Date of Award

1-2026

Document Type

Thesis

Degree Name

Master of Science (MS)

College/School

College of Science and Mathematics

Department/Program

Biology

Thesis Sponsor/Dissertation Chair/Project Chair

Kevin Bilyk

Committee Member

Kirsten Monsen

Committee Member

Charles Du

Abstract

Notothenioid fishes represent one of the most compelling examples of adaptive radiation driven by extreme environmental change, having diversified in the chronically cold waters of the Southern Ocean. While most cryonotothenioids evolved highly specialized cold-adapted characteristics, several secondarily temperate species later re-invaded warmer environments. These lineages provide a rare opportunity to examine evolutionary reversibility and the genomic mechanisms enabling escape from extreme specialization. In this study, we investigate two secondarily temperate species, Dissostichus eleginoides and Champsocephalus esox, comparing their protein-coding orthogroups to background sets from Antarctic cryonotothenioids and basal temperate teleosts. Using phylogeny based evolutionary models (BUSTED, aBSREL, and RELAX) and Gene Ontology enrichment analysis, we identify signatures of both positive and relaxed selective pressure across metabolic, structural, immune, and sensory pathways. Our results reveal that secondarily temperate species exhibit a mosaic genomic remodeling; some cold-specialized biological systems undergo relaxation or replacement, whereas some tied to membrane integrity, ion transport, and cellular organization retain Antarctic-derived features. Together, these findings highlight the complex selective landscape accompanying thermal transitions and demonstrate that even extreme physiological specialization does not preclude subsequent evolutionary flexibility. This work contributes a comparative genomic framework for understanding reversibility in cold adaptation and the molecular basis of secondary temperate evolution.

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