Rebecca Welch

  1. Evolution within an ecological time frame

  2. Species interactions as drivers of evolutionary process

  3. Dynamics and mechanisms of bacterial adaptation in heterogeneous environments

  4. Evolution of cooperation and the constraints of history

  5. Genetic mechanisms of conflict resolution in generalist symbioses

  6. Genomic plasticity involved in host switching and multi-host competence

  7. Dispersal, community structure, and gene flow in Nostocalean cyanobacteria

  8. Interactions in multi-species biofilms

  9. Spatial structure and divergence in clonal organisms

General Interest

Dissertation Project

I study the symbiosis between Nostocalean cyanobacteria and their bryophyte hornwort hosts.  In this nutritional symbiosis, hornworts fix carbon by photosynthesis and cyanobacteria fix dinitrogen, which they then exchange. Because fixed nitrogen is required to form the photosynthetic enzyme rubisco, and since most hornworts occur in nitrogen-limited environments such as soil banks, this symbiosis is, for hornworts, an essential one.  It is less clear however, what net benefits the cyanobacteria gain from the interaction, as they have the metabolic capacity to fix both carbon and nitrogen and can live freely without symbiotic partners.  Nostoc also occurs in symbiosis with a wide variety of host taxa including cyanolichens, cycads, and the angiosperm Gunnera. 

The Nostoc /hornwort symbiosis appears to be a generalist one, with low specificity of interaction between host and symbiont, with hornwort hosts, sometimes within a single thallus, containing a variety of symbiont genotypes in the genus Nostoc, (family Nostocaceae), and occasionally Calothrix (family Rivulariaceae).  Such lack of specificity on the part of both host and symbiont is theoretically surprising and raises a suite of questions.

•How is cooperation maintained when host and symbiont lack fidelity and consequent coevolution between the lineages?

•What are the relative fitness benefits received and costs incurred for both host and symbiont in this interaction?

•Does Nostoc’s jack-of-all-trade ability to switch between a variety of hosts and lifestyles have fitness consequences for their partners?  For themselves?

•Are assemblages of Nostocalean genotypes correlated with habitat, host, or resource-niche?  (i.e., do they display any degree of specialization?)

•What mechanisms allow Nostocalean cyanobacteria to thrive in the widely different hosts and lifestyles in which they are found?

•Are there genomic signatures of such lifestyle flexibility?

I’m currently studying this interaction at several different scales: the historical, through phylogenetic analysis and comparative genomics; at the population and community levels through population genetic and community analysis; and at the level of the individual in a cross-inoculation fitness experiment.  My primary field sites are in the Smith River National Recreation Area, Six Rivers National Forest, Del Norte County, California.


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