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Vertical and horizontal transmission are terms that describe the transfer of symbionts from parents to offspring and among unrelated hosts, respectively. Many symbionts, including parasites, pathogens, mutualists, and microbiota, use a combination of both strategies, known as mixed-mode transmission (MMT). Here I review what is known about the evolution, ecology, and epidemiology of symbionts with MMT and compare MMT with our expectations for single-mode strategies. Symbionts with MMT are common and, in comparison with single-mode symbionts, show many surprising features. MMT combines the best of two worlds with regard to the ecological conditions required for persistence and plays a role in the evolution of virulence and genome architecture. Even rare transmission by the minority type of these two transmission modes can make a big difference for the system. This review explores the conceptual issues surrounding the dynamics of mixed-mode symbionts by reviewing literature from the entire range of host and symbiont taxa.
Dieter Ebert (Fri,) studied this question.