Immune responses are essential for survival, allowing animals to fight infections and adapt to disease threats. By studying the genes behind immunity, scientists can better understand how species evolve and persist in changing environments.
While immune systems are well studied in mammals and birds, reptiles β particularly sea turtles β remain less explored, leaving critical gaps in scientific understanding.
New research published in helps address this gap by examining the major histocompatibility complex (MHC), a critical group of immune system genes that enables organisms to recognize and fight diseases.
The study, which examined four species β loggerheads, green turtles, Kempβs ridleys and leatherbacks β found that most sea turtles maintain high levels of immune gene variation, likely inherited from a common ancestor. However, variation differs across species and different copies of these genes can function in distinct ways.
βSea turtles are an interesting case for studying immune system evolution,β says Katherine Martin β24PhD, an integrative conservation biology alum and postdoctoral researcher at Oregon State University who led the study. βThey live for a long time and encounter many different types of pathogens across multiple habitats.β
How MHC and Genetic Variation Work Together
MHC plays a key role in identifying and flagging pathogens for destruction by the immune system.
βMHC is essentially holding a small molecular flag that says to T cells, βThis is the invader that you need to seek and destroyβ,β says Martin, who specializes in immune system genetics in sea turtles.
Because pathogens vary widely, immune defenses must also adapt, creating strong evolutionary pressure for variation in MHC genes.
βFor each different pathogen, you need a different MHC protein,β Martin says. βYou can think of it kind of like a lock and key.β
Martin adds that immune gene variation is critical for population health and studying this builds insight on how well a population might respond to disease.
Key Findings and Evolutionary Insights
The study revealed differences in genetic variation across species, with leatherbacks showing lower MHC diversity than others.
βOne of the things that can contribute to low genetic variation is low population size,β Martin says. βWe think this might be the case with leatherbacks.β
Another key finding was the presence of shared genetic variants across species, suggesting deep evolutionary roots.
βThe results indicate that shared ancestry is the most likely explanation,β Martin says. βThat likely underscores their importance and their function.β
Martin also identified balancing selection as a key evolutionary force maintaining immune gene variation.
βInstead of selecting for a single trait, itβs the variation within that trait thatβs advantageous,β Martin says.
A Comparative Approach Across Species
βThe turtle species have different diets, habitats and disease prevalence, and [these samples] provided a useful comparison of the different ways of living that sea turtles have and how that might bear out in patterns of MHC variation.”
To establish a baseline for variations, Martin analyzed MHC genes from more than 300 turtles samples collected through and collaborators, highlighting the shared effort behind large-scale conservation research.
β[The turtle species] have different diets, habitats and disease prevalence,β Martin says. β[These samples] provided a useful comparison of the different ways of living that sea turtles have and how that might bear out in patterns of MHC variation.β
Martin extracted DNA from samples across coastal nesting sites, lagoons and offshore waters. She then amplified target genes and sequenced them using next-generation DNA sequencing technology.
βIn a single sequencing run, you can analyze multiple individuals all at once,β Martin says. βWe also get high sequencing depth, meaning each bit of DNA is sequenced multiple times.β
This approach improves accuracy, especially for highly variable genes like MHC.
Expanding Studies and Conservation Efforts
Martin plans to expand her research to additional sea turtle populations worldwide rather than just the northwest Atlantic, as well as to reptiles more broadly.
βI really love being able to ask questions about how that variation arises in the first place and what forces maintain it over time,β Martin says. Understanding immune gene variation has direct applications for conservation strategies, particularly as sea turtles face increasing environmental pressures.
βIf we protect the habitats these sea turtles rely on, we can bolster population sizes and, in turn, maintain genetic variation across all genes,β Martin says.
While advanced interventions such as gene editing may be possible in the future, Martin emphasizes that habitat protection remains the most practical and effective approach.
βThe most effective solution is public advocacy for [protection of] the natural world,β Martin says.
Funding and support for this research was provided in part by the Sea Turtle Grants Program funded from the proceeds of the Florida Sea Turtle License Plate, the Sigma Xi Grants in Aid of Research Program, the NOAA Oil Spill Supplemental Spend Plan, the Florida RESTORE Act Centers of Excellence Program administered through the Florida Institute of Oceanography and the National Fish and Wildlife Foundation.
Turtle handling conducted as part of permitted research (FL-MTP-225, FL-MTP-231, NMFS 19508, and predecessors).
This project was paid for in part with federal funding from the Department of the Treasury under the Resources and Ecosystems Sustainability, Tourist Opportunities, and Revived Economies of the Gulf Coast States Act of 2012 (RESTORE Act). The statements, findings, conclusions, and recommendations are those of the author(s) and do not necessarily reflect the views of the Department of the Treasury.
