Joel L. SachsAssistant Professor of Biology
Symbiotic bacteria are a key yet poorly understood facet of our natural world. Yet humans and our food sources often depend on bacterial cooperation for health and fitness. Current research is beginning to unravel the molecular and selective basis of bacterial cooperation. In the process we are discovering that beneficial infections are often evolutionarily unstable and more dynamic than expected. In the Sachs Lab we investigate the forces that shape bacterial cooperation with hosts as well as the origins and evolution of harmful strains. Our current focus is on rhizobial bacteria that live in soils throughout the world and nodulate the roots of many legume hosts. Projects in the Sachs Lab often utilize a wide range of techniques to answer basic questions. These include field collections of wild bacteria, experimental infections and experimental evolution, sequencing and phylogenetic analysis. Below are two broad research themes that we are currently investigating:
The evolutionary origins of uncooperative symbionts
Rhizobia are bacteria that fix nitrogen in legume roots in exchange for photosynthates from their hosts. However, non-beneficial rhizobia are widespread, including non-fixing strains that appear to be cheaters and non-nodulating strains that fail to infect hosts. Recent research has shown that legumes can punish some uncooperative rhizobia and substantially reduce their fitness, but these sanctions must not be universally effective. Important questions about uncooperative rhizobia remain unresolved. (i) Is it adaptive for rhizobia to be uncooperative with hosts? (ii) Do uncooperative rhizobia evolve from cooperative ancestors? (iii) What are the mechanisms of rhizobial exploitation? We are using experimental approaches as well as phylogenetic analyses to address these key gaps in our knowledge.
Microecology of symbiotic bacteria
Symbiotic bacteria often encounter hosts from environmental sources and can exhibit multiple life histories including host-inhabiting and environmental stages. Research on host-associated bacteria -- including pathogens and beneficial symbionts -- has primarily focused on infection of hosts. In the mean time, key questions about the ecology and evolution of free-living stages has remained unanswered. For instance, is host association ubiquitous within bacterial lineages, or do host-infecting genotypes represent subsets of environmental populations? Assuming that host infection and free-living existence exert different selective pressures, do diverged bacterial lineages result? Another set of questions addresses the degree to which bacteria associate with specific host partners. Do bacterial genotypes invariably associate with specific host lineages, and is such specificity based on control by one or both partners? Alternatively, is specificity a byproduct of ecological co-occurrence among bacteria and hosts? We have been using a combination of phylogenetic and population genetic approaches to tackle these questions.
Dr. Sachs participates in the Ecology and Evolutionary Biology tracks of the Evolution, Ecology and Organismal Biology graduate program (EEOB), as well as the graduate programs in Genetics, Genomics & Bioinformatics (GGB) and Cell, Molecular, and Developmental Biology.