Characterizing immune gene diversity and function is paramount to understanding population adaptive potential and species survival in the face of selective pressure from infectious disease. In this dissertation, I examined immune gene evolution and function using sea turtles to better understand the selective forces shaping reptile immunogenomic diversity. I focused on four species from the northwest Atlantic and Gulf of Mexico: loggerheads (Caretta caretta), green turtles (Chelonia mydas), leatherbacks (Dermochelys coriacea), and Kemp’s ridley sea turtles (Lepidochelys kempii). In Chapter 2, I characterized MHC class I (MHCI) evolution and diversity in Ca. caretta and Ch. mydas, and evaluated associations between Ch. mydas MHC and the infectious tumor disease fibropapillomatosis (FP). I found weak associations between MHCI and FP, suggesting a potential relationship with viral antigen presentation, but that other immune loci and environmental factors may play larger roles in disease susceptibility. I also found that sea turtle MHCI evolves adaptively in Ca. caretta and Ch. mydas. In Chapter 3, I quantified selection and diversity in all four species at three MHC peptide-binding regions: MHCI, class II (MHCII) loci
within the MHC region, and at an isolated MHCII gene on a different chromosome. I found that shared MHC polymorphism is maintained by balancing selection, and that MHC diversity is extremely low in D. coriacea and in the unlinked MHCII gene copy. In Chapter 4, I characterized gene expression in Ca. caretta and Ch. mydas with and without FP. In the blood and skin of individuals with FP in both species, there were few DEGs and similar pathways were enriched, although regulation direction was different in skin. While inflammation and immune processes were active in tumors of both species, Ca. caretta tumors had increased cell division while cell division and angiogenesis were inhibited in Ch. mydas tumors. Overall, I found classical patterns of immune gene diversity and evolution in some species and gene copies, but dramatically low diversity in others. I also explored functional immune diversity and identified candidate disease response pathways in two species. The data herein provide a strong foundation for future work on immune function and evolution in turtles and reptiles more broadly.
Katie Martin
Advisors: Dr. Savage and Dr. Mansfield
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