Ask a friend how they feel about bacteria, and chances are they'll first think of 'germs' that cause illness. This is understandable, given the influx of new anti-microbial products that claim to make us safer. However, while some bacteria make their living as pathogens, many species perform a wide variety of beneficial functions -- e.g., nitrogen-fixation, food digestion, waste degradation, vitamin biosynthesis -- that are crucial to the health of other organisms and the environment. After 3.5 billion years of evolution, it's hardly surprising these unseen 'bugs' encompass most of Life's genetic and metabolic diversity and often play key roles in the biology of other organisms.

As an evolutionary biologist, I'm interested in understanding the processes that shape microbial variation and the wide array of interactions bacteria form with other species. I'm especially intrigued by endosymbiotic bacteria that live within the tissues or very cells of their hosts. By losing metabolic functions required in more variable external environments, these symbionts often become completely host-dependent. The radical lifestyle transition to endosymbiosis has happened several times in the bacterial world, with important implications for microbes and their hosts. Many insects have coevolved with bacterial partners for tens to hundreds of millions of years, and now rely these symbionts to produce essential nutrients. For example, aphids depend on their bacterial symbiont to produce essential amino acids that are low in plant sap. Much of the research in my lab focuses on such bacteria-insect symbioses as models to explore the ecological and evolutionary consequences of long-term species associations.

• How does an endosymbiotic lifestyle affect bacterial evolution on a genome level? We address this question by comparing bacterial mutualists of ants, aphids, and tsetse flies to each other and to their free-living bacterial relatives. Like genomes of other intracellular microbes, including obligate parasites, these insect mutualists have low G+C contents, accelerated evolutionary rates, and severe genome reduction. In fact, reductive genome evolution is most severe among nutritional endosymbionts that associate with many insect species and include the smallest bacterial genome known (0.45 Mb). We are attempting to untangle the contribution of three basic evolution processes – selection, drift and mutation – to these distinct genome features. Our projects range from inferring fitness effects of mutations using population genetic approaches, to reconstructing the metabolism of an endosymbiont genome that we've sequenced here in the Bay Paul Center.
  
  
• How do microbial associations shape the physiology and evolution of their hosts? Of course, endosymbiosis also impacts insect evolution by enabling hosts to exploit resources that would be inadequate otherwise. Using comparative genomics, we are exploring the implications of genome variation among endosymbionts on host ecology. A new project in the lab explores whether endosymbionts help their hosts cope with environmental variation. We are linking changes in the gene content and gene expression patterns of an ant mutualist called Blochmannia to natural environmental variation its hosts experience, including ecological variation among ant species and dramatic physiological variation among castes in a single ant colony.
  
  
• What genome traits underlie transitions between mutualism and parasitism? In collaboration with Seth Bordenstein, Assistant Scientist in the BPC, we are exploring lifestyle and genome evolution in a pervasive invertebrate endosymbiont, Wolbachia. This alpha-Proteobacterium acts as a reproductive parasite in arthropods but as a mutualist in certain nematodes. We are exploring the role of bacteriophage in Wolbachia genome plasticity through a combination of molecular phylogenetics, genomics, and quantitative assessment of phage and bacterial densities. In addition, an ongoing project aims to identify genes responsible for distinct host effects by comparing gene contents across diverse Wolbachia lineages.

Like many labs in the Bay Paul Center, I am delighted to pursue these and other projects as part of the new Brown-MBL joint graduate program, and invite interested students to contact Dr. Jennifer Wernegreen for more information.