Evolution

Event Date: 
Wednesday, September 30, 2015 - 19:00 - 19:45
Institution: 
University of Southern California
Title: 

Microbial evolutionary surprises in the future ocean:  Long-term adaptation of marine nitrogen-fixing cyanobacteria to high CO2

Abstract: 

The globally-distributed marine cyanobacterium Trichodesmium plays a key role in ocean biogeochemical cycles, as it is a major source of newly fixed atmospheric nitrogen to marine food webs.  Trichodesmium N2 fixation rates have been shown to increase under expected future high carbon dioxide (CO2) levels in short-term studies due to physiological plasticity, but its long-term adaptive responses to ongoing anthropogenic CO2 increases are unknown. My lab has been carrying out a nearly decade-long experimental evolution study with Trichodesmium growing under selection by projected future elevated CO2 levels.  Unexpectedly, selection under high CO2 results in large increases in nitrogen fixation and growth rates that appear to be irreversible, even after adapted cell lines are moved back to lower present day CO2 levels for hundreds of generations. This represents an unprecedented microbial evolutionary response, as reproductive fitness increases acquired in the selection environment are maintained even after returning to the ancestral environment. These constitutive rate increases are accompanied by irreversible shifts in diel nitrogen fixation patterns, up-regulation of cellular energetic pathways, elevated expression of non-coding intergenic DNA, and increased activity of a potentially regulatory DNA methyltransferase enzyme. Ongoing work in my lab is examining the consequences of multiple nutrient limitation interactions (iron and phosphorus) for the physiology, biochemistry and genetics of Trichodesmium adapted to growing in a more nutrient-limited, acidified future ocean environment.  
 

Event Date: 
Wednesday, August 26, 2015 - 19:00 - 19:45
Institution: 
CSIRO
Title: 

The evolution of mutualistic trait variation in rhizobial symbionts across genetic and geographic scales

Abstract: 

Interactions between plants and nitrogen-fixing rhizobial bacteria are characterized by high genetic diversity for traits important to the outcome of the interaction at the population and species level. However, the selective processes underpinning the generation and maintenance of genetic and phenotypic variation in such interactions are not well understood. I will present an overview of data gathered from a series of experiments using interactions between Acacia spp. and their associated rhizobia, and that address questions regarding the ecological and evolutionary drivers of trait variation across different scales.  Specifically, I will discuss how 1) phylogenetic constraint; 2) within-species local adaptation; 3) nutrient availability; and 4) partner diversity and identity, influence patterns of specialization and community structure in legume-rhizobial mutualistic interactions. Our results suggest that both host-bacterial and bacterial-bacterial interactions are important for understanding evolutionary and ecological dynamics and highlight the importance of designing experiments that span different genetic and geographic scales.

Event Date: 
Wednesday, May 29, 2013 - 18:00 - 18:15
Institution: 
CSIRO
Title: 

Genotypic associations of Borrelia burgdorferi in mammalian and avian hosts

Abstract: 

Borrelia burgdorferi s. s., the bacterium that causes Lyme disease in North America, occurs as multiple co-circulating genotypes of the ospC gene, an important antigenic outer surface protein C. The diversity of ospC genotypes is thought to arise from fairly specific associations between genotypes and vertebrate hosts, such that different host species act as different “ecological niches” for the pathogen. To evaluate the degree of specificity of B. burgdorferi-host associations, we sampled genotypes of bacteria transmitted to ticks by several mammalian and avian host species. We also examined how the subset of genotypes known to infect humans, considered as human-invasive strains (HIS), is distributed among host species and higher taxonomic levels (birds, shrews, rodents). We adapted a patch occupancy model used for species detection to test for the occurrence probabilities (ψ) and transmission efficiencies (ε) associated with each ospC type. We found that the frequency of specific ospC genotypes varied among host species, demonstrating some support for the niche concept. Indeed, examination of all ospC types concurrently indicated that the ospC frequencies associated with birds and rodents were more similar within the taxonomic group than between groups based on principal components analysis. Conversely, HIS frequencies were more similar across all host species than within a host taxonomic level, implying weaker support for the niche concept. The occupancy model suggests that HIS types occurred more frequently than non-HIS types, but that HIS types generally had lower transmission efficiencies (from hosts to ticks). In several cases, rare genotypes had higher transmission efficiencies, suggesting an evolutionary trade-off between transmission efficiency and maintenance within the host. Our study highlights the importance of American robins (Turdus migratorius) and short-tailed shrews (Blarina brevicauda) in contributing large proportions of HIS types, and offers a novel way of examining occurrence and transmission efficiencies of ospC types within tick vectors, using a likelihood approach.

Event Date: 
Tuesday, January 25, 2011 - 19:15 - 20:00
Institution: 
University of California-Davis
Title: 

Gene flow and speciation in populations of bacteria and archaea from a genomic perspective.

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