Date Thesis Awarded

5-2018

Document Type

Honors Thesis

Degree Name

Bachelors of Science (BS)

Department

Biology

Advisor

Helen Murphy

Joshua Puzey

Jelena Pantel

Leah Shaw

Abstract

Microorganisms were once assumed to live as solitary cells interacting with just their physical environment; however, like most multicellular organisms, they have been found to engage in complex social behaviors that play an important role in their ability to survive and reproduce. Unlike competitive behaviors, such as chemical warfare and antibiotic production, cooperative behaviors have been more challenging to explain from an evolutionary perspective. In multicellular organisms, most cooperative behaviors can be explained by kin selection and kin recognition. In clonally growing microbes, cooperative behaviors involving cells adhering to one another generally rely on “kind” recognition, whereby a single locus or trait, referred to as a greenbeard, is enough to signal cooperation.

Microbes frequently rely on membrane-associated proteins with variable extracellular domains to recognize one another. In the budding yeast, Saccharomyces cerevisiae, FLO11, a highly regulated gene, encodes a cell-surface adhesin that allows individual cells to attach to one another and plays a crucial role in social phenotypes, such as the formation of biofilms and other structured communities, which are critical to survival during rapid environmental changes.

To characterize the amount of genetic variation at FLO11, its regulatory and coding regions were amplified and sequenced in 78 environmental isolates that vary in their social phenotypes; de novo assemblies of the locus were generated. Population genetic analyses suggest that the precise regions implicated in cell-cell adhesion exhibit a signature of positive selection, while the rest of the gene is under purifying selection. Furthermore, a region of the upstream regulatory sequence exhibits a signal of balancing selection.

Phenotypic assays demonstrate that different natural FLO11 alleles generate diverse biofilm architectures in an otherwise constant genetic background. These assays also shed light on the complex role that natural regulatory variation and variegated expression patterns may play in the outcome of inter-clonal competitions.

Our results suggest a "shades of greenbeard" system in which Flo11p preferentially adheres to like kinds. Unlike in motile microbes where cheater avoidance is likely driving the evolution of recognition, homophillic binding of Flo11p may be selected during competition among clones. Thus, the interplay between inter-clone competition and intra-clone cooperation in spatially structured microbial communities can potentially lead to recognition systems.

Available for download on Sunday, May 03, 2020

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