Genome Size Influences Ranaviral Disease Development and Survival in a Unisexual Salamander Species

Amphibians are the most imperiled vertebrate taxa on the planet, and ranaviruses constitute a serious threat to wild and captive populations. Ranaviral disease outbreaks have been documented globally in amphibians, leading to population declines and local extinctions. Despite strong evidence that genetic factors influence susceptibility and disease severity, this area remains understudied – especially in urodeles. To address the lack of information concerning genetic factors of ranaviral disease in salamanders, we experimentally challenged larvae from a clonally reproductive salamander (Ambystoma platineum, N = 27), the sperm-host (Ambystoma texanum, N = 50), and tetraploid hybrids (A. platineum x texanum, N = 9) with an FV3-like ranavirus isolated during a prior outbreak at the wetland from which egg masses were collected. The challenge (dose administered as a three day immersion in 103 PFUs) began 30 days after the last larvae hatched and lasted for an additional 30 days. We monitored animals daily for clinical sign development (hemorrhaging, edema, skin sloughing). We estimated viral load via qPCR using DNA extracted from tail clips, and distinguished A. platineum x texanum hybrids from triploid A. platineum by PCR amplifying a microsatellite region that is only present in A. texanum. We observed an 89% mortality rate in exposed animals and found that A. platineum x texanum hybrids experienced reduced mortality risk (OR -3.39, p < 0.05) and were less likely to develop clinical signs than triploid A. platineum (p < 0.05). Rates of infection and levels of viral load were comparable between the tetraploid hybrids and triploid A. platineum and were not good predictors of survival using Cox Proportional hazards models (p > 0.05). These results indicate that genetic contribution in A. platineum x texanum hybrids was beneficial for animals exposed to an FV3-like ranavirus and emphasize the importance of studying genetic factors in disease dynamics.