Population and evolutionary dynamics of recombining genes and alleles in bacteria

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PROJECT SUMMARY/ABSTRACT Species are conventionally defined as groups of individuals that breed with each other and produce fertile offspring, but not with those of other species (according to Ernst Mayr's Biological Species Concept). However, it is difficult to apply this eukaryotic definition of species to microbes because, even if they reproduce clonally, DNA may be acquired between strains and between species. Genetic recombination allows a bacterial cell to acquire novel traits through incorporation of DNA fragments from other organisms into its own genome. Recombination influences a myriad of evolutionary and population processes, including levels of standing diversity, niche expansion, spread of resistance and virulence determinants, and rapid adaptive changes in response to new or fluctuating environmental conditions. These processes are fundamental to questions critical to society and public health, such as whether an emerging disease is caused by a new species or variants of existing ones, what factors make a strain resistant or transmissible, and how a pathogen will respond to clinical interventions and host immune system. It is often assumed that all strains recombine at a uniform frequency and randomly across the entire species. However, recent work from the PI's lab show that recombination rates of strains of the same species vary along a continuum spanning several orders of magnitude, with a unique pattern of exchange for different strains and lineages. The causes and consequences of within-species variation in recombination is poorly understood, and therefore we still lack a coherent model for genome evolution that incorporates this variation. Filling in this gap in our knowledge of recombination has important ramifications for understanding species formation in microbes and the mechanisms that keep them separate once they begin to diverge. The goal of our proposed research is to elucidate how variation in recombination rates between strains impact population structure, genome evolution and speciation in microbes. Using a combination of comparative population genomics, laboratory experiments and mathematical modeling, we will answer the following questions: (1) To what extent does the probability of recombination influenced by the genetic distance and ecology of the parental strains? (2) How do different modes and genetic units of recombination vary across a species? (3) What are the evolutionary consequences of variable recombination rates in genome structure and divergence? Output from this research will help address the fundamental question in microbiology of whether species exist and if so, what processes keep them separate and distinct. The proposed research will also be a significant step forward to developing an evolution-based taxonomical framework and species boundaries for microbes. The results of the studies proposed in this application are expected to lead to other opportunities for fruitful cross-disciplinary research at the boundary of evolutionary biology, microbial genetics, computational biology and epidemiology.