Thesis Defense: 'Dietary Protein Content Influences Use of Microbially-Liberated Urea-Nitrogen in Summer-Active Arctic Ground Squirrels' on May 11

by UAA Biological Sciences  |   

Please join M.S. candidate in Biological Sciences, Sarah Gering, for a public thesis defense via Zoom beginning at 10:30 a.m. May 11.

Description: Ground squirrels are obligate seasonal hibernators that forage over a short active season in preparation for hibernation. During hibernation, which lasts for 5 to 8 months depending on species, the squirrels neither eat nor drink, but rather rely on fat stores accumulated during their active season to meet energy demands. Despite the inactivity and long fast of hibernation, squirrels emerge in the spring with little lean mass loss or muscle atrophy, and recent studies in 13-lined ground squirrels (Ictidomys tridecemlineatus) revealed that squirrels may rely on urea-nitrogen salvage to meet their nitrogen needs. Urea nitrogen salvage occurs when urea, produced by the liver and transported to the gut, is hydrolyzed by gut microbes, and the liberated urea-nitrogen is made available for both microbial and host use. The Duddleston lab is studying urea-nitrogen salvage in arctic ground squirrels (Urocitellus parryii), the northern most and most extreme of the hibernators, during hibernation as well as the summer active season. The objectives of my thesis were to determine the effect of dietary protein content and sex on a) incorporation of microbially-liberated urea-nitrogen into arctic ground squirrel tissues, and b) gut microbial diversity and community composition. I fed yearling active season squirrels diets comprising 18% and 9% protein for eight weeks, at which time I injected urea (13C/15N labeled or unlabeled) and collected breath and tissue samples to measure 13C and 15N enrichment, respectfully. I also collected samples from the squirrel gastrointestinal tract for microbial diversity analysis via 16S rRNA amplicon sequencing. The breath of squirrels was enriched in 13CO2 after urea injection, providing evidence of urea hydrolysis in the gut. Isotope analysis of tissues revealed δ15N enrichment of tissues in all squirrels, with greater enrichment of δ15N in several tissue types collected (heart, liver, skeletal muscle, and small intestine) in squirrels fed 9% protein compared to 18% protein. Although there were no significant differences in alpha or beta diversity of gut microbiotas among squirrels on different diets or of different sexes, the cecal compartments (lumen and mucosa) and small intestinal compartments contained site specific community composition and ureolytic indicator taxa. My results suggest that arctic ground squirrels’ gut microbes hydrolyze urea nitrogen and that they incorporate a greater amount of urea nitrogen into their tissues when consuming a lower protein diet, but the gut community does not differ significantly in composition as a response to diet or sex in active season arctic ground squirrels.

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