Insect facultative symbionts: Biology, culture, and genetic modification

Maternally transmitted, symbiotic bacteria have been identified in a wide range of insect taxa (Buchner, 1965). Although a few of these symbionts are known to be parasitic (e.g., Wolbachia spp.), the majority appears to have a beneficial effect upon their hosts. These mutualistic associations can be further classified as obligate or facultative, based on the level of dependence exhibited by the host insect. In an obligate association, the symbiont is anticipated to provide benefits that are essential for host survival and reproduction. In a facultative association, the symbiont is anticipated to provide benefits that merely enhance host fitness, sometimes under specific circumstances. Often, but not exclusively, the obligate associations are based on nutrition; symbionts provide essential nutrients that are either absent or restricted in the host’s natural diet.