Rapid Changes in Thermal Sensitivity of Entomopathogenic Nematodes in Response to Selection at Temperature Extremes

Entomopathogenic nematodes (Rhabditida: Heterorhabditidae, Steinernematidae) are lethal insect parasites that have a symbiotic association with bacteria in the family Enterobacteriaceae. We evaluated the changes in thermal sensitivity of two nematodes Heterorhabditis bacteriophora and Steinernema anomali. The nematodes were genetically selected, together with their respective symbiotic bacteria, at nematodes' reproductive thermal niche breadth extremes, 15° and 30°C, by repeated passage through larvae of the wax moth Galleria mellonella. Nematode virulence (rate of insect mortality), establishment, and reproduction were evaluated after 12 passages (equal to 24-36 generations). Direct and correlated responses of the selected strains were compared with the ancestral strains, maintained at 25°C, at the selection and novel temperatures. The thermal limits for virulence and establishment were extended in both species and at both selection regimes. The thermal reproductive niche breadth of H. bacteriophora was extended from 15°-30°C to 12°-32°C after selection at either 15° or 30°C, but that of S. anomali remained unchanged. Virulence of S. anomali was enhanced across the entire thermal niche breadth, but H. bacteriophora improved in one portion of the thermal niche and declined in another portion. Enhanced virulence of S. anomali was most likely due to the improvements in growth rate of the symbiotic bacteria, Xenorhabdus sp. Establishment of both species and reproduction of H. bacteriophora improved at most temperatures. S. anomali reproduction showed no direct response to temperature selection, but decrements and/or improvements were evident at novel temperatures. Our results demonstrate that the temperature-specific virulence and thermal niche breadth of entomopathogenic nematodes are malleable. Adaptation to either cold or warm temperature enhanced fitness in both cold and warm environments. S. anomali adapted to novel temperature by enhancing virulence, whereas H. bacteriophora improved fecundity. Trade-offs in fitness were rarely observed.