Bacterial Leaf Symbiosis in Angiosperms: Host Specificity without Co-Speciation

Bacterial leaf symbiosis is a unique and intimate interaction between bacteria and flowering plants, in which endosymbionts are organized in specialized leaf structures. Previously, bacterial leaf symbiosis has been described as a cyclic and obligate interaction in which the endosymbionts are vertically transmitted between plant generations and lack autonomous growth. Theoretically this allows for co-speciation between leaf nodulated plants and their endosymbionts. We sequenced the nodulated Burkholderia endosymbionts of 54 plant species from known leaf nodulated angiosperm genera, i.e. Ardisia, Pavetta, Psychotria and Sericanthe. Phylogenetic reconstruction of bacterial leaf symbionts and closely related free-living bacteria indicates the occurrence of multiple horizontal transfers of bacteria from the environment to leaf nodulated plant species. This rejects the hypothesis of a long co-speciation process between the bacterial endosymbionts and their host plants. Our results indicate a recent evolutionary process towards a stable and host specific interaction confirming the proposed maternal transmission mode of the endosymbionts through the seeds. Divergence estimates provide evidence for a relatively recent origin of bacterial leaf symbiosis, dating back to the Miocene (5–23 Mya). This geological epoch was characterized by cool and arid conditions, which may have triggered the origin of bacterial leaf symbiosis

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 expec- tations 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 condi- tions 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