The Epidemiology and Evolution of Symbionts with Mixed-Mode Transmission

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.