The Darwin Finches of the Bacterial World
Under specific conditions, organisms with a particular ability to adapt to their environment are able to initiate an explosive process of speciation. This process, called «adaptive radiation», was originally described by Charles Darwin to explain the variety in form observed in finches on the Galápagos Islands (Darwin finches) and since then also observed in the speciation of numerous other animals and plants. That bacteria, in acquiring new traits, could also trigger adaptive radiation has been demonstrated for the first time by an international research team headed by Prof. Christoph Dehio at the Biozentrum of the University of Basel. Using the pathogen Bartonella as a model, the scientists have been able to show that bacteria, through the acquisition of a molecular injection needle to inject bacterial proteins into the host cells, can adapt much more efficiently to a new a host organism, such as man. The knowledge gained is of fundamental importance for the understanding of the evolution of new types of human pathogens. The research findings have been published in the current issue of the US scientific journal PLoS Genetics.
Adaptive radiation is the rapid origination of multiple species from a single ancestor as the result of concurrent adaption to disparate environments. This fundamental evolutionary process is considered to be responsible for the genesis of a great portion of the diversity of life. Adaptive radiations typically occur after the arrival of a founding population in a novel environment with unoccupied ecological niches ('ecological opportunity') and/or by the acquisition of a novel trait ('evolutionary key innovation') allowing the exploitation of so far unapproachable niches.
Adaptive Radiations are Well Known in Animals and Plants
The paradigm of adaptive radiation are the Darwin finches at Galápagos Islands - morphological adaption of the beak allowed the 14 known species to nourish on different food sources. Prominent examples for adaptive radiations are also described for reptiles, fishes, insects and plants, however, little is known about this fundamental process of speciation for bacteria despite their enormous biological diversity.
A Novel Example of Adaptive Radiation in Bacteria
At the University of Basel, researchers of the team of the infection biologist Prof. Christoph Dehio at the Biozentrum in collaboration with the evolutionary biologist Prof. Walter Salzburger from the Zoological Institute and further international collaborators have described for the first time a compelling example of an adaptive radiation in bacteria, and furthermore uncovered its molecular basis. They investigated speciation of the bacterial pathogen Bartonella - each of the closely related species is specifically adapted to a different mammalian host (specific host = ecological niche).
Parallel Evolution Provides Insight how Novel Pathogens Emerge
The detailed investigation of the pylogenetic relation of the Bartonella species resulted in the unexpected discovery of two parallel adaptive radiations with multiple examples of pairs of species from each radiation co-adapting to the same mammalian host. Such striking examples of parallel evolution typically facilitate uncovering of the underlying molecular evolutionary mechanism. Indeed, the parallel adaptive radiations of the bartonellae could be associated to the independent acquisition of a novel adaptive trait serving to facilitate host adaption. This evolutionary key innovation is represented by a molecular injection needle ('type IV secretion system') that injects a cocktail of bacterial effector proteins adapted specifically to the respective mammalian host of the different Bartonella species. Given that Bartonella species have repeatedly switched their mammalian hosts giving rise to at least two specifically human-adapted pathogens the knowledge gained in this study is of fundamental importance for our understanding of the evolution of novel human pathogens from animal-specific pathogens.
Philipp Engel, Walter Salzburger, Marius Liesch, Chao-Chin Chang, Soichi Maruyama, Christa Lanz, Alexandra Calteau, Aurélie Lajus, Claudine Médigue, Stephan C. Schuster & Christoph Dehio Parallel evolution of a type IV secretion system in radiating lineages of the host-restricted bacterial pathogen Bartonella. PLoS Genetics. Published online: 10. February, 2011 | doi: pgen.1001296. PDF