Microbes as Key Players During Evolution Part II- From Unicellular to Multicellular Organisms

Microbes and thereof derived structures (plasmids, plastids, transposible elements, retrotransposons) are described as key drivers in the evolution of genes, genomes and new species in metazoa. This is a change of paradigm compared to the mechanisms of mutation and selection, and Mendelian genetics. The review first describes early metazoan developments and early symbiotic interactions. It presents examples of microbial influences on medusazoans, corals and sea anemones and then continues with microbial impacts on terrestrial ecosystems: Mutualistic associations of fungi, bacteroids and bacteria with lower plants (lichens, bryophytes) and higher plants (gymnosperms, angiosperms) and the exchange of reduced carbon for reduced nitrogen. The review goes on with animals, starting with the most recent common ancestor of vertebrates about 560 million years ago. Microbial impacts are described on insects, amphibians, reptiles, birds, and mammals including homo sapiens. A further chapter deals with impacts of transposible elements. Special emphasis is directed towards the creation of new species. Chapter 6 describes bipartite and tripartite host-microbe cooperations which led to new holobiont species in metazoa. Defense systems and reticulate evolution are further topics of this review. The discussion points out that evolution is driven by net formation including multiple nods of primary, secondary or tertiary endosymbiogenesis events.