Improvement of heat and desiccation tolerance in Heterorhabditis bacteriophora through cross-breeding of tolerant strains and successive genetic selection

Genetic selection can be a powerful tool to increase beneficial traits in biological control agents. In this study the heat and desiccation tolerance of the entomopathogenic nematode Heterorhabditis bacteriophora Poinar (Rhabditidomorpha: Strongyloidea) were significantly increased by cross breeding tolerant parental strains and successive genetic selection. These strains originated from a prior screening among 60 strains for increased stress tolerance. During genetic selection, the selection pressure was constantly increased and only the most tolerant 10% of the nematode populations were propagated for further selection steps. Assessment of tolerance and selection for both traits was performed with and without prior adaptation to the stress conditions. Eleven selection steps were performed to increase heat tolerance. A final overall increase in mean heat tolerance of 5.5°C was achieved when nematodes had been adapted to heat stress. For non-adapted tolerance an increase of 3.0°C from 40.1°C to 43.1°C was recorded. For comparison, a commercial strain had a mean tolerated temperature after adaptation of 38.2°C and of 36.5°C without adaptation. For assessment of the desiccation tolerance the mean tolerated water activity (aw-value) of a population was measured. Cross-breeding most tolerant strains reduced the aw-value from 0.67 to 0.65 after adaptation and from 0.9 to 0.7 without prior adaptation. The following six selection steps could not increase the tolerance whether nematodes had been adapted to stress or not. In comparison, the commercial strain tolerated a mean aw-value of 0.985 after adaptation and 0.951 without adaptation. Further investigation will have to assess trait stability and possible trade-off effects. This study is a first important step on the road towards domestication of the entomopathogenic nematode H. bacteriophora.