From genetic management to genetic rescue: Quantifying outbreeding depression thresholds in threatened Chiricahua Leopard Frogs (Lithobates chiricahuensis)

Ball State University, Muncie IN

Investigators

Abstract

This project will provide a scientific roadmap for saving threatened species from extinction. As animal populations shrink and become isolated, they lose the genetic diversity that is essential for their long-term survival. A powerful conservation tool called "genetic rescue" can restore this diversity by mixing individuals between isolated populations. However, this action carries a risk: if the parent populations are too genetically different, their offspring may be less healthy or less able to survive, a problem known as "outbreeding depression." Because of this, conservation managers and organizations have been hesitant to use this strategy. This project will study the risk of outbreeding depression in the threatened Chiricahua Leopard Frog in Arizona and New Mexico. By carefully breeding frogs from populations with different levels of genetic relatedness, the researchers will identify the optimal genetic distance that boosts population health without negative effects. The project will also test if these genetic crosses produce frogs that are more resistant to a deadly fungal disease that threatens amphibians globally. The results will give wildlife managers the genetic rescue data they need to recover this frog species and will create a pathway for saving other endangered species using this method. This project also provides hands-on research training for university and high school students and will share the story of amphibian conservation with the public through a professional documentary film. This research will conduct a comprehensive, experimental assessment of outbreeding depression in a federally threatened species, the Chiricahua leopard frog (Lithobates chiricahuensis; CLF). Using a recent, rangewide genomic analysis that identified four primary genetic clusters and finer-scale subdivisions, the researchers will implement a multi-year, controlled breeding program at two field stations in Arizona and New Mexico. The experimental design involves a series of hierarchical crosses between CLF populations of decreasing relatedness, from within-population controls to maximally divergent inter-cluster pairings. Researchers will measure key fitness proxies in the resulting offspring, including egg hatching success, survival to metamorphosis, larval growth rate, and size at metamorphosis, to establish any threshold at which outbreeding depression occurs. To assess the applicability of these outcomes in a natural context, juveniles from successful crosses will be introduced into monitored wild populations. The project team will use capture-mark-recapture (CMR) methods for demographic monitoring and genome-wide polymorphisms for genomic surveillance, tracking changes in allele frequencies and population size over time. Finally, the project will evaluate if genetic mixing impacts disease resistance by conducting standardized laboratory inoculation experiments with the lethal amphibian pathogen Batrachochytrium dendrobatidis (Bd) on offspring from select crosses. This multi-faceted approach, integrating controlled experiments with in-situ monitoring and disease challenges, will directly inform adaptive management strategies for the CLF and provide a transferable framework for applying genetic rescue in other species of conservation concern. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.