Congratulations, Microbiologist Shawn Christensen: Best Dissertation

Congratulations to UC Davis outstanding scientist Shawn Christensen, a doctoral candidate and microbiologist who was just selected the 2024 recipient of the Merton Love Ecology and Dissertation Award, an annual award that celebrates the university's most outstanding doctoral dissertation in the fields of ecology and evolutionary biology.

A member of the Microbiology Graduate Group and the laboratory of community ecologist Rachel Vannette, associate professor and vice chair of the UC Davis Department of Entomology andNematology, he will deliver the Merton Love seminar from 3 to 4:30 p.m., Friday, May 24 in Room 1230 of Walker Hall.  The Zoom link:

The award memorializes Robert Merton Love (1909-1994), emeritus professor of agronomy and range science, who served on the UC Davis faculty from 1940-1976.

“Shawn's thesis work spans microbiology, ecology and evolutionary biology, combined with careful natural history, to document novel species interactions between hosts and microbial communities,”  said Vannette, who nominated him for the award. “Each chapter has broad implications for the ecology and evolution of host-microbe interactions. Shawn's work has already reframed the conditions under insect-microbe interactions are thought to hold relevance and evolve. His research has highlighted the utility of natural history observations of microbes and sampling understudied life history stages (overwintering developing insects).”

“Overall, Shawn's dissertation is an example of how detailed observations combined with fearless experimental dissection of interesting phenotypes can yield novel descriptions of species interactions that change the field's perception of when and where microbial communities are important,” she said.

“Shawn's first chapter describes adaptations of a flower specialist bacteria to acquiring resources from pollen—a nutrient source specific to flowers,” Vannette said. “Our lab was interested in the ecology of the flower-dwelling bacteria Acinetobacter, but Shawn took a new look at this bacterium, examining its growth morphology in nectar and in association with pollen. He noticed that this bacterium co-localized with pollen and grew exponentially more in the presence of pollen. To explore this phenotype, he designed new media and assays (microwaving pollen) to determine that Acinetobacter benefits the most from live pollen via stimulating pollen to germinate (within minutes!), then digesting it. Shawn then designed a series of experiments to examine if this ability was unique to this bacterial clade or shared among many floral microbes (it seems to be specific to Acinetobacter).”

In Christensen's last two chapters, he investigated the microbial communities of the solitary bee, Anthophora bomboides, a digger bee-bumble bee mimic which nests in the sand cliffs of Bodega Bay and Point Reyes.

“He collected brood cells (nectar and pollen balls along with developing larvae) from these locations at multiple points through bee development and examined not only the bacteria and fungal community composition through insect development, but also examined changes in microbial abundance at each life stage,” Vannette said. “In contrast to my (and the literature's) predictions, Shawn showed that microbial abundance peaks during larval overwintering, when solitary bees and other holometabolous insects are predicted to have voided their microbial gut communities. Instead, healthy A. bomboides hosts the highest abundance of fungi and bacteria during the fall—a wet season where pathogen abundance is also highest.”

Christensen received accolades and widespread media coverage for the first chapter of his dissertation, “Nectar Bacteria Stimulate Pollen Germination and Bursting to Enhance Microbial Fitness,” published in July 2021 in Current Biology.  His second thesis chapter on the microbes associated with Anthophora bomboides, has just been accepted for publication by the International Society of Microbial Ecology (ISME).

And more good news: Christensen is a" co-principal investigator on a newly awarded grant by JGI to sequence the genome of the yeast," Vannette said, "and explore its functional potential in renewable energy and is applying to continue to leverage this system's potential in antifungal chemistry and evolution of pathogen defense.”

Every time we see the nests of Anthophora bomboides stanfordiana on the sand cliffs at Bodega Head, we think of the scientists, including Shawn Christensen and Rachel Vannette, who study them. The late Robbin Thorp, UC Davis distinguished emeritus professor, UC Davis Department of Entomology and Nematology, researched them decades ago.

Check out PBS' Deep Look video, "This Bee Builds Sandcastles at the Beach," and you'll never go to Bodega Bay without thinking of these digger bees.