Chris Zhang, QBioS Thesis Proposal

Quantitative Biosciences Thesis Proposal
Christopher Zhang
School of Biological Sciences

Reexamining Fundamental Assumptions of the Type VI Secretion System
Monday, September 11, 2023, at 10:00 am
In Person Location: Marcus Nano 1116
Zoom Link: https://gatech.zoom.us/j/6787018468
Open to the Community

Advisors: 
Dr. Brian Hammer (School of Biology)
Dr. William Ratcliff (School of Biology)

Committee Members:
Dr. Sam Brown (School of Biological Sciences)
Dr. Peter Yunker (School of Physics)

Abstract:
The Type VI Secretion System (T6SS) is a key bacterial weapon that has been intensely studied, yet critical knowledge gaps remain regarding its prevalence and fitness costs in nature. This talk will challenge two major assumptions about the T6SS through bioinformatics, precise fitness measurements, and experimental evolution.

First, it is often stated that 25% of gram-negative bacteria possess the T6SS based on early genomics studies. However, I hypothesize this figure is an overestimate due to dataset biases. Through comprehensive genomic re-analysis, I will generate a more accurate estimate of T6SS frequency across bacteria. Second, the T6SS is assumed to be metabolically costly, however, I will present direct evidence showing negligible T6SS fitness costs, implying other selective forces shape its evolution.

Finally, I will introduce the concept of "evolutionary opportunity costs" as a novel mechanism constraining complex traits like the T6SS. Despite no measurable growth defects, dedicating resources to the T6SS may limit future evolutionary routes. Experimental evolution comparing T6SS+ and T6SS- strains will provide the first evidence for such opportunity costs in bacteria. Overall, this work will overhaul current assumptions about a major bacterial weapon and provide general insights into the evolutionary constraints on microbial competition.