism, maintenance of the symbiont requires either horizontal transmission or a direct benefit to infection. Whereas secondary symbionts can produce direct benefits outside of immunity (for instance, increasing the spectrum of host plants that can be utilized or improving thermal tolerance; Montllor et al., 2002; Tsuchida et al., 2004; Dunbar et al., 2007), it is quite likely that many secondary symbionts have unrecognized effects on insect resistance to pathogens and parasitoids. In some cases, there is circumstantial evidence of this from geographical association between infection frequency and frequency of para-sitization, for instance in the case of Arsenophonus in psyllids (Hansen et al., 2007).
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