Author ORCID Identifier

https://orcid.org/0000-0002-8810-225X

Date of Award

6-2024

Document Type

Thesis (Ph.D.)

Department or Program

Ecology, Evolution, Environment and Society

First Advisor

Dr. Olga Zhaxybayeva

Second Advisor

Dr. Mark A. McPeek

Third Advisor

Dr. Carey D. Nadell

Abstract

Life thrives across incredibly diverse environmental conditions, yet most organisms are restricted to growing within a narrow range around their optimum growth temperature (OGT). The evolutionary events leading to changes in OGT are poorly understood, and it is uncertain if specific genes are required to thrive at a particular temperature. The bacterial phylum Thermotogota is an excellent model for the evolution of OGT. It comprises mesophilic, thermophilic, and hyperthermophilic members that collectively grow between 20°C and 90°C.

In this work, I analyze the history of OGT in the Thermotogota phylum and show how horizontal gene transfer contributes to the evolution of OGT. I initially tested the phylogeny for topological artifacts and established a reliable reference phylogeny. An analysis of genes from Mesoaciditoagles showed that the deeply branching moderate thermophiles of the phylum contain a large proportion of genes that are more closely related to other moderate thermophiles in the phylum based on phylogenetic relationships. Next, I used ancestral sequence reconstruction to estimate that the OGT of the last common ancestor of Thermotogota was 66°C, followed by multiple OGT increases and decreases in descendants during Thermotogota diversification. Finally, I correlated the presence/absence of gene families with OGT using genome-wide association studies. I identified 158 gene families associated with OGT. While many of the OGT-associated genes are poorly characterized, most are cytoplasmic or membrane-bound, and many are involved in metabolism. I reconstructed the phylogenies of individual families and modeled their gain and loss throughout Thermotogota history. The genes were repeatedly gained and transferred within the phylum and are often more closely related to taxa outside the phylum. These findings suggest hyperthermophily is often a secondary adaptation arising from an ancestral phenotype of lower OGT. My results suggest that parallel gains via horizontal gene transfer may promote independent adaptations to similar OGTs across the phylum.

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