Rising temperatures favour parasite virulence and parallel molecular evolution following a host jump

Climate change is facilitating the poleward emergence of parasites, increasing the risk of jumping into new animal species, including humans. Whether more virulent or transmissible variants will spread during these climate-driven outbreaks is unclear. We experimentally evolved a wild parasitic bacterium, across the thermal range (20-30°C) and extremes (35°C) of Cape Verde - the site of field collection - in a novel, temperate animal host. At the parasite's typical warm environmental temperature, we found that virulence escalated across evolutionary time. Parasites evolved at hot temperatures, towards the limit of host-parasite survival, displayed a 'cryptic' virulence boost, deadlier once infecting animals at cooler temperatures. Patterns of molecular evolution were constrained to parallel changes in fewer loci at extreme temperatures. Our findings suggest that rising temperatures will leave predictable phenotypic and genomic signatures on evolving parasites as they emerge with climate change.