Grant: 22-019R
Project Title: Immunogenetics and pathogen drivers of fibropapillomatosis in stranded sea turtles
Project Manager: Erin E. Seney, Ph.D.
Organization: University of Central Florida (Research and Educational Institute)
Grant Amount: $15,059.00
Completion Date: 2024-12-10
Summary: The infectious tumor disease fibropapillomatosis (FP) occurs widely among sea turtle species and is especially prevalent in green turtles (Chelonia mydas). FP is associated with chelonid alphaherpes virus 5 (ChHV5). Consequences of FP range from mortality to complete tumor regression, and not all turtles with ChHV5 have active FP tumors. Differences in FP occurrence between and within sea turtle species remain understudied and poorly understood but likely relate to genetic differences in FP susceptibility due to high variability of immune genes and could also arise from gut microbiome differences. Thus, conservation actions aimed at mitigating sea turtle disease could benefit dramatically from genetic studies relating FP occurrence and severity to viral infection, genetic diversity, microbiome composition, and candidate resistance genes. The underlying genetic mechanisms contributing to FP in Florida's sea turtle populations should be identified, with key genetic indicators used to improve and develop the management and conservation approaches needed to treat, prevent, and mitigate FP. Despite a wealth of evidence that immune genes underlie disease susceptibility in wildlife populations and that gut bacterial communities impact host health, we know little about how genetic factors or microbiome composition impact turtle susceptibility to ChHV5 infection and development of FP. To address this knowledge gap, we will analyze over 200 kidney samples collected in 2017-2019 from stranded green turtles, loggerheads (Caretta caretta), and Kemp's ridleys (Lepidochelys kempii) to quantify herpesvirus prevalence and intensity and to examine immune gene diversity to better inform conservation, management, and rehabilitation. 
Results: Major histocompatibility complex (MHC) immune genes comprise one of the most diverse gene families in vertebrates. Several non-mutually exclusive evolutionary mechanisms, including pathogen-mediated balancing selection, favor high allele number and sequence diversity within individuals and populations. Similar and even identical MHC alleles are found in species millions of years diverged, in a phenomenon known as trans-species polymorphism (TSP). Two primary scenarios can produce TSP: 1) maintenance of ancestral polymorphism in descendant species via balancing selection, and 2) convergent evolution of similar or identical alleles in different species that confer the same pathogen resistance. Immune genes like MHC are ideal candidates for understanding selective processes that shape genetic variation across millions of years of diversification, especially in disease-threatened taxa. Sea turtles are also ideal species for studying immune system evolution given their long lifespans, worldwide conservation threats, and species-level susceptibility differences to the viral-associated tumor disease fibropapillomatosis. However, sea turtle immunogenetic variation is minimally explored. We sequenced the ?1 peptide-binding region of MHC class I genes from 88 loggerhead (Caretta caretta), 268 green (Chelonia mydas), 65 leatherback (Dermochelys coriacea), and 35 Kemp's ridley (Lepidochelys kempii) sea turtles. We reconstructed phylogenetic relationships among all sequenced MHC alleles and recovered extensive allelic variation, evidence of TSP, and significant evidence of positive selection shaping allele evolution. We present simulation analyses assessing whether identical alleles arose from common ancestry or convergent evolution. Our study is the first comparative MHC analysis across turtle families, providing insight into the evolutionary forces shaping immunity in reptiles more broadly.