Interdisciplinary PhD in Molecular Biosciences

Dr. Sarah Bergemann - Research Interests

The classic interactions between plants and fungi is a widely recognized phenomenon and an example of one of the diverse interactions that may be central to understanding evolutionary patterns and processes between fungi and their hosts.? My research has linked molecular techniques, in combination with field and laboratory experiments, to understand the evolutionary and ecological processes governing the distribution of plant pathogens and mycorrhizal fungi including: (1) What are the contributions of historical, biological or ecological factors that promote population divergence and speciation? (2) How do biotic interactions affect genetic structure of populations? (3) Are patterns of divergence linked to past ecological or selective pressures?

During the last three years at MTSU, the research in the my lab has focused on three areas: 1) a collaborative project on the worldwide distribution of Armillaria mellea species that seeks to identify the routes of introduction, divergence between geographic species, and the processes involved in shaping the genetic history of A. mellea; 2) the Moorea Biocode Project, a genetic inventory of the fungi of Moorea (French Polynesia) and; 3) the diversity of entolomatoid fungi in western Australia.?

The forest pathogen Armillaria mellea (Basidiomycota, Physalacriaceae) is among the most significant forest pathogens causing root rot in northern temperate forest trees worldwide. A present, we are working on several projects aimed at understanding the evolution history of A. mellea with particular focus on identifying mechanisms of varying levels of variation from worldwide populations.? In one study, we compared levels of genetic within and between eastern US using variable microsatellite markers.? We found evidence of genetic structure consistent with phylogenetic divergence.? Although there were no apparent restrictions to gene flow within each region, Bayesian assignment tests implemented in STRUCTURE identified contrasting patterns of intra-regional genetic diversity: i) western populations formed a single deme and, ii) eastern populations showed population structure, evidenced by three demes or clusters, in the absence of assignment based on geographic origin.? It appears that a potential source of alleles into the southeastern US may be from Mexico.? Our analyses of isolates from self-fertile strains that produce unusual, diploid spores from Africa and Japan indicate a shared history, likely mediated by transport of infected plants, and are the result of hybridization between lineages that originated in Europe and China.? At present there are also several questions that remain unanswered including the major routes of colonization from the ancestral population, and the maintenance of two divergent, mitochondrial haplotypes in western North America.? These questions, along with understanding the evolution of functional gene complexes in A. mellea, will be possible with the draft release of the A. mellea genome (see http://www.jgi.doe.gov/genome projects/pages/projects.jsf?kingdom=Fungi) for more information about the JGI genome project).

The Moorea Biocode project involves the complete genetic inventory (barcoding) of the terrestrial and marine macrobiota in French Polynesia. My current work on the project includes: (1) surveys of macrofungi (i.e., mushroom-forming fungi); (2) analyses of environmental samples (soil, leaves and wood of living plants, decaying litter and wood) for characterization of fungal communities using culture isolation and analysis of DNA clone libraries; (3) analyses of fungal communities associated with invasive plant species; (4) comparisons of endophytic fungal communities associated with dominant plant species across a land-use gradient ranging from agricultural plantations and agroforestry to endemic plant communities; and (5) characterization of mycorrhizal symbionts of an introduced tree species, Pinus caribaea.

The last project is a collaboration with Dr. Dave Largent (former master’s advisor at Humboldt State University) to understand the diversity of entolomatoid fungi in western Australia.? At present, we are currently utilizing morphological? and molecular methods to characterize species diversity within the entolomatoid fungi.? Future projects include the integration and development of additional protein coding loci to understand the infrageneric relationships within this entolomatoid fungi.