A complex journey: transmission of microbial symbionts

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Abstract
Symbiont transmission enables the host to maintain a symbiosis throughout generations. Two profoundly different strategies can be distinguished. The symbiont can be selected out of a pool of environmental bacteria (horizontal transmission), or the host offspring can take up the symbiont from the parents following a finely choreographed baton exchange (vertical transmission). There are many variations of these two modes, and transmission can also involve both vertical and horizontal transfers and intraspecific or interspecific host switching. From the moment of initial contact, the symbionts have a long journey to reach their final residence. This includes establishing contact with the host, entering the host, evading the host immune defences and travelling to the symbiont housing organ. The two main transmission modes shape the evolution of the symbiotic partners.