Speaker: Ben Kerr, Professor, University of Washington

The passage of genes from parents to offspring is a fundamental rule of heredity. However, bacteria violate this rule of strict vertical inheritance by shuttling DNA between unrelated cells through horizontal gene transfer (HGT). Common vehicles for HGT are conjugative plasmids, extrachromosomal pieces of DNA encoding the machinery for their own transfer. In addition to standard vertical transmission, genes on such plasmids can move between different strains or even different species of bacteria. This partial “uncoupling” of the evolutionary trajectory of plasmid-borne genes from the evolution of their host has interesting consequences for the ecology and evolution of mobile genes, which will be the focus of this presentation. First, I will discuss some challenges in measuring the rate of transfer, and I will present a novel estimate derived from a branching process framework and implemented through an adjustment of the classic fluctuation analysis of Luria and Delbrück. In particular, I will discuss how this new approach opens up unbiased estimates of the rate of HGT between bacterial species. Second, I will discuss the impact of cross-species HGT on protein evolution, focusing on a gene encoding an enzyme that degrades a ubiquitous class of antibiotics. In the process, we will revisit Sewall Wright’s fitness landscape metaphor. The degree of topographical alignment of landscapes for a mobile gene across different bacterial hosts can influence the nature of genetic evolution in the presence of HGT. Furthermore, such alignment affects the potential for adaptation to be “crowdsourced” to different members of a microbial community. This work offers perspectives on the spread of antibiotic resistance within bacterial communities as well as why certain genes may embrace a “mobile lifestyle” via HGT.